Developments in Palygorskite-Sepiolite Research: A New Outlook on these Nanomaterials (Developments in Clay Science 3)

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Author: Emilio Galan | Arieh Singer

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Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands First edition 2011 Copyright

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2011 Elsevier B.V. All rights reserved

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Contents

Contributors

)�XV

List of Reviewers Dedication

xix xxi

Preface

XXV

Part I

Advances in the Structure and Chemistry of Sepiolite and Palygorskite 1. The Structures and Microtextures of the Palygorskite-Sepiolite Group Minerals

3

Stephen Guggenheim and Mark P.S. Krekeler 1.1 Introduction

3

1.2 Part 1. Structure-Related Topics

4

1 .2.1

Structural Characteristics of the Palygorskite-Sepiolite Mineral Group

1 .2.2 Species and Nomenclature

4 6

1 .2.3 Structure of the Palygorskite-Sepiolite Mineral Group

and Related Minerals 1 .2.4 Substitutions in Palygorskite and Sepiolite

7 14

1 .2.5 H20, OH2, and OH Positions in Palygorskite and Sepiolite

15

1 .2.6 Genetic and Synthesis Relations

16

1.3 Discussion of Structure-Related Topics 1 .3.1

Structure Parameters Described

18 18

1 .3.2 Structure Parameters and Tetrahedral-Octahedral Misfit

18

1 .3.3 If Not Misfit, Why Do Polysomes Form?

21

1.4 Part 2: Microstructure-Related Topics 1 .4.1

22

Polysome-Width Disorder and the Transformation of Palygorskite to Smectite

22

1 .4.2 Open Channel Defects

26

1 .4.3 Stacking Errors and Planar Defects

28

Acknowledgements

29

References

29

v

�

Contents

2. Advances in the Crystal Chemistry of Sepiolite and Palygorskite

33

Mercedes Suarez and Emilia Garda-Romero 2.1 Introduction

33

2.2 Chemical Composition of Sepiolite

36

2.3 Chemical Composition of Palygorskite

44

2.4 Is there a Compositional Gap between Sepiolite

and Palygorskite?

49

2.5 Possible Structural Arrangements of the Intermediate Minerals

51

2.6 Open Questions

55

References

55

Part I I

Palygorskite-Sepiolite Geochemistry, Genesis and Deposits 3. Environmental Influences on the Occurrences of Sepiolite and Palygorskite: A Brief Review

67

69

Blair F. }ones and Kathryn M. Conko 3.1 Introduction

69

3.2 Spain

71

3.3 Turkey

73

3.4 Kenya

75

3.5 Morocco

76

3.6 Tunisia

76

3.7 Senegal

76

3.8 South Africa

77

3.9 Somalia

78

3.10 Argentina

78

3.11 United States

78

3.12 Summary

79

References

81

4. An Introduction to Palygorskite and Sepiolite Deposits- Location, Geology and Uses

85

Haydn H. Murray, Manuel Pozo, and Emilio Galan 4.1 Introduction

85

4.2 United States

87

4.2.1

South Georgia-North Florida Deposit

4.2.2 Amargosa Deposit, Nevada 4.3 Spain 4.3.1

87 89 90

Vallecas-Vicalvaro-Yunclillos Deposit (Tagus Basin)

90

Con tents

4.3.2 Mara Deposit (Calatayud Basin)

92

4.3.3 Bercimuel Deposit (Duero Basin)

92

4.3.4 Torrejon Deposit (Torrejon el Rubio Basin) 4.4 Senegal 4.4.1 Theis Deposit 4. 5 Turkey 4.5.1

Eskisehir Deposit

4.6 Other Worldwide Deposits 4.6.1

China

4.6.2 Greece

93 93 93 94 95 95 95 95

4.6.3 Ukraine

96

4.6.4 Other Occurrences

96

4.7 Summary

97

Acknowledgements

97

References

97

5. Palygorskite Clays in Marine Sediments: Records of Extreme Climate

1 01

Medard Thiry and Thomas Pietsch 5.1 Introduction

1 01

5.2 History of Deep-Sea Palygorskite Research

1 02

5.2.1

Palygorskite Clay in Continental Environments

1 02

5.2.2 Discovery of Palygorskite Clay in Marine Deposits

1 02

5.2.3 Widespread Palygorskite Clay in Deep-Sea Formations

1 02

5.2.4 Deep-Sea Palygorskite Clay Formation in Relation

to Palaeoceanography 5.3 Cretaceous Palygorskite Clay in the Central Atlantic 5.3.1

Occurrence of Palygorskite

1 03 1 04 1 OS

5.3.2 Clay Mineral Assemblage Distribution

1 08

5.3.3 Significance of Palygorskite

1 09

5.3.4 Geochemistry of Cretaceous Deep Water

1 09

5.3.5 Palaeoceanography and Palaeogeography

110

5.3.6 Conclusions

111

5.4 Early Eocene Palygorskite Clay in Gulf of Guinea and

Sargasso Sea 5.4.1

Gulf of Guinea

112 112

5.4.2 Sargasso Sea

113

5.4.3 Distribution of Early Eocene Palygorskite Clay

115

5.4.4 The Palaeoenvironment

117

5.4.5 Conclusions

117

5.5 Perspectives for Marine Palygorskite Research

118

Acknowledgements

118

References

119

Contents

6. Palygorskite and Sepiolite Deposits in Continental Environments. Description, Genetic Patterns and Sedimentary Settings

125

Emilio Galan and Manuel Pozo 6.1 Introduction

1 25

6.2 Genetic Conditions

1 30

6.2.1

Experimental and Natural Evidence of Sepiolite Formation

1 31

6.2.2 Natural and Experimental Evidence of

Palygorskite Formation

1 35

6.3 Main Genetic Characteristics of the Worldwide Deposits

and Occurrences of Sepiolite and Palygorskite in Continental Sedimentary Environments 6.3.1

Spain

1 37 1 37

6.3.2 Turkey

1 54

6.3.3 Other Deposits and Occurrences

1 57

6.4 Final Remarks 6.4.1

1 59

Continental Sedimentary Environments for Sepiolite and Palygorskite Formation

1 59

6.4.2 Origin of Deposits with Sepiolite and Palygorskite:

Lithological Associations

1 63

Acknowledgements

1 66

References

1 66

7. Sepiolite-Palygorskite Occurrences in Turkey

1 75

Hi.iseyin Yalc;in and Omer Bozkaya 7.1 Introduction

1 75

7.2 Geology and Mineralogy

1 75

7.2.1

Marine Sepiolite-Palygorskite Occurrences

1 77

7.2.2 Lacustrine Sepiolite-Palygorskite Occurrences

1 78

7.2.3 Hydrothermal Sepiolite Occurrences

1 84

7.2.4 Pedogenic Palygorskite Occurrences

1 85

7.3 Geochemistry

1 87

7.4 Genesis

1 87

7.5 Economy

1 94

7.6 Conclusions

1 94

Acknowledgements

1 97

References

1 97

8. Genesis and Distribution of Palygorskite in Iranian Soils and Sediments

201

Saeid Hojati and Hossein Khademi 8.1 Introduction

201

8.2 Palaeogeography and Palaeogeology of Iran

202

8.3 Climate of Iran

203

Con tents

8.4 Palygorskite in Western Iranian soils

204

8.5 Palygorskite in Central Iranian Soils and Sediments

205

8.6 Palygorskite in Southern Iranian Soils and Parent Rocks

208

8.7 Palygorskite in North-Eastern Iranian Soils

21 3

8.8 Conclusions

21 4

8.9 Direction for Future Research

21 5

Acknowledgement

21 6

References

21 6

9. Evidence for the Biogenic Origin of Sepiolite

219

jaime Cuevas, Santiago Leguey, and Ana I. Ruiz 9.1 Introduction 9.1.1

The Processes of Biomineralization

21 9 221

9.2 Biomorphs in the Dolomite-Sepiolite Sediments from the

Miocene in the Madrid Basin 9.2.1

Recognition of Structures Reminiscent of Microorganisms

9.3 Recognition of Mineral Formation Processes 9.3.1

Mineralization of Biomass

9.3.2 From Microfibrils to Sepiolite

223 224 228 228 230

9.3.3 Geochemical Evidence of the Biogenic Context of

Sepiolite Formation

231

9.4 Sepiolite Mineralization Hypothesis: Passive

Organomineralization of Sepiolite

232

Acknowledgements

235

References

235

10. Overview of Chinese Palygorskite Clay Resources-Their Geology, Mineralogy, Depositional Environment, Applications and Processing

239

Huitang Zhou and Haydn H. Murray 10.1

Introduction

239

10.2 Previous Work in the Area

241

10.3 Geology

243

1 0.3.1

Stratigraphy

1 0.3.2 Regional Geotectonic and Volcanic Setting 10.4 Mineralogy 10.5 Depositional Enviornment of the Guanshan Palygorskite 1 0.5.1

Source Material

1 0.5.2 Depositional Environment 10.6 Processing of Chinese Playgorskite Clay 1 0.6.1

Gel Grade

1 0.6.2 Absorbent Grades

243 244 246 247 247 253 256 256 257

10.7 Industrial Uses of Guanshan Palygorskite Clays

259

10.8 Summary

260

References

261

Contents

11. Amargosa Sepiolite and Saponite: Geology, Mineralogy, and Markets

265

William }. Miles 11.1 Introduction

265

11.2 Geographic and Geologic Settings

265

11.3 Mineralogy, Chemistry, and Functional Properties

of Amargosa Sepiolite and Saponite 11 .3.1

Sorptive Properties of Sepiolite

1 1 .3.2 Dispersal Properties of Sepiolite 11.4 Uses and Markets of Amargosa Sepiolite 1 1 .4.1

269 270 271 272

Markets for Sepiolite, and Blends of Sepiolite and Saponite

1 1 .4.2 Future and Present Markets for Saponite

272 275

11.5 Studies Related to Health and Sepiolite

276

References

276

Part Ill

Applications: Industry Environment, Advanced Materials, Others 12. Current Industrial Applications of Palygorskite and Sepiolite

279

281

Antonio Alvarez, julio Santaren, Antonio Esteban-Cubillo, and Patricia Aparicio 12.1 Introduction

281

12.2 Physico-Chemical Features

282

12.3 Special Industrial Operations to Enhance Properties and

uses: The Most Relevant Market 1 2.3.1

First Generation Products

286 287

1 2.3.2 Second Generation Products

290

1 2.3.3 Third Generation Products

293

1 2.3.4 Fourth Generation Products

293

1 2.3.5 Fifth and Sixth Generation Products

295

12.4 Health and Safety Issues

296

12.5 Conclusions

297

References

297

13. Pharmaceutical and Cosmetic Uses of Fibrous Clays

299

Alberto L6pez-Calindo, Cesar Viseras, Carola Aguzzi, and Pilar Cerezo

13.1 Introduction

299

13.2 Mineralogy, Chemistry and Habit

300

Copyrighted Material Contents

13.3 Pharmaceutical and Cosmetic Nomenclature and Specifications 303 13.4 Use as Active Substances

306

13.4.1 Antidiarrhoeals and Antacids

306

13.4.2 Adsorbents and Protectors

308

13.5 Use as Excipients

308

13.5.1 Solid Dosage Forms

308

13.5.2 liquid and Semi-solid Dosage Forms

310 312

13.6 Drug Interactions 13.7 Use of Fibrous Clays in Cosmetics

313

H.ll Health Risks of Fihrous Clays

.114

13.9 Concluding Remarks

316

References

316

14. The Effects of Palygorskite on Chemical and

Physico-Chemical Properties of Soils

325

Alexander Neaman and Arieh Singer 14.1 Introduction

325

14.2 Magnesium Chemistry of Palygorskite-Containing Soils

326

14.3 Point of Zero Charge of the Palygorskite Surface

329

14.4 Rheology of Standard Palygorskite Suspensions

331

14.5 Rheology of Palygorskite-Containing Soil Clay Suspensions

336

14.6 Flocculation of Palygorskite-Containing Clays

338

14.7 Effect of Palygorskite on D isaggregation and Colloid 340

Migration in Soils 14.8 Palygorskite-cemented Crusts (palycretes)

341

14.9 Mechanisms of Metal Sorption on Palygorskite

342

14.10 Use of Palygorskite for Metal Immobilization in Soils

344

14.11 Research Needs

346

References

346

15. Adsorption of Surfactants, Dyes and Cationic

Herbicides on Sepiolite and Palygorskite: Modifications, Applications and Modelling

351

Uri Shuali, Shlomo Nir, and Giora Rytwo 15.1 Introduction

351

15.2 Surface-Related Physico-Chemical Properties 15.2.1 Form ulae and Chemical Analyses

351 351

15.2.2 Crystal lography

352

15.2.3 Surface Area and Porosity

352

15.2.4 Adsorption Sites and lon Exchange Capacity

355

15.2.5 Thermal Analysis

355

15.2.6 Infrared Spectroscopy

356

15.3 Surface Modifications of Sepiolite by Surfactants-literature Survey

356

15.3.1 Cationic Surfactants

357

15.3.2 Anionic Surfactants

363

Copyrighted Material


�

Contents

15.3.3 Neutral Surfactants

363

15.3.4 Pesticide Formu lations

363

15.4 Model Equations

364

15.5 Results of Model Application

367 369

References

16. Sepiolite and Palygorskite as Sealing Materials for the

Geological Storage of Carbon Dioxide

375

Emilio Galan, Patricia Aparicio, and Adolfo Miras 16.1 Introduction

375

16.1.1 The Geological Storage of Carbon Dioxide

375

16.1.2 C02 Reactivity and I ntegrity of the Cap Rock

376

16.1.3 Characteristics of Palygorskite and Sepiolite

379

16.2 Interaction of Sepiolite and Palygorskite with Supercritical C02 379 16.2.1 Material Characterization and Methodology

379

16.2.2 Potential of Sepiolite and Palygorskite for the Physical and Geochemical Trapping of C02 16.3 Modelling the Role of Sepiolite and Palygorskite in the

380

Geological Storage of C02: A Task for the Future

389

16.4 Concluding Remarks

390

Acknowledgemen t

390 390

References

17. Advanced Materials and New Applications of

Sepiolite and Palygorskite

393

Eduardo Ruiz-Hitzky, Pilar Aranda, Antonio Alvarez, julio Santaren, and Antonio Esteban-Cubillo 17.1 Introduction

393

17.2 Use of Sepiolite and Palygorskite as Nanoclays

395

17.2.1 Preparatio n of Organoclays from Sepiolite and Palygorskite

395

17.2.2 Sepiolite and Palygorskite Nanocomposites Based on Thermoplastic and Thermosetting Polymers 17.2.3 Mechanical, Thermal and Fire Retardancy Propert ies 17.3 Sepiolite-Based Functional Materials: Hybrid Nanomaterials

396 405 409

17.3.1 Sep iolite-Organic Compound I nteractions

411

17.3.2 Biohybrids and Biomimetic Materials Based on Sepiol ite

416

17.3.3 Sepiolite-Carbon Materials

421

17.3.4 Sepiolite as Support of Nanoparticles

423

17.4 Conclusions

440

Acknowledgements

441

References

441


Contents

18. The Maya Blue Pigment

453

Manuel Sanchez del Rio, Antonio Domenech, Marfa Teresa Domenech-Carb6, Marfa Luisa Vazquez de Agredos Pascual, Mercedes Suarez, and Emilia Garda-Romero 18.1 History of Maya Blue

453

18.2 Experimental Techniques

457

Diffraction Studies

457

1 8.2.1

1 8.2.2 Infrared Spectroscopies

458

1 8.2.3 Raman

461

1 8.2.4 Optical Spectroscopies

463

1 8.2.5 Voltammetry

463

1 8.2.6 Nuclear Magnetic Resonance

464

1 8.2.7 Computer Modelling

466

18.3 The Syntheses, Properties and Nature Of MB

467

1 8.3.1 The Synthesis of MB

467

1 8.3.2 The Chemical Resistance of the Pigment

467

1 8.3.3 The Hue of MB

468

1 8.3.4 Structural Aspects: The Attachment of Indigo to the Clay

469

1 8.3.5 The Nature of the Palygorskite-Indigo Association

470

18.4 MB Research in Relation with the Archaeological and

Historical Contexts 1 8.4.1

Historic Relevance of Indigo

470 470

1 8.4.2 Palygorskite in Contemporary and Ancient Mesoamerica

471

1 8.4.3 Sepiolite and MB

472

1 8.4.4 The Production and Use of MB in Ancient Times

472

1 8.4.5 Trade and Distribution and of MB in Ancient Times

474

1 8.4.6 Chronology and Distribution of MB

474

1 8.4.7 Symbology of MB

475

Acknowledgements

476

References

476

Subjet Index

483

Contributors

Numbers in Parentheses indicate the pages on which the author’s contributions begin.

´ lvarez (281, 393), Technological Innovation Department, Tolsa, S.A., Antonio A Carretera de Madrid a Rivas de Jarama, 35. 28031 Madrid, Spain Carola Aguzzi (299), Departamento de Farmacia y Tecnologıá Farmaceútica, Facultad de Farmacia, Universidad de Granada, 18071 Granada, Spain Patricia Aparicio (281, 375), Departamento de Cristalografıá, Mineralogıá y Quı´mica Agrıćola, Facultad de Quı´mica, Universidad de Sevilla. C/ Prof. Garcıá Gonza´lez 1, 41012 Seville, Spain Pilar Aranda (393), Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Ine´s de la Cruz Juana de la Cruz 3. 28049 Madrid, Spain ¨ mer Bozkaya (175), Department of Geological Engineering, Cumhuriyet University, O Sivas, Turkey Pilar Cerezo (299), Departamento de Farmacia y Tecnologıá Farmaceútica, Facultad de Farmacia, Universidad de Granada, 18071 Granada, Spain Kathryn M. Conko (69), US Geological Survey, National Research Program, Reston, Virginia, USA Jaime Cuevas (219), Departamento de Geologıá y Geoquı´mica, Facultad de Ciencias, Universidad Autońoma de Madrid, Cantoblanco s/n, Madrid, Spain Marıá Teresa Domeńech-Carbo´ (453), Departament de Conservacio´ i Restauracio´ de Bens Culturals, Institut de Conservacio´ del Patrimoni, Universitat Politećnica de Vale`ncia, Camı´ de Vera 14, Vale`ncia, Spain Antonio Domeńech (453), Departament de Quı´mica Analı´tica, Universitat de Vale`ncia, Dr. Moliner, 50, Burjassot, Vale`ncia, Spain Antonio Esteban-Cubillo (281, 393), Technological Innovation Department, Tolsa, S.A., Carretera de Madrid a Rivas de Jarama, 35. 28031 Madrid, Spain Emilio Galań (85, 125, 375), Departamento de Cristalografıá Mineralogıá y Quı´mica Agrıćola, Facultad de Quı´mica, Universidad de Sevilla, Professor Garcıá Gonza´lez 1. 41012 Seville, Spain Emilia Garcıá-Romero (33, 453), Departamento de Cristalografıá y Mineralogıá, Universidad Complutense de Madrid, Facultad de Geologıá, and Instituto de Geociencias (UCM-CSIC), Ciudad Universitaria, 28003 Madrid, Spain Stephen Guggenheim (3), Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois, USA

xv

xvi

Contributors

Saeid Hojati (201), Department of Soil Science, College of Agriculture, Shahid Chamran University of Ahvaz, Ahvaz, Khuzestan, Iran Blair F. Jones (69), US Geological Survey, National Research Program, Reston, Virginia, USA Hossein Khademi (201), Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran Mark P.S. Krekeler (3), Department of Geology, Miami University Hamilton, Hamilton, Ohio, USA Alberto Lo´pez-Galindo (299), Instituto Andaluz de Ciencias de la Tierra, IACT, CSIC—Univ. Granada, Avda. Palmeras, 4. 18100 Armilla, Granada, Spain Santiago Leguey (219), Departamento de Geologıá y Geoquı´mica, Facultad de Ciencias, Universidad Autońoma de Madrid, Cantoblanco s/n, 28049 Madrid, Spain William J. Miles (265), Miles Industrial Mineral Research, Denver, Colorado, USA Adolfo Miras (373), Departamento de Cristalografıá, Mineralogıá y Quı´mica Agrıćola, Facultad de Quı´mica, Universidad de Sevilla, Profesor Garcıá Gonza´lez 1. 41012 Seville, Spain Haydn H. Murray (85, 239), Department of Geological Sciences, Indiana University, Bloomington, Indiana, USA Alexander Neaman (325), Facultad de Agronomıá, Pontificia Universidad Cato´lica de Valparaı´so, Quillota, Chile, and Centro Regional de Estudios en Alimentos Saludables, CREAS, Regioń de Valparaı´so, Chile Shlomo Nir (351), Department of Soil Science, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel Thomas Pletsch (101), Bundesanstalt fu¨r Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, Germany Manuel Pozo (85, 125), Departamento de Geologıá y Geoquı´mica, Universidad Autońoma de Madrid, Cantoblanco, 28049 Madrid, Spain Eduardo Ruiz-Hitzky (393), Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Ine´s de la Cruz 3, 28049 Madrid, Spain Ana I. Ruiz (219), Departamento de Geologıá y Geoquı´mica, Facultad de Ciencias, Universidad Autońoma de Madrid, Cantoblanco s/n, 28049 Madrid, Spain Giora Rytwo (351), Department of Environmental Sciences, Faculty of Sciences and Technology, Tel-Hai Academic College, Upper Galilee, and MIGAL, Galilee Technology Center, Kyriat Shmona, Israel Manuel Sańchez del Rıó (453), European Synchrotron Radiation Facility, BP220, Grenoble Cedex, France Julio Santareń (281, 393), Technological Innovation Department, Tolsa, S.A., Carretera de Madrid a Rivas de Jarama, 35. 28031 Madrid, Spain

Contributors

xvii

Uri Shuali (351), Department of Soil Science, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel Arieh Singer (325), Seagram Center for Soil and Water Sciences, Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot, Israel ´ rea de Cristalografıá y Mineralogıá, Departamento de Mercedes Sua´rez (33, 451), A Geologıá, Universidad de Salamanca, 37008 Salamanca, Spain Me´dard Thiry (101), Mines ParisTech, Centre des Geósciences, 35 rue St Honore´, Fontainebleau, France Marıá Luisa Va´zquez de Agredos Pascual (453), Departament de Histo`ria de l’Art, Universitat de Vale`ncia, Passeig al Mar, Vale`ncia, Spain Ce´sar Viseras (299), Instituto Andaluz de Ciencias de la Tierra, IACT, CSIC—Univ. Granada, Avda. Palmeras, 4. 18100 Armilla, Granada, Spain, and Departamento de Farmacia y Tecnologıá Farmaceútica, Facultad de Farmacia, Universidad de Granada, 18071 Granada, Spain Hu¨seyin Yalçin (175), Department of Geological Engineering, Cumhuriyet University, Sivas, Turkey Huitang Zhou (239), MinTech International, Inc., 3803 Commodore Trail, Bloomington, Indiana, USA

List of Reviewers

Faiza Bergaya, Centre de Recherche sur la Matie´re Diviseé, CNRS, University of Orleans, Orleans, France Juan Cornejo, Instituto de Recursos Naturales y Agrobiologıá, C.S.I.C., Seville, Spain Joe Dixon, Texas A&M University, College Station, Texas, USA Ray E. Ferrell, Louisiana State University, Louisiana, USA Richard L. Folk, University of Texas at Austin, Texas, USA Emilio Galań, University of Sevilla, Seville, Spain Steve Guggenheim, University of Illinois at Chicago, Illinois, USA Alberto Lo´pez-Galindo, Instituto Andaluz de Ciencias de la Tierra, CSIC – University of Grenade, Granada, Spain Luigi Marini, Universita` di Genova, Genova, Italy Alexander Neaman, Pontificia Universidad Cato´lica de Valparaı´so, Quillota, Chile Manuel Pozo, Autońoma University of Madrid, Madrid, Spain Mercedes Sua´rez, University of Salamanca, Salamanca, Spain Benny K. G Theng, Landcare Research, Manawatu Mail Centre, New Zealand Chris Vasconcelos, ETH, Zurich, Switzerland Jiaxin Yan, China University of Geosciences, Wuhan, Hubei Province, P.R. China

xix

Dedication

IN MEMORY OF ARIEH SINGER (1934–2010) This book that you have in your hands is a tribute to the memory of Prof. Arieh Singer. We started working together on its content 3 years ago, but unfortunately, he will never see it published. Prof. Arieh Singer was born in 1934. He held a master of science in agriculture and a Ph.D. in soil science from the Hebrew University of Jerusalem. Since 1968, he occupied different positions at the Hebrew University of Jerusalem. In 1989, he was nominated as full professor of soil science (1989– 2002), and after his retirement, he was nominated professor emeritus. His research interests were focused on pedology with emphasis on soil mineralogy, clay minerals in the context of soil formation, soil distribution and weathering phenomena, ecological effects on soil mineralogy, interrelationships between clay mineralogy and soil properties, atmospheric dust deposition, etc. He also developed other researching works on the origin of clay minerals in non-edaphic environments: sediments and marine/lacustrine deposits. His very last and remarkable contribution to the field of knowledge was the book entitled Soils of Israel (Springer, 2007), which describes and relates soils in desert landscapes and the geological history of Israel. Arieh was a tireless traveller. He spent sabbatical and other long stays in CSIRO, Adelaide, Australia; at the University of Sakatchewan, Canada; Institute for Soil Research of Pretoria, South Africa; University of Colorado in Boulder, USA; University of Hohenheim, Stuttgart, and University of Heidelberg, Germany, etc. as a visiting scientist. Due to his restless personality, he participated in missions in Nicaragua and China during the 15 years he acted as the consultant for Soil Survey, Evaluation and Land Use Planning, Water Planning of Israel. As a result of these stays, Arieh was able to speak English, German, Spanish, and maybe Chinese. He was a brilliant scientist and a cultured man. He was in a permanent state of excitement and wakefulness, learning from anything and publishing to share his knowledge. He published more than 300 papers, book chapters and whole books. Out of his works, I would like to point out a couple of papers on palaeoclimatic interpretation of clay minerals in soils and weathering profiles, and on sediments. Both were published in the journal Earth Science Review and became fundamental reference key works for clay geologists. His book Palygorskite-Sepiolite: Occurrences, Genesis and Uses (Elsevier, 1984), which we edited, deserves a particular mention. We both xxi

xxii

Dedication

worked together on that book though the technology at that time did not allow us to be in touch so easily. It was a bit more complicated to edit each contribution. However, that was the reason why Arieh had to spend some time in Seville and so we tightened our friendship. The last time I met him was the 19th of November of 2009. We were together in Madrid because of his nomination as honorary member of the Spanish Clay Society, and I had the pleasure to introduce him. During that meeting, we had opportunity to update the content of the book and distribute the edition work. On the following days, we exchanged a lot of ideas through e-mail and we gave shape to the whole project. On December 31st, after he wished me and my family Merry Christmas and a happy New Year, he told me he had a cancerous tumour in his lungs. However, he was hopeful that the doctors would surgically remove it without secondary consequences. On January 2010, he was still actively working though he was receiving chemotherapy treatments. On March 6th, Kamy, Arieh’s son, sent me the following note, as he dictated: “Dear Emilio, I think I ought to update you on the state of my illness. Right now I am in the hospital for all kinds of interventions. I underwent two chemotherapeutic treatments and I ought to start the third one next week. This all leaves me completely exhausted and incapable of any normal activity . . ..” Unfortunately, 20 days later, on March 26th, Prof. Shlomo Nir informed me that he passed away. As I promised Arieh’s family, I continued the work we started together to finally edit this volume. I am sure that Arieh would be happy about the outcome and the willing that led all of us to complete it. Many of the authors particularly wanted to contribute to the book as a tribute to him, sharing their knowledge the way he did, taking part of his work, being part of it, being part of him. I keep many memories of him . . . among them I still have in my office a beautiful handmade parchment he gave me in 1984 showing a picture of Jerusalem. Thank you, Arieh, for your friendship and teaching. Seville, June 2011 Emilio Galań

Dedication

xxiii

Prof. Arieh Singer receives the Diploma of Honorary Member of the Spanish Clay Society from the President, Prof. Eduardo Ruiz-Hiztky, in presence of the President of Spanish Academy of Science (left) and the President of the Spanish Mineralogical Society (right) (19th November, 2009).

Preface

The 7th International Clay Conference held in Italy in 1981 was the spawning ground for a compendium edited by Arieh Singer and Emilio Galań devoted to a review of information on the distribution, genesis and industrial applications of palygorskite and sepiolite. Eight papers from the conference were supplemented by 10 additional contributions and published by Elsevier in 1984 as Developments in Sedimentology v. 37: Palygorskite–Sepiolite; Occurrences, Genesis and Uses. The volume is partially responsible for increasing the awareness of the clay science community of the importance of these unique geological and industrial materials. According to entries in the Web of Science database, scientific publications on palygorskite and sepiolite increased from 18 and 17, respectively, in 1984–1985 to 55 and 95 in 2009–2010. The diversity of journals containing articles on palygorskite and sepiolite also expanded. This increased interest encouraged Galań and Singer to propose this volume dedicated to “advances” in our understanding of the distribution, genesis, and state of the art of industrial applications of this important clay mineral group. The new volume, Developments in Palygorskite–Sepiolite Research: A New Outlook on These Nanomaterials, containing 18 chapters authored by prominent clay scientists, provides new insights on crystal structure and chemistry, an expanded description of palygorskite and sepiolite genesis and occurrences, and much more on present and predicted uses of these nanomaterials. Peer review by a group of equally distinguished scientists ensures the quality of the presentation. This volume promises to become an important source of information for future research. The first chapter by Guggenheim and Krekeler provides a thought-provoking summary of the crystal structures and microstructures of the palygorskite– sepiolite group minerals, including palygorskite, sepiolite, falcondoite, kalifersite, loughlinite, raite, tuperssuatsiaite, and yofortierite. The relationship between crystal structure variability (revealed by X-ray powder diffraction and transmission electron microscopy) and industrial application is the chief focus of the presentation. Open channel defect (OCD) structures and wide polysomes in palygorskite and sepiolite may explain why some samples absorb large molecules and others do not. A major question concerning the continuity of chemical variations among mostly trioctahedral sepiolite and the almost dioctahedral palygorskite is addressed in the presentation by Sua´rez and Garcıá-Romero. Their analysis

xxv

xxvi

Preface

of chemical composition determined by analytical electron microscopic (AEM) techniques provides particle-specific data on palygorskite–sepiolite that reduces interparticle interference affects common in bulk mineral analysis methods. Magnesic palygorskite and Al-sepiolite can have very similar chemical compositions and all the intermediate compositions between pure palygorskite and sepiolite can be found. The observed range can be explained by a mixed polysome model involving intergrowths of sepiolite and palygorskite ribbons or polysomes. Nine reports on occurrences of palygorskite–sepiolite comprise the second section of the book. Murray, Pozo and Galań point out the Spanish and U.S. dominance of sepiolite and palygorskite production, respectively, and comment on world reserves, particularly in China. The overview of terrestrial deposits in Spain, Turkey, Greece, China, Kenya-Tanzania and other countries by Galań and Pozo summarizes the main genetic conditions leading to the formation of fibrous clays in the continental environment. Six lithologic associations observed in Spain, China, India, Greece and elsewhere create a pattern for lithologic control of fibrous clay genesis and a general genetic model based on the interaction between silica- and magnesium-bearing solutions in an environment with suitable physicochemical conditions (mostly high pH and high salinity), and on the activity of aluminium in solution or the presence of reactive Al-bearing phases in the case of palygorskite. Cuevas, Leguey and Ruiz add that microorganisms may play a role in the genesis of some Madrid Basin sepiolites. Mg-rich lithologies play an important role in the most prominent Turkish localities described by Yalçin and Bozkaya. Miles details the special Mg-rich groundwater conditions leading to the precipitation of sepiolite and saponite in the Amargosa valley of Nevada, USA. As reiterated by Jones and Conko and the other authors in Part 2 of the book, sedimentary neoformation (direct precipitation from solution) and diagenetic transformation of other Mg-rich phyllosilicates rather than inheritance of detrital materials are generally invoked to explain the origin of palygorskite and sepiolite in continental and peri-marine settings. Environments of formation may be sedimentary, hydrothermal or pedogenic. Peri-marine types generally form in the shallow-coastal lagoonal environment. Vein occurrences develop within volcanics and are mostly regarded as hydrothermal in origin. Pedogenic deposits, often rich in palygorskite, occur in modern soils, palaeosols, caliches (calcrete) and crusts. More details concerning palygorskite occurrences in soils of Iran are contained in the chapter by Hojati and Khademi. The palygorskite in the world’s largest deposit from the Guanshan area of China is reported by Zhou and Murray to have formed in a fluvial– lacustrine environment from the alteration of basalt or basaltic ash. They also provide new details on other fibrous clay occurrences in China that were unknown when the first palygorskite–sepiolite volume was published.

Preface

xxvii

Thiry and Pletsch describe a unique deep-sea origin for almost pure palygorskite clays in Middle Cretaceous and Early Eocene deep sea deposits. Warm, highly saline waters present on the seafloor during this long-lasting, extreme greenhouse period stimulated the authigenic formation of palygorskite clay, but since palygorskite contains about 10% Al2O3 and Al is poorly soluble in this type of solution, strict authigenesis is not likely. These marine palygorskite deposits developed mainly by transformation of former clay minerals, an origin consistent with formation conditions in terrestrial environments. The most expanded section of the new volume is devoted to industrial applications. The overview of the markets and applications for sepiolite and ´ lvarez, Santareń, Esteban-Cubillo and Aparicio stresses palygorskite by A the importance of particle shape and active surface area in current and predicted uses. The organization of their chapter with respect to six generations of increasingly more complex mining and processing procedures provides a useful basis for illustrating how the products have developed and suggestions for modifications to create new nanomaterials. The contribution by Lo´pezGalindo, Viseras, Aguzzi and Cerezo provides a new summary of special properties of fibrous clays in cosmetic and pharmaceutical applications. The need to conduct more field research on disaggregation potential and migration ability of palygorskite and associated minerals and organics in heavily irrigated soils is strongly recommended by Neaman and Singer. A well-organized review of sepiolite and palygorskite adsorption of surfactants, dyes and cationic herbicides by Shuali, Nir and Rytwo follows. Their emphasis on the role of modeling in assessing applications should be useful to many readers. Implications and future research directions concerning the use of sepiolite and palygorskite as sealing materials for the geological storage of carbon dioxide are presented by Galań, Aparicio and Miras. Both clays may sequester carbon dioxide by physical adsorption and mineral reactions, but their long-term stability remains unanswered. The keywords fibrous clays, nanostructured materials, polymer-clay nanocomposites, bionanocomposites, supported nanoparticles, adsorbents, catalysts, sensor devices, magnetic materials, and the title, “Advanced Materials and New Applications of Sepiolite and Palygorskite” of the contribution by Ruiz-Hitzky, ´ lvarez, Santareń and Esteban-Cubillo say it all. This summary Aranda, A should be required reading for all those interested in the present and future industrial uses of palygorskite and sepiolite. Part 3 concludes with a chapter devoted to one of the most ancient applications of fibrous clays, the production of the organoclay pigment, Maya Blue. Sańchez del Rıó, Domeńech, Domeńech-Carbo´, Va´zquez de Agredos Pascual, Sua´rez and Garcıá-Romero present new, modern analytical data on the adsorption mechanisms and discuss the archaeological significance of this historic material.

xxviii

Preface

This volume covers many topics that provide up-to-date information on the occurrences, genesis, and industrial use of palygorskite and sepiolite. It is obvious that the clay science community has learnt a lot in the intervening years since the publication of Volume 37. This book is a useful compendium, full of challenging new research ideas. It should be on the library shelves of individual clay scientists, corporations, research institutions and universities. Ray E. Ferrell Louisiana State University

Chapter 1

The Structures and Microtextures of the Palygorskite–Sepiolite Group Minerals Stephen Guggenheim* and Mark P.S. Krekeler{ *Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois, USA { Department of Geology, Miami University Hamilton, Hamilton, Ohio, USA

1. INTRODUCTION Although palygorskite–sepiolite minerals are closely related to chain silicates or represent transitional phases between chain silicates and layer silicates (e.g. the biopyriboles, Zoltai, 1981), their affinities to the (layer) phyllosilicates are a valuable asset in comparative crystal chemistry. Bailey (1980), and to a much lesser extent Guggenheim and Eggleton (1988), made comparisons of palygorskite and sepiolite to ideal phyllosilicates or modulated phyllosilicates, respectively. Since this earlier work, additional members of the palygorskite–sepiolite group or related minerals have been discovered and their atomic structures have been refined by single-crystal techniques. Authors of these single-crystal studies recognized and commented on the relationships of these structures to phyllosilicates in general and palygorskite and sepiolite in particular. However, they did not analyze in detail the relationship of these minerals as a group to relate crystal chemistry, structure, and geologic origin. This chapter, although limited by length, develops an initial cogent comparison of the palygorskite– sepiolite group minerals to modulated phyllosilicates by considering these relationships. In addition, a summary of the literature on the microstructure of palygorskite and sepiolite is included. Portions of this chapter were presented in abstract form by Guggenheim (2010).

Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00001-3 # 2011 Elsevier B.V. All rights reserved.

3

4

Developments in Palygorskite-Sepiolite Research

2. PART 1. STRUCTURE-RELATED TOPICS 2.1. Structural Characteristics of the Palygorskite–Sepiolite Mineral Group Like all ideal phyllosilicate minerals containing 2:1 layers where there is an octahedral sheet between two opposing tetrahedral sheets, the palygorskite– sepiolite minerals have continuous planes of tetrahedral basal oxygen atoms ˚ apart (e.g. Bailey, 1980). However, unlike the ideal 2:1 approximately 6.6 A phyllosilicates, the apical oxygen atoms point away from the basal oxygen atom plane in opposing directions to form ribbons of joined pyroxene-like chains (e.g. Figure 1, details of specific species given below). The apical A

[100] H2O2 H2O1

[010]

T1 T2 OH2 OH M2 M2 M3

B

M2

M1 M3 M4

OH2

T1 T2 T3

[100]

[010]

x

H O2 x 2 x

H2O3 x

H2O4 H2O1

Chapter

1

Structures/Microtextures of Palygorskite-Sepiolite Group

5

C

M1

T1 M3 M2 T2

[001]

[010] FIGURE 1 Projection of the monoclinic palygorskite (A) and sepiolite (B) structures along the [001] direction. The box in the centre of (A) illustrates a polysome. All figures show polyhedral representations. Octahedral metal cations (commonly Mg, Al) are shown as dark-fill octahedra (grey for online version), and Si-containing tetrahedra are shown as lighter-fill triangles (yellow for online version). Octahedral cation sites (M) and tetrahedral cation sites (T) are labelled. In (A), the M1 site (not shown) is located between and in front of the two labelled M2 sites in this projection; see (C) for additional site identification. Open circles are zeolitic H2O. In sepiolite (B), M2 is slightly behind M1, which is slightly behind M3 in this projection. Open circles are zeolitic H2O, with those marked “x” being labelled. (C) Projection of palygorskite along the [100] direction. The tetrahedral sheet, with sixfold rings, is an essential feature in the palygorskite–sepiolite mineral group. Coordinates from Post and Heaney (2008) and Post et al. (2007). Plotting programme ATOMS (Dowty, 2005) was used to create all figures.

oxygen atoms of the tetrahedra partially form the coordination unit of the octahedral sheet. Therefore, there is a region of the structure where a channel may form adjacent to the basal oxygen planes between two 2:1 layers, for example, in palygorskite and sepiolite. Guggenheim and Eggleton (1988) proposed a ‘modulated’ phyllosilicates classification scheme, with palygorskite and sepiolite as members because of the inverted tetrahedral arrangement and the formation of the channel where the octahedral sheet becomes discontinuous. This classification scheme was extended and adopted by the Clay Minerals Society (Martin et al., 1991) and included in a summary of recommendations of nomenclature committees relevant to clay mineralogy presented by Guggenheim et al. (2006, Table 3). The essential features of the palygorskite–sepiolite mineral group are apparently (1) the continuous tetrahedral basal oxygen planes, (2) the inverted tetrahedral arrangement that forms ribbons of joined pyroxene-like chains, and (3) the discontinuous octahedral sheet. The description of the palygorskite–sepiolite mineral group has, in the past, involved the minerals palygorskite and sepiolite as model structures because there was no need to consider other possible models. Since 1991, several minerals closely associated to palygorskite and sepiolite have been described and their structures were determined, for example, intersilite (Khomyakov, 1995; Yamnova et al., 1996), kalifersite (Ferraris et al., 1998), raite

6


(Khomyakov, 1995; Pluth et al., 1997), and tuperssuatsiaite (Ca´mara et al., 2002; Karup-Mller and Peterson, 1984). Tuperssuatsiaite and raite have all the essential features of the group, and they are clearly members. Because a structure refinement is unavailable, it is unclear if kalifersite (Ferraris et al., 1998) merits membership, but provisional analysis by Ferraris et al. (1998) suggests that it should be tentatively assigned to the group. For intersilite, the configuration of the chains is not “pyroxene-like”, and thus this mineral should be considered a “related” mineral. This chapter includes intersilite for comparison where possible, but the different chain geometry makes many comparisons difficult.

2.2. Species and Nomenclature The palygorskite–sepiolite group (Table 1) consists of palygorskite, sepiolite, falcondoite, kalifersite, loughlinite, raite, tuperssuatsiaite, and yofortierite. Intersilite is given in the table for comparison. Each mineral of Table 1 is trioctahedral, except for palygorskite and its Mn analogue, yofortierite, which are dioctahedral. “Attapulgite” was a name introduced by de Lapparent (1935) for a fibrous clay found near Attapulgus, Georgia, USA, but the name was subsequently discredited by the International Mineralogical Association

TABLE 1 Palygorskite–Sepiolite Group Minerals and Intersilite. Mineral

Formula

Reference

Falcondoite

(Ni8y zR3þy□z)(Si12xR3þx) O30(OH)4(OH2)4R2þ(x-yþ 2z)2(H2O)8

Modified from Springer (1976)

Intersilite

(Na0.80K0.45□0.75)Na5Mn(Ti0.75Nb0.25) [Si10O24(OH)](O,OH)(OH)24H2O

Yamnova et al. (1996)

Kalifersite

(K,Na)5Fe3þ7(Si20O50)(OH)612(H2O)

Ferraris et al. (1998)

Loughlinite

Na4Mg6(Si12O30)(OH)4(OH2)4

Fahey et al. (1960)

Palygorskite

(Mg5y zR3þy□z)(Si8-xR3þx) O20(OH)2(OH2)4 R2þ(x yþ 2x)/2(H2O)4

Drits and Aleksandrova (1966)

Raite

Na3Mn3Ti0.25(Si8O20)(OH)210(H2O)

Pluth et al. (1997)

Sepiolite

(Mg8y zR3þy□z)(Si12xR3þx) O30(OH)4(OH2)4R2þ(x yþ 2z)/2(H2O)8

Bailey (1980)

Tuperssuatsiaite

Na1.87Fe2.14Mn0.48Ti0.14 (Si8O20)(OH)2n (H2O)

Ca´mara et al. (2002)

Yofortierite

(Mn5y zR3þy□z)(Si8xR3þx) O20(OH)2(OH2)4R2þ(x yþ 2x)/2(H2O)4

Modified from Perrault et al. (1975)

The □ symbol denotes vacancy.

Chapter

1


7

(IMA) because palygorskite, which was reported from the Palygorsk Range, Ural Mountains, Russia, in 1862 (Ssaftschenkow, 1862), has precedence. Robertson (1962) suggested the name “hormite” for the palygorskite–sepiolite group, but this name was not accepted by the IMA. Unfortunately, the terms “attapulgite” and “hormite” are still commonly used in industry journals. The name “palysepioles” was introduced by Ferraris et al. (1998) in reference to a “palysepioles polysomatic series” with the “palysepioles” as members of the palygorskite–sepiolite series, but this name was rejected by the Association Internationale pour l’Etude des Argiles (AIPEA) Nomenclature Committee as unnecessary.

2.3. Structure of the Palygorskite–Sepiolite Mineral Group and Related Minerals Single-crystal X-ray analyses of palygorskite and sepiolite are not available, but the overall structures have been obtained from powder-data studies. These studies (Artioli et al., 1994; Chiari et al., 2003; Chrisholm, 1992; Christ et al., 1969; Drits and Sokolova, 1971; Giustetto and Chiari, 2004; Post and Heaney, 2008) confirmed the basic structure of Bradley (1940) for palygorskite (for summaries of previous models, see Bailey, 1980 and Jones and Galań, 1988). The known structural modifications are monoclinic (C2/m) and orthorhombic (Pbmn); these modifications are commonly intergrown. Rietveld refinement procedures were applied to palygorskite by Artioli et al. (1994), Chiari et al. (2003), Giustetto and Chiari (2004), and Post and Heaney (2008). Like the ideal phyllosilicates, the palygorskite–sepiolite group has an overall structure where there are infinitely extending tetrahedral sheets involving sixfold rings of tetrahedra. These tetrahedral sheets have a continuous basal oxygen atom plane but, unlike the ideal phyllosilicates, the palygorskite–sepiolite group has apical oxygen atoms pointing along either the [100] or the [ 100] direction, that is, in opposing directions (Figure 1). The apical oxygen atoms form a strip or ribbon pattern such that the strip extends along the [001] direction; the width of the apical oxygen atom strip consists of a tetrahedral ring (or two pyroxene-like chains) in palygorskite, tuperssuatsiaite, and raite, and 1.5 rings (or three pyroxene-like chains) in sepiolite. Strips with apices pointing in one direction link to metal cations (typically Mg or Al in palygorskite and sepiolite) to form a portion of the octahedral coordination. The remaining part of the coordination unit is completed by apical oxygen atoms of an opposing tetrahedral strip and by two OH groups (or by OH2 groups in special cases, see below). Thus, two apical oxygen atoms are obtained from one strip, two additional apical oxygen atoms are from the opposing strip, and two OH groups complete most octahedra. Strips that are eight octahedra wide link to tetrahedra via apical oxygen atoms in sepiolite, and strips that are five octahedra in width occur in palygorskite. Thus the octahedra do not form continuous sheets. The

8


combination of an octahedral strip and adjacent strips of tetrahedra form a “polysome” which, when compared to an ideal phyllosilicate, resembles the 2:1 layer, although more limited in lateral extent (Figure 1). The basal ˚ , which oxygen atom plane to basal oxygen atom plane spacing is about 6.5 A is similar to that found in mica. Structural information from Rietveld refinements (Table 2) is generally less precise than single-crystal refinements (see Post and Bish, 1989), and the lack of such precision is commonly observed in the reported bond distances, with associated errors about a magnitude larger than those reported from a typical single-crystal refinement; bond lengths are paramount in determining site occupancy and distortions. In part, obtaining a reasonable result in Rietveld refinements often involves fixing (or limiting the variation of) atomic parameters, which may establish the size and shape of polyhedra, and this was done to some extent for all Rietveld refinements (constraints are noted in Table 2). Although the lack of precision of individual bond distances and angles inhibits detailed interpretation of structural data, average polyhedral sizes from Rietveld refinements tend to be similar to those obtained from single-crystal data, even for polyhedra that are not constrained. Thus, consideration of structural parameters, which generally involve averaging, is probably more fruitful than direct comparisons of individual distances and angles. Single-crystal X-ray studies (Table 3) have been performed on raite (Pluth et al., 1997) and tuperssuatsiaite (Ca´mara et al., 2002). Both minerals have palygorskite-like Si tetrahedral frameworks, and both contain Na, Mn, and Ti, but tuperssuatsiaite is Fe rich. The octahedral backbone in raite (Figure 2) consists of a continuous strip of Mn octahedra (Mn1, Mn2) along the [001] direction with appendages of Na octahedra (Na1) on either side of the strip to form the octahedral part of the polysome. Polysomes are weakly connected laterally by channels containing vacant regions, isolated Na octahedra (Na2), and partially occupied (1/9 occupancy) distorted Ti octahedra linking the Na2 and the Mn2 and Na1 octahedra. Like palygorskite, strong polysome connectivity is obtained by the cross-linking tetrahedral rings with apical oxygen atoms that belong also to the coordination of the M1 and M2 octahedra. Intersilite (Yamnova et al., 1996) is similar to the members of the palygorskite–sepiolite group in that there are continuous planes of basal oxygen atoms which form polysomes involving an octahedral strip (Mn, Na, and Ti þ Nb) and coordinating tetrahedra (not illustrated). In contrast, however, the tetrahedral configuration within the polysome is composed of both six- and eightfold rings normal to the [100] direction, with polysomes connected by fivefold rings. Also, there are regions of partially occupied (with K, Na) seven- and eight-coordinated polyhedra that alternate with polysomes along both the [010] and [100] directions. Although this polysome configuration is similar to that of the palygorskite–sepiolite group minerals, the

Reference

Space group

Final Rwp (m, multiphase)a

apolysome (o)

ainterpolysome ( )

1. Sepiolite

Post et al. (2007)

Pncn

0.021

2.20

2.46

2. Palygorskite

Post and Heaney (2008)

C2/m

0.022

0.81

0.53

3. Palygorskite

Giustetto and Chiari (2004)

C2/m

0.043 (m)

12.62

10.82

4. Palygorskite

Chiari et al. (2003)

C2/m

6.33

11.79

8.28

5. Palygorskite

Artioli and Galli (1994)

C2/m

0.130 (m)

12.43

8.64

6. Palygorskite

Giustetto and Chiari (2004)

Pbmn

0.043 (m)

5.72

8.63

7. Palygorskite

Artioli and Galli (1994)

Pbmn

0.130 (m)

6.72

8.11

Average bond distances (A˚)

coctahedron ( )

Oct. thickness (A˚)

t ( )

Tet. thickness (A˚)

b/2 (A˚)c

Average

b

1. Sepiolite

M1: 2.077

T1: 1.634

M1: 58.2

M1: 1.192

T1: 111.3

M2: 2.062

T2: 1.635

M2: 58.8

M2: 2.137

T2: 113.3

M3: 2.084

T3: 1.620

M3: 58.3

M3: 2.157

T3: 110.7

M4: 57.8

M4: 2.224

M4: 2.085 2. Palygorskite

M1: 2.158

T1 1.622

M1: 61.3

M1: 2.075

T1: 110.4

M2: 1.873

T2 1.624

M2: 57.0

M2: 2.041

T2: 112.9

M3: 55.6

M3: 2.360

d

M3: 2.087

d

2.216

9.005

2.412

8.925


Species

1

Species

Chapter

TABLE 2 Powder (Rietveld) Refinements and Derived Structural Parameters.

9

Continued

TABLE 2 Powder (Rietveld) Refinements and Derived Structural Parameters.—Cont’d 10

Species


coctahedron ( )


t ( )


b/2 (A˚)c

Average

b

3. Palygorskite

M1: 2.497

T1: 1.644

M1: 64.7

M1: 2.136

T1: 106.7

M2: 2.128

T2: 1.649

M2: 58.3

M2: 2.239

T2: 106.9

M3: 57.8

M3: 2.290


M1: 2.219

T1: 1.648

M1: 63.9

M1: 1.951

T1: 113.9

M2: 2.133

T2: 1.665

M2: 59.8

M2: 2.146

T2: 107.2

M3: 64.0

M3: 2.027

M3: 2.312 M1: 2.235

T1: 1.647

M1: 61.1

M1: 2.162

T1: 119.4

M2: 2.097

T2: 1.661

M2: 57.2

M2: 2.272

T2: 106.7

M3: 62.1

M3: 2.162

M1: 2.263

T1: 1.635

M1: 58.7

M1: 2.349

T1:108.5

M2: 2.053

T2: 1.635

M2: 58.9

M2: 2.121

T2: 100.3

M3: 57.4

M3: 2.281



M1: 1.724

T1: 1.629

M1: 70.1

M1: 1.174

T1: 98.7

M2: 1.941

T2: 1.631

M2: 67.7

M2: 1.475

T2: 108.8

M3: 66.6

M3: 1.637

M3: 2.064

8.940

1.938

8.938

2.006

8.934

2.193

8.938

1.130

8.921

a Constraints/procedures used during refinement: 1 and 2, T O bonds, “soft” constraints, but not fully removed at end of refinement; 3 and 6, soft constraints (¼ constraints removed at end of refinement) on TO and MO bonds, refinement matrix uncoupled to lower correlations, hard constraints on thermal (displacement) parameters; 4, soft constraints on TO, M O bonds and angles, refinement matrix uncoupled to lower correlations; 5 and 7, constraints, if any, not given. b Includes OH, but not the oxygen atom on special position 1/4, 1/4, z, or OH2. c Sepiolite value normalized to 4 tetrahedra per polysome. Value for falcondoite, normalized to 4 tetrahedra per polysome, is 8.967; the value for yofortierite, assuming that the 4.41 diffraction line is indexed as the 040 is 8.82; and the value for loughlinite is 8.90 A˚, assuming that the 4.45 diffraction line is the 040. d M sites redefined from Post and Heaney (2008), M1 is the vacant site.


5. Palygorskite

1.874

Chapter 1

TABLE 3 Single-Crystal Refinements and Derived Structural Parameters of Palygorskite-like Minerals. Reference

Space group

# obs. reflections

Final R1

apolysome ( )

ainterpolysome ( )

Tuperssuatsiaite

Ca´mara et al. (2002)

C2/m

905

0.075

1.99

1.57

Raite

Pluth et al. (1997)

C2/m

1164

0.07

5.09

3.70

Species


coctahedron (o)


t (o)


b/2 (A˚)

Average Tuperssuatsiaite

M1: 2.063

T1: 1.615

M1: 58.2

M1: 2.174

T1: 111.5

M2: 2.038

T2: 1.610

M2: 57.3

M2: 2.204

T2: 111.6

M3: 62.6

M3: 2.214

M3: 2.406 Raite

M1: 2.074

T1: 1.623

M1: 57.0

M1: 2.259

T1: 112.4

M2: 2.154

T2: 1.622

M2: 57.2

M2: 2.337

T2: 112.8

Na1: 2.400

Na1: 58.9

Na1: 2.482

Na2: 2.401

Na2: 60.06

Na2: 2.397

Ti: 2.291

Ti: 58.84

Ti: 2.371

2.254

8.921

2.241

8.800


Species

11

12


A

T1

T2 Ti +

M2

Na2

[100]

[010]

B

[010]

[001] Na2

M1

Na1

T1

T2

M2

Ti +

FIGURE 2 Raite (atomic data from Pluth et al., 1997) projected down the [001] direction in (A) and down [100] direction in (B). Light-filled triangles (yellow for online version) are Si tetrahedra, dark-fill octahedra (grey for online version) are Mn, cross-hatched, dark-fill octahedra (purple for online version) contain Na, and line-hatched, dark-fill (green for online version) octahedra contain Ti (0.45 per cell) and vacancies (0.55 per cell, signified in the label as a box). Part (B) better illustrates the discontinuous nature of the octahedral sheet because Na2 resides in the centre of the channel, and it is poorly linked to the octahedral ribbons via a site that contains mostly vacancies and some Ti.

topology of the tetrahedral connectivity is sufficiently different that this mineral is not considered in detail here. Kalifersite is believed to be composed of two polysomes: a palygorskite- and a sepiolite-like polysome. Ferraris et al. (1998) used a distance least-squares refinement procedure to show that the palygorskite–sepiolite polysome topology is consistent with the cell dimensions of kalifersite. This procedure optimized a set of atomic coordinates to match ideal bond distances, but the refinement process did not adjust atomic coordinates to fit a calculation with observed diffraction data. Comparison of the resultant atomic coordinates to powder X-ray data was poor, and Ferraris et al. (1998) attributed this result to poor crystallinity and preferred orientation of the sample in the X-ray beam.

Chapter

1


13

However, an incorrect structure model could also explain a poor fit. Although a detailed X-ray study is probably not possible with the sample, a high-resolution transmission electron microscope study is likely to be useful. In palygorskite and sepiolite, exchangeable cations, zeolitic H2O, and vacant regions may reside in the channels in natural samples. Possible exchange reactions with organic molecules show that this exchange is dependent on the size of the organic cations because of steric constraints of the channels. One synthetic pigment (Maya Blue, used extensively by the Maya civilization because of its bright blue colour) involves the adsorption of the (organic) indigo molecule in palygorskite or sepiolite. Larger molecules also may be adsorbed by the structure, probably because of the existence of defects, which are discussed further below. In palygorskite and sepiolite, the octahedral strips are terminated at the channel by four OH2 per formula unit to form a part of the octahedral coordination polyhedron around Mg or Al (see details below). Raite is more complex and also has OH2 (and H2O) associated with octahedra and partially occupied octahedra in or next to the channel. In contrast, in tuperssuatsiaite, the octahedra at the strip edge contain Fe, Mn, or Na, and half the terminations involve OH groups and half involve OH2 (Ca´mara et al., 2002). Bailey (1980) related the two varieties of stacking (monoclinic, orthorhombic) of the palygorskite–sepiolite structures to the atomistic approach he used to derive the standard polytypes for the 2:1 layer phyllosilicates. Chisholm (1992) proposed atomic coordinates and space groups for these derived models. Within a polysome, the direction of shift of the upper tetrahedral strip may be either þ c/3 or c/3, depending on whether set I or set II octahedral cations occupy possible sites (the reader is referred to Bailey, 1980, p. 8, for the definition of set I and set II). The direction of shift or stagger of the upper tetrahedral strip in the polysome relative to the lower tetrahedral strip is based on closest packing within the polysome. Where the same set of octahedral cations is occupied from polysome to polysome (regardless if it is set I or set II), the direction of shift always remains the same and a monoclinic structure results (with an ideal b ¼ 105.2 ). Where alternation of shift directions occurs because set I alternates with set II in adjacent strips, an orthorhombic structure results (b ¼ 90 ). Because of the lack of precision of a Rietveld refinement over a single-crystal study, obtaining a reasonable structure with multiphase samples of closely related structures is especially difficult. For example, the three Oapical–T–Obasal angles describing the shape of a tetrahedron in the Giustetto and Chiari (2004) monoclinic palygorskite model vary between 83.1 and 119.8 . This is crystal chemically unreasonable because a large deviation from near 109.5 implies that the SiO bond lacks significant covalent character. In contrast, however, the average of 106.9 is reasonably close to the ideal value of 109.5 . Similar problems exist for the orthorhombic palygorskite model.

14


2.4. Substitutions in Palygorskite and Sepiolite Cation sites in the palygorskite–sepiolite group minerals are commonly defined from the centre of the octahedral strip, where there is often a special position such as a mirror plane, to the outer edge. For example, M1 is the central octahedral site on the special position, with M2 adjacent to M1, and M3 adjacent to M2 but further from the special position, etc. Tetrahedral cation sites are defined in a similar fashion. In general, sepiolite is Mg rich and palygorskite has a ratio of Mg to R3þ cations from 3:1 to 1:3 and significant numbers of vacancies. Martin-Vivaldi and Cano-Ruiz (1955) and Drits and Aleksandrova (1966) surveyed the available (early) analytical data for palygorskite and found the data consistent with an octahedral sheet with a vacant site (one site vacant per five octahedra). Thus, sepiolite is primarily trioctahedral and palygorskite approaches dioctahedral. Because samples often contain impurities, obtaining accurate analyses are difficult (e.g. Smith and Norem, 1986). However, Galań and Carretero (1999) have shown that sepiolite can obtain near Mg endmember trioctahedral compositions of Mg8Si12O30(OH)4(OH2)4(H2O)8 and that palygorskite is intermediate between dioctahedral and trioctahedral with one of five octahedra vacant and the other octahedral sites occupied by Mg, Al, and Fe with R2þ/R3þ near 1.0. Although impurities may be an issue, Newman and Brown (1987) found that octahedral occupancy varied from R3 þ2. 5 þ R2 þ1. 5 for Al-rich specimens to R3 þ0.5 þ R2 þ4.25 for Mg-rich palygorskite. Tetrahedral occupancy is very Si rich with Al substitutions limited from Al0.12 to Al0.66 per eight T sites. Serna et al. (1977), using infrared spectroscopy, and Heller-Kalai and Rozenson (1981), using infrared and Mo¨ssbauer spectroscopy, concluded that vacancies are ordered into M1 in palygorskite, and that Mg (and Fe) preferentially orders into M3. Chryssikos et al. (2009), also using infrared analysis, found that regions of the palygorskite structure were dioctahedral (with AlAlOH, AlFe3þOH, Fe3þFe3þOH interactions) and trioctahedral (MgMgOH) regions, with these interactions implying that Al and Fe3þ order into M2. Based on average bond distances (see Table 2), Post and Heaney (2008) found vacancies ordered in M1, Al in M2, and Mg in M3, in accord with the spectroscopy studies and other Rietveld refinement studies (Artioli and Galli, 1994; Chiari et al., 2003; Giustetto and Chiari, 2004). Although sepiolite is generally considered trioctahedral, sepiolite shows some octahedral vacancies (without ordering) where R3þ content is high with octahedral sums from 7.0 to 8.0 (Newman and Brown, 1987), with cations of mostly Mg and minor Mn, Fe3þ, Fe2þ, and Al. Tetrahedral content varies from (Si11.96Al0.05) to (Si11.23Fe3þ0.53Al0.24), although R3þ content has been reported to be as high as 1.3 atoms per 12 sites (Bailey, 1980). Santaren et al. (1990) found a small substitution (1.3 wt.%) of F for OH ( 25% of the OH groups) in a sepiolite from Spain.

Chapter

1


15

2.5. H2O, OH2, and OH Positions in Palygorskite and Sepiolite Hydrogen positions cannot be determined precisely from powder (Rietveld) X-ray data because the scattering efficiency of H is low for X-rays, even for higher quality material. Although this is not the case for neutron experiments, H positions may still be difficult to obtain accurately if the sample is poorly crystalline, as is generally the case for these minerals. Therefore, to determine the positions of the OH and OH2 groups, only the oxygen atom location is used in most studies. For palygorskite, there is general agreement that the OH groups are part of the octahedral anion coordination of the M1 and M2 sites, which occur well within the octahedral strips (Figure 1). The OH2 is part of the coordination unit around the M3 site along the edges of the octahedral strips, where the two hydrogen atoms are required for charge balance. For sepiolite, the OH groups are part of the inner octahedral strip coordinating to M1, M2, and M3, whereas the OH2 groups are along the edges of the octahedral strip coordinating to M4, and this pattern is similar to that of palygorskite. In contrast, locations for the zeolitic H2O may vary depending on the Rietveld refinement being considered, and this may be a result of poor crystallinity, limitations of the refinement, and/or differences in chemical composition, including relative humidity. For (monoclinic) palygorskite, Post and Heaney (2008) found locations for two zeolitic H2O molecules (or four H2O per eight tetrahedral sites), which are consistent also with the molecular modelling results of Fois et al. (2003). One of these sites (H2O2) is occupied only half the time (site locations are defined in Figure 1 and the zeolitic H2O labels do not necessarily correspond to the labels in the original paper cited), based on both the refined occupancy factor of 0.5 and because adjacent H2O2 ˚ ) together to be occupied fully. This H2O site is sites are too close (1.03 A ˚ ) of the OH2 site. The other within hydrogen bonding distance (2.79, 2.93 A H2O molecule (H2O1) resides on the mirror plane and shows significant positional disorder. In contrast, Giustetto and Chiari (2004) also located a third H2O molecule in the channels of (monoclinic) palygorskite in a neutron Rietveld study of a deuterated sample containing both orthorhombic and monoclinic polytypes. They also found a more random network of hydrogen bonded H2O molecules between the two polytypes. In summary, all refinements indicate a loosely held network of zeolitic H2O molecules which account for the low temperatures of dehydration (e.g. < 191 to < 247 C) and higher temperatures ( 354–510 C) that account for OH2 and OH loss (thermal data from palygorskite, Florida, CMS Source clay PFl-1, Guggenheim and Koster van Groos, 2001). Dehydration (zeolitic H2O loss) and dehydroxylation and OH2 loss may partially overlap in temperature. For sepiolite, Post et al. (2007) found four zeolitic H2O molecules in a powder (Rietveld) synchrotron X-ray study. Two of the H2O sites are approximately at full occupancy (H2O1, H2O2), one at near 0.5 occupancy

16


(H2O3), and one (H2O4) at about 0.3 occupancy, which produces about 15.6 zeolitic H2O per 24 T sites (per unit cell). Hydrogen bond distances can vary considerably depending on the number of oxygen acceptor atoms linking to the hydrogen. However, the H2O4 site, occupied at 0.3, is too close ˚ ) to adjacent H2O4 sites to be occupied fully. This site, along with (1.64 A ˚ ) to the OH2 sites to be the H2O2 site, is sufficiently close (2.41, 2.71 A ˚ ) to a hydrogen bonded. The H2O4 site is also sufficiently close (2.61 A framework oxygen atom to allow for hydrogen bonding.

2.6. Genetic and Synthesis Relations The occurrences of palygorskite and sepiolite are discussed throughout this volume and are only summarized briefly here for the purposes of discussion below. Palygorskite and sepiolite are characterized as crystallizing from solution (e.g. Jones and Galań, 1988; Weaver, 1984) either in lacustrine (e.g. Chahi et al., 1997) or in perimarine (e.g. Singer, 1979; Velde, 1985; Weaver and Beck, 1977) environments. Additionally, crystallization may occur during diagenesis (e.g. Couture, 1977), or hydrothermally (e.g. Imai and Otsuka, 1984), although alteration from precursors such as smectite is also known (e.g. Singer, 1979; Yaalon and Wieder, 1976). Deep-ocean authigenic palygorskite and sepiolite near active ridge zones were described by Bowles et al. (1971). Jones and Galań (1988) summarized the occurrences of palygorskite and sepiolite in soils. They also tabulated a summary of the favourable environmental conditions of formation for palygorskite and sepiolite as compared to trioctahedral smectite (Table 4). At lower pH, palygorskite may form from amorphous silica and dioctahedral smectite, whereas at slightly higher pH, sepiolite, amorphous silica, and palygorskite can precipitate. Sepiolite is favoured over palygorskite at higher pH in silica-poor solutions (Birsoy, 2002). As expected, with high values of Al, Mg, and Si activity, palygorskite is favoured over sepiolite, but temporary variations in chemistry relating to changes in environmental conditions such as evaporation, rain, freshwater flow, etc. affect the formation of palygorskite and sepiolite (Garcia-Romero et al., 2007). The rare members of the palygorskite–sepiolite group have more limited environmental conditions of formation. The type locality for kalifersite is the Khibina massif, Mt. Kukisvumchorr, Kola Peninsula, Russia, where the mineral is associated with hyperagpaitic (excess alkali) hydrothermally altered pegmatites (Ferraris et al., 1998). Ferraris et al. (1998) described the occurrence as “crystallization from residual peralkaline liquids during the hydrothermal stage of the pegmatic process”. Raite occurs in pegmatite veins crossing an agpaitic nepheline syenite at Lovozero alkaline massif, Karnasurt Mountain, Kola Peninsula (Khomyakov, 1995; Pluth et al., 1997). These veins contain mineral assemblages that crystallized during early to late hydrothermal stages and epithermal stages, with raite found in cavities believed to

Chapter

1


17

TABLE 4 Summary of Chemical Environments of Formation (after Jones and Galań, 1988) for Palygorskite (P), Sepiolite (S), and Trioctahedral smectite (TS). Environmental conditions

Extent

Mineralogy

pH, alkalinity

pH < 8.5

þP, 0S, TS

PH ¼ 8–9.5

0P, þS, 0TS

PH > 9.5

P, S, þTS

High (Mg þ Si)/Al

0P, þS, TS

High (Mg þ Fe)/Si

P, S, þTS

High

P, S, þTS

Low

þP, þS, TS

High

P, S, þTS

Intermediate

0P, þS, 0TS

Moderate

þP, 0S, TS

Major element ratios

P(CO2) of sediment water

Alkali salinity

Symbols: þ, favourable; 0, less favourable; , absent.

originate from the last epithermal stage (Pluth et al., 1997). Tuperssuatsiaite was described from the Ilı´maussaq alkaline complex in South Greenland (Karup-Mller and Peterson, 1984) and in other alkaline intrusive and extrusive rocks (Ca´mara et al., 2002). Falcondoite (Springer, 1976) was found in laterite deposits derived from a serpentinic harzburgite massif in the Dominican Republic, with veins primarily consisting of garnierite and sepiolite and limited falcondoite. In contrast, loughlinite occurs in veins in dolomitic oil shale replacing shortite, northupite, and searlesite (Fahey et al., 1960) in the Green River Formation, Wyoming, USA. This latter paragenesis suggests that loughlinite is an alteration product of the dolomitic marlstone from the saline zone of the Green River Formation. Yofortierite was described in agpaitic pegmatite veins (Perrault et al., 1975) in nepheline at Mont St. Hilaire. If intersilite (Yamnova et al., 1996) is included in the discussion for comparison, this mineral was described by Khomyakov (1995) from agpaitic–hydrothermal residual differentiates of an ultra-alkaline magma in the Lovozero alkaline massif, Kola Peninsula. Experimental studies (La Iglesia, 1977; Siffert and Wey, 1962; Wollast et al., 1968) on palygorskite and sepiolite indicate that the conditions for synthesis involve a high activity of Si and Mg and high pH, with the availability of Al, favouring palygorskite over sepiolite (Hay and Wiggins, 1980; Singer and Norrish, 1974). Sepiolite (and kerolite) tends to form at lower Al activities, whereas palygorskite (and saponite) forms at higher Al activities

18


(Birsoy, 2002). Birsoy, using equilibrium activity diagrams, determined that direct precipitation of palygorskite and sepiolite is favoured at low values for log [aAl/(aHþ)3], and he noted that palygorskite and sepiolite are more likely to be directly precipitated from solution in the presence of amorphous silica rather than quartz.

3. DISCUSSION OF STRUCTURE-RELATED TOPICS 3.1. Structure Parameters Described To develop a context for the discussion of the palygorskite–sepiolite group minerals, structure parameters (Figure 3, Tables 2 and 3) are used here much like those used to describe phyllosilicate structures. The tetrahedral rotation angle, a, quantitatively describes the in-plane rotation of adjacent tetrahedra in opposite directions around the sixfold tetrahedral ring (Radolovich, 1961; Zvyagin, 1957). Tetrahedral rotation effectively reduces the lateral dimensions of an overly large Al-rich silicate tetrahedral sheet (in palygorskite–sepiolite minerals, these tetrahedra are Si rich only) to allow a fit for the apical oxygen atoms to coordinate the octahedral cations within a layer (or polysome). For palygorskite–sepiolite group members, there are two types of rings: those that are within the polysome, have tetrahedral apices pointing in one direction, and fit onto the continuous octahedral strip along the [001] direction (“polysome a”), and those that connect polysomes and involve the reversal of tetrahedral apices such that three tetrahedra point in one direction and three tetrahedra point in the opposite direction (“inter-polysome a”). The a value can be determined directly by measurement of relevant angles between tetrahedra. Bailey (1980) described phyllosilicate structures as being composed of semi-elastic sheets. In addition to tetrahedral rotation, tetrahedral sheets further adjust their lateral dimensions either by thickening or by thinning. The value of 109.47 represents the ideal value for t (¼ OapicalTObasal): t > 109.47 (thickening) reduces the apical oxygen atom to apical oxygen atom distance and t < 109.47 (thinning) increases the lateral dimension. Likewise, an octahedral coordination around a cation, M, has a comparable set of angles designated as c (ideal 54.73 ) which is defined as the angle between the vertical (¼ octahedral thickness) and the octahedral body diagonal. Thus, cos c is calculated from (octahedral thickness)/[2(MO, OH)], where MO,OH is the average value of the bond distance for the octahedral site. The octahedral sheet is thinned, where c is greater than the ideal.

3.2. Structure Parameters and Tetrahedral–Octahedral Misfit Modulated phyllosilicates, especially those with continuous octahedral sheets, are often described in terms of a misfit that originates because the lateral dimensions of the octahedral sheet are larger than the lateral dimensions of

Chapter

1


B

19

Oapical

A a 120⬚ a a

T site

120⬚

tau

a

Obasal Obasal

b b (ideal)

Obasal C

Psi

FIGURE 3 Structure parameters are used to quantify how a tetrahedral sheet or an octahedral sheet may adjust lateral dimensions to fit together. Panel (A) shows how an in-plane rotation of adjacent tetrahedra in opposite directions can reduce the lateral dimensions of an overly large tetrahedral sheet by deforming from hexagonal symmetry to a ditrigonal shape, in this case showing a reduction along b. The angle, a, can be measured directly from the atomic structure as shown. An a of zero implies that the tetrahedral sheet is at a maximum size. In (B), the angle, tau or t, is defined. This angle is defined as the average of the three Oapical–T–Obasal angles. The ideal value in a perfect tetrahedron is 109.47 . Therefore, as this angle becomes larger than the ideal, the tetrahedron thickens (i.e. increases in height, as measured from the basal plane to Oapical). In (C), the angle, psi or c, is defined as between the octahedral body diagonal and the vertical (line with the arrow); the ideal value is 54.73 . Where this angle is greater than the ideal, the octahedron is thinner than ideal.

the adjacent (and linked) tetrahedral sheet (e.g. Bates, 1959; Guggenheim and Eggleton, 1987). As previously noted (Bailey, 1980; Guggenheim and Eggleton, 1988), misfit between these sheets does not seem to be a requirement for the tetrahedral inversions (modulations) in palygorskite and sepiolite. For example, if the channel occupancy is not considered, palygorskite (Post and Heaney, 2008) has a (weighted) average octahedral cation MO bond

20


˚ and an average tetrahedral distance, including the large vacant site, of 2.016 A ˚ cation TO bond distance of 1.623 A. These values are consistent with those ˚ , T: 1.632 A ˚ , a: 7.3 , Guggenheim, 1981; found in the micas (e.g. M: 2.019 A ˚ M: 2.063, T: 1.625 A, a: 1.4 , Toraya et al., 1976), which do not have modulations. The parameter, b/2, represents the lateral size of the polysome (and the channel) or the width of the octahedra ribbon in palygorskite or sepiolite (where b is the cell dimension). This parameter (b/2) is not as strong an indicator of octahedral composition as a, for example, see Sua´rez et al. (2007). However, this ribbon width dictates the span of tetrahedra along the direction perpendicular to the ribbon direction. For palygorskite and similar structures, the ribbon width represents a span of four tetrahedra, and for sepiolite, the span is six tetrahedra. Thus, the magnitude of b/2 is a rough estimate of misfit (assuming all tetrahedra are Si rich). A b/2 value smaller than ideal implies that an outof-plane tilt of tetrahedra occurs, in addition to any other structural compensation, such as tetrahedral rotation or sheet thinning or thickening. Palygorskite ˚ , and the structure appears to has a very uniform b/2 value, at 8.921–8.940 A have a planar basal oxygen atom plane. Tuperssuatsiaite is similar, with a b/2 ˚ . In contrast, sepiolite is slightly less compressed (at 9.005 A ˚ value of 8.921 A when the value is normalized to four tetrahedra), and raite has the greatest ˚ ) and the greatest amount of out-of-plane tilt. compression (8.800 A Although some misfit occurs in raite, it does not appear that a maximum strain (misfit) has been reached. The rotation angle in the polysome is 5.09 and the rotation angle between polysomes is 3.70 , and these values do not suggest fully extended tetrahedral rings to minimize overly large octahedra and a too small tetrahedral sheet. The c values for those octahedra linked to tetrahedral apices (M1, M2) are considerably larger than the ideal (at M1: 57.0 , M2: 57.2 ), which indicates thinning, rather than the expected thickening, and t values are larger than the ideal (at T1: 112.4 , T2: 112.8 ), which indicates thickening, rather than the expected thinning, if misfit is being minimized. In tuperssuatsiaite, the rotation angle in the polysome is 1.99 and the interpolysome rotation angle is 1.57 , and both these angles are close to the limits of fully extended rings, as noted by Ca´mara et al. (2002). However, the b/2 value is similar to that of palygorskite (no tetrahedral out-of-plane tilting), and the average octahedral size for octahedra that are linked to tetrahedral apices ˚ ), considerably smaller (M1, M2) is relatively small (M1, M2 average: 2.046 A ˚ than the relevant octahedra in raite (2.127 A). Ca´mara et al. (2002) noted that M1 and M2 show substantial substitutions of R3þ cations (¼ Fe, Mn) and M2 may also contain Ti4þ, which explains the relatively small size of these sites. The t angles (at T1: 111.5 , T2: 111.6 ) are smaller than those found in raite and therefore more ideal (tetrahedra not as thick), although the tetrahedra are certainly not thin. The c values for octahedra linked to tetrahedral apices are larger than the ideal (at M1: 58.2 , M2: 57.3 , indicating thinning), and these values in tuperssuatsiaite are larger than that in raite.

Chapter

1


21

In summary, palygorskite and sepiolite octahedral and tetrahedral sizes (i.e. effects of composition) are not much different than some micas which do not show modulations involving tetrahedral inversions. Even for the well-determined structures such as raite and tuperssuatsiaite, which are structurally similar to palygorskite, but compositionally different, structural parameters are not at their limits, which suggests that misfit does not play an important role in the formation of these minerals.

3.3. If Not Misfit, Why Do Polysomes Form? In an early publication, Martin-Vivaldi and Cano-Ruiz (1955) suggested that the palygorskite–sepiolite structure may be favoured at certain ratios of octahedral cations to vacancies, that is, 4:1 in palygorskite and from (7 to 8): (1 to 0) in sepiolite, based on the idea that there may be structural discontinuities between dioctahedral and trioctahedral phyllosilicates. However, micas are now known to readily accommodate vacancy contents between dioctahedral and trioctahedral compositions, for example, see discussion in Guggenheim (1984, pp. 72–75). Thus, the argument that vacancy content is the reason for the palygorskite–sepiolite structure is without support. Without the view that inherent differences in the lateral extent of the octahedral and tetrahedral sheets are responsible for these structures, there is no prevailing view for why the palygorskite–sepiolite structure occurs instead of more traditional phyllosilicates, such as the smectite or mica group minerals. Milton (1977) pointed out the similarity between the mineralogy of salt lakes (e.g. sepiolite and loughlinite in the Green River Formation) and alkalic igneous assemblages. Thus, the environment of formation (see above) of palygorskite–sepiolite group minerals ranges from low-temperature aqueous solutions (e.g. lacustrine and perimarine) to high-temperature hydrothermal (agpaitic). For lacustrine environments, palygorskite often forms in salt lakes from detrital material rich in aluminium, whereas sepiolite tends to precipitate further away from shore (Meunier, 2005). In a relatively Al-poor, hightemperature hydrothermal environment, the rare minerals (e.g. kalifersite, raite, tuperssuatsiaite) of the group are associated with agpaitic intrusions. These conditions indicate an alkali-rich [(Na þ K)/Al > 1] aqueous environment at near-surface to hydrothermal (< 350 C) temperatures. In support of Milton (1977), Khomyakov (1995, p. 47) stated in reference to the agpaitic post-magmatic environments and soda lakes “. . .that the crystallization of minerals during the final stages of formation of the agpaitic nepheline syenites and the salt-bearing sediments took place under quite similar physicochemical conditions, in particular, with markedly increased alkalinity of the mineralforming medium.” Although Khomyakov (1995) stresses the similarities of the “physicochemical” conditions, a more precise statement may be that there is a continuum of environments, where the activity of (the components comprising) the polysomes, apolysome, is similar.

22


Two important characteristics of the palygorskite–sepiolite group minerals are the continuous basal oxygen atom plane forming the tetrahedral linkages and the octahedral strips that forms polysomes. Of particular interest is that the octahedral strips are terminated by OH and OH2 for anion completion of the octahedra at the polysome-channel interface. These terminations are consistent with an aqueous environment with a high aOH. In contrast, trioctahedral smectite, either stevensite (Mg) or saponite (Al), forms also in aqueous environments at conditions similar to but not necessarily identical to palygorskite and sepiolite (Table 4, also see below). Clearly, the formation of smectite requires a high value of aOH also. This suggests that unknown chemical parameters may affect the continuity of the octahedral sheet to produce either a palygorskite–sepiolite polysome or a traditional layer structure like smectite. This parameter may relate to aalkali, aSiO2, and/or aOH. In addition, aMg may be important because the activity of Mg affects and is affected by OH and Cl. To resolve these issues, more detailed experimental studies are required to define the conditions for polysome formation in the palygorskite–sepiolite group.

4. PART 2: MICROSTRUCTURE-RELATED TOPICS Microtexture is a critical property of clay-sized materials because it strongly influences sorption behaviour, solubility, density, and many other fundamental properties involved in environmental interactions and industrial applications. Three broad types of defects occur in palygorskite, sepiolite, and yofortierite, and these are categorized as stacking errors, variation in the width of polysomes (and by extension transformation to montmorillonite), and omission of polysomes. These defects can be directly observed by transmission electron microscopy (TEM) techniques and to a lesser extent by atomic force microscopy (AFM) techniques. TEM of palygorskite–sepiolite group minerals for microtexture study is challenging owing to the high H2O content and the small crystallite size of most of these minerals, which make imaging and acquisition of selected area electron diffraction (SAED) data in the [100] direction difficult. Even with rapid (0.3– 0.7 s) image capture by a charge-coupled device, beam damage of the sample is common. TEM data must be acquired with very low illumination.

4.1. Polysome-Width Disorder and the Transformation of Palygorskite to Smectite Variation in the width of polysomes can commonly be observed from heavily streaked SAED patterns. Direct TEM imaging of variable width polysomes has been problematic owing to beam sensitivity of the sample. Krekeler and Guggenheim (2008) reported variable width in a sepiolite from Helsinki where quadruple-chain width polysomes were observed. Various widths of polysomes were also observed in yofortierite. Figure 4 shows images along

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B

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D E

F FIGURE 4 (A) TEM image of yofortierite along the [100] direction with significant beam damage in the right portion of the image. A parallelogram-like lattice is present where each side is ˚ , and the angles of the lattice are 72 and 108 . This spacing approximately approximately 10.5 A ˚ spacing of the (011) as determined by X-ray diffraction by Perrault corresponds to the 10.5-A et al. (1975). (B) TEM image of a single fibre in the [100] direction. The image shows parallelogram-like cleavage that is commonly observed in yofortierite. A parallelogram-like lattice is visi˚ with angles of 71.3 and 108.7 . (C) TEM image showing a ble with a spacing of 10.94 A ˚ parallelogram-like lattice is apparent with two similar but unequal spacings of 11.4 and 10.9 A with angles of approximately 107 and 73 . (D) TEM image showing structural detail along the

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the [100] direction of yofortierite with (A)–(C) showing images interpreted as being of dominantly regular polysome widths and image (D) showing a region interpreted as comprising irregular polysome widths. Although variations in polysome widths can be seen, it remains unclear whether these features are common in the palygorskite–sepiolite mineral group and correlate to geologic environments. Variation in polysome width may impact industrial mineral performance, and a better understanding of the nature of such variation is required for both mineralogical and applied reasons. The transformation of palygorskite to smectite is related to polysome width disorder and has been studied or discussed by Golden and Dixon (1990), Merkl (1989), Golden et al. (1985), Gu¨ven and Carney (1979), and Randall (1956), and the transformation of smectite to palygorskite was identified in a TEM study by Chen et al. (2004). Transformations of palygorskite to and from smectite are further evidence of the importance of chemical parameters in affecting the continuity of the octahedral sheet rather than purely structural parameters (e.g. misfit, see above). Golden and Dixon (1990) showed a close textural association of smectite and palygorskite from a series of experiments using TEM data. Their work indicated that palygorskite readily converts to smectite above 100 C, although the reaction was sluggish at room temperature (22 C). They showed that at conditions near a pH of 12, the palygorskite to smectite transformation occurs over a period of several months. Merkl (1989) investigated textural relationships of palygorskite, smectite, and kaolinite, in material from the Meigs Member and the Dogtown Clay Member of the Hawthorne Formation in southern Georgia (USA) using scanning electron microscopy (SEM) techniques. This work suggested that a transformation between palygorskite and smectite may exist in these sediments; however, the SEM data were insufficient in spatial resolution to definitively identify a transformation. Golden et al. (1985) conducted experiments with solutions at 150 C which produced smectite from palygorskite. TEM data from grain mounts showed clear alteration textures of palygorskite fibres [100] direction. The lower right portion of the image is a grain boundary between the yofortierite fibre and amorphous carbon film. The extreme upper left of the image is a beam-damaged yofortierite particle not in the same orientation as the central portion of the image. Structural information in the image is most pronounced in the upper central portion. A parallelogram-like lattice ˚ and angles of 73 and 103 . Widths pattern is present with a spacing of approximately 10.6 A of white regions in the image vary, which may be related to widths of either channels or poly˚ wide feasomes. (E) TEM image along [100] of sepiolite from Helsinki showing a strip of 18.4 A tures consistent with quadruple-chain width polysomes. (F) Image approximately along the [100] direction of a yofortierite fibre. This image has a rhombus-like lattice fringe contrast with a ˚ , a value consistent with the (011) spacing from X-ray diffraction spacing of approximately 10.8 A (Perrault et al., 1975). OCD regions (labelled O) are rhombus- and parallelogram-like in shape and vary in cross-sectional area from approximately 16 to 75 nm2. The limited selected area electron diffraction (SAED) data suggest that this is a single crystal.

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and an intimate association of smectite with reacted palygorskite fibres, and these observations suggest a dissolution–reprecipitation process. Gu¨ven and Carney (1979) found in hydrothermal experiments that NaCl increased the rate of formation of stevensite at temperatures below 260 C, and from 260 to 316 C sepiolite transformed to stevensite independent of ionic strength of the solution. This body of work clearly defines a process, although details on the specific nature of the structural transformation were not provided. The transformation of palygorskite to smectite was investigated in detail using AFM and TEM techniques on natural samples by Krekeler et al. (2005) using materials from a paleohydrologic horizon from the Meigs Member of the Hawthorne Formation, southern Georgia, USA. AFM investigation indicated that palygorskite fibres in this horizon were commonly altered. Many AFM images of the altered fibres showed an oriented overgrowth of platy morphology, which was interpreted as smectite. This latter mineral forms along the length of the palygorskite crystals with an interface parallel to {010} of the palygorskite. The resulting grains have an elongate “wing-like” morphology. TEM imaging shows smectite lattice-fringe lines that are intergrown with 2:1 layer ribbon polysomes of fibres (Figure 5). The polysomes involved in these textures commonly are of variable widths ˚ ), triple-tetrahedral that are consistent with double-tetrahedral chains (10.4 A ˚ ˚ chains (14.8 A), quadruple-tetrahedral chains (21.7 A), and quintuple˚ ). These lattice-fringe lines indicate an epitaxial tetrahedral chains (24.5 A overgrowth of smectite on palygorskite fibres. They also illustrate the structural relationship between platy overgrowths on fibres observed in AFM data. This epitaxial relationship may be described as {010} [001] palygorskite k {010} [001] smectite. The transformation of palygorskite to montmorillonite and the resulting intergrowths are expected to cause variations in bulk physical properties of palygorskite-rich clays, which may have important crystal chemical (and industrial) and geologic implications. For example, the variable widths of the polysomes would be expected to accommodate organic molecules of corresponding size. Hence, the sorptive properties of palygorskite may be affected by large molecules that are accommodated in (larger) defect interstices (see below also). Therefore, the sorptive properties may not be limited to molecules that fit only in the ideal palygorskite structure where the polysomes are two pyroxene-like chains wide. If future research shows that defects affect sorption properties significantly, then industrial applications such as pesticide carriers, filtering, cleaning products, etc. may be enhanced. The transformation may also account for the very low abundance of palygorskite found in Mesozoic and older sediments. An implication of the transformation is that palygorskite deposits may have existed in abundance in Mesozoic and perhaps even older sedimentary systems.

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A

C

5 nm

D

B

{010}

smectite

Region of Polysomes

10.4 14.8 21.7.4 24.5 16.9

Region of Smectite

palygorskite

c a

c b

a

b

FIGURE 5 TEM images of palygorskite–smectite intergrowth. (A) Unlabelled image with parallel to sub-parallel lattice fringes (montmorillonite) surrounding rectangular blocks arranged in a regular or nearly regular manner (ribbons) in the centre right portion of the image. (B) Enlargement of the transition zone between a region of smectite and a region of polysomes with variable ˚ ngstro¨ms. The image is approximately along the [100]. (C) AFM ribbons widths labelled in A image (height data) of particles dispersed on a mica substrate. Acicular crystals are interpreted as palygorskite with “wing-like” overgrowths of montmorillonite. (D) Structural schematic of the epitaxial relationship between palygorskite and smectite overgrowths; {010} [001] palygorskite k {010} [001] smectite. Figure from Krekeler et al. (2005) and reprinted with permission of The Clay Minerals Society, publisher of Clays and Clay Minerals.

4.2. Open Channel Defects In addition, open channel defects (OCDs) were commonly observed in sepiolite from Helsinki and yofortierite from Mont St. Hillaire. These defects consist of omission of single or multiple polysomes in a fibre (hereafter referred to as OCDs), both as isolated occurrences in single fibres (Figure 6) and as dense groups. The cross-sectional area of OCDs varies greatly from approximately 3.9 nm2 for a single omission to as much as 75 nm2 for multiple polysome omission in yofortierite. The specific cause of OCD formation is currently unclear; however, structures may result from rapid crystallization. OCDs may possibly arise by changes in direction of crystal growth where (011) faces coalesce during

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B

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FIGURE 6 (A) TEM image of yofortierite along the [010] direction showing examples of stacking disorder of polysomes. Inset SAED pattern shows streaking along the c axis. (B) TEM image of a palygorskite fibre from the Hawthorne Formation exhibiting a planar-angular defect near the centre of the crystal. The image is taken approximately along the [010] direction. (C) TEM image of palygorskite fibre from the Hawthorne Formation along the [010] direction showing a planarangular defect at a palygorskite fibre edge. Figure from Krekeler and Guggenheim (2008).

crystallization to produce large channels several tens of nanometres wide. Krekeler and Guggenheim (2008) interpreted this as a possible mechanism for the formation of OCDs occurring in the fibrous sepiolite from Helsinki. Progressive stages of OCD formation were observed. The presence or absence of OCDs is expected to have a strong control on the variations in H2O content in palygorskite and sepiolite. For example, differential thermal analysis (DTA) techniques show variations commonly of 2–7 wt.% H2O from palygorskite and sepiolite (e.g. Jones and Galań, 1988). The H2O molecules residing in OCD structures may account for this variation, and palygorskite and sepiolite with anomalously high H2O content may be rich in OCD structures.

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Conflicting reports exist regarding the nature of sorption and interaction of large organic molecules, such as cationic dyes and aromatic hydrocarbons, with the channels of palygorskite and sepiolite (e.g. Jones and Galań 1988; Ruiz-Hitzky, 2001). The primary issue is whether large organic molecules can fully exchange and replace cations and H2O in the channels. The organic molecule exchanged in palygorskite and sepiolite requires a seemingly ordered transfer in the linear geometry of the confined channels along the [100] direction. An efficient mechanism for transport of large organic molecules and expulsion of at least some zeolitic H2O molecules through a ˚ ngstro¨ms, fibre length (a minimum of several hundreds to thousands of A Jones and Galań, 1988) of channels seems problematic. However, large organic molecules have been reported to exchange in the channels of palygorskite and sepiolite (e.g. Jones and Galań, 1988; Ruiz-Hitzky, 2001; Serna and Fernandez-Alvarez, 1974). The determination of this exchange is primarily based on infrared spectroscopy, DTA, and other related techniques (e.g. Ruiz-Hitzky, 2001). The presence of OCD structures in palygorskite and sepiolite may partly explain why some samples absorb large molecules and others do not. OCD structures or channels resulting from the occurrence of wide polysomes would enable geometric configurations consistent with exchange. Further, OCD structures and wide channels may enable zeolitic H2O molecules to be more mobile during exchange with organic molecules, potentially affecting the kinetics of exchange.

4.3. Stacking Errors and Planar Defects Defects relating to orientation of 2:1 ribbons are very common in palygorskite and sepiolite and fall into two broad groups: stacking errors and planar defects. Stacking errors are 180 rotations of 2:1 ribbons with respect to each other along the c axis, and these errors may be observed along the [010] of fibres often as cross-fringes in the TEM. Figure 6A shows stacking errors indicated by arrows. These cross-fringes are interpreted to represent the orientation of groups of 2:1 layer ribbon polysomes. Four regions in the fibre are shown, left to right with groups of 3, 4, 1, and 3 coherently oriented polysomes. The inset SAED pattern shows streaking in the stacking direction parallel to the c* axis. Planar defects such as these are common in palygorskite and sepiolite fibres and are characterized by 2–5 offsets normal to [001]. Common lengths are 75–125 nm along the [100] direction, and displacements generally are one to four lattice fringes. These defects are most common in fibres that are 50–100 nm in width. Palygorskite fibres from the Miocene Hawthorne Formation have larger angular deviations being approximately 8–16 normal to [001]. Planar defects of this type are common and occur in approximately 10–15% of these palygorskite fibres. Figure 6C is a TEM image of an edge of a palygorskite fibre approximately along [010] with lattice-fringe spacings

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˚ . Near the centre of the image, lattice fringes of approximately 11.3 and 3.2 A indicate a planar defect, and these fringes are oriented approximately 16 from the lattice fringes of the fibre interior. Such defects are common in palygorskite fibres from the Hawthorne Formation, occurring in approximately 3–5% of fibres in the Pittman quarry samples. Establishing a more detailed understanding of the nature of defects is critical for refining and developing industrial applications. More detailed TEM investigations of the palygorskite–sepiolite group are needed for comparative study.

ACKNOWLEDGEMENTS We thank P. Heaney, Pennsylvania State University, A. F. Koster van Groos, University of Illinois at Chicago, and an anonymous reviewer for comments on the chapter.

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Gu¨ven, N., Carney, L.L., 1979. The transformation of sepiolite to stevensite and the effect of added chloride and hydroxide. Clays Clay Miner. 27, 253–260. Hay, R.L., Wiggins, B., 1980. Pellets, ooids, sepiolite and silica in three calcretes of the southwestern United States. Sedimentology 27, 559–576. Heller-Kalai, L., Rozenson, I., 1981. Mo¨ssbauer studies of palygorskite and some aspects of palygorskite mineralogy. Clays Clay Miner. 29, 226–232. Imai, N., Otsuka, R., 1984. Sepiolite and palygorskite in Japan. In: Singer, A., Galań, E. (Eds.), Palygorskite-Sepiolite. Occurrences, Genesis, and Uses. Developments in Sedimentology, vol. 37. Elsevier, Amsterdam, pp. 211–232. Jones, B.F., Galań, E., 1988. Sepiolite and palygorskite. In: Bailey, S.W. (Ed.), Hydrous Phyllosilicates (Exclusive of the Micas). Reviews in Mineralogy, vol. 19. Mineralogical Society of America, Washington, DC, pp. 631–674. Karup-Mller, S., Peterson, O.V., 1984. Tuperssuatsiaite, a new mineral species from the Ilı´maussaq intrusion in South Greenland. Neues Jahrb. Mineral. Monatsh. 501–512. Khomyakov, A.P., 1995. Mineralogy of Hyperagpaitic Alkaline Rocks. Clarendon, Oxford. Krekeler, M.P.S., Guggenheim, S., 2008. Defects in microstructure in palygorskite-sepiolite minerals: a transmission electron microscopy (TEM) study. Appl. Clay Sci. 39, 98–105. Krekeler, M.P.S., Hammerly, E., Rakovan, J., Guggenheim, S., 2005. Microscopy studies of the palygorskite to smectite transformation. Clays Clay Miner. 53, 94–101. La Iglesia, A., 1977. Precipitacioń por disolucioń homogeńea de silicatos de aluminio y magnesio a temperatura ambiente. Sintesis de la paligorskita. Estud. Geol. 33, 535–544. Martin, R.T., Bailey, S.W., Eberl, D.D., Fanning, D.S., Guggenheim, S., Kodama, H., et al., 1991. Revised classification of clay materials: report of the Clay Minerals Society Nomenclature Committee for 1986–1988. Clays Clay Miner. 39, 333–334. Martin-Vivaldi, J.L., Cano-Ruiz, J., 1955. Contribution to the study of sepiolite: II. Some considerations regarding the mineralogical formula. Clays Clay Miner. 4, 173–176. Merkl, R.S., 1989. A sedimentological, mineralogical, and geochemical study of the fuller’s earth deposits of the Miocene Hawthorne group of south Georgia-north Florida. Ph.D. Dissertation, Indiana University, Bloomington, Indiana 182p. Meunier, A., 2005. Clays. Springer, Berlin. Milton, C., 1977. Mineralogy of the Green River Formation. Mineral. Rec. 8, 368–379. Newman, A.C.D., Brown, G., 1987. The chemical constitution of clays. In: Newman, A.C.D. (Ed.), Chemistry of Clays and Clay Minerals. Monograph, vol. 6. Mineralogical Society, London, England, pp. 1–128. Perrault, G., Harvey, Y., Pertsowsky, R., 1975. La yofortierite, un nouveau silicate hydrate de mangane`se de St-Hilaire, P.Q.. Can. Mineral. 13, 68–74. Pluth, J.J., Smith, J.V., Pushcharovsky, D.Y., Semenov, E.I., Bram, A., Riekel, C., et al., 1997. Third-generation synchrotron x-ray diffraction of a 6-mm crystal of raite, Na3Mn3Ti0.25Si8O20(OH)2.10H2O, opens up new chemistry and physics of low-temperature minerals. Proc. Natl. Acad. Sci. USA 94, 12263–12267. Post, J.E., Bish, D.L., 1989. Rietveld refinement of crystal structures using powder X-ray diffraction data. In: Bish, D.L., Post, J.E. (Eds.), Modern Powder Diffraction. Reviews in Mineralogy, vol. 20. Mineralogical Society of America, Washington, DC, pp. 277–308. Post, J.E., Heaney, P.J., 2008. Synchrotron powder X-ray diffraction study of the structure and dehydration behavior of palygorskite. Am. Mineral. 93, 667–675. Post, J.E., Bish, D.L., Heaney, P.J., 2007. Synchrotron powder X-ray diffraction study of the structure and dehydration behavior of sepiolite. Am. Mineral. 92, 91–97.

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Radolovich, E.W., 1961. Surface symmetry and cell dimensions of layer lattice silicates. Nat. Lond. 191, 67–68. Randall, B.A.O., 1956. Stevensite from the Whin Sill in the region of the North Tyne. Mineral. Mag. 32, 218–229. Robertson, R.H.S., 1962. The acceptability of mineral group names. Clay Miner. Bull. 5, 41–43. Ruiz-Hitzky, E., 2001. Molecular access to intracrystalline tunnels of sepiolite. J. Mater. Chem. 11, 86–91. Santaren, J., Sanz, J., Ruiz-Hitsky, E., 1990. Structural fluorine in sepiolite. Clays Clay Miner. 38, 63–68. Serna, C., Fernandez-Alvarez, T., 1974. Adsorcion de hidrocarburos en sepiolite II: Propiedades ed superficie. Anal. Quim. 71, 371–376. Serna, C., VanScoyoc, G.E., Ahlrichs, J.L., 1977. Hydroxyl groups and water in palygorskite. Am. Mineral. 62, 784–792. Siffert, B., Wey, R., 1962. Synthe`sis d’une se´piolite a` tempe´rature ordinaire. C. R. Acad. Sci. Paris 245, 1460–1463. Singer, A., 1979. Palygorskite in sediments: detrital, diagenetic, or neoformed. A critical review. Geol. Rundsch. 68, 996–1008. Singer, A., Norrish, K., 1974. Pedogenetic palygorskite. Occurrences in Australia. Am. Mineral. 59, 508–517. Smith, D.G.W., Norem, D., 1986. The electron microprobe analysis of palygorskite. Can. Mineral. 24, 499–511. Springer, G., 1976. Falcondoite, nickel analogue of sepiolite. Can. Mineral. 14, 407–409. Ssaftschenkow, T.V., 1862. Palygorskit. Verhandlungen der Russisch Kaiserlichen Gesellschaft fu¨r Mineralogie, Sankt Petersburg 102–104. Sua´rez, M., Garcıá-Romero, E., Sańchez del Rıó, M., Martinetto, P., Dooryheé, E., 2007. The effect of the octahedral cations on the dimensions of the palygorskite cell. Clay Miner. 42, 287–297. Toraya, H., Iwai, S., Marumo, F., Daimon, M., Kondo, R., 1976. The crystal structure of tetrasilicic potassium fluor mica KMg2.5Si4O10F2. Z. Kristallogr. 148, 65–81. Velde, B., 1985. Clay minerals: a physico-chemical explanation of their occurrence. Developments in Sedimentology, vol. 40. Elsevier, New York, p. 427. Weaver, C.E., 1984. Origin and geologic implications of the palygorskite of S.E. United States. In: Singer, A., Galań, E. (Eds.), Palygorskite-Sepiolite. Occurrences, Genesis, and Uses. Developments in Sedimentology, vol. 37. Elsevier, Amsterdam, pp. 39–58. Weaver, C.E., Beck, K.C., 1977. Miocene of the S.E. United States: a model for chemical sedimentation in a perimarine environment. Sediment. Geol. 17, 1–234. Wollast, R., Mackenzie, F.T., Bricker, O., 1968. Experimental precipitation and genesis of sepiolite at earth-surface conditions. Am. Mineral. 53, 1645–1662. Yaalon, D.M., Wieder, M., 1976. Pedogenetic palygorskite in some arid brown (caliothid) soils of Israel. Clay Miner. 11, 73–79. Yamnova, N.A., Egorov-Tismenko, Yu.K., Khomyakov, A.P., 1996. Crystal structure of a new natural (Na,Mn,Ti)-phyllosilicate. Crystallogr. Rep. 41, 239–244 as translated from the Russian in: Kristallografiya (1996) 41, 257–262. Zoltai, T., 1981. Amphibole asbestos mineralogy. In: Veblen, D.R. (Ed.), Amphiboles and Other Hydrous Pyriboles. Reviews in Mineralogy, vol. 9A. Mineralogical Society of America, Washington, DC, pp. 237–278. Zvyagin, B.B., 1957. Determination of the structure of celadonite by electron diffraction. Sov. Phys. Crystallogr. 2, 388–394.

Chapter 2

Advances in the Crystal Chemistry of Sepiolite and Palygorskite Mercedes Sua´rez* and Emilia Garcıá-Romero{,{

*A´rea de Cristalografıá y Mineralogıá, Departamento de Geologıá, Universidad de Salamanca, 37008 Salamanca, Spain { Departamento de Cristalografıá y Mineralogıá, Universidad Complutense de Madrid, Facultad de Geologıá, Ciudad Universitaria, Madrid, Spain Instituto de Geociencias (UCM-CSIC), Ciudad Universitara, 28003 Madrid, Spain { Instituto de Geociencias (UCM-CSIC), Ciudad Universitara, 28003 Madrid, Spain

1. INTRODUCTION The structure and chemical composition of sepiolite and palygorskite are known from the first half of the twentieth century based on work by Caille`re (1936, 1951), Bradley (1940), Nagy and Bradley (1955), Brauner and Pressinger (1956), Martin Vivaldi and Cano Ruı´z (1953, 1955, 1956a,b), Drits and Aleksandrova (1966), and others. Later, studies on the structure and chemical composition of both minerals have found many different aspects, and logically, as more cases are studied more differences are found. Recently Mg-rich palygorskites, Fe-rich palygorskites and Al-rich sepiolites have been reported and it seems that the compositional limits accepted until now could be too narrow. The aim of this chapter is define the limit of chemical composition of both minerals, if it exists, using the data from the literature available today. Sepiolite and palygorskite are modulated phyllosilicates. The modulated components are the octahedral sheets (Guggenheim et al., 2006). Both minerals can be described as 2:1 type ribbons running parallel to the c-axis. Ribbons are connected by oxygen atoms. There are continuous oxygen planes, but the periodical inversion of the apical oxygen (each two tetrahedral chains in palygorskite and each three in sepiolite) limits the lateral dimensions of the octahedral chain (Figure 1). Ideal palygorskite has a dioctahedral character (80% of octahedral position occupied) and sepiolite is a pure trioctahedral mineral. A great number of sepiolite and palygorskite references (159 sepiolites and 216 palygorskite analyses) were compared and studied to obtain this general Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00002-5 # 2011 Elsevier B.V. All rights reserved.

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review of their composition. The number of articles offering chemical data on the fibrous clay minerals is more numerous than those used in this report, since the number of studies made on the genesis, properties and application is very abundant and is increasing yearly. However, we only consider analyses coming from pure or almost pure samples in this chapter.1 In general, sepiolite and palygorskite, as do most clay minerals, appear in natural occurrences as mixtures with other clay minerals and with impurities like carbonates, feldspar and quartz. The presence of small amounts of other clay minerals on the final data of composition could change significantly the results of analyses. As an example, palygorskite frequently has illite and montmorillonite impurities containing 30 wt% of Al2O3, that is, approximately twice of the fibrous mineral and, consequently, lower amounts of silica. If the whole sample analysis is considered, and the structural formula is fitted from it, the formula of palygorskite obtained could have more tetrahedral substitutions than it really does.

1. Akbulut and Kadir (2003), Aqrawi (1993), Pe´rez et al. (1989), Argast (1989), Arranz et al. (2008), Artioli and Galli (1994), Artioli et al. (1994), Bonatti and Joenssu (1968), Bonatti et al. (1983), Botha and Hughes (1992), Boules et al. (1971), Bradley (1940), Brauner and Pressinger (1956), Caille`re (1936, 1951), Caille`re and Henin (1957, 1961, 1972), Caille´re and Rouaix (1958), Cetisli and Gedikbey (1960), Chahi et al. (1993, 1997, 2002), Chen et al. (2004, 2008), Corma et al. (1987), Dahab and Jarjarah (1989), Drits and Aleksandrova (1966), Dromashko (1953), Ece (1989), Fahey et al. (1960), Fersmann (1913), Fleischer (1972), Frank-Kamenetskiy et al. (1969), Fukushima and Okamoto (1987), Galań (1987), Galań et al. (1975), Galań and Carretero (1999), Galań and Castillo (1984), Galań and Ferrero (1982), Galopim De Carvalho et al. (1970), Garcıá-Romero and Sua´rez (2010), Garcıá-Romero et al. (2004, 2006), Gibbs et al. (1993), Gionis et al. (2006), Giusteto et al. (2006), Gonzalez et al. (1993), Gu¨ven (1992) Gu¨ven and Carney (1979), Haji-Vassiliou and Puffer (1975), Hathaway and Sachs (1965), Hay and Stoessell (1984), He et al. (1996), Heystek and Smidh (1953), Hoe and Hayashi (1975), Huertas et al. (1971), Huggins et al. (1962), Imai and Otsuka (1984), Imai et al. (1966, 1969), Inukai et al. (1994), Jamoussi et al. (2003), Kadir et al. (2002), Kamineni et al. (1993), Kauffman (1943), Komarneni et al. (1986), Krekeler et al. (2004, 2005), Krekeler and Kearns (2008), Kulbicki (1959), Leguey et al. (2010), Li et al. (2007), Linqvist and Laitakari (1981), Lapparent De (1935, 1936), Long et al. (1997), Lo´pez Aguayo and Gonza´lez Lo´pez (1995), Lo´pez-Galindo (1987), Lo´pez-Galindo et al. (1996, 2008), Lo´pez-Galindo and Sańchez Navas (1989), Magalhaes et al. (2009), Maksimovic and Radukic (1961), Martıń Pozas et al. (1983), Martin Vivaldi and Cano Ruı´z (1953), Mayayo et al. (1996), McLean et al. (1972), Midgele (1964), Millot et al. (1977), Minato et al. (1969), Muchi et al. (1965), Muraoka et al. (1958), Nagata and Sakae (1975), Nagy and Bradley (1955), Neaman and Singer (2000), Otsuka et al. (1966), Paquet (1983), Post and Heaney (2008), Post (1978), Post and Crawford (2007), Post and Janke (1984), Pozo and Casas (1999), Preisinger (1957), Rautoureau et al. (1972, 1979), Robertson and Stot (1974), Robertson (1961), Rogers et al. (1954, 1956), Santaren et al. (1990), Shannon (1929), Serna (1973), Shimosaka et al. (1976, 1980), Siddiki (1984), Singer (1976, 1981), Singer and Norrish (1974), Singer et al. (1998), Smith and Norem (1986), Starkey and Blackmon (1984), Stoessell and Hay (1978), Stphen (1954), Sua´rez et al. (1991, 1994, 1995), Sua´rez and Garcıá-Romero (2006a, 2006b), Tauler et al. (2009), Takahashi (1956), Tien (1973), Torres-Ruı´z et al. (1994), Verrecchia and Le Coustumer (1996), Vicente Rodriguez et al. (1994), Watts (1976), Weaver (1984), Weaver and Polland (1973), Wiesma (1970), Yalçin and Bozkaya (1995, 2004), Yeniyol (1986), Zaaboub et al. (2005), Zheng (1991, 1997).

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Advances in the Crystal Chemistry of Sepiolite and Palygorskite

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The chemical composition of the two pure dehydrated minerals calculated from the theoretical formulae is very simple: 69.1 wt% SiO2 and 30.9 wt% MgO for sepiolite; 72.47 wt% SiO2, 15.37 wt% Al2O3 and 12.5 wt% MgO for palygorskite. The deviations from these values are due to three different possibilities: (1) isomorphic tetrahedral and octahedral substitutions; (2) contamination by impurities of other minerals in the sample analysed mainly other clay minerals like illite, smectites and quartz or zeolite; and (3) A

B

C

D

E

F

Ribon or polysome Unit cell M3

Coordinated water Hydroxil Oxygen Mg Al

M3

M1 M2

M2

G

M4

M2 M3

M1

H

a c

b

FIGURE 1 Structural schemes of palygorskite and sepiolite. (A and B) Tetrahedral sheet of palygorskite and sepiolite, respectively, projected on (001), black and grey mean tetrahedrons with apical oxygens pointing in opposite directions. (C and D) Tetrahedral sheet of palygorskite and sepiolite, respectively, projected on (100), grey shadow corresponds to the octahedral sheet. (E and F) Octahedral sheet of palygorskite and sepiolite, respectively, projected on (001). (G and H) Schematic view (from Sańchez del Rio et al., 2005) of a 1 1 2 supercell of palygorskite and sepiolite, respectively.

36


instrumental errors. In the past decades, the use of transmission electron microscopy to obtain analyses (AEM) of very small particles makes it possible to obtain more accurate data from the individual particles and to avoid the influence of impurities. Bound or structural water is used in many different ways in the literature; so in this chapter, most of the reported chemical data have been recalculated on an anhydrous basis to compare them. The objective of this chapter is to establish the variations in the chemical composition of natural sepiolites and palygorskites from the ideal composition which correspond to pure and “perfect” minerals of sepiolite and palygorskite without any type of isomorphic substitutions. The range and type of isomorphic compositions in the two fibrous minerals can be obtained by analysing already published data of pure or almost pure sepiolites and palygorskites. Therefore, it is possible to look for the compositional limits between the two minerals, taking into account the existence of recently reported Al-sepiolite and Mg-palygorskite.

2. CHEMICAL COMPOSITION OF SEPIOLITE Ideal sepiolite, Si12O30Mg8(OH)4(OH2)4(H2O)8, is a pure trioctahedral mineral, and the four possible octahedral positions (Figure 1) are occupied by Mg. The earliest experimental references about chemical composition of sepiolite or palygorskite are from the early decades of the twentieth century (Fersmann, 1913; Kauffman, 1943; Lapparent De, 1935; Shannon, 1929). However, the most outstanding works are those of Caille`re (1936, 1951), Bradley (1940), Nagy and Bradley (1955) and Brauner and Pressinger (1956) or the review of chemical analyses for both sepiolite and palygorskite done by Caille`re and Henin (1961). We must not forget, in this review, a special mention to the pioneer works of Martıń Vivaldi and collaborators. In 1953, Martıń Vivaldi and Cano Ruı´z reported a chemical characterization of five different Spanish sepiolites. They compared the analyses with others from the bibliography and defined the SiO2/MgO ratio for an ideal, wholly magnesian sepiolite. In 1956 (Martıń Vivaldi and Cano Ruı´z, 1956c), the same authors did a detailed study of the dehydration of sepiolite. In addition in Martıń Vivaldi and Cano Ruı´z (1956b), they suggested that the minerals of the palygorskite–sepiolite group occupy the region of discontinuity between dioctahedral and trioctahedral minerals. They affirm that there is a series of minerals. Martıń Vivaldi and Linares found fibrous clays with intermediate properties between sepiolite and palygorskite in the Cabo de Gata (Almerıá, Spain) in 1962. They interpreted that this structure could be a random intergrowth. Later, Martıń Vivaldi and Fenol Hach-Ali (1970) and Martıń Vivaldi and Robertson (1971) made an excellent revision about both fibrous fillosilicates. The first comparative study on the composition of sepiolite and palygorskite by scanning electron micrsocopy (SEM) was done by Paquet et al. (1987)

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studying 145 individual particles from palygorskite–smectite and sepiolite– smectite assemblages. They conclude that the octahedral composition fields of the smectites and fibrous clays partly overlap. The sepiolite field is clearly in the trioctahedral domain, whereas the palygorskite field is both in the dioctahedral and between the trioctahedral and dioctahedral domains. Nowadays, the ideal structural formula for sepiolite based on the model of Brauner and Pressinger (1956) is (Mg8-y-zRy3þ□z)(Si12xRx3þ) O30(OH)4(OH2)4R2þ(xyþ 2z)/2(H2O)8, where the number of octahedral cations lies between 6.95 and 8.11. The cations are mainly Mg with some Al, Fe3þ, Fe2þ, Ti and occasional Cr3þ and Ni. Mg varies between 4.96 and 8.1. According to the review by Newman and Brown (1987), tetrahedral occupancy ranges from (Si11.96Al0.05) to (Si11.23Fe0.533þAl 0.24) and the total number of octahedral cations ranges from 7.01 to 8.01 (Table 1). Afterwards, Jones and Galan (1991) affirm that the theoretical SiO2/MgO ratio of sepiolite is 2.23 with SiO2 ¼ 55.6 wt%; MgO ¼ 24.99 wt%. But usually, SiO2 falls in the range of 53.9 1.9 wt%, and MgO between 21 and 25 wt%. Galań and Carretero (1999) evaluated compositional limits for sepiolite and palygorskite and concluded that sepiolite is a true trioctahedral mineral, with negligible structural substitutions and eight octahedral positions filled with magnesium (Table 1). The most recent study about chemical composition of sepiolite was made by Garcıá-Romero and Sua´rez (2010), from a study of more than a thousand AEM analyses. They concluded that “some octahedral substitutions of Mg by Al and/or Fe are possible, which produces an increase in the number of octahedral vacancies. Sepiolite can contain large proportions of Al and be considered as Al-rich sepiolite”. They affirm that the mean MgO content in sepiolite varies greatly between 30.57% and 18.58%, and the largest proportion of Al2O3 that they found was 8.35 wt%. The content of other oxides like TABLE 1 Compositional Ranges Found for Sepiolite by Different Authors. Tetrahedral Positions Si Brauner and Pressinger (1956) Newman and Brown (1987)

11.96–11.23

Galań and Carretero (1999)

Octahedral Positions Number of Cations

Mg

6.95–8.11

4.96–8.1

7.01–8.01 8

8

Garcıá-Romero and Sua´rez (2010)

11.50–12

6.87–7.96

4.88–7.92

This study

11.16–12.05

6.7–8.03

4.28–8

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TiO2, K2O and Na2O is generally very low or zero. Exceptionally, there are mean contents somewhat greater ( 1 wt% TiO2 or 0.94 wt% Na2O), but in these cases, the standard deviation is similar to the mean value which indicates the high variability of these oxides in the spot analyses. From the published literature, SiO2 and MgO are the only essential oxides in sepiolite (Figure 2), and the other oxides could appear in different amounts or be absent. Sepiolite from Eskisehir (Turkey) is one of the purest sepiolite taking into account the isomorphic substitutions. It has been studied by different authors and they all agree that this is the ideal chemical composition (Cetisli and Gedikbey, 1960; Ece, 1989; Garcıá-Romero and Sua´rez, 2010; Yalçin and Bozkaya, 2004 among others). In general, SiO2 in sepiolite ranges from 57.96 to 74.67 wt%, but most of the data show less dispersion. In fact, 66 wt% of the references reported sepiolites with SiO2 between 68 and 71 wt% ( 2% over the theoretical content; Figure 3). The mean value is 67.59 wt%, very close to the theoretical amount of SiO2 for sepiolite. For AEM data, SiO2 ranges between narrower extremes due to the presence of impurities in the other raw samples. MgO, the most important oxide with SiO2, ranges from 13.48 to 32 wt%, but most of the data are grouped within narrower limits, 17% of the analyses have less than 20 wt% of MgO (Figure 3). The mean value is 25.57 wt%. The sepiolites with lower amounts of Mg are richer in Al2O3 and Fe2O3. Al2O3 is present in most sepiolites. Only a few references report sepiolite with no Al (Arranz et al., 2008; Imai and Otsuka, 1984; Mayayo et al., 1996; Muraoka et al., 1958). This oxide could reach as much as 8.9 wt% (Rogers 0.00 1.00

0.75 O3 e2 +F O3 AI 2

Mg O

0.25

0.50

0.50

0.75 0.25

1.00 0.00

0.00 0.25

0.50 SiO2

0.75

1.00

FIGURE 2 Proportions of the main oxides in sepiolite from the references.1

2


60

60

50

50

40

40 Frequency

Frequency

Chapter

30

30

20

20

10

10

0

0 55

70 SiO2

65

60

75

80

40

85

50

80

40 Frequency

100

Frequency

60

60

20

20

10

0 20

30

40

80

90

30

40

50

0

50

0

AI2O3

10

20 AI2O3

100

50

80

40

60

30

Frequency

Frequency

70

30

40

10

60 SiO2

120

0

50

40

20

20

10

0 0

10

20 Fe2O3

0

40

30

0

10

20 Fe2O3

30

40

0

10

20 MgO

30

40

50

40

40

Frequency

Frequency

30

20

30

20

10

10

0

0 10

15

20

25 MgO

30

35

40

FIGURE 3 Variations of the main oxides in sepiolites (left column) and palygorskites (right column) from the references,1 the double arrows show the compositional range founded by AEM of individual particles by Garcıá-Romero and Sua´rez (2010).

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40


et al., 1956). However, 88% of bibliographic data have less than 4 wt% of Al2O3, and 40% of analyses have less than 1 wt% (Figure 3). Among the Al-rich (Al2O3 > 7 wt%) are those in the Ninetyeast Ridge, Indian Ocean (Argast, 1989) and those in Polatti, Turkey (Garcıá-Romero and Sua´rez, 2010). The richest Al-sepiolite occurs (Mclean et al., 1972) in a lacustrine deposit of the Southern High Plains. This sepiolite has an Al2O3/MgO ratio 0.5, but it might have a minor amount of illite as impurities. After Al, Fe is the main element substituting for Mg in the octahedral sheet, and it can appear in different proportions. As can be seen in Figure 4, there are certain sepiolites that have no Al and are plotted on the MgO– Fe2O3 axis. There are eight locations where sepiolite has been cited with more than 10 wt% of Fe2O3 and a ratio of Fe2O3/MgO is greater than 0.5. Several of them are from Japan and they appear in dolomitic rocks. The most Fe-rich sepiolites are from Middle Atlas of Morocco (Arranz et al., 2008) and from Tyrol (Brauner and Pressinger, 1956; Preisinger, 1957). In both locations, the Fe2O3/MgO ratio is greater than 1 and the Fe2O3 content is 20 wt%. This high amount of Fe2O3 means that more than two octahedral positions are occupied by Fe. The name xylolite had been used for Fe-rich sepiolite (Brauner and Pressinger, 1956), and also for Fe-rich minerals for the serpentine group (Caille`re, 1936) however; nowadays, it is not accepted by the IMA (International Mineralogical Association). In Garcıá-Romero and Sua´rez (2010), the Fe2O3 ranges between 3.22 and 0.07 wt%, although 70% of the samples have less than 2 wt% of this oxide. That is in agreement with the references used here. Although Al and Fe are the main cations substituting for Mg, other elements can appear in the octahedral sheet of the sepiolite. Ni-rich sepiolite can exist, varying from Ni-sepiolite to falcondoite (nickel analogue with Ni > Mg; Springer, 1976). Falcondoite is a rare mineral species that has been found in the Dominican Republic in Ni laterite deposits (Lewis et al., 2006; Proenza et al., 2007). The chemical compositions of Central Dominican Republic cover a large interval of falcondoite–sepiolite solid solution (Fal3 and Fal70). These compositions suggest a complete miscibility along the sepiolite–falcondoite join. The maximum NiO content is 30.42 wt% and the (Ni þ Mg)/Si ratio varies from 0.6 to 0.73 (Tauler et al., 2009). In addition, Ni-sepiolites are common constituents of garnierite mineralization in hydrous silicate-type lateritic nickel deposits (Brand et al., 1998; Freyssinet et al., 2005). Ni-sepiolite has been described in the weathering profile of the Cerro Matoso silicate Ni laterite deposit in northwest Colombia with 10.6 wt% NiO (Gleeson et al., 2004), and in garnierites from Poro Mine nickel ore deposit in New Caledonia with 12.8 wt% NiO (Manceau et al., 1985). Sepiolite with up to 1.5wt% Ni occurs in millimetre-thick veins and has a thin coating on the slickensided surface of fractures in the lateritic deposits at Nickel Mountain, Oregon (Hotz, 1964). Sepiolite can have other elements like Ti or Mn. Very small amounts of TiO2 have been reported for around half of

Chapter

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41

0.00

A

1.00

0.25

O3 AI 2

Mg O

0.75

0.50

0.50

0.75 0.25

1.00 0.00 0.00

0.25

0.50

0.75

1.00

Fe2O3 0.00

B

1.00

0.25

Mg

O3 AI 2

O

0.75

0.50

0.50

0.75 0.25

1.00 0.00 0.00

0.25

0.50 Fe2O3

0.75

1.00

FIGURE 4 Proportions of the oxides of the three main octahedral elements in sepiolite and palygorskite from the data: (A) from the references,1 (B) from AEM analysis (Garcıá-Romero and Sua´rez (2010).

the data considered herein, but most of them have less than 0.5 wt%. Only three sepiolites, all of them Al-rich sepiolites and studied by AEM, those from Polatty, Turkey, and Batallones, Spain (Garcıá-Romero and Sua´rez, 2010) and from Oum El Kaheb, Tunisia (Zaaboub et al., 2005), contain between 1 and 2.2 wt% TiO2. MnO is rarer, appearing in the chemical analyses of only a

42


few sepiolites. It reaches a maximum of 3.7 wt% in the sepiolite from Akatani, Japan (Imai and Otsuka, 1984). Sepiolite usually has small amounts of Ca, K and/or Na, but they are always scarce, the first is frequently related to small amounts of calcite impurities. The sum of these atoms rarely reaches 0.5 atoms per half unit cell. As suggested by Newman and Brown (1987), “it is possible that part of the Ca, K and Na reported in some analyses may be enclosed within the channel frameworks and cannot be readily exchangeable”. There is a Na analogue named louglhinite which has four atoms of Na p.h. u.c. (Fahey and Axelrod, 1948; Fahey et al., 1960; Kadir et al., 2002). These references describe a fibrous mineral, similar to sepiolite, with the main X-ray ˚ and 8 wt% of Na2O which powder diffraction (XRD) reflection at 12.9 A means the Na/Mg ratio ¼ 2/3. However, they do not explain whether Na could be both in octahedral positions and inside the channels or only in the latter position. Preisinger (1963) proposes that loughlinite has two Na substituting for two Mg at the edges of the ribbons, with two Na in the channels. Regarding the sepiolite structural formulae, the number of Si atoms referred to in the literature ranges from 11.16 to 12.05 (Table 1), a little wider than the range from 11.50 to 12 found by Garcıá-Romero and Sua´rez (2010). This study, as said before, was made by AEM on a score of samples, and the results are not influenced by impurities. They conclude that only a few samples of sepiolite have very few atoms (< 0.04) of tetrahedral Fe(III). The number of total octahedral cations ranges from 6.87 to 7.96. These values correspond to 0.5–14% octahedral vacancies. Clearly, Mg is the main octahedral cation in sepiolite (4.88–7.92 p.h.u.c.), but most have more than seven atoms of Mg p. h.u.c. The new data indicate that sepiolite contains only minor amounts of Al (0.01–1.24 atoms p.h.u.c.) and Fe(III) (0.01–0.43 atoms p.h.u.c.) and very small amounts of Ti. These elements can occupy M1, M2 and M3 positions in sepiolite (Figure 1). Ca is always present as exchangeable cation, and Na and K appear in most samples. In general, all studies have found that the structural formulae of sepiolites can be affected by the presence of small amounts of impurities or by the accuracy of the method employed, and as a consequence, the dispersion of the data is higher than in AEM analyses. The number of octahedral cations reported in former studies ranges from 6.7 to 8.03 with a mean value of 7.63 (Table 1). The main octahedral cation is the Mg, which ranges from 4.28 to 8 atoms per half unit cell, with a mean value of 7.11. Sixty-six percent of the formulae have seven or more Mg atoms. The rest of the octahedral positions are filled by Al and even by Fe3þ in some occasions ( 0.5. That is to say, the majority are a few richer in Mg than the ideal palygorskite. In addition, Mg-palygorskite has been described. Magnesic palygorskites are those from Spain, in Esquivias and Los Trancos (Garcıá-Romero and Sua´rez, 2010; Garcia-Romero et al., 2004), Russia (Drits and Aleksandrova, 1966), Eastern China (Zheng, 1997), Pacific Ocean (Gibbs et al., 1993) and Silver Bell Mine (Post and Crawford, 2007). All of them have large amounts of Mg and are similar in composition to the Al-rich sepiolites. Garcıá-Romero et al. (2007) found samples very rich in Mg-palygorskite together with Al-rich sepiolite in the Allou-Kagne deposit.

9 8 7

Mg

6 5

AI-sep

4 3 2 6.0

SEP SEPB

6.5

7.0

7.5

8.0

8.5

S O.C. FIGURE 5 From Garcıá-Romero and Sua´rez (2010). Mg content (p.h.u.c.) versus total octahedral content in sepiolite, for data from the literature (○) and from AEM data (). The two kinds of data are projected in the same region of the plot.

46


0.00

1.00

0.25

0.75

Mg

O

e 2O +F O3 AI 2

0.50

0.50

3

0.75

0.25

1.00 0.00

0.25

0.50 SiO2

0.00 1.00

0.75

FIGURE 6 Proportions of the main oxides in palygorskite from references.1

4

3.5

3

Mg

T-III 2.5 T-II

2

T-I 1.5 PAL

T-IV

PALB

1 3

3.5

4 S O.C.

4.5

5

FIGURE 7 From Garcıá-Romero and Sua´rez (2010). Mg content (p.h.u.c.) versus total octahedral content in palygorskite, for data from literature (○) and from AEM data (). The two kinds of data are projected in the same region of the plot. T-I, ideal palygorskite; T-II, common palygorskite; T-III, magnesic–palygorskite; and T-IV, aluminic-palygorskite.

Chapter

2


47

Fe substituting for Al in M2 position (Figure 1) is common in palygorskite. We found only three references to analyses where Fe2O3 was not present (Drits and Aleksandrova, 1966; Fersmann, 1913; Haji-Vassiliou and Puffer, 1975). As can be seen in Figure 4, almost all points are separated from the Al2O3–MgO axis indicating the presence of Fe. The Fe2O3 content can reach 14.8 wt%, with a mean value of 4.35 wt% in the references considered, but only seven samples had over 10 wt% (Figure 3). Fe2O3 can also appear as impurities of oxides or other clay minerals, but the presence as octahedral cation in most palygorskites is unquestionable based on the results of AEM analyses and IR spectroscopy (Chryssikos et al., 2009; Frost et al., 1998, 2001; Gionis et al., 2007; Sua´rez and Garcıá-Romero, 2006a). Although Fe is almost always present in palygorskite, Fe-rich palygorskites are scarce. The term Fe-rich palygorskite is used when Fe > Al. This type has been reported by different authors (Chryssikos et al., 2009; Gionis et al., 2006, 2007; Imai and Otsuka, 1984; Stathopoulou et al., 2011). Garcıá-Romero and Sua´rez (2010) found that all samples studied by AEM contain Fe2O3 and it can reach 5.54 wt%, with a standard deviation equal to 0.51. Fe is almost always considered to be ferric, and there are a few studies in which FeO was analysed. Taking into account that palygorskite is a supergenic mineral formed in oxidizing conditions ferrous Fe cannot be abundant, although Botha and Hughes (1992) and Drits and Aleksandrova (1966) have reported it to be a major oxide in palygorskite. Almost half of the references to palygorskite contain a small proportion of TiO2. This element could be related to impurities, but it could also be an octahedral cation in palygorskite because it was found by AEM analyses of isolated fibres (Krekeler et al., 2004; Sua´rez and Garcıá-Romero, 2006b). The richest TiO2 palygorskite found is the one studied by Krekeler et al. (2004) in the sediments from the Hawthorne Formation which contains 3.5 wt%. MnO is even scarcer than TiO2 but appears in palygorskite from Georgia (Krekeler et al., 2004) and from Atlantic Ocean (Boules et al., 1971). A manganoan variety has been called yofortierite (Perrault et al., 1975) and is a fully trioctahedral mineral with four positions occupied by Mn(II). Na2O, K2O and CaO appear, as in sepiolite, as minor components; the latter, in all samples studied, is clearly related to carbonate impurities. In certain analyses of palygorskite (Weaver and Polland, 1973), K2O can be over 3 wt% due to small amounts of illite and smectite. Influence of these impurities in the analyses of palygorskite is greater than in sepiolite because they are more frequently found in the rock and cause the dispersion of the data in the central region of Figure 4B. The dependence of cell parameters in palygorskite on the octahedral composition was studied by Sua´rez et al. (2007) using high-resolution X-ray diffraction (HRXRD) with synchrotron radiation. They propose a very simple way to produce a good approximation of the palygorskite structural formula

48


(octahedral sheet) that depends on the d-spacing of the 200 reflection: Al(VI) ¼ 49.1617 7.4401 * d200 and octahedral vacancy ¼ 24.0047 3.6065 * d200. In their work, a classification of the palygorskites into three types is proposed: (Type I) Ideal palygorskite with an octahedral composition near to the ideal palygorskite, similar contents of Al and Mg and negligible substitutions. (Type II) Common palygorskite where VIAl content is less than in the ideal formula and as a consequence the Mg content is higher, but the number of octahedral cations is close to 4 (vacant octahedral positions ¼ 1). Although Al may be partially substituted by Fe(III) and/or Mg, this type of palygorskite has dioctahedral character. (Type III) Magnesic palygorskite is the most trioctahedral extreme. The number of octahedral cations is greater than 4 (vacant octahedral positions < 1), and M1 position (Figure 1) is partially occupied. Recently, Garcıá-Romero and Sua´rez (2010) from a study of more than a thousand AEM analyses of 22 very pure palygorskites from different localities corroborated the validity of this classification and showed that the most abundant are Type II varieties with an excess of Mg related to the theoretical content (Figure 7). In the same work, a new type of palygorskite was proposed for the minerals which have Al/Mg > 1. Thus Type IV or Aluminic-palygorskite is defined by a total number of octahedral cations (p.h.u.c.) < 4, with Mg < 2 and consequently (Al þ Fe3) > Mg. Gionis et al. (2007) and Chryssikos et al. (2009) proposed a more complex structural formula for palygorskite taking into account that most palygorskites have, as a result of the substitution of Al in M2 position by Mg, an excess of Mg which produces trioctahedral domains in the mineral. The structural formula has two parts, the trioctahedral fraction is represented by the coefficient y, and the content in Fe occupying M2 position in the dioctahedral component is indicated by x. The formula is yMg5Si8O20(OH)2(1 y) [xMg2Fe2(1 x)Mg2Al2] Si8O20(OH)2. Later, this same group (Stathopoulou et al., 2009) demonstrated from near infrared spectroscopy that the trioctahedral entities in palygorskite are not pure trioctahedral palygorskite but sepiolite polysomes intergrown with palygorskite. Therefore, the Mg-rich palygorskites are described by the formula yMg8Si12O30(OH)4(1 y) [xMg2Fe2(1 x)Mg2Al2] Si8O20(OH)2, where y indicates the content in sepiolite polysomes (0 < x < 0.7 and 0 < y < 0.33). There are few palygorskites with structural formulae close to the ideal, that is to say, corresponding to Type I (Garcıá-Romero and Sua´rez, 2010). One of them is from Palygorskaya (Russia), the type locality of the mineral. In general, both tetrahedral and octahedral substitutions can be found in the structural formulae. The number of Si atoms ranges from 7.20 to 8.13 atoms for half unit cell (Table 2), but the mean value is close to 8 (7.91). Eighty percent has values between 7.5 and 8 atoms per half unit cell. The number of octahedral cations ranges from 2.86 to 4.66 atoms per half unit cell, but most of them have values close to 4, thus the mean value is 3.96 p.h.u.c. The main octahedral cations are Al (0.12–2.35 p.h.u.c.) and Mg (0.00–3.91 p.h.u.c.). In the theoretical formula, both atoms have the same

Chapter

2


49

values (close to 2) although analyses in the older literature show slightly higher values of Mg. The mean value of Mg atoms per half unit cell is 2.1 and 1.47 the Al mean. Palygorskite has minor proportions of octahedral Fe(III). It could be as high as 1.31 atoms per half unit cell, with a mean value of 0.36. About 75% of them have less than 0.5 Fe(III) atoms per half unit cell. Only some samples have high Fe content. The most Fe-rich samples come from Maderuelo, Spain (1.31 and 1.16 atoms of Fe(III)) (Torres-Ruı´z et al., 1994), from Western Macedonia, Greece (1.12 Fe(III)) (Gionis et al., 2006), from Serinhisar-Acipayam Basin, Denizli, SW, Turkey (1.05 Fe(III)) (Akbulut and Kadir, 2003) and from Mayaosan, Eastern China (0.04) (Li et al., 2007). As with sepiolite, the AEM analyses of palygorskite found narrower limits (Garcıá-Romero and Sua´rez, 2010; Garcia-Romero et al., 2004; Krekeler et al., 2004; Zheng, 1997, among others). Si ranges from 7.88 to 8.06 and the total octahedral cation content ranges from 3.35 to 4.40 atoms for palygorskite. These values correspond to between 12% and 23.6% octahedral vacancies for palygorskite. Palygorskite shows greater octahedral variability than sepiolite: Mg (1.79–3.34), Al (0.92–1.99), Fe(III) (0.02–0.47). There are also very small amounts of Ti, never exceeding 0.17 atoms. Ca, Na and K appear in most samples as exchangeable cation. As with sepiolite, references about trace elements and/or rare earths in palygorskite are scarce (Jamoussi et al., 2003; Kamineni et al., 1993; Lo´pez-Galindo et al., 1996; Torres-Ruı´z et al., 1994; Zaaboub et al., 2005), and the variations are a consequence of the origin of the mineral. Sańchez del Rıó et al. (2009) found that the contents in rare earths present in palygorskite from the Yucatan Peninsula vary according to the deposition environment. Just like sepiolite, palygorskite also contains zeolitic, coordinated and structural water. As the palygorskite channel width is lower than that in sepiolite (Figure 1), the number of zeolitic water molecules is also lower and generally four molecules of zeolitc water (p.h.u.c.) are considered. However, the most recent studies based on thermal analyses and synchrotron XRD (Frost and Ding, 2003; Post et al., 2007 among others) show that the amount of zeolitic water is lower than this value. Attention was placed on the dehydration, folding and rehydration processes of palygorskites (Frost and Ding, 2003; Giustetto et al., 2005; Kuang et al., 2004; Van Scoyoc et al., 1979). However, the number of zeolitic water molecules is not considered. Only a few studies, as Hirsiger et al. (1975), quantify this type of water. They find minor amounts of zeolitic water (3.9 molecules p.h.u.c.) than in the theoretical formula studying palygorskite from Bu¨rgenstock (Switzerland).

4. IS THERE A COMPOSITIONAL GAP BETWEEN SEPIOLITE AND PALYGORSKITE? The existence of Al-sepiolite and Mg-palygorskite is unquestionable, taking into account the data presented here. As a consequence, two questions can be proposed: first, is there a compositional gap between the two minerals, as suggested in the earliest studies on the composition of sepiolite and

50


palygorskite (Martin Vivaldi and Cano Ruı´z, 1956a,b,c) and second, what is the compositional limit for both minerals? If the content in the oxides of the three main octahedral cations (Mg, Al and Fe) is taken into account (see Figure 4), there is no compositional gap and all possible compositions between the two ideal minerals exist. As stated before, small amounts of impurities can exist and influence the chemical analysis. However, the raw data confirmed which obtained by Garcıá-Romero and Sua´rez (2010) from more than 1300 AEM analysis. Both mineral data groups coming from different deposits with different origins exhibit the same trends, and it is possible to find all the intermediate compositions between the two extremes corresponding to the two pure minerals. Palygorskite can be so rich in Mg and sepiolite so rich in Al that the projections of the compositions of the two groups of minerals overlap, and a continuous trend is obtained when all samples are considered. Due to the presence of iron in most samples, the points in Figure 4 are separated from the axis MgO–Al2O3. Only the iron-rich sepiolites separate of this trend and they project on the Fe2O3–MgO axis. If we project all data considered here1 into the plot used by Martin Vivaldi and Cano Ruı´z (1956b) in which moles of MgO are plotted, versus XO moles (Al2O3 þ Fe2O3 expressed as MgO moles), the continuity of the plotted points can be observed (Figure 8) and no compositional gap can be observed. Al-rich sepiolites and Mg-rich palygorskites are not very frequent. In earlier papers, only two references to Mg-palygorskites and Al-sepiolites could be found (Rogers et al., 1954, 1956). The more samples studied, the more variability observed. If whole rock data and AEM analyses are taken into account, the existence of minerals with an intermediate composition is unquestionable. Garcıá-Romero and Sua´rez (2010) show the overlap of the composition of intermediate palygorskites and sepiolites (Figures 9 and 10). They establish that a range of compositions between the two structures is possible. If the bibliographic data (SiO2/MgO and Al2O3 þ Fe2O3/MgO ratios) are plotted in the same way that those AEM analyses, the same kinds of curve are obtained. In both cases, sepiolite and palygorskite overlap in a very wide range of compositions. Although bibliographic data are influenced by the presence of impurities, the intervals of composition in which sepiolite and palygorskite are possible are similar to that obtained from AEM analyses of particles. Both sepiolite and palygorskite are possible for SiO2/MgO between 3.5 and 5 and Al2O3 þ Fe2O3/ MgO between 0.2 and 1. In consequence, there are sepiolites so rich in Al, that they could be almost fitted as palygorskites, and Mg-palygorskites that overlap sepiolite. These authors call these minerals, which have a composition between the two ideal sepiolite and palygorskite, intermediate. Intermediate mineral could be interpreted as both Al-sepiolite and Mg-palygorskite but only if chemical data are taking into account. Horizontal lines in Figures 9 and 10 define the compositional ranges for intermediate minerals. XRD data indicate that intermediate composition samples are either sepiolite or palygorskite.

Chapter

2


0.80

51

Palygorskites Ideal palygorskite Sepiolites Ideal sepiolite

XO mol

0.60

0.40

0.20

0.00 0.00

0.20

0.40 MgO mol

0.60

0.80

FIGURE 8 Moles of MgO versus moles of XO for sepiolites and palygorskitesfrom references and ideal values.

The existence of a large amount of samples, with all possible intermediate compositions between sepiolite and palygorskite (Figures 4 and 8–10), shows that not only is there no compositional gap, but on the contrary, a continuous series exists.

5. POSSIBLE STRUCTURAL ARRANGEMENTS OF THE INTERMEDIATE MINERALS Mumpton and Roy (1958) pointed out that “in nature there does not seem to be any evidence of a continuous solid solution series between the two, although this might be expected from the postulated similarity of their structures”. Half a century later, with numerous studies performed, and the possibility of studying the chemical composition of individual particles, allows us to affirm that a continuous composition series exists. As can be seen in Figure 8, sepiolite and palygorskite overlap, and there are compositions for which both minerals are possible (Figures 10 and 11). First of all, one must make sure that the data are not influenced by impurities. When data come from a reference, it is not possible to be sure about sample purity. However, by using AEM, the composition of individual crystals is obtained. Sua´rez et al. (2007) studied several palygorskites, some

52


12

A

B

8

SEP PAL

SiO2/MgO

7

10

6 5 Palygorskite Sepiolite

SiO2/MgO

4 3

8

2 0

200

400

600

800

1000

1200

6

Palygorskite Sepiolite

4

2 0

100

200

300

400

FIGURE 9 SiO2/MgO ratios ordered from the largest to the smallest values: (A) from the References 1 and (B) from AEM analysis (Garcıá-Romero and Sua´rez, 2010).

5

2.5

A

B

SEP

(AI2O3 + Fe2O3)/MgO

2.0

AI2O3 + Fe2O3/MgO

4

3

PAL

1.5

1.0 Palygorskite

0.5

Sepiolite

0.00 0

200

400

600

800

1000

1200

1400

2

1 Palygorskite Sepiolite

0 0

100

200

300

400

FIGURE 10 Al2O3 þ Fe2O3/MgO ratios ordered from the largest to the smallest values: (A) from the references1 and (B) from AEM analysis (Garcıá-Romero and Sua´rez, 2010).

Chapter

2


A

B

C

D

E

F

53

Polysome of palygorskite Polysome of sepiolite FIGURE 11 Structural scheme for the sepiolite–palygorskite polysomatic continuous series. (A) palygorskite, (B) palygorskite with a small proportion of sepiolite polysomes, (C) palygorskite with sepiolite polysomes, (D) sepiolite with palygorskite polysomes, (E) sepiolite with a small proportion of palygorskite polysomes, (F) sepiolite.

of them very rich in Mg, both by HRXRD and by AEM, and found no evidence of impurities, especially sepiolite, in the diffraction patterns. If there are no impurities in the sample, the high Al or Mg content has to be explained due to its crystallochemical features. References to Mg-rich palygorskite are more frequent than Al-sepiolite, and the arrangement of octahedral cations has been studied by IR spectroscopy. Chahi et al. (2002) proposed a trioctahedral arrangement of Mg based on the 3680 cm1 peak in the FTIR spectra. However, Garcia-Romero et al. (2004) did not find this peak in the very rich in Mg palygorskite from Esquivias. They proposed a possible distribution of the excess of Mg without Mg3–OH bonds but with local excess of the octahedral charge. A later and more detailed study of this and other palygorskites (Chryssikos et al., 2009) showed that most palygorskites have a trioctahedral fraction that can be expressed by using the formula suggested by Gionis et al. (2007). The possibility of polysomatism in sepiolite–palygorskite was referred to by

54


Ferraris et al. (2005). In addition, Krekeler et al. (2005) proposed the existence of polysomes with different widths (2, 3, 4 or 5 tetrahedral chains) in order to explain the transformation of smectite into palygorskite. Therefore, a polysome of one of these minerals could contain a 2:1 ribbon, and it is to say half unit cell (Figure 1). Taking into account the possibility of the existence of sepiolite polysomes intergrowth in the palygorskite structure, a small portion of sepiolite polysomes produces a high portion of Mg, causing a small loss of crystallinity. If this polysomatic model, which was suggested for palygorskite with an excess of Mg, is taken into account, then, in the same way, the existence of Al-sepiolite could also be explained with the presence of palygorskite polysomes in the sepiolite structure. It is feasible to wonder about the existence of all possibilities in the continuous series, from the purest and the most exclusive magnesic sepiolite, to the purest palygorskite, with all intermediate compositions. These compositions could include palygorskite with different portions of sepiolite polysomes grading into sepiolite with all possible portions of palygorskite polysomes, as shown in the simplified scheme in Figure 11. This structural scheme for the sepiolite–palygorskite polysomatic continuous series could explain all the compositions found and all of the types of the two minerals. Type I palygorskite is a pure palygorskite without sepiolite polysomes (Figure 11A). Type II, the most common palygorskite, with a small excess of Mg, corresponds to a palygorskite with a small amount of sepiolite polysomes (Figure 11B). Type III, the Mg-rich palygorskite, is a palygorskite with a high proportion of sepiolite polysomes (Figure 11C). The Al-rich sepiolite is a sepiolite with a high proportion of polysomes palygorskite (Figure 11D). On the other hand, a small portion of palygorskite polysomes in the sepiolite (Figure 11E) is almost a pure sepiolite from chemical data because it causes a small effect on the chemical composition of the crystal. The existence of a “certain type of transition” between the two minerals has been considered since the first studies were performed on the chemical composition of these minerals. Martin Vivaldi and Linares Gonzales (1962) did not use the term polysome but referred to it when they pointed out the possible existence of a mineral formed by a random intergrowth of the two types of structures. They found a mineral “that has intermediate properties ˚ between those of attapulgite and sepiolite. . . a broad line in the 10–12 A ˚ region with a central value at 11 A. It seems, then, logical to imagine that the rotation of tetrahedra suitable to give sepiolite or attapulgite structures is produced at random, thus giving about 50 percent of each structure for the ˚ and the b axis mineral. This would account for the 110 reflections at 11 A having an average value between those for sepiolite and attapulgite”. This is ˚ , which would the only reference found to a mineral with a reflection at 11 A imply periodicity for the 2 þ 3 silicate chains unities, or polysomes, like in Figure 12A, to produce the diffraction peak. If polysomes are random, as suggested in Figure 11 B–E, the only expected effect in the XRD patterns is

Chapter

2


A

55

B

FIGURE 12 Structural schemes for (A) the regular sepiolite–palygorskite intergrowth. (B) Clusters of sepiolite and palygorskite polysomes in a crystal.

a broader peak corresponding to the loss of crystal perfection. Another possibility for the arrangement of the polysomes is to occur in clusters with discrete domains of the two minerals (Figure 12B). They would appear in XRD patterns as a mixture of the two minerals because the two regions of the crystal would diffract separately. Therefore, the overlapping chemical compositions could be considered as intergrowths of the two minerals in all possible proportions.

6. OPEN QUESTIONS All of the previously stated points to the complexity of the sepiolite and palygorskite crystallochemistry and some questions regarding the chemical composition of these minerals remain open. First, the polysomatic model before exposed has to be supported by HRTEM and electron diffraction data. In addition, Al-rich palygorskite, or palygorskite Type IV, is not uncommon in nature (Garcıá-Romero and Sua´rez, 2010; Sua´rez et al., 2009), and could be explained from the point of view of its composition, by intergrowth of pure dioctahedric polysomes in palygorskite. These new types of polysomes could be formed by units of more than three tetrahedral chains as shown by Krekeler et al. (2005). The possibility of intergrowth with laminar structures or close relationships with the inosilicates remains yet to be studied in detail.

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Neaman, A., Singer, A., 2000. Kinetics of palygorskite hydrolysis in dilute salt solutions. Clay Miner. 35, 433–441. Newman, A.C.D., Brown, G., 1987. Palygorskite and sepiolite. In: Newman, A.C.D. (Ed.), Chemistry of Clays and Clay Minerals. Monograph 6. Mineralogical Society, London, pp. 107–112. Otsuka, R., Imai, N., Nishikawa, M., 1966. On the hydration of sepiolite from the Akatani Mine, Niigata Prefecture, Japan. J. Chem. Soc. Jpn. Indus. Chem. Soc. 66, 1677–1680. Miner. Abstr. 1967, 18, 55. Paquet, H., 1983. Stability, instability and significance of attapulgite in the calcretes of Mediterranean and tropical areas with market dry season. Sci. Geol. 72, 131–140. Paquet, H., Duplay, J., Valleron-Blanc, M.M., and Millot, G. 1987. Octahedral compositions of individual particles in smectite-palygorskite and smectite-sepiolite assemblages. Proccedings of the International Clay Conference. Denver, 1985. In: Schult, L.G., Van Olphen, H., Mumpton, A. (Eds.). The Clay Minerals Society, Bloomington Indiana. 73–77. Perrault, G., Harvey, J., Pertsowsky, R., 1975. La yofortierite, un nouveau silicate de mangane`se de St. Hilaire. P. Q. Can. Mineral. 13, 68–74. Post, J.E., 1978. Sepiolite deposits of the Las Vegas, Nevada Area. Clays Clay Miner. 26 (1), 58–64. Post, J.L., Crawford, S., 2007. Varied forms of palygorskite and sepiolite from different geologic systems. Appl. Clay Sci. 26, 232–244. Post, J.E., Heaney, P.J., 2008. Synchrotron powder X-ray diffraction study of the structure and dehydration behaviour of palygorskite. Am. Mineral. 93, 667–675. Post, J.E., Janke, N.C., 1984. Ballarat sepiolite, Inyo County, California. In: Singer, A., Galań, E. (Eds.), Palygorskite–Sepiolite: Occurrences, Genesis and Uses, Developments in Sedimentology, pp. 159–167. Post, J.E., Bush, D.L., Heaney, P.J., 2007. Synchrotron powder X-ray diffraction study of the structure and dehydration behaviour of sepiolite. Am. Mineral. 92, 91–97. Pozo, M., Casas, J., 1999. Origin of kerolite and associated Mg clays in palustrine–lacustrine environments. The Esquivias deposit (Neogene Madrid Basin, Spain). Clay Miner. 34, 395–418. Preisinger, A., 1963. X-ray study of the structure of sepiolite. In: Proceedings of the 6th National Conference on Clays and Clay Minerals, pp. 61–67. Proenza, J.A., Zaccarini, F., Lewis, J., Longo, F., Garuti, G., 2007. Chromite composition and platinum-group mineral assemblage of PGE-rich Loma Peguera chromitites, Loma Caribe peridotite, Dominican Republic. Can. Mineral. 45, 211–228. Rautoureau, M., Clinard, C., Mifsud, M., Caillere, S., 1979. E´tude morphologique de la palygorskite par microscopie e´lectronique. In: C. R.104th, Congr. Nat. Soc. Sav., III. 199–212. Rautoureau, M., Tchoubar, C., Mering, J., 1972. Analyses structurale de la se´piolite a` partir des donneés de la diffraction e´lectronique. In: Proceedings International Clay Conference, Madrid, 115–121. Robertson, R., 1961. The origin of English Fullers Earths. Mineral. Mag. 4, 282–287. Robertson, R.H.S., Stot, A., 1974. Mineralogical analysis of some sepiolitic clays. Est. Geol. XXX, 347–357. Rogers, L.E., Martin, A.E., Norrish, K., 1954. The occurrence of palygorskite, near Ipswich, Queensland. Mineral. Mag. 30, 534–540. Rogers, L.E.R., Quirk, J.P., Norrish, K., 1956. Occurrence of an aluminium–sepiolite in a soil having unusual water relationships. Soil Sci. 7, 177–184.

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Sańchez del Rıó, M., Sua´rez, M., Garcıá-Romero, E., Alianelli, L., Felici, R., Martinetto, P., et al., 2005. Mg K-edge XANES of sepiolite and palygorskite. Nuclear Instruments and Methods in Physics Research B. 238, 55–60. Sańchez del Rıó, M., Sua´rez, M., Garcıá-Romero, E., 2009. The occurrence of palygorskite in the Yucatań Peninsula: ethno-historic and archaeological contexts. Archeometry 51 (2), 214–230. Santaren, J., Sanz, J., Ruitz-Hitzky, E., 1990. Structural fluorine in sepiolite. Clays Clay Miner. 1, 63–68. Serna, C., 1973. Naturaleza y propiedades de la superficie de la sepiolita. Tesis Doctoral. Universidad de Madrid. Shannon, E.V., 1929. Tschermigite, ammoniojarosite, epsomite, celestite and palygorskite from Southern Utah. U.S. Nat. Mus. Proc. 74 (13), 12. Shimosaka, K., Kawano, M., Sudo, T., 1976. New occurrence and mineralogical properties of iron sepiolite. Appl. Clay Sci. 5, 31–41. Shimosaka, K., Suzuki, S., Tatematsy, H., Otsuka, R., 1980. Iron sepiolite from the Seikan Tunnel, Japan. J. Jpn. Assoc. Miner. Petrolog. Econ. Gel. 75, 164–171. Siddiki, M.K.H., 1984. Occurrence of palygorskite in the Deccan Trap Formation in India. In: Singer, A., Galań, E. (Eds.), Palygorskite–Sepiolite: Occurrences, Genesis and Uses. Developments in Sedimentology. Elsevier, Amsterdam, pp. 243–250. Singer, A., 1976. Dissolution of two Australian palygorskites in dilute acid. Clays Clay Miner. 25, 126–130. Singer, A., 1981. The texture of palygorskite from the Rift Valley, Southern Israel. Clay Miner. 16, 415–419. Singer, A., Norrish, K., 1974. Pedogenic palygorskite occurrences in Australia. Am. Mineral. 59, 508–517. Singer, A., Stahr, K., Zarei, M., 1998. Characteristics and origin of sepiolite Meerschaum from Central Somalia. Clay Miner. 33, 349–362. Smith, D.G.W., Norem, D., 1986. The electron-microprobe analysis of palygorskite. Can. Mineral. 24, 499–511. Springer, G., 1976. Falcondoite, nickel analogue of sepiolite. Can. Mineral. 14, 407–409. Starkey, H.C., Blackmon, P.D., 1984. Sepiolite in pleistocene lake Tecopa, Inyo County, California, (1984) sepiolite in the Amboseli Basin of Kenya: a new interpretation. In: Singer, A., Galań, E. (Eds.), Palygorskite–Sepiolite: Occurrences, Genesis and Uses. Developments in Sedimentology. Elsevier, Amsterdam, pp. 137–147. Stathopoulou, E.T., Sua´rez, M., Garcıá-Romero, E., Sańchez Del Rıó, M., Kacandes, G.H., Gionis, V., Chryssikos, G.D., 2009. Composition dependence of the unit cell dimensions of palygorskite. In: Fiore, S., Belviso, C., Giannosi, M.G. (Eds.), XIV International Clay Conference Italy 2009, Book of Abstracts, I, p. 351. Stathopoulou, E.T., Sua´rez, M., Garcıá-Romero, E., Sańchez del Rıó, M., Kacandes, G.H., Gionis, V., Chryssikos, G.D. 2011. Trioctahedral entities in palygorskite: Near-infrared evidence for sepiolite-palygorskite polysomatism. Eur. J. Mineral. doi: 10.1127/0935-1221/2011/00232112. Stoessell, R.K., Hay, R.L., 1978. The geochemical origin of sepiolite and kerolite at Amboseli, Kenya. Contrib. Miner. Petrol. 65, 255–267. Stphen, I., 1954. An occurrence of palygorskite in the Shetland Isles. Mineral. Mag. 30, 471–480. Sua´rez, M., Garcıá-Romero, E., 2006a. FTIR spectroscopic study of palygorskite: influence of the composition of the octahedral sheet. Appl. Clay Sci. 31, 154–163. Sua´rez, M., Garcıá-Romero, E., 2006b. Macroscopic palygorskite form Lisbon Volcanic Complex. Eur. J. Mineral. 18, 119–126.

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Sua´rez, M., Flores, L., Anõrbe, M., Dıéz Torres, J.A., Navarrete Lo´pez, J., Martıń Pozas, J.M., 1991. Mineralogical and textural characterization of the Bercimuel palygorskite (Segovia, Spain). In: Proceedings of the 7th Euroclay Conference, p. 91. Sua´rez, M., Robert, M., Elsass, F., Martıń Pozas, J.M., 1994. Evidence of a precursor in the neoformation of palygorskite. New data by analytical electron microscopy. Clay Miner. 29, 255–264. Sua´rez, M., Flores, L., Martıń Pozas, J.M., 1995. Mineralogical data for palygorskite from Bercimuel (Segovia, Spain). Clay Miner. 30, 161–266. Sua´rez, M., Garcıá-Romero, E., Chryssikos, G.D., Gionis, V., Kakandes, G., Sańchez del Rıó, M., 2009. Structure and properties of palygorskite with excess Al. In: Fiore, S., Belviso, C., Giannosi, M.G. (Eds.), XIV International Clay Conference Italy 2009, Book of Abstracts, I 352. Sua´rez, M., Garcıá-Romero, E., Sańchez del Rio, M., Martinetto, P., Dooryheé, E., 2007. The effect of the octahedral cations on the dimensions of the palygorskite cell. Clay Mine. 42, 287–297. Takahashi, H., 1956. Occurrence of sepiolite from the Karasawa Mine, Tochigi Prefecture. J. Jpn. Assoc. Miner. Petrolog. Econ. Geol. 56, 187–190 Miner, Abstrs, 18, 242. Tauler, E., Proenza, J.A., Galı´, S., Lewis, J.F., Labrador, M., Garcıá-Romero, E., et al., 2009. Nisepiolite-falcondoite in garnierite mineralization from Falcondo Ni-laterite deposit, Dominican Republic. Clay Miner. 44, 435–454. Tien, P.L., 1973. Palygorskite from Warren Quearry Enderby, Leicestershire, England. Clay Miner. 10, 27–34. Torres-Ruı´z, J., Lo´pez- Galindo, A., Gonza´lez-Lo´pez, J.M., Delgado, A., 1994. Geochemistry of Spanish sepiolite–palygorskite deposits: genetic considerations base on trace elements and isotopes. Chem. Geol. 112, 221–245. Van Scoyoc, E.G., Serna, C., Ahlrichs, J.L., 1979. Structural changes in palygorskite during dehydration and dehydroxylation. Am. Mineral. 64, 215–223. Verrecchia, E.P., Le Coustumer, M.N., 1996. Occurrence and genesis of palygorskite and associated clay minerals in a Pleistocene calcrete Complex, Sde Boqer, Negev Desert, Israel. Clay Miner. 31, 183–202. Vicente Rodriguez, M.A., Lo´pez Gonza´lez, J.D., Banãres, M.A., 1994. Acid activation of a Spanish sepiolite: physicochemical characterization, free silica content and surface area of products obtained. Clay Miner. 29, 361–367. Watts, N.L., 1976. Paleopedogenic palygorskite from the Basal Permo-Triassic of the Northwest Scotland. Am. Mineral. 61, 299–302. Weaver, C.E., 1984. Origin and geologic implications of the palygorskite deposits of S.E, United States. In: Singer, E., Galań, E. (Eds.), Palygorskite-Sepiolite: Occurrences, Genesis and Uses, Developments in Sedimentology, Amsterdam, 39–58. Weaver, C.E., Polland, L.D., 1973. The chemistry of clay minerals. Developments in Sedimentology. Elsevier, Amsterdam, 213pp. Wiesma, J., 1970. Provenance, Genesis and Paleo-Goegraphyical Implications of Microminerals Occurring in Sedimentary Rocks of the Jordan Valley Area. Publication of Physical, Geography, Bodemkunde Laboratory, University of Amsterdam, Amsterdam. Yalçin, H., Bozkaya, O., 1995. Sepiolite–palygorskite from the Hekimhan region (Turkey). Clays Clay Miner. 32, 705–717. ¨ ., 2004. Ultramafic rock hosted vein sepiolite occurrences in the Ankara Yalçin, H., Bozkaya, O ophiolitic me´lange, Central Anatolia, Turkey. Clays Clay Miner. 52 (2), 227–239. Yeniyol, M., 1986. Vein-like sepiolite occurrence as a replacement of magnesite in Konya, Turkey. Clays Clay Miner. 34, 353–356.

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Zaaboub, N., Abdeljaouad, S., Lo´pez-Galıńdo, A., 2005. Origin of fibrous clays in Tunisian Paleogene continental deposits. J. Afr. Earth Sci. 43 (5), 491–504. Zheng, Z., 1991. Palygorskite in China. In: Sto¨rr, M., Hening, H., Adolin, P. (Eds.), Proc. 7th Euroclay Conf., Dresden, 91, Greinfswald. pp. 1209–1210. Zheng, Z., 1997. The chemistry of palygorskite clays. In: Zheng, Z.L., Song, J.X. (Eds.), Palygorskites of China. Geological Press, Beijing, pp. 26–45 (in Chinese).

Chapter 3

Environmental Influences on the Occurrences of Sepiolite and Palygorskite: A Brief Review Blair F. Jones and Kathryn M. Conko US Geological Survey, National Research Program, Reston, Virginia, USA

1. INTRODUCTION Sepiolite and palygorskite are classified within phylosilicates because they contain two-dimensional tetrahedral sheets but, differ from other layer silicates in lacking continuous octahedral sheets. The structure can be described as containing ribbons of 2:1 phylosilicate structures, one ribbon linked to the next by inversion of SiO4 tetrahedral with silica (Si) bonds. These minerals are relatively rare in nature but they have been used by man for centuries because of their diverse and useful properties. The Mg:SiO2 ratio of pure sepiolite is 2:3 and that of palygorskite ranges between 1:2 and 1:5. Other Mg-clay minerals that are commonly associated with sepiolite (and their Mg:Si ratios) are kerolite (ratio 3:4), and the Mgsmectites saponite (ratio 2:3) and stevensite (ratio 1:2). Carbonate minerals are also usually found in association with sepiolite and palygorskite, most commonly dolomite, but also calcite and even magnesite. Table 1 lists the stability relations for the sesquioxide-free hydrous magnesium silicates, showing the probable geochemical reactions of Mg2þ and SiO2 and the associated mineral phases of sepiolite and palygorskite. Millot (1964) in his early work in North Africa presented the now classic chemical scheme for closed basin clay distribution. The most Al- and Fe-rich phases tend to be at the periphery and are succeeded basin-ward by more siliceous and magnesian smectite clays, with sepiolite in the central area (Figure 1). Further refinement of this model by Eugster and Hardie (1975) described the formation of sepiolite as more likely to be found in lacustrine sediment transition facies leading to evaporate minerals, with, crystalline rocks of bordering mountain ranges providing the major source of detritus and soluble silica, along with fluctuating brackish to saline waters the primary contributors of Mg. The most effective environment for the production of Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00003-7 # 2011 Elsevier B.V. All rights reserved.

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TABLE 1 List of the Stability Relations for the Sesquioxide-Free Hydrous Magnesium Silicates, Showing the Probable Geochemical Reactions of Mg and Si and the Associated Mineral Phases of Sepiolite and Palygorskite.

1.

2Mg2þ þ 3SiO2 ðaqÞ þ 5:5H2 Osolution ¼ Mg2 Si3 O7:5 ðOHÞ 3H2 O þ 4Hþ sepiolite

2.

3Mg2þ þ 4SiO2 ðaqÞ þ 5H2 Osolution ¼ Mg3 Si4 O10 ðOHÞ2 H2 O þ 6Hþ kerolite ðtalcÞ

3.

4Mg2 Si3 O7:5 ðOHÞ 3H2 O þ Mg2þ þ 2OHsepiolite ¼ 3Mg3 Si4 O10 ðOHÞ2 H2 O þ 9H2 Okerolite ðtalcÞ

4.

1:33Mg2 Si3 O7:5 ðOHÞ 3H2 O þ 0:33Naþ þ 0:66OHsepiolite ¼ Na0:33 Mg2:67 Si4 O10 ðOHÞ2 þ 4H2 Ostevensite

5.

Mg3 Si4 O10 ðOHÞ2 H2 O þ 0:33Naþ kerolite ðtalcÞ ¼ Na0:33 Mg2:67 Si4 O10 ðOHÞ2 þ 0:33Mg2þ þ H2 Ostevensite

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Snow and rain Perennial alkaline springs

Perennial mountain streams

Block Fault Mountains

s

ial

fan

Exposed mudflats

uv All

Surface flow

Shallow lake

Groundwater flow

Bedrock

Calcitecemented gravels

Dolomitic playa muds

Cyclical playa-lake deposits

FIGURE 1 Block diagram of representative intermountain saline basin (after Eugster and Hardie, 1975) illustrating the depositional environment of playa-lake deposits.

sepiolite, particularly containing a low concentration of impurities, is associated with somewhat saline (> 2000 ppm) groundwater discharging under semi-arid climatic conditions (Mayayo et al., 1998). Examples of deposits illustrating the principle features of sepiolite occurrence as discussed above are presented here according to country of location. Details are discussed in reference to the most accessible and comprehensive reports for each area. With the exception of the United States, primarily papers published since 1988 (Jones and Galań, 1988) were used. Here, we review briefly some selected examples of sepiolite–palygorskite deposits in Spain, Turkey, Argentina, USA, and the African countries of Kenya, Morocco, Tunisia, Senegal, Somalia and South Africa.

2. SPAIN Some of the best developed accumulations of sepiolite are in the intermontane continental Tertiary strata of Spain. Probably, the most significant by size and extent are from lacustrine sediments, particularly those of the Madrid Basin, as described in detail by Galań and Castillo (1984). Additional descriptions have been offered by Calvo et al. (1986) focusing on the sepiolite and sediment facies and Leguey et al. (1989) on the petrography of the sepiolite. Mineralogy of many of the Spanish basins comprises a complete set of authigenic clay minerals ranging from sepiolite to illite, reflecting octahedral dominance from magnesium to aluminum. Particularly noteworthy was the work of Garcia-Romero et al. (2004) from the Esquivias locality, which demonstrated the most Mg-rich palygorskite material recorded at that time.

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These clays neatly filled a compositional gap that had been earlier proposed to exist between end-member sepiolite and palygorskite compositions (e.g., Paquet et al., 1987). Transmission electron microscopy (TEM) analyses by Garcia-Romero and Sua´rez (2010) indicated that no compositional gap exists between sepiolite and palygorskite, and that Al-rich sepiolites and Mg-rich palygorskites can have nearly identical chemical compositions (Figure 2) despite the differences in atomic structure. In the Madrid and several other Spanish basins, the authigenic clay spatial distribution is related to the sedimentary facies typically developed in an intermontane closed basin. This distribution is arkosic alluvial fans at the periphery, finer-grain transition facies towards the fan toes, and evaporitic materials in the central basin. Bedded sepiolite occurs primarily in transition facies but locally is also recognized in fracture fillings in the arkosic facies (Calvo et al., 1986). A number of chemical parameters have been used to determine the nature of the clay-forming environment. Differences in the concentrations of transition elements (TE) and rare earth elements (REE) have been used to distinguish detrital from precipitate phases (Torres-Ruı´z et al., 1994). Deuterium and oxygen isotopes of water, and carbon and oxygen isotopes of carbonate, have also proved useful in demonstrating evaporative concentration of pore fluids to produce elevated salinities in the sepiolite-precipitating environment. Deuterium, carbon and oxygen isotopes increase in concentration during evaporation.

AI

AI

A

B illite

Beidellite Palygorskite Ditrioct smectite Saponite

Fe

Stevensite Sepiolite

Mg

FIGURE 2 Diagram showing the (A) principal octahedral cation compositions of idealized 2:1 clay mineral types from the Mara Basin of NE Spain (modified from Mayayo et al., 1998). (B) A worldwide suite of TEM analyzed samples of sepiolite and palygorskite (Garcia-Romero and Sua´rez, 2010). Reproduced with kind permission of The Clay Minerals Society, publisher of Clays and Clay Minerals. This figure shows how the representative compositional mineral ranges compared with actual compositional data from sepiolite and palygorskite samples. Note that there is no physical separation between the mineral compositions.

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An excellent example of a sepiolitic-bearing basin illustrating the full range of mineral–chemical processes and related assemblages is the deposit of Mara. This deposit occurs in the marly-carbonate transitional facies of the Calatayud Tertiary lacustrine basin (Mayayo et al., 1998). The sepiolitebearing intervals consist of alternating strata of clays, marls and carbonates. The mineral assemblages are made up of detrital phases (illite, interstratified illite-smectite, quartz, feldspars, dioctahedral smectite, chlorite and kaolinite); neoformed phyllosilicates (sepiolite, trioctahedral smectite and palygorskite); and carbonates (calcite and/or dolomite). Sometimes heulandite–clinoptilolite, apatite and opal were also reported, the presence of these minerals indicating a source of excess silica and an alkaline environment. In this deposit palygorskite, derived from post-depositional transformation of detrital aluminosilicate minerals, has higher TE and REE concentrations that clearly distinguish it from the sepiolite and/or Mg-smectite. Low trace element data suggest that the sepiolite and/or Mg-smectite are primary phases originating by direct precipitation in a lacustrine environment. Oxygen isotope data from Mg-smectite and silica nodules indicate that the precipitating solutions were meteoric. For other Spanish sediments of a similar geologic setting to those just described, Torres-Ruı´z et al. (1994) showed that carbonates, stevensite, and sepiolite contained low TE and REE concentrations, which they attributed to direct precipitation from solution. The low REE content is in contrast to samples rich in palygorskite, which are high in REE concentration. As in the Mara deposits, detrital transformation is especially favoured when the proportion of sediment to water is high and there is considerable solute content. Such is the case in typical saline lakes or playas, where an increase of Si and Mg can occur towards the centre of the basin, reinforcing the compositional trend proposed by Millot (1964), and favouring the chemical precipitation of sepiolite. The diagenetic transformation of detrital minerals would also have been encouraged by the precipitation of carbonates, causing a decrease in permeability of the sediments, preventing colloidal transport and thus immobilizing the Al required for palygorskite formation. Oxygen isotopic data have confirmed that palygorskite and dolomite formed in equilibrium with isotopically heavier and, therefore, more evaporated solutions than the sepiolite (Torres-Ruı´z et al., 1994). Additional information on the Spanish continental deposits of sepiolite is presented in Chapter 6.

3. TURKEY One of the unique features of clay deposits in Turkey is the Mg-rich environment associated with the presence of ultramafic bedrock (Gurel and Kadir, 2006; Kadir, 2007). Clay mineral occurrence in Turkey includes sepiolite veins in ultramafic rocks (Yalçin and Bozkaya, 2004), smectites formed by

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weathering of volcanics, claystones with sepiolite that are precipitated directly from solution, and directly precipitated palygorskite during dolomitization. Locally, the formation of authigenic clay in the lacustrine environments of Turkey is by transformation of magnesite to Mg-rich clay. So, in the Hirsizdere magnesite deposit in south-west Turkey (Kadir and Akbulut, 2001), sepiolite is formed either by this transformation process or by direct precipitation from meteoric water. Akbulut and Kadir (2003) have described sepiolite intercalated with saponite, precipitated either from alkaline lake water or from interstitial pore water in dolomitic sediments. Kadir et al. (2002) noted that in this lacustrine environment with varying water depth and salinity, sepiolite and loughlinite (soda sepiolite)—both chain-structure phases—formed authigenically, but independently in different physicochemical environments (as a function of different salinities). In the Eskis¸ehir lacustrine basin, Sariiz (2000) described the main mineral assemblages to be sepiolite plus dolomite or opal, suggesting that these mineral assemblages formed at a pH range between 8 and 9 under moderately saline lake conditions. An older stratigraphic section, made up of the alternation of dolomites and dolomitic marls, had been described previously by Yeniyol (1992). The general character of sedimentation in the Eskis¸ehir and other basins in western Turkey can be illustrated in a block diagram (Figure 3). The sepiolite beds consist of sepiolite-rich layers (as much as

Basement rocks (source rocks) River

Fluviatile facies

Marginal facies 0 Plant

Dolomitic limestone Dolomitic marl Dolomitic limestone

6m

Alkaline lake inter

swam p nal fa

cies

Dolomitic marl Sepiolite Dolomitic marl Siltstone Silty limestone Conglomerate

FIGURE 3 Block diagram illustrating depositional environment for bedded sepiolite occurrences in the Mio-Pliocene strata of the Eskisehir basin, north-west Turkey (Sariiz, 2000).

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90% of the mineral) and mixed clay. These layers formed at the shallow margins of an alkaline lakes or ponds, and in marshlands. In the Neogene lacustrine basin of Konya in south-west Turkey (Karakas and Kadir, 1998) observed in scanning electron microscopy (SEM), sepiolite– palygorskite fibres and fibre bundles cover calcite and dolomite and occur as meniscus-type cement indicative of vadose solutions. Clay mineral precipitation occurred over wide-ranging climatic conditions but predominantly semi-arid. Chain-structure clay authigenesis was attributed to calcrete formation on carbonate units. In the Hekimhan region of the east, ophiolitic rocks were the detrital source of Mg for the clays (Yalçin and Bozkaya, 1995). Sepiolite was formed by diagenetic replacement of dolomite and transformation of palygorskite or by direct crystallization from solution. Cyclical variations in mineral distribution in the Yagca formation, for example, were inconsistent with the homogenous paragenesis in other units. The cycles can be explained only by decided changes in environmental conditions: sepiolite plus dolomite when Mg/Ca was high, or Mg-clay (palygorskite, smectite) and/or calcite when the ratio was lower, as shown by variations in the vertical distribution in the Hekimhan stratigraphic sections. Irkec and Unlu (1993) report on the Kibriscik sepiolite, in the Gallation Volcanic Belt (north central Turkey), formed by hydrothermal alteration of vitric tuff. The differential thermal analysis (DTA) and infrared (IR) analysis of this sepiolite were similar to palygorskite, most likely because of its high Al composition. The source of the Mg is in the leaching of basaltic and andesitic volcanics in the area. Additional information on the Turkish occurrences and deposits is collected in Chapter 7.

4. KENYA According to Stoessel and Hay (1978) and Hay and Stoessel (1984), a massive white sepiolite (meerschaum) deposit at Amboseli, Kenya, has precipitated as a result of excess Mg and silica from an earlier lacustrine deposit. In addition, kerolite (hydrous talc) formed adjacent to the massive sepiolite body by an alteration of the sepiolite when the pH fell below 8. Supersaturation with respect to sepiolite and kerolite resulted from high concentrations of Mg and SiO2 released by the weathering of alkaline olivine basalts at the foot of nearby Mt. Kilimanjaro. Nodules of carbonate with disseminated stevensite were formed by evaporation of groundwater both at or above the water table and within the pre-existing sepiolite beds (Stoessel and Hay, 1978). With fluctuating water tables, fractures and continuing carbonate crystallization, dome-shaped masses of caliche breccia formed. Next, meerschaum filled space between dolomite blocks and fractures in folded clays. This space filling occurred after growth and uplift of the caliche breccias and deformation of the clays.

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5. MOROCCO One of the most interesting and well-studied occurrences of sepiolite in the world is within the Tertiary lacustrine sequence of the Jbel Rhassoul in Morocco. Ancient lake deposits containing dolomite-rich Mesozoic carbonates and silica beds were the source of the Mg and Si for the sepiolite beds. In this alumina-poor lacustrine environment, sepiolite is characteristic of the dolomitic facies and is commonly associated with stevensite. The younger of two sedimentary stages is a fan–delta complex that pro-graded into a fresh or brackish water lacustrine (perennially wet) environment (Chahi et al., 1997). Sepiolite, stevensite and chert are confined to marly dolomitic deposits. Microscopic textures of the deposits indicate diagenetic replacement of lacustrine dolomite by stevensite. The sepiolite strata occur between palygorskite- and stevensite-rich beds. Sepiolite is typically formed during drier climatic periods, when the Mg to Si ratio is low (owing to the conservative nature of the SiO2 in the lake inflow). Wet conditions led to the formation of pure mineable beds of stevensite without any trace of sepiolite. The subsequent drier climate cycle destabilized stevensite, leading to the re-formation of sepiolite. As indicated by Chahi et al. (1999), reactive alumina, even if not abundant, may inhibit the formation of sepiolite.

6. TUNISIA Jamoussi et al. (2003) have indicated that the clay fraction of Tunisian Eocene continental sediments is dominated by aluminosilicate and can contain up to 96% palygorskite. The average chemical analysis of these clays is intermediate between dioctahedral and trioctahedral. The REE and TE concentrations of pure palygorskite are intermediate between authigenic clays (sepiolite, stevensite) and detrital clays, such as illite, mixed layers and Al-smectite. Jamoussi et al. (2003) suggested that the fibrous clay (palygorskite) originated during the first stage of diagenesis and formed by the destabilization of preexisting aluminosilicates. This dissolution–precipitation reaction may have resulted from increased solute magnesium concentration during postdepositional carbonation processes. Zaaboub et al. (2005) in an intensive study of the minerals chemistry of Paleogene sediments in central and southern Tunisia found good and widespread evidence demonstrating the negative association of REE concentrations with authigenic sepiolite and the positive correlation with detrital mineral components.

7. SENEGAL The Allou Kagne deposit in western Senegal (Garcia-Romero et al., 2007) is characterized by varying proportions of sepiolite and palygorskite. These fibrous clay minerals usually occur in horizontal beds interbedded with calcite

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and dolomite. The layers consist mainly of palygorskite mixed with lesser amounts of sepiolite. These beds were generated by sedimentation in an epicontinental marine environment. Textural and microtextural features indicate that the fibrous clays were formed by direct precipitation from solution. The relative proportions of chain-structural clays are presumably a result of chemical precipitation as solution composition evolved. Temporal shifts in solution composition, probably at the level of fine colloids, and sometimes close to ideal formulas for the authigenic minerals, precipitated sepiolite and palygorskite so closely that the two fibrous minerals sometimes form a single bundle. When palygorskite forms, Al activity is reduced and the new conditions favour the precipitation of sepiolite. Sepiolite precipitation removes Mg from the solution and increases Al activity and a new cycle begins with precipitation of palygorskite. Both minerals could have formed in the same cycle due to the proximity of the two phases in stability relations. The precipitation of a particular chain-structure clay depended on climatic changes such as evaporation, rainfall and other freshwater in-flows.

8. SOUTH AFRICA Singer et al. (1995) examined sepiolite (and palygorskite) containing soils in gently sloping landscapes formed by aeolian sands or heavily weathered pedisediments in the Namaqualand area of western South Africa. The unconsolidated sands were in various stages of calcretization (upper soil carbonate cementation). Most of the soils were shallow, and non-saline, with a pH ranging between 7.2 and 8.2. Carbonate contents generally were low, but they increase towards calcretized horizons. The clay fractions were dominated by sepiolite and palygorskite not derived from parent material but accompanied by the precipitation of illite and mixed-layer phases. The sepiolite was Fe-rich and contained organic material within the structure. The highest concentration of fibrous clay was in the basal soil horizons. It is hypothesized that the fibrous clays formed pedogenically from silicates and Mg-rich carbonates of aeolian origin, as indicated by the decrease in Mg/Ca of carbonates with soil depth. A second Mg source was sea spray. Watts (1980) held that low-Mg calcite precipitation, with the resulting relative increase in solute Mg compared to Ca, induces precipitation of authigenic palygorskite, sepiolite and minor dolomite. In the western Kalahari (Namaqualand) sepiolite has been reported (Kautz and Porada, 1976) precipitating from ground water close to the surface in a salt pan. TEM of the Namaqualand sepiolite aggregates suggested a close resemblance to a sheet clay mineral. The sepiolite laths seemed to form from, or aggregate into, smectite-like sheets, recalling the proposal by Bigham et al. (1980) that sepiolite was degrading into smectite on the Texas High Plains, under humid climatic conditions. Thus, it is suggested that Namaqualand sepiolite is, presently in a state of alteration towards a mixed layer or smectite clay, related to climatic shifts influencing calcretization.

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9. SOMALIA Singer et al. (1998) described nearly pure sepiolite clay body cropping out in a playa-like depression near El Bur, Somalia. The overall deposit includes limestone, dolomite, and gypsiferous marls, extensive anhydrite and various other evaporate phases. The soils and soil solutions are moderately to highly saline. Both sepiolite and palygorskite occur in the clay fraction. The climate is semi-arid to arid, with rainfall restricted to two short seasons. The geomorphological setting of the El Bur sepiolite resembles the Tertiary lacustrine basins of Spain and Turkey. Contrary to the Millot (1964) model, most sepiolite does not occur in the centre of the basin, but at the margins. In contrast to the paleolacustrine environments of Turkey and Spain, sepiolite may be forming at present. The absence of any detrital clay minerals indicates a lack of overland flow. This lack of a detrital precursor also supports the idea that the sepiolite formed by chemical precipitation and not by transformation of precursor minerals. An alternative and more likely source for the necessary chemical constituents, supporting this hypothesis, is groundwater. The purity of the El Bur sepiolite supports the observation that the purest sepiolite is the result of evaporative precipitation from dominantly groundwater-fed shallow water lacking reactive clay detritus (Singer et al., 1998).

10. ARGENTINA Soil-landscapes in north-east Patagonia contain fibrous clay minerals in petrocalcic horizons developed on fluvio-glacial plains (Bouza et al., 2007). The clay mineralogy is related to the age of pedogenetic periods associated with the formation of distinct geomorphic surfaces. At the present surface, the dominant clay mineral is illite, often interstratified with smectite. An older pedogenic episode was identified that contains argillic and calcic horizons, in which smectite was the dominant clay. This earlier (?) soil formation period was recorded in calcic or calcic-gypsic horizons, where palygorskite was the dominant clay mineral. Pedogenetic carbonate in the younger soils was low-magnesium calcite. The precipitation of the low-magnesium calcite resulted in increased Mg in the soil solution that in turn favoured the transformation from smectite to palygorskite. The petrocalcic horizons represent the oldest pedogenetic period, where sepiolite was the dominant clay mineral. During the calcretization processes, sepiolite precipitated from the soil solution following the formation of palygorskite. Fluorite precipitation was associated with calcite and sepiolite formation under alkaline conditions during evaporation processes.

11. UNITED STATES Some of the Pliocene–Pleistocene deposits of the Amargosa Desert in southwest Nevada are strikingly similar to those of Amboseli (Hay and Stoessel, 1984). The Amargosa area hosted an ephemeral lake basin, springs were

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the principal water supply and caliche breccia masses comparable in size to Amboseli are widespread in two parts of the area. The carbonate in the breccias is dolomite, calcite or both. The carbonates contain an average of about 10% disseminated Mg-silicate clay, principally sepiolite and stevensite, with small amounts of interlayered kerolite. Most of the clays were deposited in playa and marsh environments. The Amargosa and Amboseli caliche breccias differ principally in the amount and composition of the interstitial Mg-silicate clay. This probably reflects the difference in contributed solute composition, from volcanics versus carbonates, or perhaps the amount and nature of fine detritus (Jones, 1986). Concerning palygorskite, Weaver and Beck (1977) studied the formation in the shallow, lagoonal, peri-marine Miocene environments of Georgia and Florida, USA. Here, the Si was attributed to continental weathering and/or organisms, whereas Mg and alkalinity were ultimately derived from seaspray.This scenario has also been thought to apply to basins in France, North Africa and South Australia. Because of the large area and the mineralogical interest, the review by Jones and Galań (1988) devoted considerable attention to the investigations by McGrath (1984) of the extensive indurated calcic sediments of the Llano Estacado of west Texas. McGrath (1984) showed that the dominant chainstructure clay was palygorskite whose abundance increased with increasing calcite induration of surface soil. The subordinate sepiolite and opal content likewise increased with increasing calcite induration. In contrast, the youngest poorly indurated calcretes had no chain-structure clay at all. Sepiolite dominance in one area where it occurred was most readily attributed to the lack of solute or colloidal alumina from suitable parent material. McGrath (1984) endorsed two principle controls on the formation of the chain-structure clays originally proposed by Jones (1983): (a) direct precipitation of sepiolite by evaporative concentration of vadose solutions and (b) sepiolite formation from the absence, or immobilization, of solute or colloidal alumina. In contrast, sepiolite and palygorskite were found by McLean et al. (1972) to commonly be the major clays of the calcareous lacustrine deposits on the southern High Plains of Texas. Of particular interest were the findings of Webster and Jones (1994) in the ephemeral Double Lakes sediments (Figure 4). Here, the layered sediment mineralogy identified climate cycles in the Mg-rich clays, for example, sepiolite, Mg-smectite and palygorskite.

12. SUMMARY Based on the author’s experience and illustrated by the field examples described three major influences on the occurrences of sepiolite and palygorskite are recognized:

80


Angstroms (Å) 2.6 3.0

Sm Sp

Elevation CE10

3.6

P K

4.4

5.9

8.9

17.7

44

Cyclic repetitions Sp P

I Sm

P Sp K

P Sp Sm

3123

3123

Ephemeral

3092

Saline

3088

Brackish

3083

Saline

Borehole location

cycle 4

TB18

3092 3088

3080

Ephemeral

3080

Brackish cycle 3

3076

Ephemeral

3083

TB17

3080

3072

3080

Saline cycle 2

3076 3071

3072

Ephemeral

3069

Saline

3068

Brackish

3069

3066

Saline

3068

3064

Ephemeral

3071

cycle 1

TB11

3066 3064

34

30

25

20

15

10

5

2

Degrees 29 FIGURE 4 X-ray diffractograms of sediment layer samples from Double Lakes, Lynn County, West Texas, USA. Locations of hand dug boreholes are found in Webster and Jones (1994). Sediment separations were made at < 2 mm. This figure illustrates the cyclic nature of clay deposition and may be similar to deposits in other areas of West Texas and Turkey.

(1) Source material: The sources of sepiolite appear to require solutions providing solute Mg and Si in proportion of 2:3 to the near exclusion of other constituents. In the case of palygorskite inherited Al-rich clay minerals and Mg/Si ratios of 1:2–1:5 are also required. Common sources may include detrital facies of granitic to ultramafic parent rocks, as well as vadose zone and/or lacustrine sediment.

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(2) Climate: An arid to semi-arid climate with rainfall amounts restricted depending on the regional temperature (mild to hot). (3) Physico-chemical parameters: The optimal pH for fibrous clay minerals formation is 8–9 for sepiolite and lower than 8.5 for palygorskite. There are two major mechanisms or principal controls on the occurrence and distribution of sepiolite and palygorskite, direct precipitation from solution, and the transformation of precursor phases by dissolution–precipitation. Sepiolite is most commonly found as a result of the former, whereas palygorskite is often characterized as a product of the latter. Thus, sepiolite is typically associated with lacustrine, often somewhat saline, strata, while palygorskite most commonly is found in conjunction with soils, alluvium or calcretes.

REFERENCES Akbulut, A., Kadir, S., 2003. The geology and origin of sepiolite, palygorskite, and saponite in Neogene lacustrine sediments of the Serinisar-Acipayam basin, Denizli, SW Turkey. Clays Clay Miner. 51, 279–292. Bigham, J.M., Jaynes, W.T., Allen, B.L., 1980. Pedogenic alteration of sepiolite and palygorskite on the Texas High Plains. Soil Sci. Soc. Am. J. 44, 159–167. Bouza, P.J., Simoń, M., Aquilar, J., del Valle, H., Rostagno, M., 2007. Fibrous-clay mineral formation and soil evolution in aridisols of NE Patagonia, Argentina. Geoderma 139, 38–50. Calvo, J.P., Alonso, A.M., Garcıá del Cura, M.A., 1986. Depositional sedimentary controls on sepiolite occurrence in Paracuellos de Jarama, Madrid Basin. Geogaceta 1, 25–28. Chahi, A., Fritz, B., DuPlay, J., Weber, F., Lucas, J., 1997. Textural transition and genetic relationship between precursor stevensite and sepiolite in lacustrine sediments (Jbel Rhassoul, Morocco). Clays Clay Miner. 45, 378–389. Chahi, A., Duringer, P., Ais, M., Bouabdelli, M., Gauthier-Lafaye, F., Fritz, B., 1999. Diagenetic transformation of dolomite into stevensite in lacustrine sediments from Jbel Rhassoul, Morocco. J. Sed. Res. 69, 1123–1135. Eugster, H.P., Hardie, L.A., 1975. Sedimentation in an ancient playa-lake complex: the Wilkins Peak member of the Green River Formation of Wyoming. Bull. Geol. Soc. Am. 86, 319–334. Galań, E., Castillo, A., 1984. Sepiolite–Palygorskite in Spanish tertiary basins: genetic patterns in continental environments. In: Singer, A., Galan, E. (Eds.), Palygorskite–Sepiolite, Occurrences, Genesis and Uses. Developments in Sedimentology, vol. 37. Elsevier, Amsterdam, pp. 87–124. Garcia-Romero, E., Sua´rez, M., 2010. On the chemical composition of sepiolite and palygorskite. Clays Clay Miner. 58, 1–20. Garcia-Romero, E., Sua´rez Barrios, M., Bustillo Revuelta, M.A., 2004. Characteristics of a Mg–palygorskite in Miocene rocks, Madrid Basin. Clays Clay Miner. 52, 484–494. Garcia-Romero, E., Sua´rez, M., Santareń, J., Alvarez, A., 2007. Crystallochemical characterization of the palygorskite and sepiolite from the Allou Kagne deposit, Senegal. Clays Clay Miner. 55, 606–617. Gurel, A., Kadir, S., 2006. Geology, mineralogy, and origin of clay minerals of the Pliocene fluvial-lacustrine deposits in the Cappadocian volcanic province, central Anatolia, Turkey. Clays Clay Miner. 54, 555–570.

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Hay, R.L., Stoessel, R.L., 1984. Sepiolite in the Amboseli basin of Kenya: a new interpretation. In: Singer, A., Galan, E. (Eds.), Palygorskite–Sepiolite, Occurrences, Genesis and Uses. Developments in Sedimentology, vol. 37. Elsevier, Amsterdam, pp. 125–136. Irkec, T., Unlu, T., 1993. An example of sepiolite formation in volcanic belts by hydrothermal alteration: Kibriscik (Bolu) sepiolite occurrence. Bull. Miner. Res. Explor. 115, 49–68. Jamoussi, F., Ben Aboud, A., Lo´pez-Galindo, A., 2003. Palygorskite genesis through silicate transformation in Tunisian continental Eocene deposits. Clay Miner. 38, 187–199. Jones, B.F., 1983. Occurrence of clay minerals in surficial deposits of Southwestern Nevada. Sci. Geól. Me´m. 72, 81–92. Jones, B.F., 1986. Clay mineral diagenesis in lacustrine sediments. In: Studies in Diagenesis, U.S. Geological Survey Bulletin 1578, 291–300. Jones, B.F., Galań, E., 1988. Sepiolite and palygorskite. In: Bailey, S.W. (Ed.), Hydrous Phyllosilicates (Exclusive of Micas). Reviews in Mineralogy19, Mineralogical Society of America, Washington, DC, pp. 631–674. Kadir, S., 2007. Mineralogy, geochemistry, and genesis of smectite in Pliocene volcanoclastic rocks of the Doganbey formation, Beysehir Basin, Konya, Turkey. Clays Clay Miner. 55, 402–422. Kadir, S., Akbulut, A., 2001. Occurrence of sepiolite in the Hirsizdere sedimentary magnesite deposit, Bozkurt-Denizli, SW Turkey. Carbonate Evaporite 16, 17–25. Kadir, S., Bas, H., Karakas, Z., 2002. Origin of sepiolite and loughlinite in a Neogene volcanosedimentary lacustrine environment, Mihaliccik-Eskis¸ehir, Turkey. Can. Mineral. 40, 1091–1102. Karakas, Z., Kadir, S., 1998. Mineralogical and genetic relationships between carbonate and sepiolite–palygorskite formations in the Neogene lacustrine Konya Basin, Turkey. Carbonate Evaporite 13, 198–206. Kautz, K., Porada, H., 1976. Sepiolite formation in a pan of the Kalahari. Neues Jb Miner. Monat. 12, 545–559. Leguey, S., Pozo, M., Medina, J.A., 1989. Paleosuelos de sepiolite en el Neo´geno de la cuenca de Madrid. Estud. Geol. 45, 279–291. Mayayo, M.J., Torres-Ruiz, J., Gonza´lez-Lo´pez, J.M., Lo´pez-Galindo, A., Bauluz, B., 1998. Mineralogical and chemical characterization of the sepiolite/Mg-smectite deposit at Mara (Calatayud basin, Spain). Eur. J. Mineral. 10, 367–383. McGrath, D.A., 1984. Morphological and Mineralogical Characteristics of Indurated Caliches of the Llano Estacado. M.S. thesis. Texas Tech University, Lubbock, Texas, 123pp. McLean, S.A., Allen, B.L., Craig, J.R., 1972. The occurrence of sepiolite and attapulgite on the southern High Plains. Clays Clay Miner. 20, 143–149. Millot, G., 1964. Geologie de argiles. Masson and Cie, Paris, 510 pp. Paquet, H., Duplay, J., Valleron-Blanc, M.M., 1987. Octahedral compositions of individual particles in smectite–palygorskite and smectite–sepiolite assemblages. In: Proceedings of the International Clay Conference, Denver. pp. 73–77. Sariiz, K., 2000. The geology, mineralogy, and occurrence of bedded sepiolite deposits in the Akcayir-Yurukakcayir (Eskis¸ehir) lacustrine basin, central Turkey. Explor. Min. Geol. 9, 265–275. Singer, A., Kirsten, W., Buhmann, C., 1995. Fibrous clay minerals in the soils of Namaqualand, South Africa: characteristics and formation. Geoderma 66, 43–70. Singer, A., Stahr, K., Zarei, M., 1998. Characteristics and origin of sepiolite (Meerschaum) from central Somalia. Clay Miner. 33, 349–362. Stoessel, R.K., Hay, R.L., 1978. The geochemical origin of sepiolite and kerolite at Amboseli Kenya. Contrib. Mineral. Petrol. 65, 255–267.

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Torres-Ruı´z, A., Lo´pez-Galindo, A., Gonza´lez-Lo´pez, J.M., Delgado, A., 1994. Geochemistry of Spanish sepiolite–palygorskite deposits: genetic considerations based on trace elements and isotopes. Chem. Geol. 112, 221–245. Watts, N.L., 1980. Quaternary pedgenic palygorskite from the Kalahari (South Africa): mineralogy, genesis and diagenesis. Sedimentology 27, 661–686. Weaver, C.E., Beck, K.C. (Eds.), 1977. Miocene of the SE United States. A Model for Chemical Sedimentation in a Peri-Marine Environment. In: Developments in Sedimentology. 22, Elsevier, Amsterdam, 234pp. Webster, D.M., Jones, B.F., 1994. Paleoenvironmental implications of lacustrine clay minerals from the Double Lakes formation, southern High Plains, Texas. In: Renaut, R.W., Last, W.M. (Eds.), Sedimentology and Geochemistry of Modern and Ancient Saline Lakes, SEPM Special Publication No. 50, pp. 159–172. ¨ ., 1995. Sepiolite–palygorskite from the Hekimhan Region (Turkey). Clays Yalçin, H., Bozkaya, O Clay Miner. 43, 705–717. ¨ ., 2004. Ultramafic-rock hosted vein sepiolite occurrences in the Ankara Yalçin, H., Bozkaya, O ophiolitic me´lange, central Anatolia, Turkey. Clays Clay Miner. 52, 227–239. Yeniyol, M., 1992. Geology, mineralogy and genesis of Yenidog˘an (Sivrihisar) sepiolite deposit. Miner. Res. Explor. Bull. Turk. 114, 51–64. Zaaboub, N., Abdeljaouad, S., Lo´pez-Galindo, A., 2005. Origin of fibrous clays in Tunisian Paleogene continental deposits. J. Afr. Earth Sci. 43, 491–504.

Chapter 4

An Introduction to Palygorskite and Sepiolite Deposits— Location, Geology and Uses Haydn H. Murray*, Manuel Pozo{ and Emilio Galań{ *Department of Geological Sciences, Indiana University, Bloomington, Indiana, USA { Departamento de Geologıá y Geoquı´mica, Universidad Autońoma de Madrid, Cantoblanco, 28049 Madrid, Spain { Departamento de Cristalografıá Mineralogıá y Quı´mica Agrıćola, Facultad de Quı´mica, Universidad de Sevilla, Profesor Garcıá Gonza´lez 1. 41012 Seville, Spain

1. INTRODUCTION The name palygorskite was used in 1862 by Ssaftschenkov (1862) to describe a mineral from the Palygorsk Range, Ural Mountains, Russia. Lapparant (1935) proposed the name attapulgite for clays from Attapulgus, GA, and Mormoiron, France because he thought them to be different from palygorskite. Bailey et al. (1971) proved that the two attapulgites were the same as palygorskite. The name sepiolite was first used in 1847 by Glocker and is derived from the Greek for “cuttlefish” the bone of which is light and porous. The term palygorskite is preferred by the International Nomenclature Committee because it predates the term attapulgite (Bailey et al., 1971). However, the name attapulgite is so well entrenched in commercial circles that it continues to be used. Another complication is the use of the term “fuller’s earth”. This term is applied to any natural earthy material which will decolourize mineral or vegetable oils enough to be of economic importance. The name has no genetic or mineralogical significance. The name fuller’s earth came from its use for cleaning or fulling wool in England in the eighteenth century to remove lanolin and dirt. Palygorskite and sepiolite are surface mined in open pits, similar to bentonite. The processing involves crushing, drying, pulverization, classifying, bagging and loading. According to Murray (2002), the world production of palygorskite (attapulgite) was estimated to be about 1,000,000 tons and sepiolite about 400,000 tons. By far, the largest producer of attapulgite is the United States, which in 2000 produced 725,000 tons, or 76% of the world’s production. Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00004-9 # 2011 Elsevier B.V. All rights reserved.

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Spain is the largest producer of sepiolite and accounted for about 95% of the world’s annual production. The total world production of all fuller’s earth clays including attapulgite, sepiolite and calcium montmorillonite is estimated to be in excess of 3.3 million tons. In 2010, the annual tonnage of palygorskite was estimated to be 1,300,000 tons and sepiolite 850,000 tons. Of all the clay mineral deposits, palygorskite and sepiolite are the least common. The deposits described in this chapter are located in United States, Spain, Senegal, Turkey, Greece, Australia, Somalia, India, Ukraine, Russia and China (Figure 1). The details of the large deposits of palygorskite in China are described in Chapter 10 and those from Spanish sepiolite are described in Chapter 6. Additional information about the Eskisehir and Amargosa sepiolite is presented by Chapters 7 and 11, respectively. The major uses of palygorskite and sepiolite are in drilling muds, paints, liquid detergents, adhesives, car polish, flexiographic inks, cosmetics, floor absorbents, potting mixes, oil-spill cleanup material, carriers for fertilizers, pesticides, or hazardous chemicals, decolorize various mineral, vegetable and animal oils, as a receptor coating on carbonless copy paper, binder for pelletized animal feed, additive in cement and for pet litter. Traditional and new applications of palygorskite were discussed by Murray (2000), but in this book, there are two chapters devoted to these topics (Chapters 12 and 17).

8 6 1 5

2

7

9

13 12 3

4

10

11

Sepiolite Palygorskite

15

14

Sepiolite deposits: 1 Amargosa, Nevada (USA); 6. Vallecas-Vicálvaro-Yunclillos District and Mara (Spain); 9. Eskisehir (Turkey); 10. El-Bur (Somalia). Palygorskite deposits: 2. Meigs-Attapulgus-Quincy District, South Georgia-North Florida (USA); 3. Guatemala; 4. Theis and Nianming (Senegal); 5. Torrejón and Bercimuel (Spain); 7. Ventzia, Grevena (Greece); 8. Cherkassy (Ukraine); 11. Timsampalli-Maripalli, Andhra Pradesh, and 12 Bhawnagar, Gujarat (India); 13. Guanshan, Anhui (China); 14. Ipswich (Queensland) and15 Lake Nerramyne (Australia).

FIGURE 1 Map showing the deposit location cited in this chapter.

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An Introduction to Palygorskite and Sepiolite Deposits

87

2. UNITED STATES 2.1. South Georgia–North Florida Deposit This area is known as “The Meigs-Attapulgus-Quincy District” (Patterson, 1974). In 1893, the Owl Cigar Company attempted to manufacture bricks from some local clays near Quincy, FL. A German workman who had come from Alsace-Lorraine observed that the clay resembled fuller’s earth which he had seen in Europe. Subsequently, tests were made and the clay proved to have exceptional absorption properties. The use of fuller’s earth in the refining of oils was the major application for many years until it declined after activated bauxite was found to be a more effective substitute in 1937. The Meigs-Attapulgus-Quincy District (Georgia–Florida) is a clay-rich region chiefly of palygorskite (Weaver, 1984). Its importance justifies that this district supplied 66% of the production of fuller’s earth in the United States (Harben and Kuzvart, 1996; Patterson, 1974). The estimated resources of this area are over 3,000,000 tons. The palygorskite deposits are located in the Gulf Trough-Apalachicola embayment, an north-east–south-west trending linear structure in Southern Georgia and the eastern panhandle of Florida (Figure 2). The palygorskite

FIGURE 2 Location of the Gulf trough-Appalachicola Embayment.

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European Stage

Epoch

UPPER

Age

2

N Florida

S Georgia

Placenzian

Miccosukee Fm.

5

LOWER

PLIOCENE

System

deposits are Early to Middle Miocene in age and are part of the Hawthorn formation. A typical stratigraphic section is shown in Figure 3. According to Weaver and Beck (1977), the palygorskite formed mainly by montmorillonite alteration in shallow waters of a perimarine–lagoon environment. The paleoenvironmental interpretation has been confirmed in later, more detailed studies (Krekeler et al. 2004). Salinity, pH and temperature have been critical factors in the formation of the fibrous clay mineral (Weaver, 1984). The Gulf Trough-Apalachicola embayment was filled with sediments during the Jurassic and Early Cretaceous time and by the end of the Early Cretaceous, the filling was completed. This sediment filled the Gulf Trough so that the marine current was blocked and highly saline lagoons and embayments formed with a high magnesium content which accounted for the formation of palygorskite. In the Attapulgus–Quincy area, there are generally two strata containing palygorskite. The lower strata averages about 2 m in

Zanciean

UPPER

Messinian

Tortonian

MIDDLE

Serravallian

Altamaha Fm.

Langhian

Burdigalian

Sand and Clay Unit Dogtown Clay Mbr.

Torreya Fm.

Spochoppy Mbr.

20 m.y.

FIGURE 3 Section of Georgia–Florida palygorskite deposits.

Hawthorn Group

15

Hawthorn Group

Coosawhatchle Fm.

LOWER

MIOCENE

10

Chapter

4

CPS 44.14

89


7.37

4.04

2.79

335.6

% 100

Palygorskite

Dolomite

356.0

90 80

316.4 276.9

70

Quartz

237.3

60

197.8

50

158.2

40

118.7

30

79.1

20

39.6

10 0

0.0 2

12

22

32

FIGURE 4 X-ray diffraction pattern typical of the South Georgia–North Florida Palygorskite.

thickness and the upper layer is 1 m thick. Separating these layers is a carbonate and silty clay zone 6 m thick. The mineral content of the palygorskite beds consists dominantly of palygorskite with minor quantities of smectite, sepiolite, kaolinite, quartz and dolomite. Trace amounts of clinoptilolite occur as small euhedral crystals in association with opal-CT (Zhou, 1996). Figure 4 is an X-ray diffraction pattern typical of the South Georgia–North Florida palygorskite. Annual production is of the order of 1,000,000 metric tons. Until 1950, most of these clays were used to clean and to purify oils, but now the granular absorbents are the principal product made by palygorskite. Also, they are used in thickening, drilling muds, copy paper, light aggregates, insecticides and fungicides carriers, pharmaceuticals, agriculture, pet litter, etc.

2.2. Amargosa Deposit, Nevada The presence of sepiolite has been identified in the sedimentary filling of grabens in the southwestern U.S. Basin and Range Physiographic Province. The most interesting and exploitable deposit is located in the Amargosa Desert along the California–Nevada border (Eberl et al. 1982; Khoury et al. 1982) of Late Pliocene to Pleistocene age. Sepiolite occurs in beds as much as 1.2 m thick. In this area, there are large deposits of Mg-rich smectites (saponite) and kerolite/stevensite. In addition to these minerals, dolomite, calcite and local occurrences of detrital mica are identified.

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Based on textural evidence, Khoury et al. (1982) indicates that the sepiolite could have been formed not only as the result of direct precipitation but also from kerolite/stevensite dissolution. A secondary solution, for instance, groundwater with lower pH and sodium activity than the waters in which the kerolite/stevensite precipitated, may have been the origin of secondary sepiolite. From this deposit, bentonite (sepiolite) is used for drilling muds in freshwater, sepiolite for drilling muds in salty water, both bentonite and sepiolite for sealing joints. Amargosa sepiolite is presently mined with an annual production of more than 50,000 metric tons. Further information regarding the Amargosa deposit is included in Chapter 11.

3. SPAIN In Spain, exceptional deposits of sepiolite and palygorskite occur (Casas Ruiz, 1990; Galań and Castillo, 1984; Gonzalo Corral, 1993). The production of sepiolite is the largest in the world. Palygorskite and fuller’s earth clays (mixture of palygorskite and smectite) are also produced. These deposits are mainly located in large Tertiary continental basins (Tagus Basin, Duero Basin) or even in other smaller and narrow basins (Calatayud Basin, Torrejoń el Rubio Basin), also Tertiary in age (Figure 5). A detailed description of occurrences and deposits of sepiolite and palygorskite in Spain is included in this book (see Chapter 6).

3.1. Vallecas-Vica´lvaro-Yunclillos Deposit (Tagus Basin) The largest known sepiolite deposits are located between Madrid and Toledo. The reserves are over 20 Mt although some authors have reported up to 45 Mt (Sańchez Rodrı´guez et al., 1995), which is 70% of the world deposits. The annual production is over 600,000 tons. The most important deposit is that of Vallecas-Vica´lvaro on the outskirts of Madrid City. The sepiolite was formed mainly by neoformation in a continental environment of Miocene age (Galań and Castillo, 1984). This deposit occupies approximately 7 km2 and consists of two main lacustrine beds associated with arkosic sand facies. Sepiolite occurs forming sub-horizontal beds lenticular in shape and thickness ranging between 2 and 12 m. The beds are composed mainly of sepiolite (> 80%), smectites (15%), calcite and dolomite (2%), quartz (< 2%) and feldspars (< 1%). The sepiolite is relatively rich in Al, K, V and Ti but low in Fe. A representative chemical analysis is as follows: SiO2 (63.1%), Al2O3 (1.08%), Fe2O3 (0.27%), MgO (23.80%), CaO (0.49%), Na2O (0.09%), K2O (0.21%) and H2O (10.9%). Additionally, sepiolite has been mined together with Mg-rich bentonites associated with marginal facies in contact with alluvial arkosic deposits in

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1-Torrejón El Rubio deposit (Pk) 2-Vicalvaro-Yunclillos deposit (Sp) 3-Bercimuel deposit (Pk) 4-Mara deposit (Sp) FIGURE 5 Map of Spain showing the principal sepiolite and palygorskite deposits. For details see Figure 6 in Chapter 6.

the Yunclillos-Cabanãs de la Sagra sector (Garcia-Romero et al., 1990; Pozo et al., 1999). Yunclillos sepiolite is very pure (Figure 6). Sepiolite associated with Mg-rich bentonite (saponite, stevensite) and kerolite–stevensite-rich clays have also been exploited in the innermost zones of the basin associated with mudflat deposits (Pozo and Casas, 1999). The sepiolite has diverse uses in granular form, depending on the size of the granule (support catalysts, pet litter, absorbents, insecticides, supplements for food, among others). For granular absorbents, the sepiolite must be about 70% in purity. High purity sepiolites are used for special products (size between 40 and 60 meshes) for catalysts, rubber fillers, paints, suspensions for fertilizers, thixotropic agents and other rheological applications. Applications of the Vica´lvaro sepiolite were reported by Galań (1987). Over 90% of the world’s sepiolite usage comes from the deposit at Vica´lvaro.

92


PALYGORSKITE

Q. Quartz Do. Dolomite

Do

Pk 400

Pk Pk Pk

Q Q Pk Pk

Pk

Pk

Pk

Pk

100

0 3600

SEPIOLITE

Sp 1600

400

Sp Sp SpSp Sp

Sp

Sp

Sp

Sp Sp

Sp

Sp

Sp

Sp

0 10

20

30

40

50

60

Position [⬚2Theta]

FIGURE 6 X-ray diffraction patterns typical of sepiolite from Yunclillos (Spain) and palygorskite from Theis (Senegal).

3.2. Mara Deposit (Calatayud Basin) At Mara, Calatayud Basin (Province of Zaragoza) a sepiolite deposit is located. In this Neogene Basin, Mg-rich clay deposits (primarily sepiolite, with varying proportions of palygorskite and Mg-rich smectite) are located in a transitional marly-carbonatic facies, between fluvial detrital sediments on the edges and carbonatic-gypsiferous facies in the centre of the basin. The sepiolite bed consists of alternations of clays, marls and carbonates, with thicknesses between 10 cm and 1 m, and average values of 50–60 cm. The mineralogical associations in the sepiolite beds are complex (Mayayo et al., 1998). Both the sepiolite and the trioctahedral smectite are interpreted to have been formed by precipitation in the lacustrine basin. The palygorskite is interpreted as post-depositional resulting from the transformation of aluminosilicate phases. This deposit is presently mined with a production of more than 150,000 metric tons per year.

3.3. Bercimuel Deposit (Duero Basin) The Bercimuel (Segovia) palygorskite deposit is located in an intracratonic depression running south-west–north-west (Sepu´lveda-Aylloń) within the Duero Neogene Basin. This palygorskite deposit has been studied by Sua´rez et al. (1989). A horizontal clay unit with an average palygorskite content of 60–70% overlying a carbonate bed (crust) was commercially exploited in past years. The deposit measures more than 1500 m in length and 500 m in width. The palygorskite is associated with quartz, illite and kaolinite as detrital

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minerals, and with smectites and interstratified minerals (smectite-illite) as transformation minerals. According to Sua´rez (1992), the origin of the palygorskite is associated with the weathering of phyllosilicates and the dissolution of the quartz present in the sediment. The diagenesis producing the palygorskite formation consisted of the transformation of the original sediment in the alluvial fan. Palygorskite production can reach up to 90,000 metric tons per year.

3.4. Torrejoń Deposit (Torrejoń el Rubio Basin) The palygorskite deposit is located in a small basin filled with Tertiary sediments which has a length of 37 km and a width of 12 km (Galań et al., 1975) and is a western continuation of the Tagus Basin. The palygorskite bed has a maximum thickness of 8 m but the average thickness is 3 m. The highest concentration is in the centre of the basin, between 75% and 85%. The impurities consist of quartz, calcite, dolomite, chlorite, montmorillonite, kaolinite and mica (Galań et al., 1975). The formation of palygorskite in the Caćeres deposit was direct precipitation from silicon, aluminium and magnesium solutions coming from alteration of the basement chlorite-rich slates. A detailed description of the Caćeres palygorskite deposit is reported in Galań and Castillo (1984). Palygorskite production ranges between 2000 and 3000 metric tons per year.

4. SENEGAL The deposits of palygorskite clays are located in the so-called basin of Senegal—Mauritania (Africa), where during the Paleogene formed in an epicontinental marine environment (Wirth, 1968). The sedimentary process of the clays began in the Lower and Middle Eocene with a period of tropical humid climate that favoured the strong weathering, with accumulation of lateritic products in the continental areas and supplying of dissolved products to the sedimentary basin. In this basin by means of chemical precipitation, carbonates, silica and palygorskite formed. The exploitable deposits are located in the western zone of the country, and near to the coast, principally in the areas of Theis (NE of Dakar) and Nianming (in southern Senegal).

4.1. Theis Deposit Palygorskite is mined near the town of Theis, which is about 100 km east of Dakar. The palygorskite overlies an aluminium phosphate deposit which is also mined (Figure 7). The Senegal palygorskite is Early Eocene in age and ranges from 2 to 6 m in thickness (Wirth, 1968). The stratigraphic sequence consists of clayey marls and limestone at the base passing upward to a glauconitic phosphate bed and overlaying clays where palygorskite occurs.

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FIGURE 7 Photograph of the palygorskite and Al-phosphate deposit near Theis (Senegal).

The major mineral present is palygorskite with minor amount of quartz, dolomite, chert and sepiolite (Figure 6). The coexistence of Mg-rich palygorskite and Al-rich sepiolite has been observed by Garcia Romero et al. (2007) suggesting a possible epitactic growth. The palygorskite beds extend south–south-west from Theis to the southern border of Senegal, a distance of about 100 km. The palygorskite is mined and processed near Theis, and is transported to Dakar where it is loaded on ocean going vessels and shipped mainly to Europe. Production is higher than 200,000 metric tons per year. The palygorskite is used essentially for drilling muds in saline waters, and as additives in mortars and cements. Other applications include absorbents, catalysts, binders and feeding additives.

5. TURKEY The Turkish sepiolite deposits are important because of their extensive distribution and quality, with special mention made of the Eskisehir Basin deposits. Taking important towns as references, three sectors can be established with sepiolite deposits; Eskisehir-Konya, Denizli and Sivas. The Eskisehir–Konya sector is located as a band of lacustrine deposits running in a north-west–south-east direction in the west-central area of Turkey. The northern area contains the deposits of Eskisehir (Ece and Coban, 1994). A detailed description of occurrences and deposits of sepiolite and palygorskite in Turkey is included in this book (see Chapter 7).

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5.1. Eskisehir Deposit Ece and Coban (1994) have described the presence of sepiolite in beds and as nodules (meerschaum) in the Eskisehir Basin (Turkey). The bedded sepiolite exhibits different colours (from black to white) ranging from pure sepiolite and sepiolitic dolomite and up to 4 m in thickness. The deposit is lacustrine and dates from the Miocene, filling a graben of extensional origin (rifting). The sepiolite was formed by direct precipitation in the saline–alkaline waters of the lake with supersaturation of silica. The environment was alkaline– saline in arid to semi-arid climatic conditions with possible wet intervals owing to seasonal fluctuations. Loughlinite (sodium-rich variety of sepiolite) is currently mined at Eskisehir, where according to Yeniyol (1997) the loughlinite-bearing layers can have a thickness up to 5 m. The total reserves for meerschaum sepiolite are about 17,000 metric tons (DPT, 2001) with production up to 40 tons/year. The estimation for bedded sepiolite and dolomite-rich sepiolite reserves is around 1–2 million metric tons. The production is about 50,000 metric tons/year.

6. OTHER WORLDWIDE DEPOSITS 6.1. China According to Zhang et al. (1985) and Yang and Xu (1987), the main sepiolite deposits are located in Liling and Luiyang (Hunan Province) and Pingxiang (Jiangxi Province). The stratigraphic sequence is Permian in age and consists of marine carbonates (limestones and dolostones) with siliceous facies at the top. Sepiolite occurrences are lenticular-shaped thin beds with siliceous limestones, and both carbonate and silica nodules and talc are common. The environment inferred is perimarine where sepiolite formation by precipitation takes place. A low-grade metamorphism is responsible for the presence of talc. The sepiolite content is about 50%, up to 2 m in thickness. On the borders of Jiangsu and Anhui Provinces, more than 20 palygorskite deposits have been reported (Liu and Cai, 1993). The Guanshan palygorskite deposit in Anhui Province seems to be the most important (up to 6 m thick); the palygorskite content is higher than 50% (commonly more than 90%). A transformation from components of basaltic flows and tuffs is the genetic process inferred. The reserves are about 22 million metric tons. Further information regarding the palygorskite occurrences in China is included in Chapter 10.

6.2. Greece A large palygorskite deposit was discovered in Western Macedonia, Greece (Kastritis et al., 2003). The Ventzia Basin is north-east of the city of Grevena. The basin has a maximum width of 6 km, a length of 22 km, and an area of about 70 km2.

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The basin represents a small section of a much larger continental basin that developed during the Late Pliocene/Early Pleistocene time. The palygorskite rich beds contain concentrations ranging from 60% to 95%. The thickness of the palygorskite beds ranges between 10 and 18 m. The reserves of palygorskite are estimated to be about 6 million tons. The rheological properties of the palygorskite have been measured by Christidis et al. (2010). The palygorskite from the Ventzia Basin, Grevena, develops suspensions of high viscosity which meet API specifications even at concentrations of 5%. These suspensions are not affected by the addition of electrolytes.

6.3. Ukraine Palygorskite was discovered by Fursa in 1953, in the basin of the Gurnyitikich and Gniloitikich Rivers. This palygorskite deposit covers an area larger than 120 km2. The deposit is situated along the borders of the Cherkassy and Kiev Regions and is located in the central part of the Ukrainian crystalline massif. The age of the palygorskite beds is Tertiary and Quaternary (Fursa, 1958). The third layer in the basin consists of palygorskite (Kirichenko and Kovalenke, 1960) and is characteristically 4–5 m thick. The coarse fractions contain quartz, opal, feldspars, mica, ilmenite and hematite. Fursa (1958) believes that the palygorskite resulted in the replacement of amphiboles in the relatively high alkaline medium. The estimated reserves are 10 million metric tons (Heivilin and Murray, 1994). The Ukrainian palygorskite is used as a drilling fluid when salt bearing strata are encountered, as a plasticizer in porcelain materials, as a binder in moulding sands and as an adsorbent.

6.4. Other Occurrences Palygorskite occurs in Queensland, Australia near Ipswich and in a surface deposit at Lake Narromyne 160 km north-east of the port of Gerldton in Western Australia. The deposit is 23 km long and 55 km wide. The palygorskite is used for absorbents, insecticides, pet litter, oil refining and cosmetics. Palygorskite occurs near Kutch and Bhavnagar, Gujarat in India (Industrial Minerals Directory, 1995) with reserves of 800,000 metric tons. Dolomite is almost the only contaminant mineral. According to Siddiqui (1984), large palygorskite deposits occur in Andhra Pradesh, 80 km west of Hyderabad. The reserves in the Timsanpalli–Marepalli deposit are about 14 million metric tons. Palygorskite is used mainly for drilling muds, absorbents and pet litter (Clarke, 1985). Sepiolite occurs in Central Somalia at El-Bur, which is 120 km from Hobyo, a port on the Indian Ocean. The deposit is large and currently is used mainly for handcrafted souvenirs. There is a sizeable palygorskite deposit in Guatemala which has not yet been developed. The location is on the east coast just south of the Belize border.

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Recently, Base Investments Company in Russia discovered a palygorskite near Bay and Aevasdeve, which according to their analyses, contains 95% palygorskite and 5% quartz and dolomite. The grit content is very low 1.3%. The samples contain 7.4% MgO, 4.9% Fe2O3 and 58% SiO2. According to their tests, the drilling mud viscosity is high. The company started mining this palygorskite in 2010.

7. SUMMARY In the United States, large palygorskite deposits occur in South Georgia–North Florida. A small sepiolite deposit is located in the Amargosa desert along the Nevada–California border. The largest sepiolite deposit in the world occurs between Madrid and Toledo in Spain. Palygorskite deposits mixed with smectite are located in the Duero (Bercimuel) and Torrejoń Basins (Spain). A large palygorskite deposit is mined near Theis and Nianming in Senegal. Several small sepiolite deposits are mined in Turkey. Both sepiolite and palygorskite deposits are mined in China. The Guanshan palygorskite deposit in Anhui Province is the largest with reserves of about 22 million metric tons. A large palygorskite deposit is mined in Western Macedonia in Greece. This deposit was discovered in 2000. Palygorskite deposits are mined in the Ukraine and in Russia. Other occurrences of palygorskite are located in Australia, India and Guatemala. A large sepiolite deposit occurs in Central Somalia but is used mainly for handcrafted souvenirs. The deposits are surface mined and dry processed to produce granular and pulverized products. The major uses are for pet litter, agricultural chemical carriers, floor sweep compounds, drilling fluids, paint thickeners and tape joint compounds.

ACKNOWLEDGMENTS This work has been partially supported by Project CGL-2008-05813-CO2-02 and the Andalusian Government (PAI-RNM-135).

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Eberl, D.D., Jones, B.F., Khoury, H.N., 1982. Mixed Layer Kerolite–Stevensite from the Amargosa Desert, Nevada. Clays Clay Miner. 30, 321–326. Ece, O.I., Coban, F., 1994. Geology, occurrence, and genesis of Eskiseher Sepiolites, Turkey. Clays Clay Miner. 42, 81–92. Fursa, A.E., 1958. Sbornik Bentonitouye Glini Ukrainy, No. 5, Kiev, Izd, AN Ukr SSR. Galań, E., 1987. Industrial applications of sepiolite from Vallecas-Vicalvaro, Spain: a review. In: Proceedings International Clay Conference, pp. 400–404, Denver. Galań, E., Castillo, A., 1984. Sepiolite-palygorskite in Spanish Tertiary basins: genetical patterns in continental environments. In: Singer, A., Galań, E. (Eds.), Palygorskite–Sepiolite, Occurrences, Genesis and Uses, No. 37. Developments in Sedimentology, Elsevier, Amsterdam, pp. 87–124. Galań, E., Brell, J.M., la Inglesia, A., Robertson, R.H.S., 1975. The Caćeres Palygorskite Deposits, Spain. 1975 International Clay Conference, Mexico. Applied Publ., pp. 91–94. Garcia-Romero, E., Brell, J.M., Doval, M., Navarro, J.V., 1990. Caracterizacioń Mineralo´gica y Estratigra´fica de as Formaciones Neo´genas del Borde Sur de la Cuenca del Tajo (Comarca de la Sagra). Bol. Geol. Min. 101, 945–956. Garcia-Romero, E., Sua´rez, M., Santareń, J., Alvarez, A., 2007. Crystallo-chemical characterization of the palygorskite and sepiolite from the Allou Kagne deposit (Senegal). Clays Clay Miner. 55 (6), 606–617. Glocker, J.M., 1847. Synopsis. Halle, p190. In: Dana, J.D. (Ed.), A System of Mineralogy. fifth ed., 1868 Trunner and Co., London p. 456. Gonzalo Corral, F.J., 1993. Situacioń Actual del Sector de los Minerales Industriales en Espanã. Tierra y Tecnologia, No. 5. Madrid, Spain. Harben, P.W., Kuzvart, M., 1996. Clays: Attapulgite and Sepiolite. Industrial Minerals. A Global Geology, Industrial Minerals Information Ltd, London, UK, pp. 129–142. Heivilin, F.G., Murray, H.H., 1994. Hormites: Palygorskite (Attapulgite) and Sepiolite, No. 6. Industrial Minerals & Rocks, Littleton, CO, pp. 249–254. Industrial Minerals Directory, third ed. J. Griffith (Ed.), 553pp. Kastritis, D., Mposkos, E., Gionis, V., Kacandes, G., 2003. The palygorskite and Mg–Fe smectite clay deposits of the Ventzia Basin, Western Macedonia, Greece. In: Eliopoulos, et al., (Ed.), Mineral Exploration and Sustainable Development. Proceedings of the 7th SGA Meeting. Mill Press, Rotterdam. Khoury, H.N., Eberl, D.D., Jones, B.F., 1982. Origin of magnesium clays from the Amargosa Desert, Nevada. Clays Clay Miner. 30, 327–336. Kirichenko, N.G., Kovalenke, D.N., 1960. Sbornik, Bentonitouye Glimy Ikrainy, No. 4, 5 Kiev Ind. AN Ukr SSR. Krekeler, M.P.S., Guggenheim, S., Rakovan, J., 2004. A microtexture study of palygorskite-rich sediments from the Hawthorne Formation, southern Georgia, by transmission electron microscopy and atomic force microscopy. Clays Clay Miner. 52, 263–274. Lapparant, J. de, 1935. Sur un Constituant Essential des Terres a Foulon. C. R. Acad. Sci. Paris 201, 481–482. Liu, Ch., Cai, K., 1993. A mineralogical study of palygorskite from Guanshan, Anhui Province. Acta Mineral. Sin. 12 (1), 92–96. Mayayo, M.J., Torres-Ruiz, J., Gonzales-Lopez, J.M., Lopez-Galindo, A., Bauluz, B., 1998. Mineralogical and chemical characterization of the sepiolite/Mg-smectite deposit at Mara (Calatayud Basin, Spain). Eur. J. Miner. 10, 367–383. Murray, H.H., 2000. Traditional and new applications for kaolin, smectite, and palygorskite: a general overview. Appl. Clay Sci. 17, 207–221.

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Murray, H.H., 2002. Industrial clays case study. Min. Miner. Sustain. Dev. 64, 1–9. Patterson, S.H., 1974. Fuller’s Earth and Other Industrial Mineral Resources of the Meigs-Attapulgus-Quincy District, Georgia and Florida. Prof. Paper 828, U.S. Geological Survey, 45pp. Pozo, M., Casas, J.C., 1999. Origin of Kerolite and Associates Mg Clays in Palustrine-Lacustrine Environments. The Esquivias Deposit (Neogene Madrid Basin, Spain). Clay Miner. 34 (3), 395–418. Pozo, M., Casas, J., MartindeVidales, J.L., Medina, J.A., Martin Rubı´, J.A., 1999. Caracteristicas Texturales y Composicionales en Depo´sitos de Arcillas Magne´sicas de la Cuenca de Madrid, II. Bentonitas (sector de Cabanãs de la Sagra—Yunclillos). Bol. Geol. Min. 110–113, 273–296. Sańchez Rodrı´guez, A., Ruiz Santamaria, J., Falcoń Jimeńez, J.M., Garcia de laNoceda, C., Leoń Garrido, M., Marchań Sanz, C., et al., 1995. Libro Blanco de la Mineria de la Comunidad de Madrid. Inst. Tecnolo´g. Geomin. Espanã-Comunidad de Madrid, Madrid, Espanã, 286pp plus 2 mapas. Siddiqui, M.K.H., 1984. Occurrence of palygorskite in the Deccan trap formation in India. In: Singer, A., Galań, E. (Eds.), Palygorskite–Sepiolite: Occurrences, Genesis and Uses, No. 37. Development in Sedimentology. Elsevier, Amsterdam, pp. 243–250. Ssaftschenkov, T.V., 1862. Definition of Palygorskite. Verlag Russ., Kaiser, Gessellschaft Mineralogy, St. Petersburg, USSR, pp. 102–104. Sua´rez, M., 1992. El Yacimiento de Palygorskita de Bercimuel (Segovia). I. Mineralogıá y ´ cida. Tesis Doctoral. Geńesis. II: Caracterizacioń Fisico-Quı´mica del Mineral y Activacioń A Universidad de Salamanca, 525pp. Sua´rez, M., Armenteros, I., Martin Pozas, J.M., Navarrete, J., 1989. El Yacimiento de Palygorskita de Bercimuel (Segovia). Geńesis y Propiedades Tecnolo´gicas. Stu. Geol. Salmanticensia 26, 27–46. Weaver, C.E., 1984. Origin and geologic implications of the palygorskite deposits of SE United States. In: Singer, A., Galań, E. (Eds.), Palygorskite–Sepiolite: Occurrences, Genesis and Uses, No. 37. Developments in Sedimentology. Elsevier, Amsterdam, pp. 39–58. Weaver, C.E., Beck, K.C., 1977. Miocene of the S.E. United States: a model for chemical sedimentation in a peri-marine environment. Sediment. Geol. 17, 1–234. Wirth, L., 1968. Attapulgites du Senegal Ocidental. Rapport No. 26, Laboratoire de Geologie, Faculte´ des Sciences, Universite de Dakar, 55pp. Yang, Z., Xu, J., 1987. Diagenetic transformation of early permian sepiolite and its relationship with coal metamorphism—an example in Pingle depression and its vicinity, South China. Geochemistry 6, 65–75. Yeniyol, M., 1997. The Mineralogy and Economic Importance of a Loughlinite Deposit at Eskisehir, Turkey. In: Kodama, H., Mermut, A.R., Torrance, J.K. (Eds.), Clays for our Future, Proceedings, 11th International Clay Conference, Ottawa, Canada. pp. 83–88. Zhang, R., Qui, C., Peng, C., Day, G., Yang, Z., 1985. The characteristics of magnesium-rich clay in Liling area, Hunan Province and a discussion on its genesis. Bull. Yichang Inst. Geol. Miner. Resour. CAGS 9, 237–278. Zhou, H., 1996. Industrial Clay Mineralogy of Palygorskite from Guanshan, Anhui Province, Ph.D. Thesis. Indiana University, P.R. China 196 pp.

Chapter 5

Palygorskite Clays in Marine Sediments: Records of Extreme Climate Me´dard Thiry* and Thomas Pletsch{ *Mines ParisTech, Centre des Geósciences, 35 rue St Honore´, Fontainebleau, France { Bundesanstalt fu¨r Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, Germany

1. INTRODUCTION Palygorskite clay is a natural enrichment of the minerals palygorskite and sepiolite. These magnesium-bearing, fibrous clay minerals are rare and inhomogeneously distributed, except in arid-climate soils and lacustrine deposits, where they are common (Callen, 1984; Millot, 1970; Singer, 1984, 1989; and references within this volume). Palygorskite clay is often associated with smectite clay, with which it shares several beneficial sorptive, rheologic and catalytic properties (Galań, 1996). Current palygorskite clay production is almost entirely provided from lacustrine or lagoonal deposits that are rarely older than of Miocene age (25 Ma). These deposits have been intensively studied in recent years. Other occurrences, notably those formed in the early Eocene ( 55–49 Ma) and in the middle to late Cretaceous ( 100–65 Ma) times, have received much less attention. Therefore, the focus of this study is on palygorskite clays in deep-marine sediments of mid-Cretaceous to early Eocene age from Atlantic Ocean drill cores, originating from the Deep-Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). The data are regarded in the context of the physicochemical conditions of the deep ocean during these time periods. We propose that palygorskite clays were formed authigenically, in relation to (WSBW) circulation on the seafloor.


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2. HISTORY OF DEEP-SEA PALYGORSKITE RESEARCH 2.1. Palygorskite Clay in Continental Environments During the 1950s and 1960s, the time of the earliest clay mineral studies, palygorskite clay was mainly described from recent and ancient continental environments; first in lacustrine deposits (Millot et al., 1957; Trauth, 1977) and later in soils (El Prince et al., 1979; Mashhady et al., 1980; Paquet, 1970; Singer and Norrish, 1974). The development of palygorskite clay in continental deposits has been related to an evaporative palaeomilieu under dry palaeoclimate by virtue of their analogies with present-day palygorskitebearing deposits (Blanc-Valleron and Thiry, 1997; Colson et al., 1988; Paquet, 1983). Here, terrestrial palygorskite is only stable under arid conditions with less than 300 mm precipitation/year (Paquet, 1970; Singer, 1984).

2.2. Discovery of Palygorskite Clay in Marine Deposits Among the first to describe palygorskite in marine environments, there are Slansky et al. (1959) who studied the Eocene phosphate-bearing deposits of West African coastal basins. Similar occurrences have been described during the 1970s from several peri-marine phosphate deposits (Boujo, 1976; Kauwenbergh et al., 1990; Millot et al., 1960; Weawer and Beck, 1977). During that time, the first publications on palygorskite occurrences in the deep ocean emerged (Berger and von Rad, 1972; Bonatti and Joensuu, 1968; Chamley and Millot, 1970; Esteoule et al., 1970; Heezen et al., 1965; Timofeev et al., 1977). Although these deposits were often explained as being redeposited from confined peri-marine environments, the prevailing hypothesis was that palygorskite clays formed by some authigenic process within the deep ocean (Bowles et al., 1971; Church and Velde, 1979; Couture, 1977; Nathan and Flexer, 1977; Stoffers and Ross, 1974; Timofeev et al., 1977).

2.3. Widespread Palygorskite Clay in Deep-Sea Formations Palygorskite clay in deep-sea deposits was regarded as an uncommon feature, until about the 1980s. Several hypotheses were suggested as to its formation (Figure 1). The first palygorskite discovered in deep-sea deposits was interpreted as hydrothermal in origin (Bonatti and Joensuu, 1968; Karpoff et al., 1989). In the following, as numerous other occurrences were described, the hydrothermal hypothesis became less prevalent. Several authors suggested the development of fibrous clay minerals during early diagenesis (Berger and von Rad, 1972; Couture, 1977). The need for a Mg- and silica-rich environment often led authors to invoke alteration of volcanic materials (Kurnojov et al., 1977; Lomova, 1975; von Rad and Ro¨sch, 1972).

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FIGURE 1 Schematic sketch showing the different hypotheses for explaining the occurrence of palygorskite clay in the deep-sea realm.

Inheritance from continental evaporitic deposits and arid soils have also been evoked by some authors (e.g. Beck and Weaver, 1978; Chamley, 1989; Robert, 2009), either by fluvial or aeolian pathways. The latter interpretation relies on the idea that clay minerals provide information about continental climate and marine dispersal, rather than on oceanic environments, which are considered as relatively inert with respect to clay minerals. It has often been argued that the marine deposits of palygorskite are detrital, inherited from land, and that the presence of palygorskite in deep-marine sediments could been used as proxy evidence for an arid continental climate (e.g. Beall et al., 1973; Chamley et al., 1977; Coude´-Gaussen and Blanc, 1985; Debrabant et al., 1991; Sirocko and Lange, 1991; Weser, 1974). The major occurrences of palygorskite clay, however, are difficult—if not impossible—to explain with terrestrial formation and detrital transport to the deep ocean. Alternatively, it has been proposed that these palygorskite clay deposits formed on the deep seafloor in areas with reduced sediment accumulation rates, elevated ambient temperatures, high magnesium and silica contents, and high alkalinity (Kastner, 1981; Lo´pez Galindo, 1987; Pletsch, 1998, 2001). Non-detrital palygorskite clay has been described mainly from distal locations in the Atlantic and Pacific Oceans (Bonatti and Joensuu, 1968; Bowles et al., 1971; Church and Velde, 1979; Karpoff, 1992), but both, their age dating and genetic interpretation, were rarely convincing. As a result, the origin and the relevance of marine palygorskite and sepiolite authigenesis remain a matter of an ongoing debate (Beck and Weaver, 1978; Callen, 1981; Couture, 1978; Singer, 1979; Thiry and Jacquin, 1993).

2.4. Deep-Sea Palygorskite Clay Formation in Relation to Palaeoceanography Because the physical and chemical conditions of the oceans in the past were transitory, any record of those conditions must be checked. Analyses of stable carbon and oxygen isotopes in the calcareous tests of microfossils have become a routine tool in palaeoceanographic research, because these isotopes

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are intimately related to some of the most important properties of seawater, such as productivity, temperature and salinity. However, the interpretation of isotope data is often difficult or impossible due to secondary alteration or dissolution of the tests. Silicate minerals, including clay minerals, in contrast, are thought to be more stable in the water column and at moderate burial depths. Recent studies of mid-Cretaceous to Eocene ( 120–40 Ma) climate and palaeoceanography have shown that ancient ocean circulations were quite different from the cold and ion-depleted settings of the present-day deep ocean waters. The extended Cretaceous/Palaeogene greenhouse period was punctuated by several drastic changes in oceanic circulation and some of the warmest periods in the geologic record. Frequently cited examples of these events are the early Eocene Climatic Optimum (EECO), the Palaeocene–Eocene Thermal Maximum (PETM) and some of the Cretaceous Oceanic Anoxic Events (OAEs; Douglas and Savin, 1975; Kennett and Stott, 1990; Schlanger et al., 1987; Shackleton, 1986; Zachos et al., 2001). Stable isotope studies of these ‘periods of extreme warmth’ (Kroon et al., 2000) have revealed the existence of unusually warm deep-water temperatures and indications of an oceanic circulation pattern that was very different from that of the present oceans. These intervals were characterised by the formation of deep water in low latitudes in marginal shallow seas where ion concentrations by evaporation were so high as to reduce the buoyancy of surface water, in spite of its elevated temperature. Eventually, these brines were transported down the continental slope to become the WSBW that is required to explain the isotopic patterns typical of warm water environments (Brass et al., 1982; Huber et al., 2002, Kennett and Stott, 1990; Pak and Miller, 1992; Wilson et al., 2002; Woo et al., 1992). Strikingly, marine sediments from the periods of extreme warmth frequently contain palygorskite clay (Pletsch, 1998, 2001). In the following, we present two case studies of marine palygorskite clay formation and their relation to the contemporaneous oceanic environment: in the Central Atlantic during the mid-Cretaceous (Thiry and Jacquin, 1993; Pletsch, 1997), and in the Gulf of Guinea and Sargasso Sea during the early Eocene (Pletsch, 1998, 2001, 2003).

3. CRETACEOUS PALYGORSKITE CLAY IN THE CENTRAL ATLANTIC Palygorskite is frequently present in the Cretaceous sediments of the Atlantic Ocean. Here, we will examine palygorskite clay distribution in Albian and Aptian sediments in three DSDP drill sites (370, 417 and 105) that are situated on an approximately East–West transect through the Central Atlantic Ocean (Figure 2).

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FIGURE 2 DSDP sites (triangles) in the Central Atlantic Ocean for which clay mineral assemblages were analysed. The East–West transect comprising Sites 370 and 105 is the reference section for the setting of the mid-Cretaceous oceanography.

3.1. Occurrence of Palygorskite 3.1.1. DSDP Site 370 DSDP Site 370 (Lancelot et al., 1978) was drilled off Morocco, at 4214 m water depth, on the foot of the African continental slope. The sediments show a typical mineralogical succession of the Cretaceous Atlantic series (Brosse, 1982; Thiry and Jacquin, 1993) and provide information on depositional environments (Graciansky et al., 1987). The Cretaceous series are relatively thick (Figure 3). Two main sedimentary series can be distinguished. The lower unit, up to the early Albian, consists mainly of detrital deposits. The upper unit, from Albian to Cenomanian, consists of nannofossil-bearing claystone, black shale and dark gray marlstone. Black shales show thin laminations, without any bioturbation and with only few foraminifers and radiolarians. Glauconite grains are present in all of these facies. The black shales were deposited under severe anoxic conditions, in a lower bathyal environment and below the carbonate compensation depth (CCD). The clay minerals are grouped into three distinct assemblages (Figure 3). Hauterivian–Barremian detrital deposits predominantly contain illite and illite–smectite (I–S) mixed-layer minerals, with minor amounts of kaolinite, and sporadically smectite in the clay size fraction. Within the upper Albian (Vraconian) section, palygorskite forms up to 50% of the clay minerals, together with I–S minerals and chlorite. In the upper Cretaceous section,

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FIGURE 3 Whole rock and clay size fraction of Cretaceous sediments from DSDP Site 370, off Morocco. Palygorskite occurs with a sharp onset and constitutes the main clay mineral in the middle/late Albian. The stratigraphy is adapted after Mu¨ller et al. (1983).

smectite makes up almost the entire clay fraction. Dolomite appears with the palygorskite association and is also present during the early Cenomanian.

3.1.2. DSDP Site 417 DSDP Site 417 (Donnelly et al., 1979) lies on the southernmost part of the Bermuda Rise, at 5482 m water depth. The mid-Cretaceous section was deposited on basaltic basement. There are two main sedimentary units: black and greenish siltstones in the lower part including some black shales and upper Cretaceous variegated claystones. Both units are nearly carbonate-free, which implies a deposition below the CCD. Again, the clay minerals are grouped into three distinct assemblages (Figure 4). Aptian detrital and volcanoclastic deposits predominantly contain illite and I–S minerals. Smectite appears in the lower Albian and increases in importance until comprising almost the only clay mineral in upper Albian

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FIGURE 4 Whole rock and clay size fraction of Cretaceous sediments from DSDP Site 417, on the southern flank of the Bermuda Rise. Palygorskite occurs only in a narrow interval during the middle Albian. The stratigraphy is adapted after Mu¨ller et al. (1983).

(Vraconian) and lower Cenomanian. Palygorskite occurs only in a very narrow, middle Albian interval. Opal-CT appears together with cristobalite in the lower Cenomanian.

3.1.3. DSDP Site 105 DSDP Site 105 (Hollister et al., 1972) is located in the Cap Hatteras Basin, on the foot of the American continental slope, at 5251 m water depth. The midCretaceous section is made up of black and greenish claystones, typical for black shale facies deposited below the CCD. The mineralogical composition is very monotonous (Figure 5). The clay fraction is almost exclusively formed by smectite. Additional illite and kaolinite constitute less than 10% of the assemblage. As at many other drill sites on the western margin of the Central Atlantic, the abundance of opal and clinoptilolite (zeolite), together with the lack of palygorskite, are the characteristic features (Berger and von Rad, 1972).

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FIGURE 5 Lithology and mineralogy of the Cretaceous formations from DSDP Site 105 at the foot of the North American continental slope. No palygorskite was observed, in contrast, opal-CT and clinoptilolite are abundant at this site. Stratigraphy after Mu¨ller et al. (1983).

3.2. Clay Mineral Assemblage Distribution The sharp onset of palygorskite appearance during the Albian is known from several DSDP sites off-shore Africa and Spain (Sites 137, 369, 370, 386, 417, 545, 598 and 641; Figure 6). It has to be emphasised that the diagrams of Figure 6 give mean values of the clay mineral assemblages. Short intervals and individual samples may reach even higher values. This is particularly the case for Albian strata at DSDP Sites 386, 137 and 417. Correlative sections on the western and northern sides of the Central Atlantic Ocean (DSDP Sites 105, 387, 391 and 549) do not display any palygorskite. In these latter sites, above the basal detrital paragenesis (with illite and I–S mixed layers), smectite is nearly the only clay mineral, together with clinoptilolite and opal-CT (radiolarians) that may reach 30% of the bulk sediment (Thiry and Jacquin, 1993). Palygorskite- and the opal-CT/clinoptilolite-bearing assemblages exclude each other in appearance. Coeval deposits in continental basins around the Atlantic display entirely different clay mineral assemblages: no palygorskite was detected in the Paris and Aquitanian basins (Debrabant et al., 1992; Louail, 1984; Sladen, 1983), Spanish (Berthou et al., 1982; Floquet, 1991), Moroccan (Daoudi et al., 1989) and Senegalese basins (Chamley, 1989). Kaolinite and illitic minerals

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FIGURE 6 Clay mineral distribution in mid-Cretaceous deep-sea and in platform deposits of the Central Atlantic. Diagrams display mean values of the clay mineral assemblages. Short intervals and individual samples may reach even higher palygorskite percentages. Note the contrast between deep-sea assemblages dominated by smectite and palygorskite versus platform assemblages in which illite and kaolinite are dominant.

are dominant in these basins and result from continental inheritance during this period (Thiry and Jacquin, 1993).

3.3. Significance of Palygorskite The opposition between palygorskite occurrence in the deep-sea and its lack in correlative peri-Atlantic marine and continental deposits does not argue in favour of a detrital origin in the mid-Cretaceous of the Central Atlantic. Occurrence of individual layers with high palygorskite content at sites far away from the continents, such as in DSDP Site 417, cannot be explained by detrital input. In this latter case, palygorskite would have been much stronger diluted. In situ, development of palygorskite in the Atlantic Ocean during this period has to be taken into consideration. This would require a silica- and Mg-rich palaeoenvironment, as is often testified to by the presence of dolomite within the same, palygorskite-bearing deposits.

3.4. Geochemistry of Cretaceous Deep Water Oxygen isotopes and the distribution of certain nannofossils reveal that, during the middle/late Cretaceous period, the bottom water of the Atlantic Ocean has been warm and hypersaline (WSBW; Brass et al., 1982; Hay, 1988; Huber

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et al., 2002; Woo et al., 1992). According to the low oxygen solubility in such waters (Weiss, 1970), they are probably partly responsible for the development of the Cretaceous anoxic events. Such brines are also thought to be related to the dolomitisation of the Jurassic substratum off-shore Portugal during the same period (Haggerty and Smith, 1988). The hypersaline bottom water of this period had geochemical characteristics similar to those in evaporitic environments, where palygorskite and dolomite usually develop in continental and marginal marine environments. Under these peculiar conditions, palygorskite and dolomite can develop in deep-sea environments. According to the fact that palygorskite holds about 10% of Al2O3 and that alumina is poorly soluble in this type of solution, a strict authigenesis is practically excluded. Palygorskite is more likely to have developed by transformation of former clay minerals. Exchange with the oceanic brines is necessary, and transformation has to occur at the water/sediment interface or during early diagenesis, before compaction has restricted exchange with ocean water. The magnesium was supplied for both palygorskite and dolomite by the hypersaline bottom water fed from the evaporative platform. The origin of silica remains to be determined. Organic productivity is an obvious source for the western sites, where radiolarians are frequently preserved (Brosse, 1982). The eastern area had also a high organic productivity, as testified by the thick black shales (Graciansky et al., 1987); the biogenic silica bound to this organic productivity contributed probably to the development of palygorskite. In this way, both palygorskite–dolomite and clinoptilolite-opal-CT paragenesis reflect silica-rich environments: The first system is Mg-rich, the second one is Mg-depleted. Such a geochemical environment, with increased water temperatures during the Cretaceous and biogenic silica, was presented as early as the 1970s for the palygorskite and zeolite occurrences in Pacific Ocean (Couture, 1977) and southern Israel (Nathan and Flexer, 1977).

3.5. Palaeoceanography and Palaeogeography The salinity of the WSBW is thought to result from the outflow of dense brines towards the deep-sea. The brines developed on evaporative platforms and tropical seas (Brass et al., 1982; Busson, 1989; Roth, 1986; Woo et al., 1992). During the mid-Cretaceous, the Central Atlantic Ocean was relatively narrow, restricted ocean basin. Its north-eastern rim was dominated by a wide, shallow platform that covered northern Africa from Morocco to Libya. Dolomitic and gypsiferous deposits on this platform testify to evaporitic environments, but these have not led to the final halite or other salt deposits. Huge volumes of brines (rich in Cl, Na, K and Mg), with a density of about 1.2, must have been generated on this platform and were most probably exported to the Atlantic Ocean (Busson, 1984a,b). The Bermudas formed a natural barrier restricting the outspreading of these brines fed from the Sahara platform

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FIGURE 7 Schematic sketch of the palaeoceanography of the North Atlantic Ocean during the mid-Cretaceous. (A) Brine flow from the Sahara evaporitic platform towards the Atlantic Ocean and the Bermuda Rise formed a ridge that restricted the out spreading of the WSBW. (B) It resulted in a geochemical differentiation between the eastern and western regions according to availability or not of Mg. These WSBW environmental conditions have no present analogue.

(Figure 7). This barrier also coincided with the geographic extension of the anoxic black shales to the West (Graciansky (de) et al., 1982). The palaeogeographic arrangement explains the mineralogical and geochemical differences between the eastern and western Cretaceous deposits. Biogenic silica production occurred in the surface water all over the Central Atlantic Ocean. In the West, on the Blake Plateau, and in the absence of Mg-rich brines, this silica production led to opal and clinoptilolite formation. In the East, where WSBW spread out, the Mg-rich brines with the addition of silica led to the development of dolomite and palygorskite. In the eastern region, silica has been fully exhausted by the development of palygorskite.

3.6. Conclusions (1) Huge amounts of palygorskite exist in the mid-Cretaceous (Albian) deepsea deposits of the Central Atlantic. (2) Palygorskite is unlikely to be inherited from neritic, peri-marine or continental formations, because the deposits of the surrounding basins are usually devoid of palygorskite. The majority must have developed in situ in the deep sea. (3) Palygorskite developed at the contact with hypersaline and warm brines which formed the bottom waters of the Atlantic Ocean during the midCretaceous. They likely result from transformation or aggradation of inherited clay minerals at the seafloor and from early diagenesis, before compaction prevented further exchange with the oceanic brines.

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(4) Palygorskite distribution is restricted to the eastern part of the Central Atlantic Ocean. This corresponds to the maximum outspreading of the brines generated on the evaporitic Saharan platform, this westerly flow stopped again the Bermuda Rise.

4. EARLY EOCENE PALYGORSKITE CLAY IN GULF OF GUINEA AND SARGASSO SEA 4.1. Gulf of Guinea ODP sites 959 through 962 in the Gulf of Guinea are situated along the Marginal Ridge of the Coˆte d’Ivoire-Ghana Transform Margin (Mascle et al., 1996). Lower Eocene sediments were recovered at burial depths ranging between 60 and 800 metres below the seafloor (mbsf; Figure 8). Sedimentation rates are

FIGURE 8 Generalized stratigraphy of palygorskite clay intervals (grey shade) from selected drill holes in the Gulf of Guinea. Depth in section is given in mbsf (¼ metres below seafloor). Biostratigraphic data given as nanno (¼ calcareous nannofossil) zones. Note different burial depths but correlative stratigraphic position of palygorskite clay intervals.

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high landwards (Sites 959 and 960) and strongly decrease towards the basin, at the foot of the ridge (Site 962). Palygorskite and sepiolite were detected in various concentrations in sediments ranging from mid-Cretaceous to early Eocene age. Particularly, pure (only minor admixtures of smectite, opal, clinoptilolite and rarely calcite) blue palygorskite clay containing with intercalated chert, barite sand and barite nodules of early Eocene age was recovered at Sites 960 and 961 (Pletsch, 1998). The two sites lie on an elongate sedimentary basement high about 200 km off the African shelf at water depths of approximately 2000 and 3300 m. Palaeo-water depths were shallower but certainly in the range of several thousands of metres (Mascle et al., 1996). The palygorskite clays are underlain by limestone (Site 960) and siliceous marl (Site 961) and overlain at both sites by Eocene to Miocene chert and porcellanite. It is important to note that the purest palygorskite clay is limited to a narrow stratigraphic interval of about 3 My in the middle part of the lower Eocene (calcareous nannofossil zones CP9b to CP10). There are no signs of reworking of the underlying sediment or of contemporaneous detrital input at any scale. Rather, the palygorskite clay consists of a microscopic mesh of delicate elongate fibres, some of which pierce calcareous nannofossils, that indicates post-depositional crystal growth. The purity of the blue palygorskite clays also argues against a detrital origin, because it is difficult to conceive how an almost monomineralic sediment could be deposited at such a short distance from the African continent. Moreover, geochemical reconnaissance and stable oxygen isotopes from the palygorskite intervals of ODP Leg 159 are compatible with formation of palygorskite clay under conditions of elevated temperature and alkalinity, sufficient Mg and Si supply, and the incorporation of seawater into the authigenic silicate structure (Pletsch et al., 1998).

4.2. Sargasso Sea Additional evidence, supporting the widespread occurrence of authigenic palygorskite clay deposited during the early Eocene extreme warmth period, comes from a regional study of palygorskite clay in the Sargasso Sea (Figure 9). Three localities were chosen to represent different palaeowater depths and rates of sediment supply. During ODP Leg 171B, a palaeoceanographic drilling transect was sampled along the crest of Blake Nose, which projects into the western Central Atlantic. Minor palygorskite clay showing fragile, authigenic microstructures was found at 2670 m water depth in lower Eocene calcareous nannofossil ooze containing intercalated volcanic ash at the deepest ODP Sites 390 and 1049. No relationship was observed between the palygorskite clay and the ash layers (Pletsch and Reicherter, 2001). Palygorskite clay was not found at any of the drill sites landward of ODP Site 1049. Significant amounts of palygorskite clay containing authigenic microstructures were only detected at basinal DSDP Site 417, where terrigenous sediment accumulation was very low. In contrast, virtually no palygorskite clay was

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FIGURE 9 Generalized stratigraphy of the palygorskite clay intervals (grey shade) of selected Sargasso Sea drill holes. Palygorskite occurrences in the Eocene formations from DSDP Sites 386, 390 and 417, and ODP Site 1049 are restricted to a short interval of early Eocene deposits. For explanations see Figure 8. Modified from Pletsch (2001).

found in coeval sediments at basinal DSDP Site 386, which received abundant terrigenous material from the American continental rise, in spite of its position on a ridge in this basin. In only one sample from this section, the X-ray diffraction pattern did show any weak indication of palygorskite clay. The stratigraphic age of palygorskite clay in the Sargasso Sea sites is slightly younger than at the sites in the Gulf of Guinea but lies still entirely

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within the early Eocene. The maximum abundance in the Gulf of Guinea is 1–3 million years earlier than in the Sargasso Sea. Whether this difference reflects variations in the intensity and/or distribution of WSBW, and hence, whether peak WSBW conditions in the western Central Atlantic were reached later than in the Gulf of Guinea, remains to be explored.

4.3. Distribution of Early Eocene Palygorskite Clay A compilation of palygorskite clay occurrences in the early Eocene helps us to understand the processes that controlled the distribution of the constituent minerals. For this reason, the DSDP/ODP literature and ancillary sources with an adequate stratigraphic and mineralogic treatment were checked for the occurrence of palygorskite and sepiolite in early Eocene sediments (Figure 10). The compilation is still preliminary and is biased towards sediments in and around the Atlantic and western Tethys Oceans. No conclusions should be drawn at this stage from distribution data for the Pacific and Indian Oceans, because sampling density is poor and the stratigraphy of several palygorskite-bearing deposits is vague at best. In spite of these restrictions, the distribution map reveals several patterns in the Atlantic and Tethyan realm that may be extrapolated to other regions once the database becomes more complete. Palygorskite clay has been found in lower Eocene oceanic sediments in a belt between about 30 North and 30 South of the palaeoequator. This general latitudinal distribution pattern of palygorskite clay in the early Eocene

FIGURE 10 Distribution of lower Eocene palygorskite clays plotted on a palaeogeographic map for the Eocene (53 Ma). Note that lack of palygorskite clay in terrigenous deposits adjacent to studied locations in the Gulf of Guinea and in the Sargasso Sea. Palygorskite clay occurrences along the Iberian continental margin are located at suspected outflow of warm, saline bottom water masses from Tethys Ocean. Modified from Pletsch et al. (1998).

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may at the first glance fit with a model in which palygorskite clay formed in desert soils, coastal sabkhas, or lagoons and was reworked and deposited in deep-sea realm. However, this model cannot explain a number of small-scale distribution patterns. Most strikingly, the studied occurrences in the Gulf of Guinea and in the Sargasso Sea are seaward of terrigenous sediments in which palygorskite clay was not detected. The Guinean coastal region is known for its Cretaceous to Eocene bauxite deposits (Valeton, 1999) which are incompatible with the formation and the preservation of palygorskite clays (Paquet, 1970; Singer, 1984). The overall west Africa shield was at Eocene time almost covered with thick kaolinitic soil profiles (Beauvais et al., 2008; Heńocque et al., 1998). In a similar way, palygorskite clay was neither found at any of the drill sites landward of the Blake Nose transect, nor is it known to occur in the (particularly well studied) sediments on the eastern U.S. seaboard. Further to the continental source areas, kaolinite is a main component of the Palaeocene/ Eocene neritic deposits of north-western U.S. margin (Gibson and Bybell, 1994), and kaolinite was the predominant clay mineral in the palaeosols and continental deposits of early Tertiary in the Gulf Coastal Plain of North America (Gordon et al., 1958), where several economic kaolin deposits relate to early Tertiary (Al Sanabani, 1991; Cramer, 1974). Palygorskite clay minerals are never quoted in early Eocene continental and margino-marine deposits, and thus detrital origin of palygorskite clay is unlikely at any of the studied drill sites. Several other occurrences of palygorskite clay are situated outside the areas indicated by the climate model to have suffered from the strongly arid conditions that are required to form and preserve palygorskite clay on land, such as for the sites just north of South America, and along the western and southern Iberian margin (Figure 10). The case of the palygorskite clays in marine deposits North of South America can be argument further. Bauxitic and kaolinitic deposits are well known in Suriname (Aleva, 1979) as well as deep lateritic (kaolinitic) weathering profiles dating back to lower Tertiary in Guyana (The´veniaut and Freysinnet, 2002) and on the Amazonian shield (Ruffet et al., 1996; Vasconcelos et al., 1994). During early Eocene, the hinterland of the palygorskite clay deposits North of South America was covered by kaolinitic soils and thus palygorskite cannot be inherited from land areas. A similar situation exists with palygorskite clay deposits off the west European continent. Early Eocene continental deposits of the Spanish, English, German and French basins are predominantly formed of kaolinite along with Al-smectite (Blanc-Valleron and Thiry, 1997; Brosius and Gramann, 1958; Dubreuilh et al., 1984; Garcia-Talegon et al., 1994; Isaac, 1983; Molina and Blanco, 1980). Palygorskite clay deposits appear only after the middle Eocene and become widespread only during the late Eocene in these continental deposits.

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4.4. The Palaeoenvironment The palaeoclimate simulations for the end of the Palaeocene ( 55 Ma) show that the zones of net evaporation fall mostly into marine areas rather than into land areas (O’Connell et al., 1996). Marine regions have the highest rates of evaporation, which in turn favours the formation of warm, saline brines. These brines are thought to have been the driving force of the early Eocene bottom water circulation. Many of the palygorskite clay occurrences in the Atlantic and western Tethys Ocean are close to these potential sources of WSBW. The palygorskite occurrence on the Iberian margin may have formed during the warm inflow of deep-water masses into the Atlantic from the Tethys Ocean, similar to the present-day injection of warm, saline Mediterranean water into the Atlantic (Tomczak and Godfrey, 1994). Lower Eocene deep-marine palygorskite clay deposits often occur distally to correlative deposits on land or in shallow-marine environments, especially at subtropical latitudes. These deposits were supplied, at least partly, from continental and proximal marine environments via detrital input. However, the lack of landward continuity, the remarkable purity and the microscopic textures of palygorskite from several localities suggest that authigenesis at the seafloor is the origin processes of a large part of the palygorskite clay formed during the early Eocene. The acceptance of the authigenic formation of palygorskite clay at the seafloor compromises a straightforward use as a proxy parameter for aridity on land. But, the acceptance of deep-sea authigenic palygorskite clay formation is hampered by the absence of unequivocal examples from the present time and by the lack of a satisfactory thermodynamic model (Jones and Galań, 1988; Kastner, 1981). These arguments can be neglected if the WSBW is taken into account. Although palygorskite clay may be formed diagenetically by the conjunction of saline brines providing Mg, inherited clay minerals supplying Si and Al, and addition of biogenetic silica (Badaut and Risacher, 1983; Michalopoulos et al., 2000). Actualism cannot be applied here, as there is no WSBW in the present-day oceans, and thermodynamic models have to be tested by taking into account geochemical characteristics of the WSBW.

4.5. Conclusions The formation of lower Eocene palygorskite clay in the deep-sea correlates in time with the wide distribution of WSBW during the EECO. Elevated temperatures, alkalinity and ion concentrations were all favourable for the formation of these particular silicates at the seafloor. Palaeontological and isotopic evidence indicate that early Eocene bottom water was 8 C warmer than today (Zachos et al., 2001). At places where this warm water entered the deep-sea, temperatures and ion concentrations may have been even higher, which lowered the kinetic restrictions that prevent the formation of palygorskite clay on

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the present ocean floor. Authigenic deep-sea palygorskite clay may therefore be considered as a vestige of past greenhouse oceans that have no modern analogue.

5. PERSPECTIVES FOR MARINE PALYGORSKITE RESEARCH Palygorskite clays developed in the deep ocean at specific time intervals throughout geologic history. The discrepancy between the clay mineral assemblages of these deep-oceanic deposits and those of the correlative neritic, peri-marine and continental deposits are not in favour of a detrital supply of palygorskite from the continental evaporative environments to the deep ocean. Authigenesis of palygorskite clay in oceanic environments has to be taken in account. Palaeontological and isotopic evidence indicate that bottom water of the Atlantic Ocean has been warm and hypersaline (WSBW) during mid-Cretaceous (Brass et al., 1982; Hay, 1988; Huber et al., 2002; Woo et al., 1992) and early Eocene (Zachos et al., 2001) periods, during which palygorskite clay has developed in the Atlantic Ocean. The hypersaline bottom waters, issued from brines formed in evaporative areas, had geochemical characteristics similar to those in evaporitic environments, where palygorskite usually develops in continental and marginal marine environments. Under these peculiar conditions, palygorskite can develop in deep-sea environments. The case study of the mid-Cretaceous palygorskite clay in the Atlantic Ocean points out that biogenic silica addition to the brines may be a major contribution to the development of these clays. More regional studies along transects from ancient continental to deepmarine settings are needed to investigate the geochemical environments. It has to be questioned why only palygorskite develops during the midCretaceous WSBW episode in the Atlantic, whereas palygorskite plus sepiolite develop during the early Eocene episode? This may be due to the Mg/Si ratio or any other geochemical characteristic (salinity, concentration, temperature, etc.) of the saline bottom water. As these clay minerals appear as authigenic, the study of their stable isotope composition may help to specify the geochemical characteristics of these palaeoenvironments. Moreover, these palygorskite clays may be a major proxy for tracing occurrence and palaeogeographic distribution of WSBW during these specific climatic periods.

ACKNOWLEDGEMENTS For this project, samples were provided by the Deep-Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). The German Research Council (DFG) and the Conseil National de Recherche Scientifique (CNRS) provided funding to the authors and to their institutions.

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Pletsch, T., 1997. Clay minerals in Cretaceous deep-water formations of the Rif and the Betic Cordillera (northern Morocco & southern Spain). Soc. Geól. Nord Publ., vol. 26, 1–106. Pletsch, T., 1998. Origin of Lower Eocene palygorskite claystones on the Coˆte d’Ivoire-Ghana Margin (Leg 159, eastern Equatorial Atlantic). In: Mascle, J., Lohmann, G.P., Moullade, M. (Eds.), Scientific Results, vol. 159, Ocean Drilling Program. College Station, TX, pp. 137–152. http:// www-odp.tamu.edu/publications/159_SR/CHAPTERS/CHAP_15.PDF. Pletsch, T., 2001. Palaeoenvironmental implications of palygorskite clays in Eocene deep-water sediments from the western Central Atlantic. In: Norris, R.D., Kroon, D., Klaus, A. (Eds.), Western North Atlantic Palaeogene and Cretaceous Palaeoceanography, Geological Society of London Special Publication, 183, 307–316. Pletsch, T., 2003. Authigenic palygorskite in deep-sea sediments—mineral deposits formed in ancient greenhouse oceans? Z. Angew. Geol. 2, 22–29. Pletsch, T., Reicherter, K., 2001. Hole-to-hole correlation of Eocene volcanic ash layers from the Blake Nose depth transect, Leg 171B. In: Kroon, D., Norris, R.D., Klaus, A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 171B1–10. http://www-odp.tamu.edu/ publications/171B_SR/VOLUME/CHAPTERS/SR171B08.PDF. Pletsch, T., Botz, R., Hisada, K., Kajiwara, Y., Wray, D.S., 1998. Oceanographic Controls on the Authigenesis of Palygorskite in Lower Eocene Deep-Sea Clays from the Eastern Equatorial Atlantic (ODP Leg 159). Reúnion spećialiseé ASF-SGF, Argiles: se´dimentologie, diagene`se, environnement, livre des re´sume´s, N 30, Association des Se´dimentologistes Français, Paris, pp. 79–80. Robert, C., 2009. Global sedimentology of the ocean: an interplay between geodynamics and paleoenvironment. Developments in Marine Geology, vol. 3. Elsevier, Amsterdam 487 pp. Roth, P.H., 1986. Mesozoic palaeoceonography of the North Atlantic and Tethis Oceans. In: Summerhayes, C.P., Shackleton, N.J. (Eds.), North Atlantic Palaeoceanography, Geological Society of London Special Publication, vol. 21, 299–320. Ruffet, G., Innocent, C., Michard, A., Fe´raud, G., Beauvais, A., Nahon, D., et al., 1996. A geochronological 40Ar/39Ar and 87Rb/87Sr study of K-Mn oxides from the weathering sequence of Azul, Brazil. Geochim. Cosmochim. Acta 60 (12), 2219–2232. Schlanger, S.O., Arthur, M.A., Jenkyns, H.C., Scholle, P.A., 1987. The Cenomanian-Turonian Oceanic Anoxic Event, I. Stratigraphy and distribution of organic carbon-rich beds and the marine @ 13C excursion. In: Brooks, J., Fleet, A.J. (Eds.), Marine Petroleum Source Rock, Special Publication, vol. 26, Geological Society, London, pp. 371–399. Shackleton, N.J., 1986. Paleogene stable isotope events. Palaeogeogr. Climatol. Ecol. 57, 91–102. Singer, A., 1979. Palygorskite in sediments: detrital, diagenetic or neoformed—A critical review. Geol. Rdsch. 68, 996–1008. Singer, A., 1984. The paleoclimatic interpretation of clay minerals in sediments—a review. Earth Sci. Rev. 21, 251–293. Singer, A., 1989. Palygorskite and sepiolite group minerals. In: Minerals in Soil Environments. SSSA Book Series, 1. Soil Science Society of America, Madison/Wisconsin, pp. 829–872. Singer, A., Norrish, K., 1974. Pedogenic palygorskite occurences in Australia. Am. Mineral. 59, 508–517. Sirocko, F., Lange, H., 1991. Clay-mineral accumulation in the Arabian Sea during the late Quaternary. Mar. Geol. 97, 105–119. Sladen, C.P., 1983. Trends in early Cretaceous clay mineralogy in NW Europe. Zitteliana 10, 349–357. Slansky, M., Camez, T., Millot, G., 1959. Se´dimentation argileuse et phosphate au Dahomey. Bull. Soc. Geólogique Fr. 1, 150–155, se´rie 7.

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Stoffers, P., Ross, D.A., 1974. Sedimentary history of the Red Sea. Initial Reports of the Deep Sea Drilling Project, 23, pp. 849–965. The´veniaut, H., Freysinnet, P., 2002. Timing of lateritization on the Guiana shield: synthesis of paleomagnetic results from French Guiana and Suriname. Palaeogeogr. Palaeoclim. Palaeoecol. 178, 91–117. Thiry, M., Jacquin, T., 1993. Clay mineral distribution related to rift activity, sea-level changes and oceanography in the Cretaceous of the Atlantic Ocean. Clay Miner. 28, 61–84. Timofeev, P.P., Eremeev, V.V., Rateev, M.A., 1977. Palygorskite, sepiolite and other clay minerals in Leg 41 ocean sediments: mineralogy, facies and genesis. In: Lancelot, Y., Seibold, E. et al., (Eds.), Initial Reports of the Deep Sea Drilling Project, 41. U.S. Government Printing Office, Washington, pp. 1087–1101. Tomczak, M., Godfrey, J.S., 1994. Regional Oceanography: An Introduction. Pergamon, London 422 pp. Trauth, N., 1977. Argiles e´vaporitiques dans la se´dimentation carbonateé continentale et e´picontinentale tertiaire. Bassin de Paris, de Mormoiron et de Salinelles (France). Jbel Ghassoul (Maroc). Sci. Geól. Me´m. 49, 195 pp. Valeton, I., 1999. Saprolite–bauxite facies of ferralitic duricrusts on palaeosurfaces of former Pangaea. In: Thiry, M., Simon-Coinçon, R. (Eds.), Palaeoweathering, Palaeosurfaces and Related Continental Deposits. Special Publication of the International Association of Sedimentologists 27, Blackwell, Oxford, pp. 153–188. Vasconcelos, P.M., Renne, P.R., Brimhall, G.H., Becker, T.A., 1994. Direct dating of weathering phenomena by 40Ar/39Ar and K-Ar analysis of supergene Mn-oxides. Geochim. Cosmochim. Acta 58, 1635–1665. von Rad, U., Ro¨sch, H., 1972. Mineralogy and origin of clay minerals, silica and authigenic silicates in Leg 16 sediments. In: Hayes, D.E., Pimm, A.C. et al., (Eds.), Initial Reports of the Deep Sea Drilling Project, vol. 14. U.S. Government Printing Office, Washington, pp. 727–751. Weawer, C.E., Beck, K.C., 1977. Miocene of the S.E. United States: a model for chemical sedimentation in a peri-marine environment. Developments in Sedimentology, vol. 22. Elsevier, Amsterdam, 234 pp. Weiss, R.F., 1970. Solubility of nitrogen, oxygen and argon in water and seawater. Deep Sea Res. 17, 721–735. Weser, O.E., 1974. Sedimentological aspects of strata encountered on Leg 23 in the northern Arabian Sea. In: Whitmarsh, R.B., Weser, O.E., Ross, D.A. et al., (Eds.), Initial Reports of the Deep Sea Drilling Project, vol. 23. U.S. Government Printing Office, Washington, pp. 3–519. Wilson, P.A., Norris, R.D., Cooper, M.J., 2002. Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise. Geology 30 (7), 607–610. doi:10.1130/0091-7613(2002) 0302.0.CO;2. Woo, K.S., Anderson, T.F., Railsback, L.B., Sandberg, P.A., 1992. Oxygen isotope evidence for high-salinity surface seawater in the mid-Cretaceous Gulf of Mexico: implications for warm, saline deepwater generation. Paleoceanography 7, 673–685. Zachos, J.C., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 292, 686–693.

Chapter 6

Palygorskite and Sepiolite Deposits in Continental Environments. Description, Genetic Patterns and Sedimentary Settings Emilio Galań* and Manuel Pozo{ *Departmento de Cristalografıá Mineralogıá y Quı´mica Agrıćola, Facultad de Quı´mica, Universidad de Sevilla, Professor Garcıá Gonza´lez 1. 41012 Seville, Spain { Departamento de Geologıá y Geoquı´mica, Universidad Autońoma de Madrid, Cantoblanco, 28049 Madrid, Spain

1. INTRODUCTION On the basis of their origin, two main types of clays are differentiated in the sedimentary environment: detrital and authigenic clays (Figure 1). Detrital clays are composed of inherited clay minerals as a result of exogenic processes including weathering, erosion, transport and clastic deposition. Authigenic clays are formed in situ through direct precipitation from solution (neoformation), reaction of amorphous gels, or by transformation of precursor minerals, mainly pyroclastics and detrital clays (Jones, 1986). The most important processes forming authigenic clays are neoformation and transformation. Neoformation is commonly the crystallization of a new mineral structure from simple or complex ions, in which there is no inheritance of a pre-existing mineral structure. However, transformation is the formation of a new mineral in which part or all of the pre-existing structure is inherited. In the sedimentary environment, transformation is mostly related to diagenesis, whereas neoformation can take place in both syngenetic (depositional) and diagenetic environments. Sepiolite and palygorskite are two excellent examples of authigenic clays. Palygorskite and sepiolite are relatively rare in nature; however, their origins have been studied frequently owing to the unique conditions required for their formation and stability, and the need to find new commercial deposits. According to Callen (1984), major deposits of these minerals were formed Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00006-2 # 2011 Elsevier B.V. All rights reserved.

125

126


SEDIMENTARY (depositional)

DIAGENETIC (post-depositional)

Evaporative

Intrasedimentary

Water

Clayey Sediment (detrital or chemical)

AUTHIGENIC CLAY

AUTHIGENIC CLAY

SEDIMENTARY NEOFORMATION

TRANSFORMATION

(direct precipitation from ionic or colloidal solutions)

(dissolution-precipitation from previous phases or epitaxial growth)

AUTHIGENIC CLAYS

DETRITAL CLAYS

SEDIMENTARY CLASTIC DEPOSITION (Inheritance)

Water

DETRITALCLAY

FIGURE 1 Sketch showing the origin of authigenic clays and their relationship with detrital clays in sedimentary and early diagenetic environments. Evaporative conditions commonly provoke the sedimentary neoformation (precipitation) of sepiolite. On the other hand, the early diagenesis of fine-grained sediments is often related to palygorskite formation by means of transformation of inherited Al-bearing clays. Under subaerial exposure Mg-clay paleosoils or reworked deposits can occur.

in three different environments: (1) in epicontinental and inland seas and lakes as chemical sediments or by reconstitution of former sedimentary clays; (2) in the open ocean in association with fore-arc basins and ocean ridges by hydrothermal alteration of basaltic glass, volcaniclastic sediments or existing clays; and (3) in calcareous soils by direct crystallization. With respect to their geotectonic setting, neoformed palygorskite and sepiolite are predominant in shallow shelf basins on passive margins or intraplate basins. Continental rift basins, continental “sag” basins or some perimarine basins, tend to contain significant deposits of these minerals, commonly formed in saline or hypersaline conditions (Figure 2; Merriman, 2005).

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Palygorskite and Sepiolite Deposits in Continental Environments

127

Neoformed ± Inherited clays Inherited + Neoformed clays Neoformed clays

Continental ‘Sag’ Basin

Mid-ocean ridge and abyssal plain

Passive Margin

Continental Rift Basin

FIGURE 2 Neoformed clays at shallow shelf basins on passive margin, continental rift basins and continental “sag” basins (Merriman, 2005).

In soils of arid regions, palygorskite and sepiolite are common neoformed minerals (Singer, 1979), but traces of palygorskite in some arid soils may be inherited (Mackenzie et al., 1984; Shadfan and Dixon, 1984). The instability of these minerals in wet climates favours their preservation in dry or semi-arid climates. According to Singer (1984), palygorskite in soils is associated with one of the following conditions: (i) modern soils that, at present or in the past, were affected by rising groundwater of pH 7–8 and high salinity; (ii) in soils with distinct and sharp textural transitions because these minerals accumulate in the coarse fraction (this group includes many palaeosols); and (iii) in calcretes (caliches). In all these cases, palygorskite (and very rarely sepiolite, when Al is absent or immobilized) is precipitated by evaporation of the vadose water (Jones and Weir, 1983). Soils (duricrusts) can be cemented by palygorskite (palycretes; Rodas et al., 1994). Palygorskite is more readily formed in weathering environments than sepiolite, or else sepiolite is less stable under supergene conditions and, hence, is rarer. In the sedimentary environment, palygorskite of detrital origin can be found in oceans, having come from the continent, as in the case of palygorskite in the Atlantic close to Morocco, which was transported by SW winds from near-shore. Additionally, palygorskite may occur by recrystallization during diagenesis (e.g. Couture, 1977), or transformation of smectite in the marine environment (Lo´pez Galindo, 1986), sometimes in association with marine phosphorites (Chahi et al., 1999). However, these minerals can crystallize directly from solution (e.g. Jones and Galań, 1988; Weaver, 1984), either in lacustrine (e.g. Chahi et al., 1997; Galań and Castillo, 1984) or in perimarine (e.g. Singer, 1979; Velde, 1985; Weaver and Beck, 1977) environments. In this environment, sepiolite can also be of diagenetic origin through the transformation of magnesite at pH 10.5–11.5 in silica-rich lake waters (Ece, 1998).

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Jones and Galań (1988) summarized the occurrences of palygorskite and sepiolite and tabulated a summary of the favourable environmental conditions for palygorskite and sepiolite formation as compared to trioctahedral smectite (Table 1). As expected, with high values of Al, Mg and Si activity,

TABLE 1 Environmental Conditions of Formation of Magnesian Clays (Modified After Jones and Galań, 1988). Palygorskite

Sepiolite

MgSmectite

Moderate pH < 8,5

þþþ

þþ

Intermediate pH ¼ 8–9.5

þþ

þþþ

þþ

High pH > 9.5

þþþ

Major constituent ratios

High MgþSi/Al

þþ

þþþ

High MgþFe/Si

þþþ

Sediment-water pCO2

High

þþþ

Low

þþþ

þþþ

Alkali salinity

High

þþþ

Intermediate

þþ

þþþ

þþ

Moderate

þþþ

þþ

Siliciclastic or arkosic matrix

þþþ

þþ

Carbonate or mafic matrix

þþ

þþþ

Groundwater input dominant

þþþ

þþ

Surface runoff dominant

?

þþþ

Hypersaline

þþþ

Lagoon or tidal

þþþ

þþ

Deep sea

þþ

þþ

Chemistry pH and alkalinity

Environment Pedogenic calcrete or alluvial

Closed basin lacustrine

Marine

þþþ, favoured; þþ, less favoured; , not favoured.

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palygorskite is favoured over sepiolite. However, besides the presence of Al-rich clay phases (e.g. smectites), temporary variations in chemistry related to changes in environmental conditions such as evaporation, rainfall, freshwater flow, etc. affect the formation of palygorskite and sepiolite. Palygorskite originates in many cases from the transformation of smectite via a dissolution–precipitation process (Chen et al., 2004; Galań and Ferrero, 1982; Jones and Galań, 1988; Lo´pez-Galindo et al., 1996; Sańchez and Galań, 1995; Sua´rez et al., 1994). In some shallow restricted basins with evaporitic conditions, fibrous clay minerals and Mg-rich smectites (saponite, stevensite and kerolite) form as authigenic minerals, usually together with sulphate and carbonate minerals (Pozo and Casas, 1999). Sepiolite has also been formed in Japan from low-temperature hydrothermal solutions (Imai and Otsuka, 1984) and by the hydrothermal alteration of ¨ nlu¨, 1993). Vein-type sepiolite formed by hydrothermal volcanics (I˙rkeç and U processes is abundant in ophiolite complexes in Turkey (Yeniyol, 1986). The stability of palygorskite and sepiolite is mainly a function of pH. The transformation of palygorskite to smectite has been suggested by Golden and Dixon (1990), Merkl (1989), Golden et al. (1985), and Gu¨ven and Carney (1979). Golden and Dixon (1990) used transmission electron microscopy (TEM) data to show a close textural association of smectite and palygorskite in a series of experiments. Their work indicated that palygorskite readily converts to smectite above 100 C, although the reaction was sluggish at room temperature (22 C). They showed that at conditions near a pH of 12, the palygorskite to smectite transformation occurs over a period of several months. The transformation of palygorskite from the Meigs Member of the Hawthorne Formation, in southern Georgia, USA to smectite was analysed in detail using atomic force microscopy (AFM) and TEM techniques by Krekeler et al. (2005). AFM analysis indicated that palygorskite fibres in this horizon were commonly altered. Many AFM images of the altered fibres showed an oriented overgrowth on particles with platy morphology, which were interpreted as smectite. This transformation to smectite also accounts for the very low abundance of palygorskite in Mesozoic and older sediments. An implication of the transformation is that palygorskite deposits may have existed in abundance in the Mesozoic and perhaps even older sedimentary systems. However, Tertiary sedimentary rocks have different proportions of palygorskite. Orogenic activities, which resulted in the Tethys Sea being cut off during the late Cretaceous, gave rise to the development of shallow saline lakes during the Tertiary that were chemically favourable for the formation of fibrous clay minerals. In an evaporative environment, these conditions promoted the formation of gypsum and resulted in an increase in the Mg/Ca ratio that brought about the authigenic formation of a large amount of palygorskite, particularly in Neogene sediments. The positive correlation between the occurrence of palygorskite/sepiolite and gypsum and carbonates in sediments supports this hypothesis.

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The geochemistry of the post-Tethys Sea environment, which was significantly affected by climatic conditions and orogenic events during the Tertiary, controlled the formation of palygorskite and sepiolite. The present-day arid to hyper-arid environments prevailing in many areas have caused the preservation of these minerals (Singer, 1984). The deep-sea environment of the Tethys Ocean (geological formations older than Cretaceous) does not appear to have been suitable for the formation of palygorskite and sepiolite. The possible environments for palygorskite and sepiolite formation range from soils to marine and lacustrine deposits, hydrothermal veins in serpentinite and dolostone and the weathering of volcanic rocks. The primary question addressed by Singer (1979)—that is, whether these minerals owe their origin to transformation of precursor clay minerals or to precipitation directly from solution—continues to be the most controversial aspect in concerning the origin of these minerals. Nevertheless, their formation from a dissolution–precipitation mechanism that incorporates components (primarily sesquioxides) of pre-existing detrital material seems to be inescapable. These phases can therefore contain significant geochemical information regarding the precursor and the formation environment (Millot, 1970). Studies of sepiolite and palygorskite occurrences and origin in continental deposits are well supported by abundant literature. However, studies about their genetic relationships with other Mg-clays including saponite, stevensite, kerolite and associated mixed layers are relatively scarce, although fibrous and non-fibrous Mg-clays have been recognized, in the same deposit, suggesting a genetic relationship. Within sedimentary deposits, sepiolite and palygorskite occur commonly in beds, but locally also intrasedimentary (e.g. minerals that have precipitated within the sediment from the pore water after evaporation or because of transformation during diagenetic process) or forming clay clasts levels, the late as a result of reworking of desiccated and bioturbated beds (Bellanca et al., 1992; Trauth, 1977). The main aim of this contribution is to model the pathways leading to sepiolite–palygorskite (and other Mg-clays) formation based on worldwide continental occurrences. Also to establish the sedimentological models and related lithological associations, summarizing and updating the available experimental and geological data. For this purpose, the main characteristics of the most important sepiolite and palygorskite deposits of continental origin will be reviewed.

2. GENETIC CONDITIONS From a sedimentological perspective, the best conditions for the formation of fibrous clay minerals seem to occur in lacustrine evaporative environments where pre-existing sediments become exposed and diagenetic and soil

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131

processes alter the materials as described in a number of reviews (Calvo et al., 1999; Galań and Castillo, 1984; Jones, 1986; Jones and Galań, 1988; Singer, 1979). Palygorskite and sepiolite are common in lacustrine environments in arid or semi-arid climatic conditions when the interstitial and overlying fluids are alkaline solutions with high Si and Mg activity. The high Mg and Si may be the result of groundwater concentration near the surface, or of cyclical flooding and subsequent concentrations of the solution by evaporation. The fibrous clay materials are associated with carbonates (dolomite in particular), evaporites (gypsum, halite, thenardite) and chert. They are also often associated with other Mg-rich clays such as saponite, kerolite and stevensite. Depending on the regional geology, the source of Si, Al and Mg may be volcanic tuff (McLean et al., 1972; Starkey and Blackmon, 1984) or weathering of igneous and sedimentary rocks (Galań and Castillo, 1984; Hay and Stoessel, 1984). Direct precipitation and the transformation of precursor phases (by dissolution–precipitation) have been proposed as formation mechanisms in this setting (Table 2).

2.1. Experimental and Natural Evidence of Sepiolite Formation Experimental synthesis of sepiolite has been carried out under various conditions and the results generally agree (Couture, 1977; La Iglesia, 1978; Siffert and Wey, 1962; Wollast et al., 1968). Wollast et al. (1968) were able to produce sepiolite by evaporating seawater while controlling the pH and silica content. Sepiolite precipitates at pH > 8.2, and the activity of Mg2þ, OH and SiO2 is governed by the equilibrium constant k ¼ (a16Hþ)/(a8Mg2þ þ a12SiO2(aq)) ¼ 10 75.12. In other words, sepiolite can be formed at high aMg2þ/a2Hþ ratios when the SiO2 activity is

TABLE 2 General Criteria for Discriminating Between Neoformation and Transformation. Neoformation

Transformation

Low content in trace element (F is an exception) especially REE and TTE

Moderate to high content in trace elements (depending of the transformed previous phases)

Commonly detrital minerals are absent or in low content

Detrital minerals are common both in clay and higher grain-size fractions

Generally precursor minerals absent

Precursor minerals present

Clean textures in thin section

Often dirt textures in thin section

Locally lamination

Lamination absent or inherited

132


low, while sepiolite is formed at low aMg2þ/a2Hþ ratios when SiO2 is high. They also observed that aluminium must not be present in solution or in reactive phases, a requisite later stressed by other authors (Birsoy, 2002; Jones, 1983; Starkey and Blackmon, 1979; Webster and Jones, 1994). Experiments performed by Siffert and Wey (1962) by evaporating solutions with fixed chemical ratios and varying pH were very productive. Sepiolite begins to precipitate at a pH of 8.5, although if pH is held at 9, smectite and talc can precipitate. In other words, an increase in Mg2þ is observed in precipitated solid phases when pH is increased, even leading to the formation of brucite at a pH of 10. Similar tests were carried out by La Iglesia (1978) where sepiolite was synthesized by homogeneous precipitation, with the observation that “crystallinity”, particle size and stability are greater when pH is increased, depending on the silica activity. The stability diagram produced by Jones (1986) shows that sepiolite, stevensite and kerolite equilibria depend on salinity (Na), pH and Mg concentration (Figure 3). High salinity favours stevensite formation, while sepiolite and

20

18

Talc (Kerolite)

ed Mix

ers lay

16

log a Mg2+/a 2H+

14 Sepiolite

Stevensite

12

10 Solution

8

6

4 −2

0

2

4

6

8

10

log a Na+/a H+ FIGURE 3 Stability of hydrous magnesium silicates as estimated from thermodynamic data and natural occurrences. Labels indicate fields of supersaturation of the appropriate mineral phase (adapted from Jones, 1986). The diagram explains the formation of kerolite at higher Mg/Si than sepiolite so as the predominance of stevensite at higher salinities.

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kerolite are formed in less saline conditions, with high Si/Mg ratios favouring sepiolite formation. Sepiolite stability is limited to solutions with log aMg/ a2H between about 12.5 and 16.1 and log aNa/aH ratios between 2 and 1. According to Jones and Galań (1988), the formation of sepiolite is controlled by several physico-chemical parameters including pH (alkalinity), pCO2 and salinity. A pH between 8 and 9.5, moderate salinity and low pCO2 seems to be the most favourable. More recently, Birsoy (2002) produced numerous stability diagrams for palygorskite and sepiolite and their associations with other minerals at 25 C in a system containing seven components (Figure 4; MgO–CaO– Al2O3–SiO2–H2O–CO2–HCl). She pointed out that when a solution has insignificant or very low aluminium activity (log [aAl3þ/(aHþ)3] < 7.5), direct precipitation of sepiolite occurs, diminishing as the activity ratio increases. Direct sepiolite precipitation was enhanced in solutions that were very rich in silica (log[aH4SiO4] 4.75), while in silica-poor solutions, sepiolite requires a higher pH for its formation than that required by palygorskite. In nature, sepiolite may precipitate from a water mass that is evaporating, but it may also precipitate from interstitial fluids in sediments subject to evaporation, or it may even be formed from magnesian-silicate substrates or pre-existing minerals during diagenesis. Leguey et al. (2010) recently proposed a relationship between dolomite biomineralization and sepiolite formation. They suggested a general process for dolomite dissolution as source of Mg favouring the fibrous clay neoformation.

A

B 21

21

chlorite

20

20

log [aMg2+/(aH+)2]

18

sepiolite

17 brucite talc

16 15

solution

magnesite

14

palygorskite dolomite

13 12 11 10 9

sepiolite

19

chrysotile

18 log [aMg2+/(aH+)2]

19

chlorite palygorskite

17 brucite 16 15

talc magnesite

14 13

solution dolomite

12 11

amorphous Mg-mont silica −8 −7 −6 −5 −4 −3 −2 −1 0 1 2 log [aH SiO ] 4

4

10 9

Mg-mont −8 −7 −6 −5 −4 −3 −2 −1 0 1 2 log [aH SiO ] 4

4

FIGURE 4 Representative 25 C and 1 bar phase diagrams of a seven component system (MgO– CaO–Al2O3–SiO2–H2O–CO2–HCl) with log aH2O ¼ 1, log aCO2 ¼ 3.5 and log[aCa2þ/ (aHþ)2] ¼ 13.06. Figures A and B show the stability fields of sepiolite and palygorskite for log [aAl3þ/(aHþ)3] ¼ 4.5 and log[aAl3þ/(aHþ)3] ¼ 9.2, respectively (adapted from Birsoy, 2002).

134


Indeed, the most widely accepted interpretation for sepiolite formation is that of direct precipitation from a solution containing dissolved ionic species (silica and magnesium; Jones and Galań, 1988; Trauth, 1977). Sepiolite may also form from colloidal silica in a Mg2þ-rich environment, giving rise to a hydrated magnesium silicate precursor being deposited on a mineral support that would form sepiolite in periods of desiccation (Leguey et al., 1985; Pozo et al., 1990; Williams et al.; 1985). The role of aqueous CO2 in the precipitation of magnesium silicates under evaporative conditions has been reported by Deocampo (2005). Model calculations show that pCO2 is an important control on pH, thus affecting Mg-clay mineral stability. Geochemical analysis applied to water and sediments of African lakes indicates that a minimal evaporative concentration is required for Mg-silicate supersaturation, and a strong correlation (R2 ¼ 0.7, p < 0.001) is found between pCO2 and its solubility, independent of brine evolution. Obviously, initial supersaturation of water with respect to Mg-clays generally requires elevated pH, and subsequent pH suppression due to biotic or abiotic CO2 can prevent mineral precipitation (Jones, 1986). The influence of salinity as an inhibitor for sepiolite formation was described by Darragi and Tardy (1987). Aragonite and stevensite formed in alkaline salt lakes but not sepiolite. When pH and ion activity in solution are controlled by carbonate precipitation (calcite, dolomite), only limited evaporation is necessary for sepiolite to be formed from groundwater springs (Khoury et al., 1982). A d18O isotopic investigation of Mg-rich clays showed that the highest ratios are observed when stevensite is predominant, and the lowest are reached when kerolite predominates (Hay et al., 1995). The conclusion reached was that high salinity favours stevensite and low salinity favours kerolite. Values obtained for sepiolite (meerschaum type) are midway between the two. Sepiolite genesis may also be associated with the dissolution of Mg-rich clay minerals such as stevensite, saponite and kerolite. In this case, variations in salinity may give rise to ideal thermochemical and kinetic conditions for the formation of the fibrous mineral by means of a dissolution–precipitation transformation process (Chahi et al., 1997; Eberl et al., 1982; Khoury et al., 1982; Post and Janke, 1984; Pozo, 2000; Pozo and Casas, 1999). In laboratory dissolution experiments of sepiolite and kerolite (Stoessell, 1988), sepiolite is favoured in solutions in equilibrium with, or oversaturated with quartz, and that it is metastable with regard to kerolite at lower silica concentrations. In summary, according to the experiments and calculations cited, the physico-chemical conditions for sepiolite precipitation under normal conditions of temperature and pressure require relatively high alkalinity (low pCO2), with pH ranging between 8 and 9.5, medium salinity (brackish) and sufficient silica (log[aH4SiO4] 4.75), and magnesium activity. It is also essential

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that aluminium should be absent in solution (log[aAl3þ/(aHþ)3] < 7.5) or in reactive phases in the described conditions.

2.2. Natural and Experimental Evidence of Palygorskite Formation The models for palygorskite formation have been reviewed and discussed by different authors (Isphording, 1973; Jones and Galań, 1988; Paquet, 1983; Singer, 1979). Palygorskite is commonly formed in lacustrine–palustrine environments (Millot, 1970; Saéz et al., 2003; Trauth, 1977; Webster and Jones, 1994). However, unlike sepiolite, the largest number of references to the distribution and genesis of palygorskite is in association with palaeosols and carbonate crusts (Paquet, 1970; Singer, 1984; Sua´rez et al., 1993). There have been considerable differences of opinion regarding palygorskite formation, with two models being established. One considers palygorskite to have originated from aluminous precursors (Trauth, 1977; Weaver and Beck, 1977), while the other considers it to be the result of direct precipitation from solution (Singer, 1979, 1984; Singer and Norrish, 1974). According to Trauth (1977), the mineral precursor for palygorskite is an aluminous-magnesian smectite in basic environments rich in Mg2þ and silica. Weaver and Beck (1977) in turn proposed montmorillonite as the aluminous precursor. For these authors, palygorskite occurs in environments with sufficient silica and Mg2þ in solution as well as a pH range between 8 and 9. Under these conditions, Mg2þ would migrate to the octahedral layer in the smectite, producing a distortion that would force the inversion of the tetrahedra, producing palygorskite. The stability relationships of palygorskite–smectite reported by Weaver and Beck (1977) show that for a fixed concentration of aluminium (log[Al(OH4)-)] ¼ 5.5), the stability of one phase or the other at 25 C depends on the activity of silica and Mg2þ, and pH. Velde (1985) explains that the reactions between silicates containing alumina and the elements in solution, particularly silica and Mg2þ, play a very important role in the genesis of palygorskite. Thus, if the solubility of an ionic species is low (as occurs with aluminium), and the activities of silica and Mg2þ are high, a rapid process of transformation by dissolution–precipitation is possible. In other words, the aqueous dissolution of the precursor phase (smectite) would be enhanced so that a new phase (palygorskite) could be generated in a short time. Numerous researchers interpret the formation of palygorskite from a smectite precursor, both in soils (Abtahi, 1977; Yaalon and Wieder, 1976) and in sediments (Couture, 1977; Decarreau et al., 1975; El Prince et al., 1979; Giresse, 1980; Jamoussi et al., 2003). More recently, experimental studies by Birsoy (2002) show that the system MgO–CaO–Al2O3–SiO2– H2O–CO2–HCl (Figure 4), at a temperature of 25 C with high silica activity

136


(either in the form of quartz or in the form of amorphous silica), does not easily form palygorskite by direct precipitation from solution. Rather, it does so by the transformation of certain aluminous precursors such as dioctahedral smectite. She also observed that palygorskite formation is favoured when the solution has an increasing aluminium activity (log[aAl3þ/(aHþ)3] from 4.5 to 9.2). Different investigators have shown the existence of a precursor in palygorskite formation by means of TEM (Sua´rez et al., 1994; Tazaki et al., 1986, 1987). The precursor phase generally proposed is montmorillonite, but beidellite (Sautereau and Decarreau, 1973), mica (Galań and Ferrero, 1982), chlorite (Galań et al., 1975) and primary silicates (Paquet, 1983) have also been implicated. As previously described, a model of direct precipitation from solution has also been proposed for palygorskite genesis. The work by La Iglesia (1977) demonstrated that palygorskite can be obtained experimentally by homogeneous precipitation. Singer (1979, 1984) also supports the precipitation model, while ruling out the previous transformation model based on crystal-chemical criteria and on the absence of an intermediate structure. This author states that the stability of palygorskite in solution depends on pH and Mg2þ and silica activity. For fixed Al and Fe activities, palygorskite can even be stable at a pH of 6, if silica and Mg2þ activities are high. However, when Mg2þ is scarce, high pH (> 8.5) and high silica concentrations are required. This model has also been proposed by Vanden Heuvel (1966), Singer and Norrish (1974), Watts (1976, 1980), Callen (1977), Hutton and Dixon (1981) and Esteóule-Choux (1984), mostly in calcretes. In conditions of high alkalinity, direct palygorskite precipitation would be inhibited, as the aluminium would coordinate tetrahedrally instead in octahedra as required by the mineral (De Kimpe et al., 1961), unless the octahedrally coordinated aluminium was inherited directly from the smectite precursor. Current investigators suggest that the main mechanism for palygorskite formation is transformation by dissolution–precipitation, given the abundance of fine detrital materials supplying aluminium. But, they have not ruled out that local neoformation by direct precipitation also takes place under more restricted conditions. The phase diagrams prepared by Birsoy (2002) show that precipitation of palygorskite can take place but only for log[aAl3þ/ (aHþ)3] value around 5.5 and a log[aMg2þ/(aHþ)3] value between 11 and 13. In the review by Jones and Galań (1988), it was established that the most favourable conditions for palygorskite formation were produced at pH < 8.5 with high Mg2þ, silica and aluminium activity and moderate salinity. Controversy exists concerning the most favourable salinity for palygorskite formation since against moderate salinity, Webster and Jones (1994) have proposed ephemeral-playa conditions.

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3. MAIN GENETIC CHARACTERISTICS OF THE WORLDWIDE DEPOSITS AND OCCURRENCES OF SEPIOLITE AND PALYGORSKITE IN CONTINENTAL SEDIMENTARY ENVIRONMENTS The main characteristics of the worldwide sepiolite and palygorskite deposits of continental origin are summarized in Tables 3 and 4, respectively. Concerning sepiolite the most important deposits are in Spain and Turkey, thus special attention will be paid to sepiolite in these two countries. On the contrary, the most important palygorskite deposit (Georgia–Florida, USA) will be not described here because of its perimarine origin. In relation with the Guanshan (China), probably the present biggest deposit of palygorskite is only briefly cited because it is described in detail in Chapter 10. Therefore, only some smaller deposits but of particular genetical features will be cited. Some X-ray diffraction (XRD) patterns of sepiolite and palygorskite from Vica´lvaro (Spain), Eskisehir (Turkey), Guanshan (China) and Torrejoń (Spain) are shown in Figure 5. Sepiolite samples show a high purity (only traces of calcite in Eskisehir sepiolite) and high ordering. Palygorskite from Torrejoń exhibits higher ordering than Guanshan sample, both with traces of associated minerals. Some representative chemical analyses from 10 most deposits cited in this section are displayed in Table 5. Sepiolite samples from Eskisehir and Vicalvaro are the purest samples showing the lowest Al2O3 and K2O content. Palygorskite samples display variation of MgO and Al2O3 content, being noteworthy the higher Fe2O3 and K2O percentages in the sample from India.

3.1. Spain Sepiolite and palygorskite are widely distributed in continental deposits in Spain (Figure 6), with special concentrations in the Tagus and Duero basins. However, the most noteworthy deposits are those located in the Tagus Basin.

3.1.1. Tagus Basin 3.1.1.1. Geological Setting The Tagus Basin is an intracratonic basin that was formed as a result of differential tectonic strains within the Iberian microplate during the Alpine orogeny. The lithostratigraphy of the Miocene epoch of the Neogene period is divided into three major units: Lower, Middle and Upper, which are separated by regional disconformities (Alberdi et al., 1984; Junco and Calvo, 1983). The Altomira Range (Mesozoic) divides the basin into two sub-basins named the Madrid Basin and Loranca Basin, located in the west and east of the Tagus Basin, respectively. The geological map of Madrid Basin and a representative lithological section are shown in Figure 7.

TABLE 3 Summary of Worldwide Sepiolite Deposits. Deposit (Country, Age)

Fibrous Clay Minerals (Thickness)

Lithofacies Assemblage

Mineralogical Association

Environment

Origin

Vicalvaro-VallecasCabanãs de la Sagra (Spain) Miocene

Sepiolite Lower Unit. 1–5 m Upper Unit. Up to 10 m Two main beds

Arkose, clayey arkose, Mg-clay Sepiolite,dolomite, calcite, chert

Sepiolite (> 95%) (saponite, stevensite, illite, calcite, dolomite, quartz, feldspar)

Lacustrine/alluvial

Depositional neoformation Diagenetic

Batallones (Spain) Miocene

Sepiolite Lower Unit. Up to 9 m Upper Unit. Up to 2 m Two main beds

Sepiolite, sepiolite– palygorskite clay, Mgsmectite clay, calcrete, chert

Sepiolite (palygorskite, saponite, illite, quartz, feldspar, calcite)

Palustrine


Eskisehir (Turkey) Miocene

Sepiolite 0.5–5 m Beds and nodules

Calcareous and gypsiferous clay, sepiolite,dolomite, conglomerate, magnesite, tuff, gypsum

Sepiolite (up to 90%): (dolomite, quartz, illite, feldspar, VFR)

Lacustrine


Eskisehir (Turkey) Miocene

Loulinghite 0.6–5 m

Lutite, tuff, bentonite, chert

Loulinghita (sepiolite, analcime, smectite, illite, calcite, feldspar, palygorskite, opal)

Lacustrinevolcanosedimentary

Diagenetic

Mara-Orera (Spain) Miocene

Sepiolite 0.5–0.6 m (up to 1 m) Inserts

Carbonate, dolomitic marl, sepiolitic marl, chert

Sepiolite: (palygorskite, Mg-smectite, calcite, dolomite, zeolites, opal)

Palustrine– lacustrine

Depositional neoformation

Amboseli (Kenya–Tanzania) Pleistocene

Sepiolite 1–3 m One bed and nodules (meerschaum)

Sepiolite, dolomite

Sepiolite (dolomite, kerolite-stevensite, calcite, feldspar)

Lacustrine (margin)

Depositional neoformation

Amargosa (USA) Pliocene–Pleistocene

Sepiolite: Up to 1.5 m Two beds

Silt, silty clay, sand

Sepiolite (saponite, illite, kerolitestevensite, dolomite, calcite)

Lacustrine (playa)


TABLE 4 Summary of Worldwide Palygorskite Deposits. Deposit (Country, Age)

Fibrous Clay Minerals (Thickness)

Lithofacies Assemblage

Mineralogical Association

Environment

Origin

Bercimuel (Spain) Miocene

Palygorskite 1–1.5 m Two beds

Silt, clay, calcrete

Palygorskite (illite, kaolinite, quartz, smectite, mixed layers)

Alluvial

Diagenetic (dissolution– precipitation), Al-smectite

Torrejoń el rubio (Spain) Paleogene

Palygorskite 0.5–4 m One bed

Marl. Palygorskitic clay,arkose, gravel

Palygorskite (illite, sepiolite, chlorite, dolomite, saponite, quartz, feldspar)

Lacustrine– palustrine (alteration profile)

Diagenetic (dissolution– precipitation), chlorite

Andhra Pradesh (India) (Paleogene)

Palygoskite 0.5–3 m

Limestone, chert, marl, sandstone

Palygorskite (?)

Lacustrine

Diagenetic (dissolution– precipitation), Illite Depositional neoformation

Lake Nerramyne (Australia) Garford Paleochannel (Australia) (Miocene)

Palygorskite 4–9 m Palygorskite Up to 2 m

Clay, dolomite

Palygorskite (?) Palygorskite (illite, smectite, dolomite)

Lacustrine Lacustrine

No data

Guanshan (China) (Miocene)

Palygorskite 3–6 m

Clay, basaltic ash

Palygorskite (smectite > quartz sepiolite, mica, dolomite)

Lacustrine–fluvial (alteration profile)

Diagenetic (basaltic ash and basalt))

Grevena (Greece) (Pliocene– Plesitocene)

Palygorskite 10–18 m

Clay, sand

Palygorskite (?)

Lacustrine

Diagenetic (saponitic sands, ultramafic rock)

140


ESKISEHR

S = Sepiolite C = Calcite

S

900

12.06 Å

S

S S

S

C S S

S S

S

S

100 0

S

S

400

S

S

S

S

S S

S S

VICALVARO

3600

S 12.01 Å

S

1600 S S

400

S

S

S

S

S

S

S

S

S

S S

S

S S S

S

S S

S

0 10

20

30 40 Position [⬚2theta]

GUANSHAN

P P

P P

100

Q P P

P

70

P P

Q

Q

TORREJON

P 900

60

Q P 10.47 Å

0

50

P 400

P = Palygorskite Q = Quartz D = Dolomite

Q

10.49 Å

P P

Q P

D

P P

P

P

P Q

P P

100

P

Q P

P

P

0 10

20

30 40 Position [⬚2Theta]

50

60

70

FIGURE 5 XRD patterns of sepiolite and palygorskite from representative deposits.

Sepiolite is commonly found in the Madrid Basin (Brell et al., 1985; Calvo et al., 1999; Galań and Castillo, 1984; Ordon˜ez et al., 1991). Although not as common as sepiolite, the presence of palygorskite in the Neogene lacustrine and alluvial sequences of the Madrid Basin has often been reported (Galań and Castillo, 1984; Garcıá-Romero et al., 2004; Leguey et al., 1985; Pozo et al., 1985). The sedimentological analysis of the lacustrine deposits and their lateral relationship with alluvial facies that border the basin margin suggests that the lake systems underwent significant changes throughout the Miocene (Calvo et al., 1989).

TABLE 5 Representative Chemical Analysis of Sepiolite and Palygorskite from Selected Worldwide Deposits. Sepiolites

SiO2

Al2O3

Fe2O3

Vallecas (Galań and Castillo, 1984)

63.10

1.08

0.27

Amboseli (Hay and Stoessel, 1984)

53.17

1.76

0.99

Mara (Arauzo et al., 1989)

58.29

2.32

Batallones (Pozo et al., 2010b)

56.46

Eskisehir (Ece and C ¸ oban, 1994)

FeO

H2O

CaO

Na2O

K2O

23.80

0.49

0.16

0.21

24.70

0.23

0.45

0.97

0.17

8.29

1.24

22.21

1.73

0.13

0.15

0.62

14.06

1.17

0.37

22.67

0.19

0.06

0.18

0.08

18.57

56.95

1.05

0.93

23.35

2.45

0.11

0.41

0.15

14.60

Andhra Pradesh (Siddiqui, 1984)

53.70

7.78

7.96

8.45

0.92

0.14

1.57

1.23

18.13

Guanshan (Zhou and Murray, chapter 10 of this book)

55.21

8.16

4.05

12.52

0.24

0.03

0.83

0.50

18.40

Bercimuel (Castillo, 1991)

60.00

12.40

4.80

7.80

1.68

0.90

0.90

Torrejoń (Galań et al., 1975)

51.50

10.03

2.36

0.04

TiO2

H2Oþ

MgO

LOI

10.88 8.79

Palygorskites

0.52

12.28

11.93 14.43

7.36

142


FIGURE 6 Tertiary basins in the Iberian Peninsula (Spain and Portugal).

The Mg-clay deposits (sepiolite, Mg-bentonites and kerolitic clays) are located in a transition zone between alluvial and marginal lacustrine facies within the Miocene Middle Unit (Ordon˜ez et al., 1991). According to Doval et al. (1985) and Garcıá et al. (1990), the distribution of Mg-rich clays in the Madrid Basin follows a pattern in which sepiolite is present in the marginal zones closest to detrital inputs, while Mg-rich smectites are found in the central areas. This palaegeographical distribution of clay minerals is not in accordance with the Millot’s model (Millot, 1970), that is, the most magnesian clays do not occur at the centre of the basin, as Millot proposed, but mostly in marginal sites. The facies bearing Mg-clays was named “Magnesian Unit” by Pozo and Casas (1995) in reference to its anomalous Mg content. A relationship between

0

MADRID

50 100

5 VICALVARO

V

UU

200 m

Arkoses

Legend K 5

VICÁLVARO

QUATERNARY

SD

UPPER LIMESTONE ARGANDA

4

IU

LIMESTONES CLAYS AND CHERT GREENISH CLAYS AND SANDS

N

VACDEMORO

LU

ST

B

GYPSUM

BATALLONES

RED CLAYS

MA

3

100 m

IU

RO

IC

ESQUARAS

2

EB

DUERO

RAR

ESQUIVIAS

HES

E

JAR A

Sp

NEOGENE

MAGNESIC UNIT

ARKOSES

drid TAJO

SIF

MAS

Ma

C

LU

C 0m

SP SD ST I K

ARANJUEZ

CABAÑAS DE LA SAGRA

1

C

M

sepiolite AI-smectite Mg-smectite illite kaolinite

MAGÁN

Deposit ER

O TAJ

RIV

30 km

VICALVARO BATALLONES ESQUIVIAS CABAÑAS MAGÁN

Main clay occurrence Sepiolite Sepiolite Kerolite-stevensite Bentonite Bentonite

TOLEDO

FIGURE 7 Geological map and representative lithological section and mineralogy of Neogene units at the Madrid Basin (LU, Lower Unit; IU, Intermediate Unit; UU, Upper Unit). The location of Mg-clay deposits including sepiolite, Mg-bentonite and kerolite-stevensite mixed layers is also showed (modified after Brell et al., 1985).

144


occurrences of Mg-clays, source areas and depositional systems has been suggested by Doval et al. (1985) and Calvo et al. (1989). The mineralogical assemblages are composed of both detrital (inherited) and authigenic minerals. Authigenic silicate minerals include sepiolite, Mg-smectite (stevensite, saponite), mixed-layer kerolite/Mg-smectite, quartz (nodules) and zeolites (clinoptilolite–heulandite). Calcite, dolomite and barite were also detected. The Magnesian Unit facies are interpreted as alluvial-related, dry mudflat or palustrine deposits formed in a saline-alkaline lake margin. In this environment, alternating expansion/contraction episodes of detrital input (sand intervals) and sub-aerial exposure (palaeosols) can be determined. Granitic and metamorphic rocks from the bordering Central System and Toledo Mountains are the major source of inherited minerals, colloids and silica-laden waters. Fluctuating brackish to saline lake waters are the principal contributors of Mg, probably recycled from Mesozoic evaporitic successions. According to Castillo (1991), the sepiolite deposits present in the Madrid Basin are in two main palaeogeographic situations, one associated with the distal facies of the alluvial fans (e.g. Vica´lvaro-Cabanãs de la Sagra; Figure 7) and another associated with predominantly palustrine conditions (e.g. Cerro de los Batallones). A third, highly subordinated type is associated with the central lacustrine facies where it occurs with carbonates and is not suitable for exploitation. Several authors have also described sepiolite palaeosols in the Madrid Basin, both in marginal areas influenced by arkosic deposits (Calvo et al., 1986; Leguey et al., 1989) and in shallower internal areas of the basin (Leguey et al., 1985; Martıń de Vidales et al., 1988). The existence of sepiolite with deposits of other Mg-rich clays is very common in the Madrid Basin (Martıń de Vidales et al., 1991). According to Pozo (2000), the diversity of these Mg-clay occurrences and their geochemistry can be interpreted as a result of genetic processes controlled by the existence of precursor clay phases (inherited and/or authigenic), water sources (runoff, groundwater, lake water) and their hydrochemistry (e.g. Esquivias Mg-clay deposit). The geochemical variability of Mg-rich clays in the Madrid Basin has been described by a number of different authors (Pozo et al., 1999a,b; Torres-Ruı´z et al., 1994). These authors observed generally significant geochemical differences between sepiolite and magnesium smectite with respect to palygorskite, justifying the differences by the type of mineral genesis. The different types of clays can be clearly separated by F-Mg and Li-Mg ratios (e.g. Vicalvaro and Magań Mg-clay deposits). More recently, Pozo et al. (2005, 2010a) studied the geochemistry of sepiolite from different environments in the Madrid Basin, including alluvial fan facies (Vica´lvaro, Cabanãs de la Sagra), a palustrine complex (Cerro de los Batallones) and mudflat environments (Esquivias, Magań). The results allow sepiolites to be differentiated by the abundance of trace elements, or on the

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basis of whether they originated through precipitation from solution or under diagenetic conditions. In the case of diagenetic origin, the geochemistry of authigenic sepiolite would be related to the Mg-clay/carbonate support involved and the composition and pH of the groundwater input. The abundance of palygorskite in the Madrid Basin is much lower than sepiolite (Pozo et al., 1985). Until now, the most important occurrence of palygorskite is at Tabladillo (province of Guadalajara), in Miocene beds belonging to the so-called Loranca Basin, between the fluvial (detrital) and lacustrine (evaporitic) facies (Martıń Pozas et al., 1981). The palygorskite beds are 0.4–2 m in thickness, intercalated between detrital, marly and gypsiferous facies. The palygorskite can reach a proportion of 70%, accompanied by sepiolite, mica, quartz, calcite, dolomite and opal. This has been interpreted as the result of transformation from precursor aluminous phases. 3.1.1.2. Vallecas–Vica´lvaro and Yunclillos Deposits These sepiolite deposits, the world’s most important, are associated with alluvial deposits in a band running in a NE–SW direction. The Vallecas-Vica´lvaro deposits, close to Madrid, are located at the northern, and the Cabanãs-Yunclillos deposits, in the province of Toledo, are at the south (Figure 7). The sepiolite occurs in two stratigraphic levels that are topped by arkosic filling materials originating in distal alluvial facies (Figure 8). In the exploitation of the Vallecas-Vica´lvaro area, two lacustrine units with sepiolite mineralization are distinguished (Figure 9). They form lenticular subhorizontal layers with thicknesses varying between 2 and 12 m (Castillo, 1991; Galań and Castillo, 1984). These units are separated by arkosic sands forming coarsening-upward sequences of alluvial origin with the absence of erosive bases or channels, and with a thickness varying between 10 and 50 m (Baltuille et al., 1996). The sepiolite-rich beds mostly consist of sepiolite (> 80 wt%) and smectites (15 wt%), with traces (< 5 wt%) of calcite, dolomite, quartz and feldspars. The first sepiolite-bearing unit (lower lacustrine sequence) is formed by a main bed of poorly ordered sepiolite (XRD; 1–5 m thick) with a lenticular morphology and with intercalations of dolomitic carbonates, Mg-smectite clays and silex on the margins. The unit ends locally in a laminated bed of marly smectite-rich clays. The second sepiolite-bearing unit (upper lacustrine sequence) has at its base a bed of laminated sepiolite that is rich in detrital minerals (quartz and feldspar) and reddish in colour. The sepiolite bed has a lenticular morphology and presents carbonate, smectite and silex intercalations at its margins. It becomes separated into two exploitable layers, reaching a thickness of 10 m, high purity (up to 95 wt%) and good XRD ordering (see Figure 5). Towards the top and laterally, it grades into smectite-rich clays, calcareous marls and marly limestone finishing in a laminated clay bed formed by

146


DISTAL ZONE INTERMITTENT LAKES (PLAYA – LAKE)

GRANITIC BED ROCK

PROXIMAL ZONE

PERENNIAL LACUSTRINE ZONE

B SEPIOLITE

A ALLUVIAL FAN

MUD FLAT

IV III II

I

SO.

10 m.

0 m.

E. Vertical

0

1

2 Km.

E. Horizontal

Sepiolite

Mg/AI Semectite

Arkose

Mg-Smectite, Sepiolite

Calcrete

Gypsum

Lutite/Sepiolitic arkose

Dolocrete

Chert nodule

Black clay

Silcrete

Zeolite

FIGURE 8 Block diagram showing the Madrid Basin sedimentary environments during Neogene and representative cross section of the Vicalvaro sepiolite deposit. The main sepiolite beds are labeled A and B. Roman numbers indicate boreholes position (adapted from Galań and Castillo, 1984 and Sańchez Rodrı´guez et al., 1995).

smectite and/or sepiolite. The stratigraphic succession ends with an interval of up to 20 m of arkose in fining-upward sequences, with thin inserts of clays and a basal bed of gravel. The primary mechanism for the formation of the sepiolite seems to be neoformation by precipitation. The differences in the mineralogical associations between the two established sepiolite-bearing beds have been related to the locally greater availability of magnesium and higher pH, which would explain the greater presence of Mg-rich smectites in the lower sepiolite-bearing unit (pH close to 9) and the better ordering of the fibrous mineral in the upper sepiolite-bearing unit (Galań and Castillo, 1984). A detailed study of the beds containing smectite phases in the Vica´lvaro sepiolite deposit (Cuevas et al., 2003)

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147

YUNCLILLOS SECTION 0 m.

B

2 m.

Arkoses

SEPIOLITE–SMECTITE CLAY WITH CHERT SAND AND CLAYEY SAND SEPIOLITE ILLITE–SMECTITE CLAY AND SEPIOLITE

12 m.

A 15 m.

ILLITE–SMECTITE CLAY

20 m.

Arkoses

VICALVARO SECTION

SAND AND CLAYEY SAND CHERT AND SILICEOUS LIMESTONE SMECTITE–SEPIOLITE CLAY

20 m.

SEPIOLITE

B 25 m.

ILLITE–SMECTITE CLAY

Arkoses

DOLOMITE AND SMECTITE– SEPIOLITE CLAY GRAY LIMESTONE

33 m.

A

FIGURE 9 Representative lithological sections from the Yunclillos and Vicalvaro sepiolite deposits. The main sepiolite beds are labelled A and B (adapted from Galań and Castillo, 1984).

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reviews a mixture of stevensite, saponite and a micaceous mineral. Sepiolite is interpreted as the result of a diagenetic alteration in the Mg-rich smectites. At Yunclillos-Cabanãs de la Sagra area (province of Toledo), a sepiolite deposit of truly great economic interest (Galań and Castillo, 1984; Pozo et al., 1999a,b) is associated with Mg-rich bentonites. In this deposit, two sepiolite beds can be defined (see Figure 9). The upper bed has smectites and chert nodule impurities, while the lower one is thicker (3 m) and of higher purity. The sepiolite occurs in a unit of fine-grained sands with chert and carbonates (Garcıá et al., 1990). The sepiolite-bearing beds can also contain Mg-rich smectite (saponite) in varying proportions, although the sepiolite bed can reach several metres thick and with high purity. The sepiolite is considered to be authigenic. The general sedimentary environment for the formation of the sepiolite would have been related to flooding events over the fringe facies between distal alluvial fan deposits and those corresponding to lacustrine mudflats of a saline–alkaline lake (Figure 10). In this situation, the sepiolite also occurs in sediments of the alluvial fringe and associated with calcretes (Calvo et al., 1986). 3.1.1.3. Batallones Deposit Sepiolite associated with palustrine environments appears to be well represented in the Valdemoro-Esquivias sector. The sepiolite-bearing unit extends in a NE–SW direction for over 20 km. Only one mineralized bed is recognized where sepiolite is the major mineral; however, it can be accompanied towards the top of the unit by palygorskite. The Batallones sepiolite deposit is located near the villages of Torrejoń de Velasco and Valdemoro (province of Madrid; Figure 7). Both mapping and sedimentological analysis allow three main lithological units to be distinguished (Pozo et al., 2004): (I) mudstones, dolomitic mudstones and Mg-bentonites; (II) sepiolite and opal-CT; and (III) limestones, marls and siliciclastic sediments (Figure 11). Mudstones from Unit I are interpreted as mudflat deposits associated with a Mg-rich lake margin. The sepiolites and opals of Unit II are interpreted to represent thick polyphasic sepiolite palaeosols developed in a similar alkaline lake margin environment undergoing periods of prolonged sub-aerial exposure and groundwater inputs. Deposited in disconformity, the Unit III comprises associated siliciclastic and carbonate deposits forming sequences that are interpreted to represent deposition in a freshwater palustrine to shallow lacustrine environment. The sepiolite unit in Batallones consists of three sedimentary sub-units forming sequences II.1, II.2 and II.3, each with distinct mineralogical characteristics. Sequence II.2 contains sepiolite of the highest purity and XRD ordering (Pozo et al., 2010b). A noteworthy fact of these sequences is the existence of at least three main textures: laminated, massive and brecciated-intraclastic.

Lake margin

Open lake

Alluvial Groundwater recharge

Pond

Mudflats

Evaporation Oscillation of lake water level

Carbonate facies

Clay mineralogy Carbonate mineralogy

Massive bedded Calcretes/ Nodular Nodular Stromatolites, Bedded limestones dolostones and dolostones, dolocretes limestones dolostones tepees gypsum molds Mg-smectites Polygorskite/ Sepiolite (stevensite, saponite) Illite sepiolite Kerolite/stevensite Calcite/ Calcite Dolomite Dolomite Calicite/dolomite dolomite

PALYGORSKITE

SEPIOLITE

Mg-SMECTITE

FIGURE 10 Idealized sketch of marginal to open lake environments (palustrine–lacustrine) in the Madrid Basin during Miocene. The whole clay minerals displayed are authigenic and commonly associated with carbonate facies (adapted from Calvo et al., 1999).

150


UNIT III. DETRITAL FACIES AND CARBONATES

II.3

Spain

Madrid

N

Sp-Pk

Sp

UNIT II. SEPIOLITE II.2 LEGEND

Sp

Limestones, chert,

U.3 mudstones, marls and sandy clays

U.2 Sepiolites and opal Green mudstones

U.1 and bentonites

II.1

Transition

Mudstones and gypsum

Sp-SmT

Lithological sections

200 m m 1

SmT

UNIT I. GREEN CLAYS, SILTS AND DOLOMITIC MARLS

1.LADERA 2.POSTE 3.BALLO 4.JF2 5.BAT8!9

0

FIGURE 11 Geological map and representative lithological section of the Batallones sepiolite deposit (modified after Pozo et al., 2004).

The appearance of these textures both in optical microscopy and in scanning electron microscopy is shown in Figure 12. Massive texture shows a chert-like groundmass locally with detrital grains and bioturbation features. Under the scanning electron microscope (SEM), sepiolite randomly oriented fibre bundles are displayed. Laminated texture exhibits thin lamination, sometimes containing intraclasts and/or detrital grains. Under the SEM, laminar microfabric with orientation of fibre aggregates is observed. Brecciated-intraclastic texture (Figure 12) shows several degree of brecciation sometimes forming intraclasts. Under the SEM, the intraclasts are commonly groundmass-supported indicating at least two sepiolite genetic stages. Pozo et al. (2009) suggest that the sepiolite was formed from the destabilization of Mg-rich smectite (saponite) by means of an intrasedimentary mechanism with the subsequent action of neoformation processes from solutions or Si–Mg gels. A reduction in the salinity and silica input from groundwater would have favoured these processes. In fact, an association between groundwater discharge and sepiolite formation was reported by Pozo et al. (2006) in the vicinity of the Batallones deposit. In contrast to sepiolite formation, the presence of palygorskite in sequence II.3 would have been the result of the transformation of inherited clays from the input of surface waters and detrital sediments in an environment favouring the formation of calcretes (Pozo et al., 2010c).

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FIGURE 12 Thin section (crossed polars) and SEM images of main textures in the Batallones sepiolite deposit (Madrid Basin). Massive and brecciated-intraclastic textures are the most common often forming sequences. Laminated textures are more subordinated and have been observed mainly at the base of sepiolite unit.

3.1.2. Other Spanish Deposits Sepiolite and palygorskite are also found in the Duero Basin and in small basins, particularly those of Torrejoń el Rubio, Calatayud, As Pontes and Campo de Calatrava and locally in a brackish lacustrine environment of perimarine origin, in the Guadalquivir basin (Lebrija) (Figure 6). Most of the occurrences of palygorskite and sepiolite in the Duero Basin correspond to materials included in the Cuestas Facies composed of marls, dolostone, limestone, clays and gypsum (Armenteros et al., 1989; Pozo, 1987) in the central areas of the basin. The only significant accumulation is located on the south-eastern edge of the basin in the Bercimuel palygorskite

152


deposit where the fibrous clay mineral is associated with fine-grained detrital alluvial fan deposits and carbonate crusts (Armenteros, 1986; Martıń Pozas et al., 1983). Sepiolite and palygorskite content in the Cuesta Facies can reach 28 and 73 wt%, respectively, accompanied by illite, smectite and traces of kaolinite (Pozo and Carame´s, 1983). Their formation in the carbonate-rich lacustrine environments has been interpreted as the result of neoformation from silica and magnesium gels both by intrasedimentary and by evaporitic processes. The palygorskite formed from silico-aluminous precursor phases (Pozo et al., 1990). Vivar (2010) recently studied different outcrops and cores of this facies establishing sequences starting with inherited mineral associations (illite, smectite and kaolinite) and finishing with neoformed minerals (Mgsmectite, palygorskite and sepiolite). Sepiolite was mostly neoformed, while palygorskite and magnesium smectite would be the result of transformation processes. The Bercimuel palygorskite deposit is associated with Neogene detrital materials deposited by alluvial fan systems that change laterally to lacustrine facies. This palygorskite deposit has been studied by Sua´rez et al. (1989, 1993) and Sua´rez (1992). A horizontal clay unit with average palygorskite content of 60–70 wt% overlying a carbonated bed (crust) is commercially exploited in Bercimuel. The unit consists of two beds 1–1.5 m thick. The palygorskite is associated with quartz, illite and kaolinite as inherited minerals, and with smectites and interstratified minerals (smectite–illite) as transformation minerals. According to Sua´rez (1992), the origin of the palygorskite is associated with the weathering of phyllosilicates and the dissolution of the quartz present in the sediment and by the action of solutes dissolved in surface runoff resulting from the weathering of pre-Neogene silicate and carbonate rocks. The palygorskite deposit at Torrejoń el Rubio (province of Caćeres, Spain) contains Paleogene palygorskite-rich marls overlying the basement made of Cambrian slates. This deposit is located in a small Tertiary fault basin limited by quartzite and granite outcrops to the west of the Tagus Basin (see Figure 6). According to Galań and Castillo (1984), the maximum thickness of the clay unit ranges between 6 and 50 m. The detrital-clay unit has two mineralized beds of which only the lower one (0.5–4 m thick), overlying the slate basement, is exploited (Figure 13). The palygorskite content can reach 70 wt% and is associated with illite and varying quantities of sepiolite, chlorite, dolomite, smectite (saponite), feldspars and quartz. The lower bed is the result of weathering of the slate basement in a lacustrine–palustrine environment. The fluvial–alluvial conditions would have originated with the exposure of slate areas that were previously weathered and exposed as the consequence of movement by the faults limiting the basin. Thus, the continuity between slates and palygorskite-rich clays observed at a number of points suggests the formation of palygorskite in association with weathering of the metamorphic

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153


N Sra.

R IV E

de

las

Co

rch

R

uela

s

JO TA

TORREJON EL RUBIO

S-274 S-365

ST-385 ST-301

340 319

ST-342

367

ST-199

ST-289 ST-266

351 322

389 356 347 ST-415

394

EXPLANATION − “RANA” AND/OR PEDIMENT − “RANA” DETRITAL-CLAYEY BED

0 5 10m

QUARZITE

0

1

2Km.

SLATE (BASEMENT)

Pa > I I >> Pa DETRITAL MINERALS ST-385 = LOG NUMBER MINE 319 = ELEVATION IN M

FIGURE 13 Geological map and correlation of cores from the Torrejoń el Rubio Basin. It is noteworthy the thickness variability of the palygorskite-rich bed.

rock in an acidic environment. Weathering of chlorite contained in the slate would have provided the necessary input of components required for the formation of palygorskite (Galań and Castillo, 1984; Galań et al., 1975). Other authors (Fernandez Macarro and Blanco, 1990) suggest the palygorskite would have originated from smectite in palaeosols formed on a stable alluvial plain. The Mara sepiolite deposit located in the Calatayud Basin province of Zaragoza (see Figure 6) originated during the upper Miocene in a palustrine– lacustrine environment (Arauzo et al., 1989). The sepiolite bed consists of alternating clays, marls and carbonates, with thicknesses between 10 cm and 1 m, and average values of 50–60 cm. The mineralogical associations in the sepiolite beds are complex: detrital minerals (illite, interstratified illite-smectite, quartz, feldspars, dioctahedral smectite, chlorite and kaolinite); neoformed phyllosilicates (sepiolite, trioctahedral smectite, palygorskite); carbonates (calcite and/or dolomite); and, also occasionally, heulandite– clinoptilolite, opal-A and opal-CT (Mayayo et al., 1998). Both the sepiolite and the trioctahedral smectite formed by precipitation in the lacustrine basin. The palygorskite is post-depositional resulting from the transformation of aluminosilicate phases. The volcanic region of Campo de Calatrava is located in the south-central zone of the Iberian Peninsula (province of Ciudad Real) and belongs to the Cenozoic Manchegan Basin (see Figure 6). The presence of palygorskite in

154


this basin has been reported by Pozo et al. (1986), Pozo and Martıń de Vidales (1989), and Sańchez and Galań (1995). The genesis of the palygorskite is a result of dissolution–precipitation phenomena at the expense of the dioctahedral smectites and/or illite during retractive stages of the marginal lacustrine system. The origin of the Mg is interpreted as the result of hydrolytic alteration of chloritic slates from the basement, from which this element would have been released. The weathering of volcaniclastic materials, however, would have been responsible for releasing Mg, Ca and silica into the aqueous phase. The main characteristics of the Spanish sepiolite–palygorskite deposits are summarized in Tables 3 and 4.

3.2. Turkey 3.2.1. Geological Setting References to the presence of sepiolite and palygorskite in continental deposits in Turkey have grown notably in recent years. Although there is a common presence of both fibrous clay minerals, the sepiolite deposits acquire special relevance owing to their extensive distribution and their quality, with special reference to the Ezkisehir Basin deposits. Three sectors can be established with sepiolite deposits or containing sepiolite: Eskisehir–Konya, Denizli and Sivas (Figure 14). The Eskisehir–Konya sector is a band of lacustrine deposits running in a NW–SE direction in the west-central area of Turkey. The northern area contains the deposits of Eskisehir (Ece and Çoban, 1994) and Yenidogan (Yeniyol, 1992), both in the Neogene Eskisehir Basin. Because of the economical interest of the first one, it will be described in Section 3.2.2.

FIGURE 14 Geographic map showing the main zones with sepiolite deposits in Turkey. 1. Eskisehir-Konya sector, 2. Denizli sector, 3. Sivas sector.

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155

Sepiolite and palygorskite have also been reported in the Neogene Konya Basin to the southeast, but the presence of sepiolite is less significant in the Konya Basin than in the other sector. Here, three types of sepiolite associated with carbonates have been identified (Karakaya et al., 2004). The first is a brown sepiolite, rich in organic material with common occurrences of bioclasts; another is poor in organic material with associated dolomite (50 wt%); and the third type with only 5–20 wt% sepiolite is present in light-coloured carbonates (dolomite, calcite). The presence of chart is common as nodules or lenses. The presence of palygorskite together with sepiolite has also been recognized in nearby areas (Asagi Pinarbasi Basin) in carbonate units (calcite and dolomite; Karakas¸ and Kadir, 1998). The Denizli sector is located southwest of Eskisehir–Konya with sepiolite having been discovered in the Neogene Serinhisar-Acypayam Basin. Akbulut and Kadir (2003) describe the presence of sepiolite, palygorskite and saponite in the Neogene lacustrine deposits of the basin. The deposits occupy the depression formed by a graben resulting from extensional stress. The sedimentary filling is mainly clayey material deposited in fluvio-lacustrine environments. The alkaline lake palustrine environments favoured the periodic development of sepiolite, palygorskite, saponite and dolomitic sepiolite. At some points, the sepiolite predominates but with intercalated saponite. At other points where there is evidence of volcanic activity, the saponite accompanies palygorskite. In all cases, the contact between the fibrous clay minerals is clear, which can be interpreted as rapid changes in the physicochemical conditions in the environment. The source of Fe and Al is inferred to be synsedimentary basaltic volcanism, while the SiO2 and Mg would have originated in the surrounding lithological units including ultrabasic rocks, detrital rocks and perhaps volcanic rocks. The conclusion reached is that the Mg-rich clays were formed either through the direct precipitation in the alkaline waters of the lake or in the pore water existing between the dolomite crystals. The Sivas sector is located to the east of the previously described sectors. The presence of sepiolite and palygorskite has been cited for the Neogene Kengal Subbasin that is a part of the Sivas Basin (Yalçın et al., 2004). These authors describe the presence of sepiolite and palygorskite in lacustrine facies (20–60 m in thickness) consisting mainly of carbonates (calcite, dolomite and magnesite) with alternating layers of clays and chert. The palygorskite, sepiolite and dolomitic sepiolite are present in layers with varying thicknesses of 0.1–1 m. A detailed description of occurrences and deposits of sepiolite and palygorskite in Turkey is included in this book (see Chapter 7).

3.2.2. Eskisehir Deposit The presence of sepiolite in beds and as nodules in the Eskisehir Basin (Turkey) has been described by Ece and C ¸ oban (1994) (see Figure 14). The sedimentary record is complex and is represented by calcareous clays, clayey carbonates,

156


dolomite, gypsiferous calcareous clays, siliceous tuffs, sepiolite, sepiolite-bearing dolomite and ultramafic conglomerates. The basement is an ophiolitic complex, and intercalations of weathered bentonitic tuffs are common occurrences in the stratigraphic succession. Four types of sepiolite are distinguished: one nodular and three bedded. The nodules appear in a bed (50–100 cm) of poorly sorted conglomerate from ultramafic rocks that overlie the ophiolitic basement. The sepiolite-bearing beds can be black (0.5–2 m); passing upward to brown (0.5–3.5 m); or white dolomitic sepiolite (4–5 m). The deposit is lacustrine and dates from the Miocene, filling a fault basin or graben of extensional origin (rifting). The existence of accumulations of stockwork magnesite very near the lacustrine deposits acted as the source for the formation of the sepiolite nodules (meerschaum), which would have formed as the result of diagenetic alteration of magnesite blocks under alkaline conditions. The sepiolite formed by direct precipitation in the saline–alkaline waters of the lake, with supersaturation of silica. The environment would have been alkaline–saline in arid to semi-arid climatic conditions with possible wet intervals owing to seasonal fluctuations. Yeniyol (1992) studied the geology, mineralogy and genesis of the Yenidogan sepiolite deposit, near Eskisehir (50 km to the SE) in the same basin. According to this author, this is the most important deposit in the Neogene Eskisehir Basin. The stratigraphic record shows a thickness of about 200 m in three lithological associations, of which the third contains the sepiolite deposit. The sepiolite occurs in two beds in the upper interval of the Pliocene sequence, which consists of an alternation of dolostone and dolomitic marls. The lower bed (3 m thick) is formed of sepiolite and dolomitic sepiolite. The upper bed has a thickness of up to 10 m and a broad lateral extension. It is formed by an alternation of sepiolite with sepiolite-rich layers and lenses. The sepiolite (up to 90%) is accompanied by quartz, feldspar, illite and grains of pumice. Dolomitic sepiolite is particularly abundant in the upper bed, but it does not exceed 50%. The formation of the sepiolite is associated with the shallow margins of an alkaline lake, ephemeral flooding events and wetlands (marshes), resulting from direct precipitation of the lake water. A diagenetic origin from solutions circulating through the intergranular porosity and along the desiccation cracks is also suggested. The primary source of Mg would have been the weathering of ultramafic rocks (serpentinite), common in the basement. Loughlinite (sodium-rich variety of sepiolite) is currently mined at Eskisehir. According to Yeniyol (1997), the loughlinite-bearing layers are present in Miocene deposits where layers can have thickness of 0.6–5 m, intercalated with lutites, volcanic tuffs, chert and bentonites. The loughlinite beds can be accompanied locally by analcime, smectite calcite and illite. Kadir et al. (2002) associate loughlinite in the same area to a lacustrine-volcano-sedimentary unit of highly variable sediment composition (clastic,

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clayey, evaporitic, dolomitic and siliceous sediments) and volcanic tuff deposits. In sharp contact with the sepiolite, the loughlinite contains dolomite, opalCT and analcime, especially in weathered tuffs and yellow or green lutites. At other times, the loughlinite is intercalated as lenses in the sepiolite. In this case, the associated materials are palygorskite, analcime, feldspar, opal-CT and calcite. According to Yeniyol (1997), loughlinite formed by the reaction of Naþ and Mg-rich waters with the vitreous components of pyroclastic deposits.

3.3. Other Deposits and Occurrences 3.3.1. Amargosa Desert Deposit (USA) Khoury et al. (1982) described the presence of Mg-rich clay deposits of economic interest in the Amargosa desert of southern Nevada. Sepiolite (up to 1.2 m in thickness) is associated with extensive kerolite/stevensite deposits and travertine accumulations of Plio-Pleistocene age (Eberl et al., 1982). The sepiolite is present in two main stratigraphic intervals and overlies a unit formed by sepiolite and Mg-rich smectites. In addition to trioctahedral smectite (kerolite/stevensite) and sepiolite, the minerals identified are dolomite, calcite and local occurrences of detrital mica. Based on textural evidence, Khoury et al. (1982) indicate that the sepiolite could have been formed not only as the result of direct precipitation but also from kerolite/stevensite dissolution. According to Hay et al. (1986), these deposits were deposited in the lacustrine environment of a playa-lake, with caliche, in areas influenced by groundwater infiltration. The review by Miles, Chapter 11, presents complementary information on the Amargosa deposits in Nevada, USA. 3.3.2. Amboseli Deposit (Kenya–Tanzania) According to Stoessel and Hay (1978), the Amboseli sepiolite deposit is located in the Pleistocene formation known as the Sinya Beds, characterized by the common occurrence of elongated mounds or domes. Lithologically, the deposit consists of massive white dolostone conformably overlain by green sepiolitic clays ending with a level of clays and silts with calcrete. Hay and Stoessel (1984) differentiate the sepiolite from the deposit into two varieties: waxy (green) sepiolite and meerschaum type (massive, light and porous) in cavities. Dolomite is brecciated in layers 1.5–5 m in thickness, and it is overlaid with 1.3 m of waxy green sepiolite accompanied by calcite, dolomite and potassic feldspar. Most of the meerschaum is located in the dolomitic breccia and this is the sepiolite of economic interest. The carbonate and sepiolite beds formed by deposition in marshland, or shallow lakes or ponds (palustrine). The aSiO2/aMg2þ ratio of the pore fluids would have been the main control on precipitation of sepiolite and kerolite (Hay et al., 1995).

158


3.3.3. Guanshan Deposit (China) In China, there are more than 20 palygorskite deposits on the border of Jiangsu and Anhui provinces. The Guanshan palygorskite deposit in Anhui province seems to be the most important. This palygorskite is present in a unit varying in thickness between 18 and 54 m overlying the Miocene Huaguoshan Formation. The base of this unit consists of volcanic rocks (olivine basalt and basalt ash), transitioning to palygorskite-rich clays towards the top with a thickness varying between 3 and 6 m. The palygorskite content ranges from 55 wt% to more than 90 wt% accompanied by low quartz content (< 10 wt %) and traces of sepiolite, mica and dolomite (Liu and Cai, 1993). A model is proposed explaining the formation of the palygorskite as the result of the transformation of components of the volcaniclastic deposits in a lacustrine environment. Further information regarding the palygorskite occurrences in China is included in Chapter 10. 3.3.4. Ventzia Basin Deposit (Greece) In Greece, Kastritis et al. (2003) describe the existence of a deposit of palygorskite and palygorskite and saponite-rich clays in the Ventzia Basin. This basin is located in the vicinity of Grevena in the Greek region of Macedonia. The basin is 22 km in length, has a maximum width of 6 km and was formed at the end of the Pliocene and beginning of the Pleistocene. The palygorskite is present in beds with thicknesses varying between 10 and 18 m, with 60– 90% purity. The formation of the palygorskite would have been associated with the diagenetic transformation of saponite-rich sands resulting from the weathering of ultramafic rocks (Vourinos ophiolitic complex), which would justify the high iron content (up to 11% Fe2O3) of this palygorskite (Kastritis et al., 2005). 3.3.5. El Bur Deposit (Somalia) Singer et al. (1998) describe the abundant presence of sepiolite (meerschaum) in central Somalia (El Bur). The sepiolite is associated with limestone, dolostone, gypsiferous marls and evaporites (gypsum and anhydrite) facies. Mineralogically, the sepiolite shows a high grade of purity with traces of associated minerals: calcite, quartz and halite, and traces of illite in the clay fraction. According to these authors, the characteristics of the deposit are similar to those of Spanish and Turkish deposits. The exception is its age (Quaternary), which links them in time with the Amboseli (Hay et al., 1995) and Amargosa Desert (Hay et al., 1986) deposits. The environment is interpreted as an ephemeral lacustrine and evaporitic one, where groundwaters play an important role. Based on the absence of other phyllosilicates, they conclude the formation of the sepiolite to be due to chemical precipitation.

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4. FINAL REMARKS 4.1. Continental Sedimentary Environments for Sepiolite and Palygorskite Formation A schematic sedimentary model for palygorskite formation is given in Figure 15. The image shows a cross section where the transition from an alluvial fan environment to a palustrine–lacustrine one can be observed, taking into account the presence of a source area comprised igneous and metamorphic rocks and the interaction of three water types: groundwater, runoff and lacustrine water. The rocky substrate of the parent area is fundamental in order to justify inputs of Mg or Si (Figure 16; Table 6). The silica input can come from any magmatic rock, particularly those with a felsic composition, or from silica-rich metamorphic rocks and siliciclastic or biosiliceous (diatomite) sedimentary rocks. The Mg source can be any type of rock containing Mg-rich minerals, including magmatic rock (basalt, gabbro and peridotite), metamorphic rock (slate, serpentinite) or sedimentary rock (dolomite, magnesite and magnesian clays). The Si and Mg released in these zones can travel distances and accumulate in flooded lacustrine zones. As previously mentioned, given its low solubility except with very alkaline pH levels, aluminium

+ + +

+

+

Alluvial fan system

+

+

Lacustrine–palustrine

+ +

Runoff waters

+

Lake waters

Lacustrine deposits Fine-grained alluvial fan facies Medium-grained alluvial fan facies

Groundwaters

Coarse-grained alluvial fan facies Parent rock (igneous) Parent rock (metamorphic)

Calcretes

Palygorskite

Saponite

Al-smectite illite Palygorskite Weathered clays from the parent rock

FIGURE 15 Sketch of palygorskite deposit formation in continental sedimentary environments. The transformation of Al-bearing clays to palygorskite can take place close to the parent rock supplying Mg2þ, Si(OH)4 and Al-rich colloids but also related to marginal alluvial–palustrine–lacustrine environments.

160


PALYGORSKITE FELSIC-INTERMEDIATE MAGMATIC ROCKS METAMORPHIC ROCKS Al-rich silicates

DIATOMITES Si-rich sediments

Mg2+

DOLOMITE MAGNESITE

Si(OH)4

Al from Al-clays

Mg-rich carbonates MAFIC-ULTRAMAFIC MAGMATIC ROCKS METAMORPHIC ROCKS Mg-CLAYS

SEPIOLITE Mg-rich silicates

FIGURE 16 Scheme showing some lithological sources delivering Mg2þ and Si(OH)4 in the sedimentary environment. The presence of inherited Al-rich clays favoured palygorskite over sepiolite formation.

TABLE 6 General Sources of Magnesium and Silica in the Sedimentary Environment. Si(OH)4 Sources

Mg2þ Sources

Runoff waters

Runoff and lake waters

Groundwaters

Groundwaters

Alteration/dissolution of detrital minerals (quartz, feldspars, micas)

Mineralizing fluids from calcite and gypsum formation (Mg/Ca increase)

Alteration of clay minerals (both detrital and authigenic)

Alteration/dissolution of Mg-rich minerals (silicates, carbonates)

Dissolution of biosiliceous facies (diatomites)

Dedolomitizacioń

will tend to be less mobile and remain in the vicinity of its parent rock area, or it may be transported as sediment in aluminous clay particles or colloids. Phenomena occurring in the parent rock area can favour the formation of aluminous clay minerals and the release of Mg and Si, which would favour palygorskite formation in lacustrine conditions, mainly by transformation. Detrital aluminous clay minerals are abundant in distal alluvial fan facies. Palygorskite may be formed there as the result of the transformation of the inherited aluminous clays in the vicinity of a lacustrine environment, particularly during periods of exposure. This process favours calcite precipitation.

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In areas nearest to a lake, more specifically in the zone containing intertidal mudflats where the water is more alkaline, the transformation of aluminous smectite into saponite would be favoured instead of forming palygorskite. The starting sedimentary model for sepiolite genesis is similar to that for palygorskite (Figure 17). Nevertheless, sepiolite in this case is formed in the water column or in the pore fluids in a lacustrine wetland, where inputs of silica-bearing groundwater would play a significant role. The type of magnesian clay formed will be conditioned by the pH, salinity and Si/Mg ratio. Consequently, in conditions of high salinity, pH equal to or higher than nine and relatively low Si/Mg ratios, the formation of magnesian smectite is favoured. If there is moderate salinity and the pH is between 8 and 8.5, the magnesian phase formed will depend on the Si/Mg ratio, with kerolite precipitated where this is low and sepiolite the result of a high ratio. In other words, environments that are very rich in Mg will favour the formation of kerolite or magnesian smectite depending on the pH and salinity of the environment, with transitions from one to the other by means of kerolite–smectite interstratified minerals. Sepiolite is precipitated with moderate salinity and slightly alkaline pH in a Si- and Mg-rich environment.

+ + +

+ + +

Alluvial fan system

+

+ + +

+

Lacustrine–palustrine

+ +

+

Runoff waters

+

Lake waters

Lacustrine deposits Fine-grained alluvial fan facies Medium-grained alluvial fan facies Groundwaters Basement (igneous and metamorphic rocks) Coarse-grained alluvial fan facies

Neoformation

high salinity, pH ³ 9 Lower salinity, pH = 8–8,5 Si/Mg¯

Mg-smectite

Freshening

SYNGENETIC (evaporative neoformation)

Sepiolite

Si/Mg

Transformation Sepiolite (kerolite, Si/Mg¯) Mg-smectite, kerolite-stevensite

Groundwaters (SiO2)

DIAGENETIC (replacement and/or diagenetic neoformation)

FIGURE 17 Sketch of sepiolite deposit formation in continental sedimentary environments. Salinity, pH and Mg/Si ratio play an important role in the syngenetic formation of sepiolite, kerolite or stevensite. A drastic change in salinity–pH (freshening) during early diagenesis of Mgclays favours the intrasedimentary formation of sepiolite.

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During diagenesis, changes in the hydrochemistry of runoff, lacustrine and groundwater, can cause sepiolite formation by transformation at the expense of other magnesian clays. Likewise, it could precipitate directly from solution in pores. This can give rise to the formation of several generations of sepiolite. In exposed sedimentary environments, intrasedimentary sepiolite can form in fine-grained dolomitic facies (dolomicrites). On occasions, after the dolomitization process, sepiolite can be arranged by covering the dolomite crystals or by cementing the intercrystal porosity. Sepiolite can also be formed in the presence of silica as the result of dedolomitization processes with the development of calcite crusts and the release of Mg. Palygorskite can also be formed by these processes but only if there are particles of aluminous clays mixed with the dolomite. Representative field photographs of quarry faces with sepiolite beds and nodules deposited in different continental sedimentary environment are shown in Figure 18.

FIGURE 18 Representative field photographs of sepiolite deposits in different sedimentary environments from the Madrid Basin. Sepiolite is associated to other Mg-clays (saponite, kerolite–stevensite, stevensite) well bedded (1) or intrasedimentary (3, 4) in palustrine and mudflat facies, respectively. Associated to alluvial fan deposits sepiolite occurs inserted between detrital facies (2).

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4.2. Origin of Deposits with Sepiolite and Palygorskite: Lithological Associations In lacustrine environments, fibrous clay minerals can occur near to or overlying materials supplying the elements and particles required for their formation, or faraway from them through the involvement of a sedimentary transport system. Near parent rocks the development of palygorskite will be rather more favourable than for sepiolite. This is because there will be a high proportion of Al-rich particles in weathering profiles, which would favour its formation over that of sepiolite. This occurs in the deposits at Torrejoń (Spain), on a slate substrate, or Guanshan (China) and Andhra Pradesh (India) over a mafic volcanic substrate. Obviously, if the bedrock lacks Al and is rich in Mg, the fibrous mineral that could be formed is sepiolite, provided that pH and salinity conditions and Si/Mg ratio are adequate as in the nodular sepiolite at Eskisehir. After transport, Al-clay particles accumulate in alluvial or lacustrine deposits where, in the presence of Mg, they can form palygorskite, with post-depositional calcretes (e.g. Bercimuel, Spain). Palygorskite was developed by diagenetic transformation. The arrival of Si and Mg in swamp environments will enable sepiolite formation to take place in different sub-environments within an alluvial–palustrine–lacustrine setting. With regard to distal alluvial deposits, sepiolite will form during ephemeral flooding events where inputs of silica-bearing groundwater play a significant role. The result can be thick accumulations (> 10 m) of sepiolite mainly produced through precipitation (neoformation). The most noteworthy example of this is the sepiolite deposit at Vica´lvaro (Spain). The importance of groundwater in the formation of nodular sepiolite has also been proposed for El Bur (Somalia) and Amboseli (Kenya–Tanzania). In zones near to a palustrine–lacustrine wetland with fluctuations in the lacustrine water mass, mudflat deposits may contain sepiolite beds alternating with detrital facies, carbonates and even evaporites (mainly gypsum). The sepiolite beds have moderate thicknesses (1–3 m), in cyclical sequences in correlation with the evolution of the lake. Precipitation in conditions of evaporation would be the cause for their formation under suitable physico-chemical conditions. Exceptionally, in extensional tectonic regimes, sepiolite could also be formed in deeper water where the presence of organic material would indicate anoxic conditions. The sepiolite deposits of the Eskisehir Basin (Turkey), Mara (Spain) and Amargosa (USA) are examples of the previously mentioned environments. At other times, sepiolite can be formed in palustrine conditions, that is, wetland zones subject to frequent flooding and frequent exposure episodes. In this case, an overlapping with edaphic processes is produced. Owing to amalgamation of the sepiolite beds, important thicknesses (up to 10 m) are achieved. The Batallones (Spain) sepiolite deposit would be representative of this type.

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As previously indicated, sepiolite and palygorskite can be formed between dolomite crystals during diagenesis. The presence of intrasedimentary sepiolite and/or palygorskite associated with dolomitic facies is common in the Tertiary basins of Spain and Turkey, associated in some cases with sepiolite deposits. On the basis of the above, six different lithological associations have been established that are representative of sepiolite–palygorskite deposits (Figure 19).

4.2.1. Bedrock: Fibrous-Clay Mineral Association (LA-1, LA-2, LA-3) The fibrous clay mineral is present in the vicinity of or overlying the parent rock area supplying the basic components. This is particularly interesting in the case

Sp Ke/stev

Sp

Magnesite

Dolomite Sp

LA-3

Gypsum

Detrital facies

Saponite

Sedimentary rocks

Pk

Detrital facies

LA-6

Chemical facies (Mg–clays) Slate/serpentinite

LA-2

Sp

Limestone

Metamorphic rocks Detrital facies

Pk Mudstones

Pk

Sp

LA-4

Dolomite

Detrital facies Gypsum

Tuff/lava flow

LA-1

LA-5

Chemical facies (carbonate–gypsum)

Volcanic rocks

FIGURE 19 Idealized sketch of stratigraphic sections showing the different lithological associations (LA) established. LA-1 and LA-2 show the occurrence of palygorskite in palustrine–lacustrine conditions near or overlying the parent rock source of Si, Mg and Al. In the case of LA-3, the presence of sepiolite is the result of replacement of magnesite in a silica-rich medium. Relatively far away from parent rock supplying elements, the formation of sepiolite and palygorskite can be related to marginal lacustrine facies where detrital input from alluvial fan deposits should take place, playing groundwaters an important role. Lithological association LA-4 displays the presence of sepiolite and palygorskite associated to sandy and muddy facies, respectively. LA-5 and LA-6 show the occurrence of sepiolite as a consequence of chemical precipitation and diagenetic replacement of previously formed Mg-clays (saponite, kerolite-stevensite).

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of palygorskite, where situations can be differentiated depending on whether the bedrock is mafic magmatic rock (basalts, tuffs), as is the case of the Guanshan (China) and Andhra Pradesh (India) deposits, or metamorphic rock (slate), as occurs at Torrejoń (Spain), or ultramafic rocks and saponitic sands, as at Grevena (Greece). In all of these cases, the bedrock and the palygorskite are in direct contact and can reach thicknesses of up to 6 m, although thicknesses of up to 18 m have been reported in Greece (possibly referring to the unit containing the palygorskite). With sepiolite, only the Eskisehir (Turkey) nodular sepiolite deposit would fall within this lithological association.

4.2.2. Detrital Facies: Fibrous-Clay Mineral Association (LA-4) The sepiolite layers occur between carbonate-poor detrital facies. This is the case of the Vica´lvaro (Spain) deposit. Palygorskite can also form deposits associated with the clayey detrital facies of alluvial deposits, as occurs at the Bercimuel (Spain) deposit. Finally, with regard to detrital mudflat facies, sepiolite can also form in carbonate-poor palustrine conditions but with abundant chart nodules and beds, as is the case of the Batallones (Spain) sepiolite deposit. 4.2.3. Carbonatic Chemical Facies: Fibrous-Clay Mineral Association (LA-5) Sepiolite predominates in these facies associations, intercalated between dolomite, dolomitic marl, limestone and gypsum and showing certain cyclicity. Typical examples of this are the Eskisehir (Turkey) and Mara (Spain) sepiolite deposits. 4.2.4. Clayey Chemical Facies: Fibrous-Clay Mineral Association (LA-6) Sepiolite is formed more locally as the result of diagenetic replacement of other magnesian clays, leading the sepiolite to form nodules or discontinuous beds in a lateral transition (towards the lake) to non-fibrous magnesian clays. This is the case of the sepiolite occurring at the Magań and Cabanãs (Spain) bentonite deposit and the Esquivias (Spain) kerolite–stevensite deposit. To summarize script, sepiolite and palygorskite are the result of the interaction between silica- and magnesium-bearing solutions in an environment with suitable physico-chemical conditions (mostly pH and salinity), and in the presence of aluminium in solution or reactive phases in the case of palygorskite. The physico-chemical conditions of the environment control the formation of the magnesian clay mineral, also determining its fibrous or laminar nature and, consequently, its composition. As seen throughout this chapter, these conditions occur in specific continental sedimentary environments where they give rise to sepiolite and palygorskite deposits of commercial interest. The lithological characteristics of the parent area supplying Si, Mg

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and aluminous phases, and the hydrochemistry of the participating water (runoff, groundwater and flooding events), play an important part in the formation of these fibrous clay minerals and in the complexity of their deposits.

ACKNOWLEDGEMENTS This work has been partially supported by the Project CGL-2008-05813-CO202 and by the Government of Andalusia through the Research Group Applied Mineralogy (RNM135). We would like to thank Prof. Ray E. Ferrell for his revision, discussion and comments of this chapter, which improved the manuscript.

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Chapter 7

Sepiolite–Palygorskite Occurrences in Turkey ¨ mer Bozkaya Hu¨seyin Yalçin and O Department of Geological Engineering, Cumhuriyet University, Sivas, Turkey

1. INTRODUCTION Ophiolitic units of mainly Cretaceous age are widespread in Turkey (Figure 1), and the North and South-east Anatolian Ophiolite Belts extend 100 km (Go¨ncuög˘lu et al., 1997). Therefore, various industrial minerals such as sepiolite, magnesite, phlogopite, talc and/or serpentine-asbestos together with metallic mineralizations (mainly Fe, Cr, Ni, PGE) could be developed relating to pre-/syn- and/or post-serpentinization processes of ultramafic rocks (e.g. Bas¸ıbuÿu¨k et al., 2009; Ece et al., 2005; Yalçın and Bozkaya, 2006). Upper Cretaceous serpentinite-hosted sepiolite occurrences appear to have a limited economic potential for now due to a few and small outcrops, but they have yet to be effectively prospected throughout the ophiolitic bodies in Turkey. Further, new occurrences can probably be expected within the siliciclastic-clayey carbonate rocks of basins adjacent to serpentinized ultramafic rocks which commonly provide Mg-rich materials to the Tertiary marine and continental environments as notably reported by Yalçın et al. (2004) and Yalçın and Bozkaya (2006). In addition, sepiolite, loughlinite and/or palygorskite clays may be found in the volcano-sedimentary basins and the hydrothermalized volcanic systems as well as in deposits of zeolite (Yalçın, 1997), kaolin (Yalçın and Bozkaya, 2003) and bentonite (Yalçın and Gu¨mu¨s¸er, 2000). The objective of this chapter is to serve as a useful, basic reference on the topic of Turkish sepiolite–palygorskite occurrences including geological setting, mineralogical and geochemical features, usage and genesis.

2. GEOLOGY AND MINERALOGY The stratigraphic successions and lithological variations of Turkish sepiolite– palygorskite formations are given in detail and explained below by grouping on the basis of the sedimentary environment. Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00007-4 # 2011 Elsevier B.V. All rights reserved.

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176 Developments in Palygorskite-Sepiolite Research

FIGURE 1 Geographic distribution of ophiolitic rocks and sepiolite–palygorskite-bearing basins and areas (modified from 1:500.000 maps of MTA, 2002).

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Sepiolite–Palygorskite Occurrences in Turkey

2.1. Marine Sepiolite–Palygorskite Occurrences In Turkey, there is only one example of this type that is found in a conformable sequence of Upper Cretaceous–Lower Miocene sediments from Hekimhan district, located in north-west Malatya (Bozkaya and Yalçın, 1991; Yalçın and Bozkaya, 1995). Upper Cretaceous is represented by clayey limestone with palygorskite (Figure 2(IIa)) and it also includes economic iron oxide and carbonate mineralization at the lower part of the section. Palygorskite occurs concentrated in the uppermost parts of the Upper Cretaceous unit where it is associated with other phyllosilicates (smectite, chlorite, serpentine and illite), calcite and feldspar. Palaeocene deposits, where fibrous clay minerals are most abundant, consist of evaporative facies containing gypsum, dolomite, dolomitic marl/ claystone, limestone, cherty dolomite or limestone and minor celestite deposited in a shallow marine-coastal lagoon environment. Pure sepiolite levels are only found at the Mezgi ridge in the south of the basin (Figure 2(IIb)) that occurs in three zones 15–20 cm thick, intercalated with siliceous dolomite units reaching up to 40–50 cm thickness. Pure palygorskite clay fractions are observed in the south of the basin that rests on 5 m of a sepiolite level within the dolomitic marls (30 cm) intercalated with dolomites (1 m). These are also associated with the pinkish-orange coloured dolomitic marls intercalated with cherty dolomite. Variable proportions of carbonate (dolomite,

50 m

5m

0

0

Limestone

Clayey limestone

Sandy limestone

Cherty limestone

Fossiliferous limestone

Cherty dolomite

Claystone/ marl

Dolomitic marl

Conglomerate Sandstone

lld Kumgedik hill

Upper Oligocene–Lower Miocene

M.Eocene

U.Cretaceous

Lower Paleocene

Lower Paleocene

llc Gelnek ridge

Upper Eocene–Lower Oligocene

llb Mezgi ridge

lla Uzunkaya hill

Dolomitic limestone

30 m

40 m

0

0

Dolomite

Gypsum Sandy dolomitic marl

Celestite bands

Clayey dolomite

Sepiolite/ palygorskite

Pure clay levels

FIGURE 2 Representative vertical sections of marine sepiolite- and/or palygorskite-bearing lithologies from central eastern Anatolia.

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calcite), sulphate (gypsum, celestite), clay (sepiolite, palygorskite, smectite, minor amounts of serpentine and chlorite) and silica (quartz/chalcedony) minerals, as well as plagioclase and Fe-oxide minerals of detrital origin were determined in the evaporite facies. The Eocene to Miocene units that contain palygorskite are composed of limestone–dolomitic limestone–cherty dolomite, clayey limestone–limestone and sandstone–conglomerate–dolomitic limestone–marl (Figure 2(IIc and d)). In all places of the basin, pure palygorskite clay fractions were obtained from the greyish-green laminated dolomitic marl occurring in these units. These formations contain carbonates (mostly calcite and minor dolomite) and clay minerals such as palygorskite, smectite, chlorite and serpentine.

2.2. Lacustrine Sepiolite–Palygorskite Occurrences Western and Central Anatolian continental basins, especially Eskis¸ehir, Konya, Denizli, Ankara and Sivas surroundings, cover world’s largest sepiolite–palygorskite occurrences as beds and nodules extending over large areas. These deposits display similar stratigraphy, lithology and mode occurrence, and their type localities are presented as follows. In the areas of about 20–60 km south of Sivrihisar, from Eskis¸ehir city ¸ oban, 1994), three different types of sepiolite beds (Figure 3(IIIa): Ece and C are recognized within clayey–cherty–carbonate rocks with a thickness of 32–40 m in the central parts of the basin: (1) organic matter-rich black sepiolite with up to 2.75% total organic carbon; (2) organic matter-poor brown sepiolite containing about 5% dolomite; (3) white, cream, pale yellow dolomitic sepiolite with 20–40% dolomite. Black sepiolites were deposited in the deeper part and are relatively thinner (0.5–2.0 m) than the other types. These are overlain by brown, discontinuous dolomite laminae bearing sepiolite beds. White sepiolite beds are commonly associated with dolomites (20–40%) and opal lenses (2–20 cm), and rarely gypsum crystals. Opal-cristobalite/tridymite (Opal-CT) levels are distributed irregularly and discontinuously in and overlying the sepiolitic clay beds. The sepiolite nodules, pebbles and concretions are known as ‘lu¨letas¸ı’ in Turkey meaning pipestone in Turkish (meerschaum ¼ cuttle-fish ¼ seafoam) within the strata of about 0.5–1.0 m thick. They are found in poorly sorted conglomerate beds that are composed of gravels and small blocks of ultramafics, and matrix is made up of brown and strongly altered ultramafic rocks. However, palygorskite is the second most common clay mineral, and pure palygorskite lenses occur in the northern area of Sivrihisar that are associated with smectite and rarely illite (Gençog˘lu and ˙Irkeç, 1994). Meerschaum sepiolite and palygorskite occurrences are also located at ¨ çkuyular which is 150 km south-east of Eskis¸ehir and 9 km north-east of U Konya–Yunak (Yeniyol, 1995). The sepiolite deposits are observed in a conglomerate unit with predominantly serpentinite clasts (5–50 cm in size)

Miocene

Sandstone

Claystone/marl Sandy claystone

llle SE Denizli (Kadir and Akbulut, 2004)

20 m

0

Conglomerate

llld E Konya (Karakaya et al., 2004)

lllc N Konya (Karakaş and Kadir, 1998)

Upper Miocene–Lower Pliocene

lllb SW Eskişehir (Yeniyol,2011)

Lower Pliocene

Middle to Upper Miocene

llla SE Eskişehir (Ece and Çoban, 1994)

20 m

179


Upper Miocene–Pleistocene

7

Upper Miocene–Pliocene

Chapter

5m

4m

0

0

Dolomite

Clayey dolomite

Limestone

Magnesite

Tuff/tuffite

Gypsum

10 m

0

Clayey limestone Sandy–clayey limestone

Fossil/Plant

Iron oxide

0

Nodular chert

Sepiolite/palygorskite

FIGURE 3 Representative vertical sections of lacustrine sepiolite- and/or palygorskite-bearing lithologies from western Anatolia.

extending for more than 50 m along. The conglomerate bed in which meerschaum sepiolite occurs is at the base of the sequence. Its visible thickness is nearly 4 m and it can be traced for over 500 m horizontally. Besides rock fragments, this bed also involves abundant sepiolite together with magnesite pebbles. Palygorskite is one of the principal clayey cementing materials in the sepiolite-bearing bed as well as along the entire sequence. It occurs alone or together with dolomite. Calcite, quartz and chalcedony are also cementing materials. About 20 km south-west of Eskis¸ehir (Figure 3(IIIb): Yeniyol, 2011), the Lower Pliocene sediments containing sepiolite–palygorskite overlie in unconformity with the Miocene conglomerate unit. In the Yo¨ru¨kakçayır Village, the sediments are represented by an alternation of clays, dolomite bearing clays, dolomitic marls and dolomite. Sepiolite–palygorskite occurs as lenses and relatively continuous layers, in alternation with dolomitic marls and dolomites. At the top of Miocene unit, reddish-brown clay beds with fine-grained detrital material occur as thick beds (up to 10 m). These consist predominantly of palygorskite, minor

180


dolomite and small quantities of kerolite and detrital quartz. In addition, some sepiolite beds contain irregular opal-CT inserts. In the Kepeztepe area, thick clay-rich beds occur in colours varying from light to dark green along an approximately 14 m thick succession at base. They are massive in appearance, but in some levels, fine even submicroscopic rounded or subangular detrital clay or dolomite fragments occur. In this part, rare dolomitic marl and tiny dolomite layers with light beige colour occur in alternation with clay-rich beds. Except a 40 cm thick layer at bottom which contains sepiolite together with palygorskite, the clay beds at the lower part consist only of saponite, or even both saponite and palygorskite in varying proportions. Dolomite content is found in low amounts throughout this part; however, it increases in dolomitic marls and in thin dolomite layers. Minor kerolite as authigenic mineral and low amounts of quartz, illite, chlorite and serpentine as detrital minerals are found in some beds as well. In the middle part of the sequence, clay beds are beige, light brown and even green coloured, and they appear in alternation with more frequent dolomite and clayey dolomite layers. The clay-rich beds with 0.3–1 m thicknesses contain sepiolite or sepiolite and palygorskite in high quantities with accessory kerolite and/or detrital quartz and feldspar. The upper part contains mainly thick massive dolomite beds in which a few thin clay layers are also found that consist of sepiolite, or both sepiolite and palygorskite. Loughlinite, a Na-rich variety of sepiolite subgroup, has been reported in the north-east of Mihaliççik, Eskis¸ehir (Yeniyol, 1997). Loughlinite-bearing beds comprise a 110 m thick part of the Middle–Upper Miocene sequence, covering an area of more than 3.5 km2. The thickness is ranging from 0.6 to 5 m, interbedded with claystone, tuff, chert and bentonites. Some beds contain loughlinite, smectite and analcime as the chief mineral constituents in varying proportions. Other beds also contain calcite, illite and probably soluble Nacarbonates in lesser amounts. In the other study in the same area (Kadir et al., 2002), it has been reported that the Middle to Upper Miocene lacustrine volcano-sedimentary units are generally composed of clastic, clayey, dolomitic, tuffaceous, evaporitic and silicified sediments. The yellowish-green or darker green altered tuff and claystone units are dominated by sepiolite and loughlinite. The contact between sepiolite and loughlinite bearing claystones is sharp. Sepiolitic claystone is generally widespread and characterized as pale yellowish-green, hard, friable and occasionally plastic. Sepiolite is associated with dolomite, opal-CT and analcime. However, loughlinite appears as greenish, massive, soapy lenses and layers within the sepiolitic claystone in the Killik area. In this case, loughlinite is mainly associated with calcite, opal-CT, analcime, feldspar and palygorskite. In the Polatlı area, south-western Ankara, the volcano-sedimentary rocks are generally composed of clastic, clayey, dolomitic and calcitic calcareous rocks and tuffaceous, evaporitic and silicified sediments (Çelik Karakaya

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and Karakaya, 2008). The cyclic sedimentation of green and yellow illitic and smectitic facies with different sized gypsum (1–10 cm) crystals and limestone bands is alternately exposed at the base in general. Black and organic matterrich sepiolite beds are overlain by brown dolomite layers. White sepiolite beds are commonly associated with dolomites that range from 20% to 40%, and irregular shaped opal-CT lenses (2–20 cm) and nodules (up to 3 cm in size). The composition of the samples is relatively homogenous, although the mineral phases are found in extremely variable proportions that can be classified in three groups (1) detrital silicates: illite, smectite, I-S, kaolinite, quartz and feldspar; (2) neoformed phyllosilicates: mainly sepiolite and rarely palygorskite and saponite; (3) carbonates: dolomite and/or calcite and rarely aragonite. Opal-A, gypsum and strontianite may also appear sporadically in a few samples. In the Beypazarı trona deposits of western Ankara, sedimentary sequence with a thickness of 345–780 m integrates Middle to Upper Miocene siliciclastic, carbonate, evaporitic and pyroclastic rocks that include authigenic minerals such as pirssonite, nacholite, dolomite, calcite, magnesite, K-feldspar, searlesite, analcime, clinoptilolite, pyrite, saponite, Al-smectite, sepiolite, kaolinite and illite besides trona (Gu¨ndog˘du et al., 1985). Sepiolite occurrences are found within the carbonate-laminated claystone intercalations of the trona-bearing unit (100–150 m thick). The Neogene lacustrine basin in the As¸ag˘ı Pınarbas¸ı of northern Konya is an area where sepiolite and palygorskite are found associated with carbonates with a thickness of 25–85 m, although their distribution varies significantly in both vertical and horizontal directions (Figure 3(IIIc): Karakas¸ and Kadir, 1998). Stratigraphic section begins with conglomerates and laminated finegrained sandstones that alternated with mudstones. The upper lithologies are white to cream coloured limestone, dolomitic limestone, clayey limestone, sandy limestone and claystones. All the facies include sepiolite–palygorskite, associated clays (smectite, chlorite, illite) and non-clay minerals (calcite, dolomite, quartz, feldspar). Three types of sepiolite–carbonate beds are recognized in the sediments (79–117 m) of the Upper Miocene to Pleistocene at the Karapınar area of the western Konya basin (Figure 3(IIId): Karakaya et al., 2004): (1) organic matter-rich brown sepiolite (with gastropods, ostracods and plant fossils) overlain by light-brown, discontinuous dolomitic layers; (2) organic matterpoor sepiolite (with about 50% dolomite) and (3) white–cream-coloured carbonate layers (with generally dolomite and/or calcite and lesser amounts of sepiolite (about 5–20%)). These sepiolite-carbonate beds have been observed in the upper parts of the sequences and contain opal lenses (5– 120 cm) or silica nodules of various shapes. The rims of the silica nodules and opal lenses are composed of thin (0.5–1 cm) coatings of carbonate minerals, mainly dolomite. The silica-rich layers are 10–80 cm thick. Palygorskite and aragonite occur in some layers.

182


The Serinpınar-Acıpayam basin from the Denizli province is filled with fluvial and lacustrine sediments, fan delta, shallow littoral and open lake and/or swamp sub-environments, of the Upper Miocene–Lower Pliocene, where three facies with lateral and vertical transitions are observed (Akbulut and Kadir, 2003): The green claystone facies, the dolomitic-claystone and sepiolite–palygorskite facies and the red mudstone–palygorskite facies. The first facies is brown locally, and becomes sandy mudstone that tends to be dominated by saponite and contains a small amount of dolomite and intercalated conglomerate, sandstone and dolomite layers. Locally, Chara remains and root imprints were observed. The second facies is intercalated with conglomerate, sandstone and dolomite. A few sections of brown-beige sepiolite–palygorskite occur as distinguishable layers. Saponite and, in some places, palygorskite and sepiolite are the dominant clay minerals. Saponiterich dolomitic claystone locally containing white dolomitic palygorskite is exposed at Kocapınar area and continues upwards as greenish-white dolomitic claystone, pale purple dolomitic sepiolite and brown, nodular sepiolite–palygorskite. Finally, these facies continue upwards as a pure, green saponite bed and white dolomitic claystone and dolomite bed. Basaltic and vitric tuff and syn-sedimentary volcanic emanations cut and enclose sepiolite, dolomite and dolomitic claystone clasts. The third facies start with a few accumulated oncolitic mounds of partially siliceous clayey dolomite and continue with an alternation of red mudstone, clayey sandstone and conglomerate, which change laterally into dolomitic claystone–clayey dolomite and palygorskite facies. Palygorskite predominates in the dolomitic claystone, while red, soapy saponite-rich claystone appears as brown and green–brown clay inserts with root marks in these facies. The existence of a magnesite deposit containing sepiolite is found in the Upper Miocene–Lower Pliocene lacustrine formations in the Denizli–Bozkurt area (Figure 3(IIIe): Kadir and Akbulut, 2001). The sedimentary section is divided into four different lithofacies: Rare blocky and pebbly marl, conglomerate–sandstone; a small amount of dolomite and dolomitic marl levels intercalated with conglomerate, sandstone and mudstone; alternation of dolomite, dolomitic marl and magnesite, sandstone and claystone of the lower carbonate facies and dolomite and magnesite dominated upper carbonate facies. Sepiolite is found in the last facies, which show lateral and vertical transitions with marls. The fibrous clay consists of 3–5 cm thick, yellow to greenish-brown coloured veins, but also as thin films lining parallel to vein axis within the magnesite unit. The Middle Miocene–Pliocene aged sedimentary sequence in the Kangal sub-basin of Sivas basin containing palygorskite and sepiolite beds has been divided into three facies from bottom to top according to the main lithological features and depositional environments (Figure 4(IVa–c): Yalçın et al., 2004): Fluvio-lacustrine clay facies, playa coaly tuffite facies and lacustrine carbonate facies. The first facies vertically and laterally grades to the second facies,

183


IVa

IVb

IVc

IVd

Cetinkaya-Sivas

Middle Miocene–Pliocene

Tecer-Sivas


Havuz-Sivas

Susehri-Sivas

Upper Miocene

7


Chapter

50 m

10 m

10 m

10 m

0

Conglomerate Sandstone

0

Dolomite

Clayey dolomite

Limestone

0

Clayey limestone

Sandy–clayey Claystone/ limestone marl

0

Magnesite

Sepiolite/ palygorskite

FIGURE 4 The typical vertical sections of lacustrine sepiolite- and/or palygorskite-bearing lithologies from central eastern Anatolia.

and each facies is locally made up of sub-facies. Fluvio-lacustrine clay facies is composed of red–brown to green claystone and marl with subordinate channel filling sandstones and conglomerates, and carbonate rock intercalations with a total thickness of about 200 m. Playa coaly tuffite facies consists of four zones that can be observed only in open pits. Tuffaceous claystones bearing abundantly mollusc shells are the dominant lithology in all zones. Rhizolitic and pisolitic limestone intercalations (1–5 m) are also encountered in the upper tuffite zone. Lacustrine carbonate facies mainly consist of yellow to white carbonate rocks with partly clayey, cherty and/or fossiliferous (limestone, dolomite and magnesite) horizons with thickness varying from 20 to 60 m. Pure palygorskite, nearly pure sepiolite and dolomitic sepiolite beds are observed as intercalations with 0.1–1 m thickness. The magnesites occur as nodular (1–20 cm) and beds (0.5–1.5 m). The main minerals found in the clayey–calcareous rocks are clay, carbonate (calcite, aragonite, dolomite, magnesite) and to a lesser extent, quartz, opal (both A and CT) and feldspar. Clay fractions consist of palygorskite, serpentine, Al–Fe saponite, sepiolite, illite and chlorite in order of abundance. Mixed-layers C-S and I-S, and talc are also detected in some samples. The Upper Miocene lacustrine sequence around Kızıldag˘ between Sus¸ehri and ˙Imranlı to the 120 km north-east of the Sivas city (Figure 4(IVd): Yalçın

184


et al., 2006), which is composed of an alternation of thin-medium bedded, white to yellow limestone–dolomite–claystone/marl including grey coloured sandstone intercalations with a 50–210 m thickness. Considering the magnesite, silicified magnesite, dolomite with sepiolite and palygorskite-rich claystone beds, the thickness is ranging from 1.5 to 10 m. The diagnostic minerals for serpentinite-fed basins are calcite, aragonite, dolomite, magnesite, opal-CT, sepiolite and palygorskite. However, for volcanic-fed basins, the representative minerals are hematite and goyazite, kaolinite, C-S and I-S.

2.3. Hydrothermal Sepiolite Occurrences 2.3.1. Volcanic-Hosted Occurrences Differing from the sedimentary sepiolite deposits, mostly associated with the carbonate/evaporite sequences, volcanic-related sepiolite formations are located at the district of the Us¸akgo¨l plain, about 25 km south of the Kıbrıscık town, 70 km south-east of Bolu city, occurs in the Gallatian Volcanic Belt and has formed by the hydrothermal alteration of the vitric tuff unit of Middle ¨ nlu¨, 1993). Miocene volcanites (I˙rkeç and U Tuffs of the Deveo¨ren volcanites bear significance, considered the occurrence of sepiolite within them. Thickness of the unit ranges around 40–80 m. Volcanites are composed of basalt, andesite and dacitic lava, tuff and agglomerate. Tuffs are white to pinkish, and agglomerates reddish in colour. Volcanoclastics show a sequence of crystalline and vitric tuffs, resedimented tuff/tuffite transported by small braided streams, in ascending order. Vitric, ash and pumiceous tuffs are slightly compacted and show yellow, dirty white and whitish colours. Argillization is common in millimetric scale along fractures. Zeolitization, recognized by its pale green colour is also common. Pumice fragments (< 1 cm), unevenly shaped and white silica nodules of variable size, are occasionally identified. Angular chert fragments may sometimes reach coarse block size. Sepiolite occurrences are observed within the vitric tuff where it is associated with quartz, opal-A/CT, zeolites (clinoptilolite/heulandite, mordenite), montmorillonite, plagioclase and K-feldspar. They are irregularly distributed showing gradation into tuff, mostly discontinuous and occasionally silicified with variable dimensions. Within the sepiolite occurrences, manganese dendrites and staining are also observed. 2.3.2. Serpentinite-Hosted Occurrences The outcrops of vein-type sepiolites are expected to be abundant and they must be researched deeply. Some examples in the ophiolitic suites of Turkey are given below. A vein-like formation of sepiolite is located in the Çayırag˘zı magnesite deposit, 20 km south-west of Konya (Yeniyol, 1986). The magnesites in the serpentinites are microcrystalline types and occur as veins ranging from a few millimetres to 30 cm in thickness. There is a gradual transition

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185

from magnesite to sepiolite in these veins. Sepiolite is also present in intersection monomineralic veins that vary in thickness from a few to 15 cm and that cross-cut the magnesite. All veins have a sharp contact with the host serpentinite; however, no alteration of the serpentinite was noted. A 2-m-thick brecciated zone containing magnesian minerals is present at the Elmadag˘ area, 15 km east of Ankara (Yalçın and Bozkaya, 2004). An alteration zone with vein-type bedding comprises four different levels; from bottom to top, they are (1) green–brown serpentinite with dolomite (0.9 m), (2) light greenish-white dolomite with serpentine (0.5 m), (3) white dolomite with sepiolite (0.4 m) and (4) greenish-white dolomite with smectite–chlorite (0.2 m). The first level has a mineral association of serpentine þ dolomite calcite aragonite; the second level consists of dolomite þ serpentine calcite or dolomite þ magnesite þ serpentine; the third level comprises dolomite þ sepiolite and the fourth level is made up of dolomite þ chlorite þ smectite þ serpentine. Dolomite is the main mineral of the alteration zone, occurs as coarse crystals in fractures and as small crystals in the matrix. Sepiolite has developed at the edges and surfaces of dolomite and as fibrous aggregates in voids. The second and third levels are possibly lateral transitions to serpentine magnesite with dolomite. Commercial magnesite deposits in the region are hosted by serpentinites and occur as fracture fillings and masses. Another sepiolite occurrence in the ophiolitic series of Turkey is as joint infillings (up to 1 cm) in the serpentinites (Bas¸ıbuÿu¨k et al., 2009; Yalçın et al., 2006). In the ophiolitic belt around Sivas, the layered serpentinites have numberless veins with millimetres to 15 cm thick that are usually parallel to each others and crosswise in places. They contain the assemblages of ophisilicates such as serpentine, pectolite, xonotlite, quartz, opal-CT, talc, chlorite and/or smectite; ophicarbonates such as calcite, dolomite, aragonite, magnesite and/or hydromagnesite; and oxides (brucite and hematite).

2.4. Pedogenic Palygorskite Occurrences Ever sepiolite could not be detected whereas palygorskites could be commonly identified in a wide variety of soils such as modern soils, palaeosols and sediments and caliches and crusts in Turkey, which are given below. The sepiolite and/or palygorskite minerals are not easily encountered although soils formed on volcanic parent material in Turkey are quite common (Kılıç and Yalçın, 2009). In spite of this, in the Central Anatolian terrestrial regions adjacent to ophiolitic suites, red to brown coloured soils and sediments of Upper Pliocene to Quaternary age contain significantly palygorskite and associated minerals such as calcite, dolomite, quartz, feldspar and almost all other clay species (Yalçın et al., 1994, 2000). The superficial deposits of the Adana basin mostly comprise Holocene alluvial smectite-rich soils to Pliocene marine smectitic soils as well as

186


Pleistocene-aged basalts (Kapur et al., 1989). They form smectitic vertisols on the west, whereas the eastern coast is mostly surrounded by Holocene alluvial smectitic soils as well as minor parts of regosols on Mio-Pliocene materials and Plio-Pleistocene red soils. The land and marine surface sediments of the Iskenderun Bay and also the ancient terrace and former mouth of the Ceyhan River are characteristics from the point of the presence and distribution of palygorskite (Kapur et al., 1989; Yalçın et al., 2001). Caliche in various forms, namely powdery, nodule, tube, fracture-infill, laminar crust, hard laminated crust (hardpan) and pisolitic crust, is widespread in the Mersin area in southern Turkey (Eren et al., 2008; Kadir and Eren, 2008). It generally occurs within and/or over the reddish-brown mudstone of the Upper Miocene and alluvial red soils of the Quaternary. In the caliche profiles, calcite is the most abundant mineral associated with minor amounts of palygorskite, whereas smectite is prevalent mainly in the reddish-brown mudstone and alluvial red soils of the caliche parent materials and is associated with appreciable amounts of palygorskite. These minerals are also accompanied by trace amount of illite, quartz, feldspar and a poorly crystalline phase. Siliciclastic red mudstones within alluvial-fan deposits of the Middle Miocene locally contain dolocretes in various forms (powdery, nodular and fracture filling) and scarce matte-brown, authigenic clay lenses in the C ¸ anakkale region in north-western Turkey (Kadir et al., 2010). The dolocretes are indeed predominantly dolomite, coexisting with variable amounts of palygorskite. A section at Kızıldere in the Misis-Adana basin of Turkey reveals relations among mature massive caliche and palaeosolic immature caliche and the Tertiary clay and colluvial deposits (Kapur et al., 1993). It also defines a gradational sequence from unweathered Tertiary clay deposits to massive caliches. From older to younger, the section comprises Tertiary clays, palaeosolic caliche, colluvial material, massive caliche and lenticular caliche. The two massive caliche beds have similar assemblages of carbonate minerals, whereas the palaeosolic caliche contains more clay minerals and most probably represents an earlier stage of massive caliche formation. The variability of types of calcite, the quantity and crystallization state of palygorskite and the presence of kaolinite and degraded smectite support that the massive caliche is the end product of a degradation sequence from the Tertiary clay deposit to palaeosolic caliche and the lenticular caliche is the well-known petrocalcic horizon. The most typical toposequences representing the Quaternary landscape evolution in/on calcretes are located in the south-Adana, and west I˙zmir of Anatolia (Kapur et al., 2000). The calcretes are formed on the Pliocene clay materials, Pleistocene conglomerates and Lower Mid-Miocene glacis with karstic features and sandstones in the Adana region and on the mudflow deposits transported from Neogene in the Bornova-I˙zmir. The smectite,

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palygorskite and kaolinite suite are typical for calcretes and column horizons. Rhombohedral calcite is also determined as aggregates cemented with palygorskite in calcite columns. Pure palygorskite develops into bundles of wedges or interwoven mats together with smectite in between the rhombohedral or micrite matrix of the calcite columns.

3. GEOCHEMISTRY The bulk chemical compositions of sepiolite–palygorskite clays with different genetic types in Turkey are selected and summarized in Table 1. The negligible numbers of samples analysed in the literature seem to be admixtures of minerals such as other clays, feldspars and/or carbonates with trace or minor quantities (< 2%). By a direct comparison, it is apparent that excess CaO, Na2O and K2O come from the contaminating minerals associated rather than to structural features of sepiolite–palygorskite. The following assessments could be made despite of these impasses. Bedded sepiolites and particularly palygorskites contain notably Al2O3 and Fe2O3 (Figure 5A). Taking the tetrahedral and octahedral cations into account, sepiolites are very close to those of the theoretical composition, whereas tetrahedral and expressly octahedral substitutions of Mg, Al and Fe atoms in palygorskites are partly high (Figure 5B). On the basis of the compositions of the octahedral sheets of sepiolite– palygorskite on the binary diagram by adding the discrimination lines proposed by Foster (1960), Weaver and Pollard (1973) and Paque´t et al. (1987; Figure 6), sepiolites are present within the trioctahedral domain, but palygorskites are intermediate between dioctahedral and trioctahedral domains, as previously reported by Galan and Carretero (1999). In this study, discrimination limits can be declared as VI[R2þ/(R2þþ R3þ)] ¼ 0.75 for intermediate-trioctahedral and VI[R3þ/(R2þþ R3þ)] ¼ 0.50 for intermediatedioctahedral in the half unit cell.

4. GENESIS Sepiolites and palygorskites in Turkey comprise different occurrences and genesis that have been documented by X-ray diffraction, optical and scanning electron microscopies and chemical analyses in the literature. A geologic model (Figure 7) is proposed to explain the deposition environments, source terranes and rocks, origin and evolution of these minerals, on the basis of all the reported data concerning the sepiolite–palygorskite occurrences in Turkey. In all the facies of the marine and lacustrine environments, it is suggested that occurrences of sepiolite and palygorskite and associated minerals (smectite, magnesite, dolomite, calcite and aragonite) were controlled by pH, salinity and/or alkalinity, cation relationships (Si/(Al þ Fe), Ca/Mg and Mg/H) and

TABLE 1 Major Element Compositions (wt%) of Sepiolites and Palygorskites from Turkey. Oxide

Sepiolites MS (n ¼ 3)

LNS (n ¼ 6)

Palygorskites LBS (n ¼ 26)

SHS (n ¼ 4)

MP (n ¼ 4)

LP (n ¼ 6)

Range

Mean

Range

Mean

Range

Mean

Range

Mean

Range

Mean

Range

Mean

SiO2

60.16–60.30

60.23

52.87–60.20

57.03

47.80–59.35

55.32

55.95–62.14

59.01

56.73–58.85

57.95

50.32–61.57

53.68

TiO2

0.01–0.10

0.05

0.02–0.07

0.06

0.03–0.32

0.12

0.00–0.02

0.01

0.36–0.55

0.47

0.19–0.59

0.34

Al2O3

0.67–0.81

0.72

0.25–0.35

0.31

0.36–5.68

2.15

0.17–1.54

0.88

6.72–10. 07

8.52

4.95–8.59

6.52

tFe2O3

1.64–2.38

2.05

0.04–0.40

0.15

0.10–5.30

1.33

0.01–1.88

0.50

5.78–10.41

7.76

4.17–9.30

6.78

MnO

0.01–0.01

0.01

0.20–0.20

0.02

0.01–0.10

0.01

0.00–0.02

0.01

0.02–0.02

0.02

0.02–0.10

0.04

Cr2O3

0.38–0.43

0.40

0.01–0.01

0.01

0.00–0.05

0.01

0.00–0.01

0.01

0.06–0.10

0.08

0.04–0.07

0.06

NiO

0.23–0.25

0.24

0.17–0.17

0.17

0.00–0.12

0.01

0.03–0.15

0.09

0.11–0.25

0.19

0.02–0.15

0.08

MgO

23.50–24.67

24.16

23.21–25.91

24.75

18.07–24.42

21.81

20.09–26.52

24.24

9.78–12.60

11.27

9.79–13.49

11.25

CaO

0.13–0.20

0.15

0.01–0.80

0.54

0.01–3.17

0.59

0.06–0.17

0.13

0.41–0.56

0.49

0.01–1.10

0.26

Na2O

0.04–0.050

0.04

0.03–0.24

0.14

0.03–4.75

0.41

0.00–0.52

0.13

0.12–0.25

0.17

0.03–0.86

0.21

K2O

0.06–0.070

0.06

0.02–0.27

0.15

0.01–0.70

0.29

0.00–0.15

0.04

0.40–1.07

0.58

0.01–1.85

0.55

P2O5

0.06–0.06

0.06

0.02–0.02

0.02

0.01–0.20

0.03

0.02–0.09

0.04

0.04–0.11

0.07

0.05–0.12

0.08

n, Sample number; MS, marine sepiolite (Yalçın and Bozkaya, 1995); LNS, lacustrine nodular sepiolite (Ece, 1998; Yalçın and Bozkaya, 1995); LBS, lacustrine bedded sepiolite (Akbulut and Kadir, 2003; Kadir et al., 2002; Yalçın et al., 2004; Yeniyol, 2010); SHS, serpentinite-hosted sepiolite (Yalçın et al., 2006; Yalçın and Bozkaya, 2004; Yeniyol, 1986); MP, marine palygorskite (Yalçın and Bozkaya, 1995); LP, lacustrine palygorskite (Akbulut and Kadir, 2003; Yalçın et al., 2004; Yeniyol, 2011).

Chapter

A

7

189


TiO2 + Al2O3 + tFe2O3

MgO

B

50%

AIVI 60%

60% Pl

Sp

Marine sepiolites Nodular sepiolites Bedded sepiolites Serpentinite-hosted sepiolites Marine palygorskites Lacustrine palygorskites

50%

Sp

Pl

FeVI

SiO2

MgVI

FIGURE 5 Ternary diagrams for chemical compositions of sepiolites and palygorskites from Turkey, (A) major oxides, (B) octahedral cations.

Marine sepiolites Nodular sepiolites Bedded sepiolites Serpentinite-hosted sepiolites Marine palygorskites Lacustrine palygorskites

Dioctahedral domain

1.5

Weaver and Pollard (1973) 1.3

Paquét et al. (1987) This study

1.0

Intermediate domain 0.5

0 0

0.5

1.0

Trioctahedral domain

Foster (1960)

(AI + Fe)VI/half unit cell

2.0

1.5

1.83

2.0

2.5

3.0

MgVI/half unit cell

FIGURE 6 Octahedral compositions of sepiolites and palygorskites from Turkey (discrimination lines proposed by Foster, 1960; Paque´t et al., 1987; Weaver and Pollard, 1973).

activities and concentration of H4SiO4 in the solution (e.g. Birsoy, 2002; Yalçın and Bozkaya, 1995, 2004). The cyclical variations in the vertical distributions of sepiolite, palygorskite and/or smectite with carbonates in almost all the sedimentary units can be explained by changes in the physicochemical conditions of the environment. In other words, the variation of Mg/(Al þ Fe) ratio may be responsible for the formation of sepiolite, palygorskite or smectite. These elements (Si, Mg, Al and Fe) may have been derived in solution from ophiolitic suites (chiefly peridotites and rarely volcanic products) as a result of hydrolysis of ferromagnesian minerals (especially serpentine) in the basic environment (pH > 8.5). Silica availability is sourced by the dissolution of diatom tests (Ece and Çoban, 1994) and of volcanic glasses in the palaeo-lakes. Occurrences of chert in the carbonate rocks indicate that the environment was silica saturated.

190

Intra-continental lac ust

rine basins Shallow marine env

ironments

2 3 4

5

Serpentinite

Dolomite

Metamorphites Conglomerate

Clayey dolomite

Volcanics

Sandstone

Claystone/ marl

Pyroclastics

Gypsum

Limestone

Magnesite

Reefal limestone

Magnesite and sepiolite nodules

Clayey limestone

Cherty limestone/dolomite

Magnesite and sepiolite veins

Coal

FIGURE 7 Schematic model for various sedimentary environments of sepiolite–palygorskite occurrences in Turkey (1: Malatya–Hekimhan/Marine type, 2: Eski¨ çkuyular and Konya–Yunak/Meerschaum type, 3: Eskis¸ehir–Sivrihisar, Konya–Karapınar, Denizli–Bozkurt and Sivas–Kangal/Lacustrine type, 4: Bolu– s¸ehir–U Kıbrıscık/Volcanic-hosted type, 5: Ankara–Elmadag˘ and Konya–C ¸ ayırag˘zı/Serpentinite-hosted type).


1

Chapter

7


191

In the detrital phyllosilicates-free samples, pure sepiolite beds may have been formed by direct crystallization, as previously stated by many writers working in several environments (Ece and Çoban, 1994; Millot, 1970; Torres-Ruiz et al., 1994; Yalçın and Bozkaya, 1995), formulized in the next (1: Jones, 1986): 8Mg2þ þ 12H4 SiO4 þ 16OH ! Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ 18H2 O

ð1Þ

The evidence of fibrous aggregates of sepiolite coating carbonate grains that seem to have partially formed by displacement of dolomite was also observed by many authors (Ece and C ¸ oban, 1994; Esteóule-Choux, 1984; Hassouba and Shaw, 1980; Karakas¸ and Kadir, 1998; Karakaya et al., 2004; Yalçın and Bozkaya, 1995). Reaction of pore waters rich in silicic acid with dolomite suggests the diagenetic transformation as shown below (2: Yalçın and Bozkaya, 1995): 8CaMgðCO3 Þ2 þ 12H4 SiO4 ! Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ 2H2 O þ 8Ca2þ þ 16HCO3 ð2Þ In the vein- and stockwork-type magnesite deposits within the ophiolitic complex, magnesites are the source materials of the sepiolite nodules in the conglomerates and they partially or even totally replacement took place in the shallow lake environment, under alkaline and saline conditions during the diagenesis (Ece; 1998; Ece et al., 2005; Ece and C ¸ oban, 1994; Yeniyol, 1995), according to the following volume expansion reaction (3) reported by Ece and Çoban (1994): 8MgCO3 þ 12H4 SiO4 ! Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ 8H2 CO3 þ 2H2 O

ð3Þ

The sepiolite þ palygorskite sample without other clay minerals suggested that sepiolites may have been formed from palygorskites. This mechanism should have taken place as a diagenetic transformation (4: Yalçın and Bozkaya, 1995): Mg2 ðAl, Fe3þ Þ2 Si8 O20 ðOHÞ2 ðOH2 Þ4 4H2 O þ 4H4 SiO4 þ 6Mg2þ þ 6OH ! Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ ðAl, Fe3þ Þ2 ð4Þ Vein sepiolites in the ultramafic rocks are epigenetic in origin. The serpentinization of forsterite by the action of water is the first stage in the alteration process (5). Sepiolite is the late mineral phase that develops in a weathering environment under basic conditions. The dissolution of serpentine by means of meteoric and/or groundwater (containing carbon dioxide or carbonic acid) moving along thrust and shear planes during or after

192


me´lange formation is the second stage of the alteration process. The occurrences of various carbonate and/or sepiolite associations are the third stage of the alteration process (6, 7, 8, 9, 10). From that fact, the following equations can be deduced easily (Yalçın and Bozkaya, 2004): 2Mg2 SiO4 þ 3H2 O ! Mg3 Si2 O5 ðOHÞ4 þ Mg2þ þ 2OH

ð5Þ

Mg3 Si2 O5 ðOHÞ4 þ 6H2 O þ 7CO2 þ 4Ca2þ ! 3CaMgðCO3 Þ2 þCaCO3 þ 2H4 SiO4 þ 8Hþ

ð6Þ

Mg3 Si2 O5 ðOHÞ4 þ 4H2 O þ 5CO2 þ 2Ca2þ ! 2CaMgðCO3 Þ2 þMgCO3 þ 2H4 SiO4 þ 4Hþ

ð7Þ

3Mg3 Si2 O5 ðOHÞ4 þ 6H4 SiO4 þ H2 O þ 6CO2 þ 3Caþ2 ! 3CaMgðCO3 Þ2 þ Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ 8Hþ 8CaMgðCO3 Þ2 þ 12H4 SiO4 ! Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ16CO2 þ 2H2 O þ 16OH þ 8Ca2þ

ð8Þ ð9Þ

6Mg3 Si2 O5 ðOHÞ4 þ 2H2 O þ 10CO2 ! 10MgCO3 þ Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O ð10Þ Unlike vein sepiolites, joint infilling type of sepiolite occurrences within the serpentinites seems to be developed from serpentines rather than carbonates (11): 6Mg3 Si2 O5 ðOHÞ4 þ 12Hþ ! Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ 10Mg2þ þ 8OH ð11Þ Loughlinites in the volcano-sedimentary basins developed as a network of fibres on the inner surfaces of dissolution voids in domains of volcanic glass, in contrast to sepiolite, which occurs as bridging fibres between crystallites of dolomite, indicating that these minerals are formed authigenically under different physicochemical environmental conditions rather than resulting from a transformation process from one to the other (Kadir et al., 2002). In other words, glassy pyroclastic products are the parent material of loughliniteand smectite-bearing clays and analcimic zeolites that were probably formed by the reaction of Mg2þ- and Naþ-rich lake water with volcanic glass during diagenesis (Yeniyol, 1997). Hydrated silicate gel forms clays or zeolites in highly basic environments (e.g. Gu¨ndog˘du et al., 1996; Yalçın and Gu¨mu¨s¸er, 2000). In other words, Na-clays (loughlinite, searlesite, Na-smectite) should be formed instead of Na-borate and -carbonate minerals in the volcano-sedimentary basins of

Chapter

7

193


western Anatolia. An aqueous environment is required for hydration of ash or tuff and the chemical reaction related to their occurrence is given as follows (12): 12SiO2 6MgO 2Na2 O þ 30H2 O ! 2Na2 O6MgðOHÞ2 12H4 SiO4 ! Na4 Mg6 Si12 O30 ðOHÞ4 12H2 O þ 16H2 O ð12Þ Volcanic-hosted sepiolites occurred from volcanic glass passing into an interphase of hydrated Mg-silicate gel as similar to that of loughlinite (13): 12SiO2 8MgO þ 32H2 O ! 8MgðOHÞ2 12H4 SiO4 ! Mg8 Si12 O30 ðOHÞ4 ðOH2 Þ4 8H2 O þ 18H2 O ð13Þ The samples containing dolomite þ palygorskite showed that palygorskite commonly occurs as mats of interwoven fibres coating dolomite rhombs (1) and in the matrixes (2) corresponding to replacement and authigenic or direct precipitation mechanisms, respectively (Yalçın et al., 2004; Yalçın and Bozkaya, 1995). In the association of palygorskite þ saponite in some lacustrine beds, palygorskite is formed by diagenetic transformation from precursor saponitic smectites (Singer, 1984; Yeniyol, 2010) (3), according to the following equations (14, 15, 16): 2CaMgðCO3 Þ2 þ 8H4 SiO4 þ 2OH þ 2ðAl, Fe3þ Þ ! ðAl, Fe3þ Þ2 Mg2 Si8 O20 ðOHÞ2 ðOH2 Þ4 4H2 O þ 4CO2 þ 8H2 O þ 2Ca2þ ð14Þ 2Mg2þ þ ðAl, Fe3þ Þ2 þ 8H4 SiO4 þ 10OH ! Mg2 ðAl, Fe3þ Þ2 Si8 O20 ðOHÞ2 ðOH2 Þ4 4H2 O þ 12H2 O

ð15Þ

NaCaðMg4 FeAlÞ½Si8 O20 ðOHÞ4 H2 O þ 5H2 O ! Mg2 ðAl, Fe3þ Þ2 Si8 O20 ðOHÞ2 ðOH2 Þ4 4H2 O þ 2Mg2þ þ Ca2þ þ Naþ ð16Þ Palygorskite fibres and fibre bundles were developed authigenically on euhedral or subhedral calcite crystals of the caliche units and at the edges of smectite flakes in the caliche host rocks or sediments (Kadir and Eren, 2008). Intense, continuous evaporation of subsurface soil–water resulted in an increase in pH and the dissolution of detrital smectite within the red mudstones and alluvial red soils that enclose the isolated caliche forms, and caused an increase in the Al þ Fe and Mg/Ca ratio, favouring the formation of palygorskite under alkaline conditions. The calcium required for caliche formation may have originated from eolian dust, detrital carbonate minerals and/or other caliche materials, which are dissolved by carbonic acid.

194


Finally, due to very complicated processes, palygorskites are frequently the in situ formed minerals from the other clay and/or carbonate aggregates and in the micropores by neoformation and transformation mechanisms in the weathering conditions as in the similar pedogenic environments (Kadir et al., 2010; Kapur et al., 1993, 2000; Singer, 1979, 1989).

5. ECONOMY The application and uses of sepiolite–palygorskite clays have attracted much attention in the world due to their suitability of structural features and physicochemical behaviour (Galan, 1996). Sepiolite nodules have been exploited in the Eskis¸ehir province since the time of Roman Empire, and considerable progress has been made in many phases of industrial and agricultural technology in Turkey since 1990s. The total reserves for nodular sepiolite only in the Eskis¸ehir province are about 17,000 tons (DPT, 2001). Approximately 1.5 million tons of reserves of the bedded pure and dolomitic sepiolites (> 50% sepiolite) are determined, and several million tons of dolomites with sepiolite (< 50% sepiolite) are the estimated amounts in the central Anatolia. The pure and nearly pure nodular sepiolites are produced as 20–40 tons/ year and are consumed as the making of pipe, pipe lining and work of arts, ornaments and press materials (e.g. Is¸ık et al., 2010). As for the bedded sepiolites, their productions vary from year to year and reached roughly 20,000–60,000 tons/year in 1990s and they are used in limited amounts as pet-litter, absorbent and petrochemical industry. Sepiolite exports are variable for each year and are around 1–30 tons. In Turkey, today there is no production and consumption of palygorskite, although many outcrops are present in the Central Anatolian lacustrine basins.

6. CONCLUSIONS Turkish sepiolite/palygorskite occurrences can be concluded under genetic types whose principal features are summarized in Table 2. Sepiolite–palygorskite minerals as pure and/or mixed with other components are commonly found in the clayey-carbonate rocks and deposited in the closed lacustrine basins and shallow marine environments. The thickness of sepiolitic and/or palygorskitic beds ranges from centimetre to decimetre dimensions within the total sections of few decimetre to a few hundred metres thick. They are also seen as veins (millimetre–centimetre) in the ophiolitic and volcanic sequences. Unique palygorskite in the soils of Upper Pliocene–Holocene age along the Mediterranean coast is widely available. The chemical compositions of sepiolites are generally very close to ideal formulas, except for some samples, but the octahedral occupants in the palygorskites have variable quantities of Mg, Al, Fe, Ti, Cr and Ni in order of abundance.

Age

Environment

Facies

Thickness of sequence (m)

Mineralogical assemblage

Origin

Reference

Malatya– Hekimhan

Upper Cretaceous– Lower Miocene

Shallow marinecoastal lagoon

Clayey carbonate Evaporative

40–240

Sep þ Dol Sep þ Pal/Cal/ Dol/Sm Pal þ Cal/Dol/ Sm

Diagenetic neoformation and transformation of dolomite and/or Mg clay

Yalçın and Bozkaya (1995)

Eskis¸ehir– ¨ çkuyular U Konya– Yunak

Middle– Upper Miocene

Saline– alkaline lake and/or playa

Stratified conglomerate

0.5–4

Sep þ Mgs

Hydrothermal transformation of magnesite

Ece and Çoban (1994), Yeniyol (1995)

Eskis¸ehir– Sivrihisar

Middle– Upper Miocene

Cherty clayey dolomite Evaporative

32–40

Sep þ Dol Sep þ Dol/Cal/ Sm/O-CT

Diagenetic neoformation and transformation of dolomite and/or Mg clay

Ece and Çoban (1994)

Eskis¸ehir– Yo¨ru¨kçayır– Kepeztepe

Lower Pliocene

Clayey dolomite

28–38

Sep þ Dol Sep þ Pal/Dol/ Cal/Sm/O-CT

Yeniyol (2011)

Konya– As¸ag˘ı Pınar

Upper Miocene– Pliocene

Clayey dolomite

25–85

Sep þ Dol Sep þ Pal þ Cal/ Dol

Karakas¸ and Kadir (1998)

7

Location

Chapter

TABLE 2 A Summary Description of the Representative Sepiolite–Palygorskite Occurrences in Turkey.


195

Continued

A Summary Description of the Representative Sepiolite–Palygorskite Occurrences in Turkey.—Cont’d

Location

Age

Konya– Karapınar

Environment

Thickness of sequence (m)

Mineralogical assemblage

Upper Miocene– Pleistocene

Cherty clayey carbonate Evaporative

79–117

Sep þ Dol Sep þ Pal þ Cal/ Dol/ O-CT

Karakaya et al. (2004)

Denizli– Bozkurt

Upper Miocene– Lower Pliocene

Clayey carbonate

10–78

Sep þ Dol Sep þ Pal þ Sm/ Cal/Dol/Mgs

Kadir and Akbulut (2001)

Sivas– Kangal– Sus¸ehri

Middle Miocene– Pliocene

Clayey carbonate

50–210

Sep þ Dol Sep þ Pal þ Sm/ Cal/Arg/Dol/ Mgs/O-CT

Yalçın et al. (2004, 2006)

Bolu– Kıbrıscık

Middle Miocene

Volcanogenic

40–80

Sep þ OCT þ Zeo

Hydrothermal transformation of volcanic glass

˙Irkeç and ¨ nlu¨ (1993) U

Ankara– Elmadag˘

Upper Cretaceous

Serpentinite

0.5–2

Sep þ Dol Sep þ Sm/Cal/ Arg/Dol/Mgs

Hydrothermal transformation of dolomite and/or serpentine

Yalçın and Bozkaya (2004)

Konya– C ¸ ayırag˘zı

Upper Cretaceous

0.1–0.3

Sep þ Mgs

Hydrothermal transformation of magnesite and/or serpentine

Yeniyol (1986)

Continental

Origin

Reference

Sp, sepiolite; Pal, palygorskite; Sm, smectite; Cal, calcite; Arg, ragonite; Dol, dolomite; Mgs, magnesite; O-CT, Opal-cristobalite/tridymite; Zeo, zeolite.


Facies

196

TABLE 2

Chapter

7


197

These clays occurred as sedimentary and diagenetic neoformations (precipitation from solution) and partly diagenetic and hydrothermal transformations of precursor materials, such as phyllosilicates (clay and serpentine) and carbonate minerals (dolomite, magnesite), and volcanic glass in the highly basic environments. The dissolved volcanic glass shards and pumice fragments contributed additional silica in the palaeo-lakes as well as diatoms. Serpentine and Mg-bearing carbonates are the only significant source of Mg available. Turkey supplies sepiolites in the limited production, consumption and use, though a commercial activity is not yet about palygorskites.

ACKNOWLEDGEMENTS This study is reviewed by using the papers selected from national and international journals, and unpublished reports of current authors relating to sepiolite–palygorskite occurrences in Turkey. We are deeply indebted to Arieh Singer and Emilio Galań for their editorial suggestions that greatly encouraged us to write our chapter of this book. The authors are also grateful to anonymous reviewers for careful and constructive comments.

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Kapur, S., Saydam, C., Akça, E., C ¸ avus¸gil, V.S., Karaman, C., Atalay, I., et al., 2000. Carbonate pools in soils of the Mediterranean: a case study from Anatolia. In: Lal, R., Kimble, J.M., Eswaran, H., Stewart, B.A. (Eds.), Global Climate Change Pedogenic Carbonates. CRC, Lewis Publ., Washington, DC, pp. 187–212, Chapter 12. Karakas¸, Z., Kadir, S., 1998. Mineralogical and genetic relationships between carbonate and sepiolite-palygorskite formations in the Neogene Lacustrine Konya Basin, Turkey. Carbonates Evaporites 13, 198–206. Karakaya, N., Çelik Karakaya, M., Temel, A., Ku¨peli, S¸., Tunog˘lu, C., 2004. Mineralogical and chemical characterization of sepiolite occurrences at Karapınar (Konya Basin, Turkey). Clays Clay Miner. 52, 495–509. Kılıç, K., Yalçın, H., 2009. Mineralogic characteristics and genesis of volcanic soils related to Erciyes Stratovolcano, Kayseri province, Turkey. In: Book of Abstracts, XIV International Clay Conference. Castellaneta Marina, Italy, 14–20 June 2009, V. II, 614 pp. Millot, G., 1970. Geology of Clays. Springer, Berlin, 429 pp. MTA, 2002. 1:500.000 Scaled Geological Map Series of Turkey. General Directorate of Mineral Research and Exploration, Ankara. Paque´t, H., Duplay, J., Valleron-Blanc, M.M., Millot, G., 1987. Octahedral composition of individual particles in smectite-palygorskite and smectite-sepiolite assemblages. In: Schultz, L.G., van Olphen, H., Mumpton, F.A. (Eds.), Proceedings, International Clay Conference, Denver, 1985. The Clay Mineral Society, Bloomington, Indiana, pp. 73–77. Singer, A., 1979. Palygorskite in sediments: detrital, diagenetic or neoformed—a critical review. Geol. Rund. 68, 996–1008. Singer, A., 1984. Pedogenic palygorskite in the arid environment. In: Singer, A., Galan, E. (Eds.), Palygorskite-Sepiolite: Occurrences, genesis and Uses. Development in Sedimentology, vol. 37, Elsevier, Amsterdam, pp. 169–176. Singer, A., 1989. Palygorskite-sepiolite and related minerals in the soil environment. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environments, Soil Science Society of America Publication, Madison, WI, U.S., Series 1, pp. 829–872. Torres-Ruiz, J., Lopez-Galindo, A., Gonzalez-Lopez, J.M., Delgado, A., 1994. Geochemistry of Spanish sepiolite-palygorskite deposits: genetic considerations based on trace elements and isotopes. Chem. Geol. 112, 221–245. Weaver, C.E., Pollard, L., 1973. The Chemistry of Clay Minerals. Elsevier, Amsterdam, 217 pp. Yalçın, H., 1997. Central North Anatolian zeolite occurrences related to Eocene submarine volcanism in Turkey. Bull. Fac. Eng. Cum. Univ. Serie A Earth Sci. 14, 43–56, (in Turkish, English abstract). ¨ ., 1995. Sepiolite-palygorskite from the Hekimhan region (Turkey). Clays Yalçın, H., Bozkaya, O Clay Miner. 43, 705–717. ¨ ., 2003. Mineralogy and geochemistry of hydrothermal kaolinite and I-S Yalçın, H., Bozkaya, O occurrences (Yıldızeli-Akdag˘madeni) W-Sivas. Geol. Bull. Turk. 46, 1–23, (in Turkish, English abstract). ¨ ., 2004. Ultramafic-rock-hosted vein sepiolite occurrences in the Ankara Yalçın, H., Bozkaya, O ophiolitic me´lange, Central Anatolia, Turkey. Clays Clay Miner. 52, 227–239. ¨ ., 2006. Mineralogy and geochemistry of ultramafic- and sedimentaryYalçın, H., Bozkaya, O hosted talc deposits of Paleocene in the southern part of the Sivas basin, Turkey. Clays Clay Miner. 54, 333–350. Yalçın, H., Gu¨mu¨s¸er, G., 2000. Mineralogic and geochemical characteristics of Late Cretaceous bentonite deposits at the north of Kelkit valley, Northern Turkey. Clay Miner. 35, 807–825.

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¨ ., Poisson, A., I˙nan, S., 1994. Lithological and mineralogical Yalçın, H., Kavak, K.S¸., Bozkaya, O ˘ characteristics of the Agcakıs¸la subbasin (Sivas basin). Bull. Fac. Eng. Cum. Univ. Earth Sci. 11, 87–95 (in Turkish, English abstract). ¨ ., Poisson, A., 2000. Relationship of clay mineralogy to Oligo-Miocene Yalçın, H., Bozkaya, O Paleo-environments in the central part of the Sivas Basin, Turkey. Bull. Fac. Eng. Cum. Univ. Earth Sci. 17, 53–62, (in Turkish, English abstract). Yalçın, H., Ergin, M., Eryılmaz, M., Eryılmaz, F.Y., 2001. Bulk and clay mineralogy of surficial marine sediments of the Gulf of I˙skenderun, Eastern Mediterranean. Bull. Fac. Eng. Cum. Univ. Earth Sci. 18, 71–78. ¨ ., Bas¸ıbuÿu¨k, Z., 2004. Mg-mineral occurrences in the Central Anatolian Yalçın, H., Bozkaya, O Neogene intra-cratonic basins related to neotectonic regime: an example from Kangal basin, Sivas, Turkey. In: Proceedings, 5th International Symposium on Eastern Mediterranean Geology (5th ISEMG), pp. 1473–1476, Thessaloniki, Greece, 14–20 April, 2004. ¨ ., Karayel, A., 2006. Mg-mineral occurrences in the serpentinite- and Yalçın, H., Bozkaya, O volcanic-fed lacustrine basins in the I˙mranli-Sus¸ehri region, northeastern Turkey. In: Abstracts, 4th Mediterranean Clay Meeting, pp. 137–138, Ankara, Turkey, 5–10 September 2006. Yeniyol, M., 1986. Vein-like sepiolite occurrence as a replacement of magnesite in Konya, Turkey. Clays Clay Miner. 34, 353–356. Yeniyol, M., 1995. Meerschaum sepiolite and palygorskite occurrence in Central Anatolia, Turkey. In: Churchman, G.J., Fitzpatrick, R.W., Eggleton, R.A. (Eds.), Clays the Controlling the Environment, Proceedings, 10th International Clay Conference, pp. 378–382, July 18–23, 1993, Adelaide, Australia. Yeniyol, M., 1997. The mineralogy and economic importance of a loughlinite deposit at Eskis¸ehir, Turkey. In: Kodama, H., Mermut, A.R., Torrance, J.K. (Eds.), Clays for Our Future, Proceedings, 11th International Clay Conference, pp. 83–88, June 15–21, 1997, Ottawa, Canada. Yeniyol, M., 2011. Geology and mineralogy of a sepiolite-palygorskite occurrence from SW Eskis¸ehir (Turkey). Clay Miner. (in review).

Chapter 8

Genesis and Distribution of Palygorskite in Iranian Soils and Sediments Saeid Hojati* and Hossein Khademi{ *Department of Soil Science, College of Agriculture, Shahid Chamran University of Ahvaz, Ahvaz, Khuzestan, Iran { Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran

1. INTRODUCTION Palygorskite is a widely distributed fibrous clay mineral in soils and sediments of arid regions (Singer, 1989). When chronological and geographical distribution of palygorskite and sepiolite are taken into consideration, the quantities of these minerals appear to increase in the Upper Cretaceous and especially in Tertiary-aged sediments (Akbulut and Kadir, 2003; Aqrawi, 1993; Kadir and Eren, 2008; Namik Cagatay, 1990; Shadfan and Dixon, 1984; Shadfan and Mashhady, 1985; Singer, 1981; Singer et al., 1995; Yalcin and Bozkaya, 1995), in zones as wide as 30 to 40 N and S latitudes (Yalcin and Bozkaya, 1995). These minerals are stable only under alkaline conditions with high Mg activity (Singer, 1980). Palygorskite is unstable in areas with more than 300 mm of mean annual rainfall and may be weathered to other clay minerals such as smectite (Paquet and Millot, 1972). Palygorskite in soils and sediments in the Middle East generally occurs in areas once covered by the post-Tethyan intermontane shallow lagoons (Callen, 1984). Identifying the origin and distribution pattern of this mineral is, therefore, essential for the reconstruction of the environments of the post-Tethyan era. Iran is located in the mid-latitude belt of arid and semi-arid regions of the earth (Modarres and Silva, 2007). The Iranian plateau is located in the ancient Tethys Seaway (Krinsley, 1970). This seaway was cut off from the ocean in the late Cretaceous (Sengo¨r et al., 1988). As a result, shallow intermontane lakes and lagoons evolved during the Tertiary (Zahedi, 1976). These shallow Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00008-6 # 2011 Elsevier B.V. All rights reserved.

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hypersaline water bodies appear to have been chemically suitable for the formation of palygorskite. Palygorskite affects soil properties when it constitutes a significant component of the soil clay fraction. When palygorskite-containing soils are irrigated, Mg is released from palygorskite into the soil solution. Palygorskite has the strongest disaggregation potential and the highest ability to migrate in the soil among the common phyllosilicates such as smectite and kaolinite. Its particles are thus likely to move preferentially over smectite and kaolinite downwards in the soil profile (Neaman and Singer, 2004). Palygorskite and sepiolite have unique colloidal properties. They do not easily flocculate even at high concentration of electrolytes. The elongate shape and small size of these minerals (from about 1 to 10 mm) result in their high surface area and also high porosity when thermally activated (Murray, 2007). This review aims (i) to summarize previous studies that have been carried out on the occurrence and genesis of palygorskite in Iranian soils and in associated sediments and rocks and (ii) to identify the priorities for future research required on fibrous clays in Iranian soils and sediments.

2. PALAEOGEOGRAPHY AND PALAEOGEOLOGY OF IRAN The geology of Iran and its tectonic style, in particular, were highly influenced by the development and history of the Tethyan region including the Iranian Plate, and the adjacent areas underwent three major evolutionary stages. The first stage, the Gondwana break-up, was associated with tensional basins and basement highs. The central Iranian segment was separated from the Arabian Plate along the line of the present high Zagros zone (Alsharhan et al., 2001; Figure 1). The result of this process was the opening of the Neo-Tethys. The second stage, closure of the Neo-Tethys, started in the late Cretaceous and proceeded into Cenozoic times. The final orogenic phase was due to the Arabia–Eurasia convergence and took place first in southern Iran at the end of Eocene (Hessami et al., 2001). This process affected the Arabian and Iranian Plates and resulted in the Zagros orogenic phase of late Miocene to Pliocene age. The latest Tertiary to Holocene events included strong uplift processes, magmatism and volcanism, erosional processes, and the associated deposition of extensive alluvial fans from the uplifted mountains (Alsharhan et al., 2001). As a result of uplift and folding, Tethys Sea, which once covered the entire Iran, was closed. With frequent uplifting, a large number of shallow water bodies were created. Weathering at higher elevations in the surrounding area released elements essential for the formation of evaporite minerals. Due to the warm and arid climate of the Tertiary period, evaporation of closed lakes caused more soluble minerals such as gypsum and carbonates to crystallize, which increased the Mg/Ca ratio and pH values in the host water

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IN TA UN

BU R

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36N

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Mashhad

Tehran

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Kavir Desert

AFGHANISTAN IRAQ

Isfahan

CENTRAL IRAN RO G ZA

Ahvaz Abadan 30N

S

KU

WA I

Shiraz Bushehr

T

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RS

QATAR

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48E

MO

UN

30N

Kerman

TAI N

PAKISTAN

S

Bandar Abbas

IA

SAUDI ARABIA

Lut Desert

N

GU

0

200 km

LF U.A.E

GULF OF OMAN

FIGURE 1 Map of Iran showing Elburz and Zagros mountains (adapted from www.geoexpro.com).

bodies and, therefore, encouraged the formation of palygorskite (Khademi and Mermut, 1998).

3. CLIMATE OF IRAN Iran is located in the mid-latitude belt of arid and semi-arid regions of the Earth. Eighty-five per cent of the total area in Iran is characterized by an arid to semi-arid climate (Banaei et al., 2005). Arid and semi-arid regions of Iran lie approximately between 50 and 64 E longitude and between 25 and 37 N latitude (Modarres and Silva, 2007). Most of the relatively scant annual precipitation falls from October to April with yearly averages of 250 mm or less in most parts. The major exceptions are the higher mountain valleys of the Zagros and the Caspian coastal plain, where precipitation averages at least 500 mm annually. In the western part of the Caspian, rainfall exceeds 1000 mm annually and is distributed relatively evenly

204


Caspian Sea

Isfahan

Fars Pe rs i

an

Hyper-arid

G

Kerman

ul

f

Arid FIGURE 2 Distribution of arid and hyper-arid areas in Iran (adapted from Food and Agriculture Organization, country pasture profiles in Iran).

throughout the year. This contrasts with some basins of the Central Plateau that annually receive 100 mm or less precipitation annually (Figure 2). Temperature in Iran is related to latitudinal position, the distribution of land surfaces relative to altitude and to the importation of air masses which are warmer or colder than those of Iranian Plateau. The annual range of temperature generally increases with latitude and distance from large water bodies (Krinsley, 1970). The present arid to hyper-arid environment prevailing in the most parts of Iran (Figure 2) has led to the preservation of palygorskite in the soils and sediments of these areas.

4. PALYGORSKITE IN WESTERN IRANIAN SOILS Henderson and Robertson (1958) were the first researchers who reported the occurrence of palygorskite in Iran. They showed that the insoluble fraction of an Eocene limestone from west of Kermanshah and the Asmari dolomite in west of Khorramabad, Luristan, are rich in palygorskite. Burnett et al. (1972) also reported the occurrence of palygorskite in limestones of Kermanshah. However, they did not find any traces of this mineral in the associated alluvial soils in their study area.

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Amirinejad and Baghernejad (1999) reported the co-occurrence of palygorskite, smectite, illite and chlorite along a toposequence in Kermanshah, west of Iran. They showed higher amounts of palygorskite in old plateau and piedmont alluvial soils, compared to the amounts of alluvial and colluvial fans. Pedogenic alteration of illite and chlorite was proposed as the main mechanism controlling the formation of palygorskite and smectite in the soils studied. No evidence for such transformation was shown. Salehi et al. (2003) reported pedogenic formation of palygorskite in soils of Farokhshahr area in Chaharmahal and Bakhtiari Province, west of Iran. The main reasons they provided to support the pedogenic origin of palygorskite were (i) lack of palygorskite in nearby rock formations, (ii) elongate morphology of palygorskite fibres observed by transmission electron microscope (TEM) and (iii) coexistence of palygorskite with pedogenic calcites. They proposed palygorskite formation through direct precipitation from soil solution.

5. PALYGORSKITE IN CENTRAL IRANIAN SOILS AND SEDIMENTS Khademi and Mermut (1998) investigated the source of palygorskite in gypsiferous Aridisols and associated sediments in central Iran. They found palygorskite, sepiolite and an appreciable amount of mica and smectite in the clay fractions of Oligocene–Miocene limestone but only traces of palygorskite in Cretaceous limestone. Palygorskite was found to be highly associated with gypsum in the gypsiferous soils studied (Figure 3). They suggested that palygorskite probably formed after the initial precipitation of gypsum which created a high pH and Mg/Ca ratio. Both pedogenic and in herited origins were suggested for the occurrence of palygorskite in these soils. Using acid dissolution and electron microscopic examinations, Khademi and Mermut (1999) proved that 8–19% of impurities present in secondary carbonates of selected Iranian Aridisols consisted of palygorskite bundles (Figure 4). While neogenesis of palygorskite during the development of calcic horizons was reported to be a possible mode of formation, illuviation of geologically formed palygorskite was proposed as a more likely reason for the presence of palygorskite in the soft pedogenic carbonate nodules in central Iranian Aridisols. Eftekhari and Mahmoodi (2002) reported large amounts of palygorskite in gypsiferous and calcareous soils of Salafchegan Plain in Qom Province, but the origin of this mineral was not discussed. Farpoor et al. (2002) found a close relationship between palygorskite morphology and geomorphic positions of two transects in Rafsanjan area. Upper geomorphic surfaces (rock pediments) were reported to contain palygorskite crystals greater in size and number than those in lower positions where palygorskite fibres could only be observed by electron microscope. They reported an inherited origin for palygorskite in soils of lower positions including mantled pediments and playas but a pedogenic origin for those formed on upper geomorphic surfaces.

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A

B

1 mm

1 mm

D

C

1 mm

E

0.5 mm

F

1 mm

1 mm

FIGURE 3 Transmission electron micrographs showing the distribution of palygorskite in the surface and subsurface soils from different landforms in central Iran. (A and B) Fine clay fraction of the A and Btky1 horizons from the colluvial fan; (C and D) clay fraction of the Ay and By horizons from plateau and (E and F) fine clay fraction of the Azy and Bw horizons from the alluvial plain (Khademi and Mermut, 1998).

The clay mineralogy of gypsiferous soils developed on different landforms in eastern Isfahan including piedmont plains, old river terraces and alluvial fans was investigated by Karimzadeh et al. (2004). Palygorskite was present in all the soils studied. This clay mineral was found to increase with depth in the alluvial fans, whereas, in the old terraces, it decreased with depth. Based on the finding that a large amount of palygorskite was present in the upper horizons, they concluded that palygorskite in the old terraces must have

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B

A

0.5 mm

0.5 mm

FIGURE 4 (A and B) Transmission electron micrographs of the ultramicrotome cut of pedogenic carbonate from the Btky horizon of a Typic Gypsiargid soil in the Isfahan Province, central Iran, showing the association of palygorskite with pedogenic carbonates (Khademi and Mermut, 1999).

been mostly detrital and of eolian origin. They claimed that palygorskite in alluvial fan soils formed authigenically when the basin was covered with shallow hypersaline lagoons towards the end of the Tertiary. Farpoor and Krouse (2008) found large amounts of palygorskite in Neogene sediments of Loot Desert, central Iran (Figure 5). In this study, palygorskite crystals were observed covering gypsum crystals. They suggested the formation of palygorskite after the crystallization of gypsum particles. Tertiary sediments are widely distributed in central Iran and, to a large extent, in other parts of the country (Llewellyn, 1973; Manouchehri, 1987; Samadian, 1996; Zahedi, 1976, 1993). Khademi and Hojati (2010) examined the distribution of this mineral in selected Tertiary sediments of central Iran. They showed that palygorskite was absent in Palaeocene, Eocene and Oligocene samples collected in Isfahan Province. In contrast, large amounts of this mineral were found in samples belonging to Miocene and Pliocene ages (Figure 6). Palygorskite was found to be highly associated with smectite in the sediments studied. The stability diagrams depicted for palygorskite– smectite system showed that in Neogene samples, smectite was unstable and theoretically transformed into palygorskite. However, no textural evidences of solid–solid transformation of palygorskite were observed in this study. Based on electron microscopy examinations and geochemical analyses of the sediments, the authors proposed the in situ formation of palygorskite in the studied sediments. Hojati et al. (2010) studied the effects of a saline and alkaline groundwater on the formation of palygorskite along a calcareous catena in the Great Kavir Basin in central Iran. They found higher quantities of smectite in the near surface horizons (lacking palygorskite), but plenty of palygorskite in partially or permanently submerged horizons that contained a much lower quantity of smectite (Figure 7). Palygorskite was, therefore, reported to be partly

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B

A Palygorskite

Det

Palygorskite

Acc.V Spot Magn Det WD 20.0 kV 4.0 13010x SE 10.2

C

5 mm

500 mm

500 mm

D

500 mm

Palygorskite

Palygorskite

FIGURE 5 (A) Scanning electron micrograph showing palygorskite bundles around gypsum crystals; (B–D) transmission electron micrographs showing palygorskite bundles in different layers of yardangs in central Iran (Farpoor and Krouse, 2008).

inherited from the Miocene sediments as the main soil parent materials. Growth of palygorskite fibres on halite and calcite crystals observed by scanning electron microscope (SEM) clearly indicated that palygorskite was pedogenic within the groundwater zone (Figure 8). They, therefore, concluded that palygorskite must be a stable mineral in the groundwater zone and form, pedogenically perhaps at the expense of smectite which is unstable in the same zone. This suggests the weathering of smectite and the neoformation of palygorskite in the groundwater zone.

6. PALYGORSKITE IN SOUTHERN IRANIAN SOILS AND PARENT ROCKS Salt-affected soils are widely distributed throughout arid to semi-arid regions of Iran where they occupy about 15% of the land area, almost equal to the total arable land. These soils occur in flat to depressional landform positions under the influence of a fluctuating groundwater table (Dewan and Famouri,

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B

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10 mm

D

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5 mm

F

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FIGURE 6 Scanning electron micrographs showing the distribution of palygorskite in selected Tertiary sediments of central Iran. (A–C) Lack of palygorskite in Palaeocene, Eocene and Oligocene samples, respectively, (D) long palygorskite fibres in Oligocene–Miocene sediments, (E) interwoven bundles of palygorskite covering aggregates of Miocene-aged sediments and (F) delicate networks of palygorskite fibres in Pliocene sediments (Khademi and Hojati, 2010).

1964). Abtahi (1977) studied the effects of a saline and alkaline groundwater on soil genesis in semi-arid regions of southern Iran. Two soil series, namely Marvdasht and Korbal, developed along a toposequence in southern Iran were studied. He showed a clear difference in the relative abundance of palygorskite and smectite as palygorskite is replaced by smectite in the drier climatic zones of the toposequence studied. A pedogenic origin was proposed for the

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1 mm

1 mm

C

2 mm

FIGURE 7 Selected transmission electron micrographs of carbonate-free clay fraction of a Typic Aquisalid in the Great Kavir Basin in central Iran. (A–C) Refer to the Az, Bz and Bzg horizons, respectively (Hojati et al., 2010).

occurrence of palygorskite in the soils studied. Similarly, Givi and Abtahi (1985) reported the pedogenic formation of palygorskite under the influence of rising groundwater in Marvdasht area. However, Abtahi and Khormali (2001) reported the formation and dominance of palygorskite in well-drained soils of Darab Plain but found smectite to be the dominant clay mineral in poorly drained soils. The nature and genesis of some salt affected soils in southern Iran were studied by Mahjoory (1979). He proposed that part of palygorskite and smectite in the Natrargids of southern Iran likely formed due to the high electrolyte content in the solutions of the soils studied. Abtahi (1980) reported a clear gradation in the relative content of palygorskite and smectite along a chronosequence in Sarvestan intermontane basin in Fars Province. Higher amounts of palygorskite were found in older soils where lower amounts of smectite were observed. The highest amount of palygorskite was found in a petrocalcic horizon (> 70%). However, calcic (50–60%) and cambic horizons (< 50%) had lower amounts of palygorskite. The origin of palygorskite in this study was mainly attributed to pedogenic processes. Baghernejad (2000) studied the variation in soil clay mineralogy of semiarid regions of Fars Province, southern Iran. The soils they studied had developed on limestones of late Tertiary and Quaternary ages. Different physiographic units including alluvial–colluvial fans, plateaus, piedmont alluvial plains, flood plains, upper terraces, river terraces and lowlands were sampled and studied. Results indicated the occurrence of illite, chlorite,

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B

A

10 mm

10 mm

D

C

10 mm

5 mm

FIGURE 8 (A) Growth of palygorskite fibres showing the neoformation of this mineral in the Bzg horizon of a Typic Aquisalid soil in the Great Kavir Basin, central Iran, (B and C) calcite and halite grains of the Bzg horizon covered by palygorskite fibres, (D) formation of palygorskite fibres in pore spaces between aggregates of the same soil (Hojati et al., 2010).

smectite, vermiculite, palygorskite and interstratified illite–smectite and chlorite–smectite in the soils studied. The higher physiographic units contained more illite and chlorite, whereas the lower ones had more smectite and palygorskite. Illite and chlorite were found to be of inheritance origin, but which transformed pedogenically to smectites. This study reported the neoformation of palygorskite by precipitation from Mg-rich solutions. Jamshidi and Abtahi (2001) studied the genesis of calcic, gypsic and salic horizons in selected soils of arid and semi-arid regions in Fars Province. They found a positive correlation between the amount of palygorskite and those of gypsum and calcite. They proposed palygorskite to have formed due to the transformation of other clay minerals but they failed to provide relevant data to support this idea. Khormali and Abtahi (2001) studied the clay mineralogy of soils and of the main calcareous sedimentary parent rocks of arid and semi-arid regions in southern Iran (Fars, Bushehr and Khuzestan Provinces) to determine their origin and the factors controlling their distribution pattern in the soils studied. They found an inverse correlation between palygorskite and smectite with soil available moisture (Figure 9). Palygorskite was found to be scarce and

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A 80

R 2 = 0 .56**

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70 60 50 40 30 20 10 0 0.2

0

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0.6

0.8

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Palygorskite (%)

60 50 40 30 20 10 0 0

10

20

30 Smectite (%)

40

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FIGURE 9 Percentage of palygorskite versus P/ET (A), and versus percentage of smectite (B) in selected soils and sediments of Fars Province, southern Iran. **Statistically significant at P 0.01. (Khormali and Abtahi, 2003).

unstable when the ratio of mean annual precipitation to mean annual reference crop evapotranspiration (P/ET) exceeded 0.4. Khormali et al. (2005) studied the late Mesozoic–Cenozoic clay mineral successions in part of Zagros orogenic area in southern Iran. Results showed a large amount of kaolinite but the absence or rare occurrence of chlorite, smectite, palygorskite and illite in the lower Cretaceous sediments. Towards the beginning of Tertiary, the gradual disappearance of kaolinite coincided with an increase in smectite and palygorskite in the sediments studied. They suggested that the occurrence of palygorskite and smectite in the late Palaeocene sediments was probably due to the increase in aridity which has continued until the present time. Results also showed that detrital input was possibly the main source of kaolinite, smectite, chlorite and illite in the study area. However, in situ neoformation of palygorskite was proposed for the occurrence of this mineral (Table 1).

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TABLE 1 Most Likely Origin of Clay Minerals in the Parent Rocks of Southern Iran Reported by Khormali et al. (2005). Clay Mineral

Detrital (Inherited)

Diagenesis

In situ Neoformation

Kaolinite

þþþ

þ

Smectite

þþþ

þ

þ

Palygorskite

þþþ

Illite

þþþ

Chlorite

þþþ

þþþ, Major importance; þþ, moderate importance; þ, minor importance; , no importance.

Fars Province in southern Iran is also known to be the largest area with Vertisols in the country. Heidari et al. (2008) reported that the fine and total clay fractions of southern Iranian Vertisols were dominated by palygorskite as a low activity clay mineral. They proposed an inherited origin from the neighbouring Tertiary sediments for the occurrence of palygorskite in these soils. Owliaie et al. (2006) reported the neoformation of palygorskite as a result of calcite and gypsum precipitation in south-western soils of Iran. Compared to calcareous soils, gypsiferous soils were found to have more pedogenic palygorskite. Three morphological arrangements of palygorskite were identified in terms of degree of weathering: bundles or sheaves, split bundles and sharp-pointed crystals and individual bundles with rounded tips (Figure 10).

7. PALYGORSKITE IN NORTH-EASTERN IRANIAN SOILS Few studies have reported the presence of palygorskite in soils and sediments of north-eastern Iran. Haghnia (1982) was first to report the occurrence of palygorskite in soils of Mashhad, north-eastern Iran. The presence of palygorskite in these soils was mainly attributed to the alkaline conditions provided by large amounts of gypsum and carbonates in the studied soils. Recently, Karimi et al. (2009) investigated the occurrence of palygorskite and its formation conditions along a transect covering granitic and marly hilly lands in southern Mashhad. Based on formation conditions, amount and morphology of palygorskite in the soils studied, three kinds of this mineral were identified: (1) palygorskite in the basal part of saline loess deposits, (2) palygorskite associated with calcic and gypsic horizons and (3) detrital palygorskite in unaltered loess deposits. To the best of our knowledge, no other study has reported palygorskite occurring in this region.

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A

B

2 mm

C

1 mm

D

0.5 mm

0.5 mm

FIGURE 10 Transmission electron micrographs showing different forms of palygorskite. (A) Bundles of palygorskite in the Bk horizon, (B) sharp-pointed palygorskite in the Bk1 horizon of an Aridic Calciustept, (C) individual fibres with rounded tips in an A horizon of an Aridic Calciustept, (D) short and broken fibres of palygorskite in the Ay horizon of an Aridic Ustorthent developed on colluvial fan (Owliaie et al., 2006).

8. CONCLUSIONS Palygorskite is a widely distributed clay mineral in soils and sediments of Iran. Most of previous studies have been focused on the distribution and genesis of palygorskite in central and southern Iranian soils and sediments. Only few studies reported occurrence of palygorskite in other parts of the country. Cretaceous or older formations contain none or only rare occurrences of detrital palygorskite. However, sedimentary rocks of Tertiary ages have different proportions of authigenically formed palygorskite. These results agree with those reported in similar studies, especially those focusing on the Middle East and the neighbouring areas which are characterized by palygorskite in their Tertiary rocks and sediments. The deep sea environment of the Tethys Ocean (geological formations older than Cretaceous)

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does not appear to have been suitable for the formation of palygorskite. Orogenic activities during the Tertiary, as a result of which the Tethys Sea was cut off during the late Cretaceous, gave rise to the development of shallow saline lakes during the Tertiary which were chemically favourable for the formation of fibrous silicate clays. These conditions led to the formation of gypsum and resulted in an increase in the Mg/Ca ratio which, under an evaporative environment, brought about the authigenic formation of a large amount of palygorskite especially in Neogene sediments. This hypothesis is supported by the positive correlation between the occurrence of palygorskite and the gypsum/carbonate contents in Iranian soils and sediments. Palygorskite in the studied soils seems to be both pedogenic and inherited. Transformation from smectite, direct precipitation from Mg-rich solution and formation under the influence of rising ground water have been reported as likely causes for the pedogenic formation of palygorskite in Iranian soils. It is proposed that the geochemistry of post-Tethys sea environment which was significantly affected by climatic conditions and orogenic events during the Tertiary controlled the formation of palygorskite. The present arid to hyper-arid environment prevailing in the most parts of Iran must have preserved this mineral in Iranian soils and sediments.

9. DIRECTION FOR FUTURE RESEARCH 1. Despite the numerous studies about the distribution and genesis of palygorskite in Iranian soils and sediments, more detailed studies are needed to identify the areas predominantly characterized by the occurrence of palygorskite. 2. More detailed studies are needed focusing on the chronostratigraphic distribution of palygorskite in western, south-western and north-eastern Iranian parent rocks and soils. 3. Despite the fact that salt-affected soils are widely distributed in Iran (about 15% of the total area), the genesis of palygorskite in saline and alkaline environments in Iran is not well understood. 4. Palygorskite is a predominant mineral in clay fractions of many soils in central and southern Iran. However, no report is available on the effects of this clay mineral on soil properties. 5. Although deposits of fibrous clay minerals have been reported in Iran, there are no detailed studies about the genesis, distribution and characteristics of these deposits. 6. The contribution of eolian processes to the occurrence of palygorskite in Iranian soils needs to be investigated. Besides, information about the contribution of Iranian soils and sediments to fibrous clays in the atmospheric dust would be greatly needed by both pedologists and environmental scientists.

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ACKNOWLEDGEMENT Comments on an earlier version of this chapter by an anonymous reviewer are greatly appreciated.

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Heidari, A., Mahmoodi, S., Roozitalab, M.H., Mermut, A.R., 2008. Diversity of clay minerals in the Vertisols of three different climatic regions in western Iran. J. Agric. Sci. Technol. 10, 269–284. Henderson, S.G., Robertson, R.H.S., 1958. A Mineralogical Reconnaissance in Western Iran. Resource Use Ltd., Glasgow, UK. Hessami, K., Koyi, H.A., Talbot, C.J., Tabassi, H., Shabanian, E., 2001. Progressive unconformities within an evolving foreland fold-thrust belt, Zagros Mountains. J. Geol. Soc. Lond. 158, 969–981. Hojati, S., Khademi, H., Faz Cano, A., 2010. Palygorskite formation under the influence of a saline and alkaline groundwater in central Iranian soils. Soil Sci. 175, 303–312. Jamshidi, A., Abtahi, A., 2001. Formation of calcic, gypsic and salic horizons in selected soils of southern arid and semi-arid regions of Fars Province. Iran. J. Soil Water Sci. (Special issue), 95–107. Kadir, S., Eren, M., 2008. The occurrence and genesis of clay minerals associated with Quaternary caliches in the Mersin area, southern Turkey. Clays Clay Miner. 56, 244–258. Karimi, A., Khademi, H., Jalalian, A., 2009. Genesis and distribution of palygorskite and associated sediments of southern Mashhad. Iran. J. Crystal. Miner. 16, 545–558. Karimzadeh, H.R., Jalalian, A., Khademi, H., 2004. Clay mineralogy of gypsiferous soils developed on different landforms in the eastern part of Isfahan. J. Sci. Tech. Agric. Nat. Res. 8, 73–92. Khademi, H., Hojati, S., 2010. Distribution and genesis of palygorskite in selected Tertiary deposits of central Iran. Iran. J. Crystal. Miner. 18, 113–124. Khademi, H., Mermut, A.R., 1998. Source of palygorskite in gypsiferous Aridisols and Associated sediments from central Iran. Clay Miner. 33, 561–578. Khademi, H., Mermut, A.R., 1999. Submicroscopy and stable isotope geochemistry of carbonates and associated palygorskite in Iranian Aridisols. Eur. J. Soil Sci. 50, 207–216. Khormali, F., Abtahi, A., 2001. Soil genesis and mineralogy of three selected regions in Fars, Bushehr and Khuzestan Provinces of Iran, formed under highly calcareous conditions. Iran Agric. Res. 20, 67–82. Khormali, F., Abtahi, A., 2003. Origin and distribution of clay minerals in calcareous arid and semi-arid soils of Fars Province, southern Iran. Clay Miner. 38, 511–527. Khormali, F., Abtahi, A., Owliaie, H.R., 2005. Late Mesozoic-Cenozoic clay mineral successions of southern Iran and their paleoclimatic implications. Clay Miner. 40, 191–203. Krinsley, D.B., 1970. A Geomorphological and Paleoclimatological Study of the Playas of Iran. Geological Survey, United States Department of Interior, Washington, DC 488 pp. Llewellyn, P.G., 1973. Dezful Geological Compilation Map No. 20507. Scale 1:250000. Geological and Exploration Division, Iranian Oil Operating Companies, Tehran. Mahjoory, R.A., 1979. The nature and genesis of some salt affected soils in Iran. Soil Sci. Soc. Am. J. 43, 1019–1024. Manouchehri, M., 1987. Mashhad Geological Quadrangle Map of Iran no. K4. Scale 1:250000. Ministry of Industry and Mines and Geological Survey of Iran, Tehran. Modarres, R., Silva, V.P.R., 2007. Rainfall trends in arid and semi-arid regions of Iran. J. Arid Environ. 70, 344–355. Murray, H.H., 2007. Applied Clay Mineralogy: Occurrences, Properties and Applications of Kaolins, Bentonites, Palygorskite-Sepiolite, and Common Clays. Elsevier, Amesterdam, 179 pp. Namik Cagatay, M., 1990. Palygorskite in the Eocene rocks of the Dammam Dome, Saudi Arabia. Clays Clay Miner. 38, 299–307. Neaman, A., Singer, A., 2004. The effects of palygorskite on chemical and physico-chemical properties of soils: a review. Geoderma 123, 297–303.

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Owliaie, H.R., Abtahi, A., Heck, R.J., 2006. Pedogenesis and clay mineralogical investigation of soils formed on gypsiferous and calcareous materials, on a transect, southwestern Iran. Geoderma 134, 62–81. Paquet, H., Millot, G., 1972. Geochemical evolution of clay minerals in the weathered products in soils of Mediterranean climate. In: Serratosa, J.M. (Ed.), Proceeding of International Clay Conference 1972. 199–206, Madrid. Salehi, M.H., Khademi, H., Karimin Eghbal, M., 2003. Identification and genesis of clay minerals in soils of Farokhshahr area, Chaharmaham Bakhtiari Province. J. Sci. Tech. Agric. Nat. Res. 7, 73–89. Samadian, A., 1996. Chabahar Geological Quadrangle Map of Iran No. 8140. Scale 1:100000. Ministry of Industry and Mines and Geological Survey of Iran, Tehran. Sengo¨r, A.M.C., Altiner, D., Cin, A., Ustao¨mer, T., Hsu¨, K.J., 1988. Origin and assembly of the Tethyside orogenic collage at the expense of Gondwana. In: Audley-Charles, M.G., Hallam, A. (Eds.), Gondwana and Tethys. Geological Society Special Publication No. 37. Oxford University Press, Oxford, pp. 119–181. Shadfan, H., Dixon, J.B., 1984. Occurrence of palygorskite in the soils and rocks of the Jordan Valley. In: Singer, A., Galań, E. (Eds.), Palygorskite-Sepiolite: Occurrences, Genesis and Uses. Developments in Sedimentology, vol. 37. Elsevier, Amsterdam, pp. 187–198. Shadfan, H., Mashhady, A.S., 1985. Distribution of palygorskite in sediments and soils of eastern Saudi Arabia. Soil Sci. Soc. Am. J. 49, 243–250. Singer, A., 1980. The palaeoclimatic interpretation of clay minerals in soils and weathering profiles. Earth Sci. Rev. 15, 303–326. Singer, A., 1981. The texture of palygorskite from the Rift Valley, southern Israel. Clay Miner. 16, 415–419. Singer, A., 1989. Palygorskite and sepiolite group minerals. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environments. Soil Science Society of America, Madison, WI, pp. 829–872. Singer, A., Kristen, W., Bu¨hmann, C., 1995. Fibrous clay minerals in the soils of Namaqualand, South Africa: characteristics and formation. Geoderma 66, 43–70. Yalcin, H., Bozkaya, O., 1995. Sepiolite-palygorskite from the Hekimhan region (Turkey). Clays Clay Miner. 43, 705–717. Zahedi, M., 1976. Explanatory text of the Esfahan Quadrangle Map 1:250000. Geological Survey of Iran, Tehran. Zahedi, M., 1993. Shahrekord Geological Quadrangle Map of Iran No. E8. Scale 1:250000. Ministry of Industry and Mines and Geological Survey of Iran, Tehran.

Chapter 9

Evidence for the Biogenic Origin of Sepiolite Jaime Cuevas, Santiago Leguey and Ana I. Ruiz Departamento de Geologıá y Geoquı´mica, Facultad de Ciencias, Universidad Autońoma de Madrid, Cantoblanco s/n, 28049 Madrid, Spain

1. INTRODUCTION The formation of various authigenic minerals, such as dolomite, magnesite, barite, celestine, Fe-oxides, pyrite and chert, has been described in the lacustrine and fluvio-lacustrine Miocene sediments of the Madrid and Duero Basins (Sanz-Montero and Rodrı´guez-Aranda, 2008; Sanz-Montero et al., 2009a,b). The mineralogical, textural and geochemical data collected for these previous studies have provided evidence of the role of microbial activity in the formation of these authigenic minerals. Leguey et al. (2010) also found indirect evidence of sepiolite formation by biogenically related mechanisms in the dolomite–sepiolite materials that were formed in shallow lacustrine environments from the intermediate unit of the Miocene in the north-eastern area of the Madrid Basin. These sediments contain dolomite aggregates that are reminiscent of mineralized microorganisms with barrel-like or doubledumbbell-shaped morphologies and spherical to tubular voids (termed biomorphs). Additionally, fibrous sepiolite–dolomite intergrowths have been frequently observed to integrate these biomorphs. The aim of this study was to determine similarities between sepiolite or sepiolite–dolomite aggregation morphologies at the micro-nanometre scale and to distinguish biologically originated microstructures. For example, in Figure 1, fossilized cyanobacteria outer membranes (OMs) and sheats are compared to living counterparts with several layers of heteropolymer envelopes (Golubic et al., 2006). The parallel arrangement of sepiolite and dolomite microcrystalline coatings is similar in size and shape to biological structures (Leguey et al., 2010). We cannot take this as definitive proof of the biogenic origin of these minerals, but this similarity can be used, together with additional morphological and chemical data, to contribute to the discussion of the biogenic origin of the studied clay sediments.


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A

B

10 mm C

30 mm

D

10 mm

FIGURE 1 Comparison of fossilized cyanobacteria envelopes (A) and living cyanobacteria heteropolymer sheets (B), both shown by Golubic et al. (2006; Decastronema kotori gen); (C and D) representative biomorphs found in dolomite–sepiolite Miocene sediments in Madrid Basin (Leguey et al., 2010).

Previous studies have suggested a relationship between sepiolite and palygorskite minerals and biological activity (Folk and Rasbury, 2007). Several morphological and biogeochemical signatures had already been determined, including a rosary-like chain of beads forming palygorskite filamentous and tubular forms of silica, the precipitation of sepiolite by the reaction of Mg ions in interstitial water with biogenic silica (Yan et al., 2005) and the sepiolitefilled boxwork textures in coccolith-laminated aggregates that are found in mid-ocean ridge environments (Hathaway and Sachs, 1965). These minerals can be classified as Phanerozoic minerals (Callen, 1984), suggesting that they developed parallel to the diversification of life (Nealson and Conrad, 1999). Considering that ageing and recrystallization change the order and morphology of such minerals, it is not easy to associate these minerals with biological activity. Further, their crystal structure results in a fibrous form that is similar to hairy, organic forms. Therefore, recognition of fossil structures in biomorph arrangements is not simple.

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Sepiolite rock has remarkable strength-to-weight properties and the versatility to perform diverse functions, including absorption-structured support or filtration (Galan, 1996; Ruiz-Hitzky, 2001). These features are often significantly enhanced by an assemblage of polysaccharides, proteins and other biopolymers (Darder et al., 2007). Dove (2010) stressed that this synergistic organization is a unique feature of biominerals. To determine the biogenic origin of sepiolite, we focused on the Mg–Si precipitates in microbial environments. First, we had to assess how bacteria and other microorganisms activate mineralization.

1.1. The Processes of Biomineralization Mineral synthesis by living organisms can be grouped as biologically induced mineralization (BIM; Frankel and Bazylinski, 2003) or biologically controlled mineralization (BCM; Lowenstam, 1981). The BCM process is mainly linked to the ability of organisms to synthesize skeletons and functional biominerals (Dove, 2010), whereas the BIM process is related to the uncontrolled consequences of metabolic byproducts that are primarily produced in prokaryotes. The BIM process has been recently referred to as ‘organomineralization sensu lato’ (Dupraz and Visscher, 2009; Dupraz et al., 2009). This term provides a clearer distinction between BIM and BCM, or ‘biomineralization sensu stricto’. Organomineralization can be (1) active, when microbial metabolic reactions are responsible for the precipitation (BIM) or (2) passive, when mineralization within a microbial organic matrix is environmentally driven (e.g. through degassing or desiccation). In this sense, the direct role of specific biomolecules, such as exopolymeric substances or cell-wall constituents, in mineralization has been recently emphasized (Bontognali et al., 2010). Bacterial surfaces present a complex arrangement of macromolecules. Bacterial cell walls are based on a peptidoglycan (PG) scaffold, with a repeating disaccharide (N-acetyl glucosamine (NAG)–N-acetyl muramic (NAM)) that has a pentapeptide branch attached to each NAM (Meroueh et al., 2006). This macromolecule shows a periodic array of carboxylic and amino functional groups and, in general, is negatively charged (Douglas, 2005; Hunter and Beveridge, 2008). This polymer builds a multilayer envelope from the inner cytoplasmic membrane, whose thickness varies from 20–40 nm in Gram (þ) to 1–3 nm in Gram () bacteria. Porins, made of polypeptides across the PG layer, become the channels for the uptake of nutrients. Cyanobacteria, the most diverse group of bacteria that are adapted to every terrestrial environment, posses a cell envelope that is a combination of these Gram (þ) and Gram () cellwall multilayers (Hoiczyk and Hansel, 2000). The PG layer is overlain by a lipopolysaccharide (LPS)/protein layer OM, which is often covered with S-layer glycoproteins. In response to environmental factors (e.g. UV light, dehydration), many bacteria produce diverse external carbohydrate structures, exopolysaccharides and other extracellular heteropolymers. These exopolymeric

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substances (EPS) are the main ‘cement’ for these cells, forming biofilms with a majority of polyanionic constitutive macromolecules (Sutherland, 2001). Negative charges in bacterial cells and EPS can result in cation binding by non-specific electrostatic interactions, contributing to the catalysis of dissolution–precipitation reactions as a result of reduced energy barriers for nucleation (Beveridge, 1989). The degradation of such macromolecular arrays that are full of binding sites, or the change in their chemistry in response to dryness, drives periodic events of cation release and supersaturation. Wright and Wacey (2005) described this situation in the Australian Coorong lagoons in relation to dolomite mineralization by means of sulphate-reducing bacteria (SRB). Such situations can be generally defined as passive biomineralization processes as long as the ionic properties of these biomass materials are maintained in living and dead cells (Hunter and Beveridge, 2008). In contrast, active mineralization generally occurs by the direct redox transformation of surface-bound metal ions and by the formation of anionic byproducts of metabolic activities on the bacterial surfaces. SRB bacteria oxidize soluble organic compounds via sulphate respiration (sulphate acts as the electron acceptor), thus producing H2S and CO2. The inhibition of MgSO40 pair formation and the increase of carbonate anions in solution favour dolomite or Mg-calcite precipitation (Sanchez-Roman et al., 2008; Van Lith et al., 2003; Vasconcelos and McKencie, 1997). Organomineralization patterns are mediated by the stabilization of a biofilm community distributed in organomineral layers called microbial mats. Phototrophs are located near the surface because they require light for metabolic activity. They provide organic carbon in the form of SRB or methanogens for heterotrophs or strict anaerobes located in subsurface environments (Visscher et al., 2010). This layered community is responsible for stromatolite formation by progressive lithification due to the precipitation of fine-grained minerals in association with cells and heteropolymer substances. Carbonates (mainly calcite) are the minerals that form modern microbialites situated in tropical marine, quasi-marine or extreme environments, such as alkaline or hypersaline lakes and thermal springs (e.g. Lo´pez-Garcıá et al., 2005). Magnesium–silicate mineral phases are seldom related to cyanobacterial mats or benthic communities in saline or saline alkaline wetlands, but they are closely related to EPS material (Benzerara et al., 2010; Bontognali et al., 2010). Magnesium–silicate layers (kerolite) and amorphous coatings remain in carbonate microbialites (Arp et al., 2003; Le´veille´ et al., 2002) and have been localized in association with extracellular polymeric substances during the fossilization of green algae and cyanobacteria cell walls (Pacton et al., 2009; Souza-Egypsy et al., 2005). Moreover, the concentration of Mg and Si in solution drops dramatically during the organomineralization of Mg-carbonates (Power et al., 2007). Previous studies have suggested that the presence of these Si–Mg coatings within carbonate precipitates is a signature of microbiological activity (Souza-Egypsy et al., 2005). In conclusion,

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these precipitates, under certain circumstances, can drive precipitation and the concentration of Mg-silicates as sepiolite or palygorskite minerals.

2. BIOMORPHS IN THE DOLOMITE–SEPIOLITE SEDIMENTS FROM THE MIOCENE IN THE MADRID BASIN Leguey et al. (2010) studied two sepiolite layers that represented an alternating sedimentary sequence of clastic detrital sediments that were mainly composed of lutites (fine-grained illite–smectite, quartz and feldspars) overlain with either a black chert or a dark green lutite (Figure 2). These materials are rich in organic debris and were interpreted to have formed in palaeosoil environments. Above these layers, dolomitic (stoichiometric dolomite) sediments, corresponding to ephemeral lake environments, are overlain on the

Mineralogy 556 44°N

z (m) 555

554

42°N

40°N

552 38°N

550 upper level

N 8°O

4°O

3 550

0°O

sepiolite

Sep > 80

2 1

sepiolite 545 smectite

cota (m)

548

546

lllite di-, tri 540

dolomite

sepiolite clay

538

3

536 lutite 535 lower level 530

quartz marl chert dolostone

534

Ephemeral lakes

2

Paleosoils/ponds

532

1 530

Gypsum–dolomite sediments

Sep60 -D30

Clastic sediments: mud flat/floodplain

0 20 40 60 80 100%

FIGURE 2 Lithology and mineralogical features from a sedimentary sequence sampled from boreholes in the north-eastern region (Barajas, Madrid) of the intermediate unit of the Miocene in the Madrid Basin. Cenozoic Basins are marked with yellow in the Iberian Peninsula map (Alonso-Zarza et al., 2004). Sepiolite (Sep > 80), dolomitic marls (Sep60-D30) and lutitic sediments (brown regions in figure) alternate in the sequence (from Leguey et al., 2010). Photographs outline the contrast of black or dark green chert and lutites, respectively, to pale brown sepiolite materials. Photographs are taken from hand specimens. 1, 2 and 3 refers to the sequence of clastic sediments (1), palaeosoils (2) and ephemeral lakes (3).

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sepiolitic sediments, which are ultimately buried by illite–smectite lutites (mainly dioctahedral) and capped by fine sands or arkosic sediments. There is evidence that the dolomite sediments from lacustrine sediments in Spain experienced biological activity. This evidence is based on the identification of domal stromatolitic structures, the presence of nanometre-scale spheroids (Pacton et al., 2010; Sanz-Montero et al., 2008) and the relatively low d13C values in the dolomite replacing the gypsum (Sanz-Montero et al., 2006). The microbial origin of dolomite in modern lacustrine sediments from La Roda (Albacete, Central Spain) has been documented by Garcia del Cura et al. (2001). Dolomite–gypsum associations in Miocene evaporitic lake deposits from the Madrid Basin have attributed to microbial communities (Sanz-Montero et al., 2006). However, the possible link between Madrid sepiolite deposits and biological activity has not been investigated.

2.1. Recognition of Structures Reminiscent of Microorganisms The base of the sedimentary sequence starts with a dolocrete–gypsum evaporitic sediment (Figure 2). The first locus that shows sepiolite within a typical microbial-mediated mineral system is the interface between gypsum fibrous crystals and dolomitic mud relicts. These mud-filled pores break the fibre alignment of crystals, indicating that the pores are produced by dissolution of gypsum (Figure 3). At the gypsum-pore interface, a small fibrous net, identified as magnesium silicate (sepiolite), can be observed. The formation of dolomite at sulphate-reducing interfaces (sulphides and organic matter) in the presence of SRB has been previously demonstrated (e.g. Douglas, 2005; Van Lith et al., 2003) and is connected with the development of slightly alkaline media. The presence of sepiolite could be based on the anomalous association of a clay-rich environment and an evaporitic environment. These environments are not commonly held together in actual dolomite evaporitic environments (e.g. Wright and Wacey, 2005 in Coorong, Australia; De la Penã and Marfil, 1986 in saline lakes from La Mancha, Spain). The first layer in which sepiolite was concentrated at above 50% of the weight in the studied sediments was located above a black chert layer (10 cm in thickness) that lies over a clastic biotitic silt bed. The chert, crossed by chalcedony recrystallized veins (submillimetric), includes cavities (centimetres in size) with sepiolite coatings. In contrast, the chert nodules are inserted in a massive sepiolite matrix in higher layers. We recorded several singular microtextures and biomorphs in the chert–sepiolite interface. Microquartz rosary bead chain fabrics were observed in some chert surfaces at the cavities (Figure 4A). Silica–sepiolite hollowed cylinders or worm-like aggregates (< 10 mm size; Figure 4B) were commonly found. Additionally, barrel-like forms made of oriented quartz crystals of micron sizes were grouped randomly in the chert pores (Figure 4C). These crystals were composed of pyramidal quartz, with intergrowths of sepiolite filaments

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FIGURE 3 Si–Mg filaments at the gypsum–dolomite interface. The SEM device was a PHILIPS-XL 30 electron microscope equipped with an energy-dispersive X-ray (EDX) analyzer. Samples were coated with Au in a vacuum device for 30 s at 1.5 kV.

terminating in nanometric silica caps (Figure 4D). Magnesium and Si mapping at the microcrystal scale confirmed the presence of sepiolite. This diversity of crystal growth styles is related to the presence of selective chemical inhibitors, micro-textured substrata (presumably organic) or skeletal biomineralization (i.e. Xu et al., 2007). The formation of exotic self-assembled filament forms (dendritic to helicoidal) in carbonate–silica chemical environments was documented by Garcıá-Ruiz et al. (2009). The presence of diatoms in nearby sediments was documented by Calvo et al. (1988) and Pozo and Lo´pez (2004). The formation of extensive micro-networks of EPS chitinbased heteropolymers has been found in barrel-like diatoms (Brunner et al., 2009). Although all these possibilities can occur in this silica-rich environment, specific common morphological features were found in the sepiolite– dolomite marl located above the chert. Hollow or not, cylindrical biomorphs, similar to the microquartz barrels (Figure 5A), were located above the chert in the sepiolite–dolomite marl. In these samples, the biomorphs were composed of stoichiometric dolomite and filled a subparallel network of white veinlets. The biomorphs consisted of micron- to submicron-sized dolomite crystals. Some showed structures of layers with sepiolite coatings resembling concentric blankets (Figures 1D and 5B). We found a predominance of ovoid aggregate forms (Figure 5C)

226


FIGURE 4 Biomorphs at the chert–sepiolite interface: (A) quartz rosary bead chains intercrossed, (B) hollowed cylinders and worm-like forms wrapped by sepiolite, (C) barrel-like microquartz aggregates and (D) detail of the barrel surfaces. Images (C) and (D) were taken from samples that were observed using FESEM NOVA NANOSEM 230 equipment at low-voltage conditions. Samples were coated with Au in a vacuum device for 30 s at 1.5 kV.

in the sepiolite matrix and found wavy parallel coatings of sepiolite (shown in Figures 1 and 5D). The cylinder or barrel-like biomorphs were often isolated in the sepiolite matrix of the sepiolite layer on top of the studied sedimentary sequence. All of the biomorphs were hollowed, sometimes preserving the microcrystalline oriented structure of dolomite (Figure 5E) or filled with tiny sepiolite networks, but always showing evidence of the recrystallization of dolomite in the outer envelope (Figure 5F).

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227

FIGURE 5 Biomorphs found in the sepiolite layers: (A) dolomite aggregates in double-barreldumbbell forms, (B) sepiolite-parallel coating in the dolomite biomorph, (C) ovoid sepiolite aggregates in the sepiolite upper layer (SUL), (D) parallel accumulation of sepiolite nets in SUL, (E) the recrystallized surface of dolomite biomorph in lutites near the SUL and (F) hollowed, altered and recrystallized biomorph within the SUL. The elongated, white crystals corresponding to Sr-rich barite in (B) were in agreement with similar forms recorded by Sańchez-Pastor et al. (2006). Samples were coated with Au in a vacuum device for 30 s at 1.5 kV.

228


The oriented growth of individual micron-sized crystals, structured as ‘mesocrystals’, is typically found in processes in which organic polymers control the growth direction and shape of individual crystals (Co¨lfen and Antonietti, 2008). These crystals are not structured as spherical peloids, in which concentric mineral rims are expected (Bosak et al., 2004). In contrast, they maintain a cylindrical (barrel-like or dumbbell-like) shape, regular size and the shape of biomorphs with submicron platy dolomite crystals (surface area of dolomite: 20–30 m2/g; Cuevas, J., unpublished data) with internal cellular textures. This organization is clearly more ordered than a random assembly of organomineral coatings. These morphologies are typical of oriented intergrowths of carbonates combined with EPS (e.g. Mg-calcite and dolomite, Van Lith et al., 2003; vaterite, Rodriguez-Navarro et al., 2007; and calcite– aragonite, Yin et al., 2009). The existence of previous structured substrata, such as a bacterial colony (filamentous) or EPS extracellular envelopes, could cause the bioreminiscent arrangement. Evidence for this may be the relative non-specificity of nucleating silica or carbonates to produce similar aggregation morphologies. In contrast, the sizes of individual quartz or dolomite crystals and internal pores in the biomorphs are compatible with microorganism sizes (< 2 mm). In dolomite, interwoven fibres of sepiolite are displayed linearly or resemble the extracellular material lining that has been observed to form around cyanobacterial extracellular envelopes (Si–Mg coatings, Souza-Egypsy et al., 2005).

3. RECOGNITION OF MINERAL FORMATION PROCESSES 3.1. Mineralization of Biomass In this study, we were searching for organics or biomass relics to determine the role of biomass degradation in the biomorph-forming process. The sediments located near the upper sepiolite layer (> 90% Sep in Figure 2) contained abundant particles of organic debris, most in the form of fibrous strands. More than 100 scanning electron microscope (SEM)–energy-dispersive X-ray (EDX) analyses were performed on filament-like fibres and related organic relics (Figure 6). Organic particles have a complex chemistry, and significant contributions of Na, Ca, K, Cl, S and Fe need to be determined. Alkaline elements, together with chloride and sulphate, were the signatures of the saline mineralizing fluids that were present in the lacustrine environment. In contrast, most filaments were made of sepiolite compositions, but Si/Mg in the biomass was significantly higher than in the sepiolite filaments. The surface of the fibrous organic strands was structured as parallel microfibrils, similar to the cellular walls of algae or plants (Figure 7). The surface of this organic material contained groups of a few individual crystals of dolomite, which included a microfibril matrix during their growth. The formation of dolomite during the degradation of organic matter is a typical signature of bacterial mineralization (e.g. SRB).

Chapter

9

229


75

Biomass Weight% of elements as oxide components

Filaments 60

45

30

15

0 Na2O MgO Al2O3 SiO2 SO42–

Cl–

K2O

CaO

TiO2 Fe2O3 PO43–

FIGURE 6 Average composition of EDX analyses taken from biomass relics and surrounding filament or fibrous forms. Analysis results are expressed as weight percentage of oxide components or salt anions Leguey and Cuevas, 2010.

FIGURE 7 Biomass strands and dolomite nucleation, with details of the microfibrils within the dolomite crystals. Punctual analysis showed stoichiometric dolomite with small quantities of Si ( 5 mm). There is inadequate evidence in experimental animals for the carcinogenicity of short fibres (< 5 mm). IARC’s overall evaluation concluded that ‘sepiolite cannot be classified as to its carcinogenicity to humans (Group 3)’. For occupational exposures, sepiolite is regulated by the United States Occupational Safety and Health Administration (OSHA) (1995) under the inert or nuisance dust standard. Its permissible exposure limits are 15.0 mg/ m3 total dust and 5.0 mg/m3 respirable particles. Presently, neither Mine Safety and Health Administration (MSHA) nor OSHA treats sepiolite as a toxic material.

REFERENCES Dudley, W.W., Larson, J.D., 1976. Effect of Irrigation Pumping on Pupfish Habitats in Ash Meadows, Nye County, Nevada. U.S. Geological Survey Professional Paper 927, pp. 1–52. Eberl, D.D., Jones, B.F., Khoury, H.N., 1982. Mixed layer kerolite/stevensite from the Amargosa Desert, Nevada. Clays Clay Miner. 30, 321–326. Environmental Protection Agency (EPA), 1982.

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Amargosa Valey Sepiolite and Saponite

277

Galan, E., 1996. Properties and applications of palygorskite–sepiolite clays. Clay Miner. 31, 443–453. Grim, R.E., 1968. Clay Mineralogy, second ed. McGraw-Hill Book Company, New York. Hay, R.L., Peston, R.E., Teague, T.T., Kyser, T.K., 1986. Spring-related carbonate rocks, Mg clays, and associated minerals in pliocene deposits of the Amargosa Desert, Nevada and California. Geol. Soc. Am. Bull. 97, 1488–1503. International Agency for Research on Cancer, 1997. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Silica, Some Silicates, Coal Dust and Para-Aramid Fibrils. World Health Organization, Lyon, France, pp. 1–508. Khoury, H.N., Eberl, D.D., Jones, B.F., 1982. Origin of magnesium clays from the Amargosa Desert. Clays Clay Miner. 30, 327–336. Miles, W.J., 2010. Unpublished data. Naff, R.L., Maxey, G.B., Kauffmann, R.F., 1974. Interbasin Ground Water Flow in Southern Nevada. Nevada Bureau of Mines Report. 20, p. 1–28. Occupational Safety and Health Administration (OSHA), 1995. General Industry - Safety and health Standards (29 CFR 1910). Papke, K.G., 1970. Montmorillonite, bentonite, and Fuller’s earth deposits in Nevada. Nevada Bur. Mines Bull. 76, pp. 1–48. Papke, K.G., 1972. A sepiolite-rich playa deposit in Southern Nevada. Clays Clay 20, 211–215. Papke, K.G., 1999. Site Geology and Mineral Deposits. Notes and data not published. Post, J.L., 1978. Sepiolite deposits of the Las Vegas, Nevada area. Clays Clay Miner. 26, 58–64. Regis, A.J., 1978. Mineralogy, physical, and exchangeable chemistry properties of bentonites from the Western United States, exclusive of Montana and Wyoming. U.S. Bur. Land Management Tech. Note 315. Santaren, J., Alvares, A., 1994. Assessment of the health effects of mineral dusts. The sepiolite case. Ind. Miner. 319, 101–114. Winograd, I.J., Thordarson, W., 1975. Hydrogeologic and hydrochemical framework, SouthCentral Great Basin, Nevada-California with special reference to the Nevada test site. U.S. Geological Survey Professional Paper.

Chapter 12

Current Industrial Applications of Palygorskite and Sepiolite Antonio A´lvarez*, Julio Santareń*, Antonio Esteban-Cubillo* and Patricia Aparicio{ *Technological Innovation Department, Tolsa S.A., Carretera de Madrid a Rivas Jarama, 35. 28031 Madrid, Spain { Departamento de Cristalografıá, Mineralogıá y Quı´mica Agrıćola, Facultad de Quı´mica, Universidad de Sevilla. C/ Prof. Garcıá Gonza´lez 1. 41012 Seville, Spain

1. INTRODUCTION The various uses of sepiolite are suggested in Figure 1 from Robertson (1957). This ‘wish-list’ of varied uses (not all were actually realized at the time of compilation) is mostly dependent on the sorptive, optical, rheological and molecular sieve properties of the mineral. The applications were divided into two broad categories, colloidal and non-colloidal, by Haden (1963). Colloidal properties result when the particles are dispersed in a liquid medium to the extent that the individual elongated needles are capable of more or less independent motion relative to one another. In the non-colloidal case, the needles form rigid particles, each of which comprises many discrete needles. ´ lvarez (1984) indicated that with the appropriate pretreatments, sepiolite A could be useful in many applications: absorbents, environmental deodorants, catalyst carriers, polyesters, asphalt coatings, paints, pharmaceutical uses, discolouring agents, filter aids, anticaking agents, phytosanitary carries, cigarette filters, plastisols, rubber, animal nutrition, detergents, cosmetics, agriculture (soil conditioning, fluid carriers for pre-germinated seeds, seed coating, fertilizer suspensions), grease thickeners, NCR paper, drilling fluids, etc. Table 1 provides a recent summary of the many uses of palygorskite and sepiolite (Galań, 1996; Murray, 2005). During the last part of the twentieth century and the beginning of this century, the applications usually required special pretreatments to improve their properties and develop new applications. The aim of this chapter is to give an overview of the applications and markets of the products that are currently manufactured with sepiolite and to a lesser extent palygorskite, with special emphasis on the new applications and those that are being developed and are expected to become a reality in Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00012-8 # 2011 Elsevier B.V. All rights reserved.

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Purifying petroleum products, viz, lubricating oil, transformer and other electrically insulating oils, shale oil, waxes, used lubricating oil

Glucose refining. sugar refining including Desiccant, waste sugar recovery, e.g, for wine and cyder fining, coccooning, vinegar, gelatine laboratory and glycerine balances, purifying food packages, sweet jars Conditioning Aqueous Non-aqueous agent for media media salt, polyMoistSolvent are phosphates, recovery Aerogels vapour fertilisers, Adsorptive and etc. Organic aerosols Pesticides, Sorptive vapours especially AbsorptSeparation of waxy types ive gases and vapours Sieving of Tobacco Massive including SEPIOLITE molecules and types hydrocarbons, toughen ions Epoxy and ions from -ing Rheologi Filtration, resins solution -cal flocculation & Suspending etc. Opticlarification Removal of cal Chemical Pharmaceutinickel and colour and Gelling cals cosmetics from hardened oils Pigmentary thermal paints Removal of pitch end flatting inks from paper stocks properties polishes Holder of chemicals in no-carbon paper and other copying systems

Greases

Paints Wax polishes

Refractory and ceramic bodies Insulating blocks

FIGURE 1 Utilization diagram for sepiolite (Robertson, 1957).

the near future. In this report, a review of principal physico-chemical properties will be presented and the most important applications of sepiolite and palygorskite will be classified in six categories according to the type of industrial processing required to prepare them for market. The last part of the chapter will be devoted to health and safety issues.

2. PHYSICO-CHEMICAL FEATURES Sepiolite and palygorskite are considered to be ‘special’ clays as a consequence of their modulated, fibrous structures. This unusual crystal structure is mainly responsible for their unique physico-chemical properties (Table 2), and important characteristics related to surface area, porosity, dehydration and high-temperature phases and sorption active centres. The wide range of industrial applications of sepiolite and palygorskite can be classified as sorptive, rheological and catalytic.

Chapter

12

Current Industrial Applications of Palygorskite and Sepiolite

TABLE 1 Applications of Palygorskite and Sepiolite. Pet Litter

Drilling Muds

Industrial and floor absorbents

Fluid suspension fertilizers

Agricultural carrier

Liquid animal feeding

Catalyst supports

Filler for polymers and elastomers

Animal feed binders

Cosmetics

Free flowing agent

Paints and coatings

Anticaking additive

Mineral greases

Premix carrier

Mortars

Pharmaceuticals

Tile adhesives

Bleaching earth

Shotcrete

Adsorbents

Gypsum compounds

Moisture control

Joint compounds

Waste treatment

Bitumens

TABLE 2 Properties of Palygorskite and Sepiolite. Particle Shape

Needle Like

Length (mm)

0.2–2.0

Width (A˚)

100–300

Thickness (A˚)

50–100

Channels dimensions (A˚)

Palygorskite: 3.7 6.4 Sepiolite: 3.7 10.6 2

Specific surface, BET N2 (m /g)

Palygorskite: 150 Sepiolite: 320

3

Specific gravity (g/cm )

2.0–2.3

Cation exchange capacity (meqiv./100 g)

5 mm) are classified in Group 2B (possibly carcinogenic to humans) because of sufficient evidence in experimental animals. The most important commercial palygorskite deposits have a sedimentary origin and occur in ancient soils, lakes or shallow seas associated mainly with semi-arid to Mediterranean-type climates. The most important deposits are located in the Meigs-Attapulgus-Quincy district of Southwestern Georgia and Northern Florida, USA, followed by the deposits located in

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297

Seńe´gal. Palygorskite from these deposits have a short fibre length (< 5 mm). This type of palygorskite with short fibres has not produce any increased incidence of tumours in animal studies (IARC, 1997). Palygorskite is used in several countries for the treatment of diarrhoea (Engle, 1994). In France, preparations containing palygorskite are used for the treatment of the symptoms of gastroduodenal ulcer or gastritis (Vidal, 1996).

5. CONCLUSIONS Sepiolite has generated over the past years a growing family of unique and interesting minerals additives. The shape of the particles and the large and active surface area are a source of physico-chemical and rheological characteristics useful in a wide variety of systems. The particular surface of this silicate influenced by six generations of processing methods opens new windows for applications that provide new solutions for industrial problems. The wish list compiled in 1957 by Robertson has become a reality by the incorporation of new industrial treatments. Sepiolite is still now, more than ever, a versatile raw material that will continue to play an important role in the decades ahead. New materials based on palygorskite and sepiolite, such as nanoclays, hybrids, bio-hybrids, biomimetic and functional materials, are being developed and studied for different uses that require new and better performance advanced materials.

REFERENCES ¨ ., Celikcapa, S., Dogan, M., 2004. Sorption of acid red 57 from aqueous Alkan, M., Demirbas, O solution onto sepiolite. J. Hazard. Mater. B116, 135–145. ´ lvarez, A., 1984. Sepiolite: properties and uses. In: Singer, A., Galan, E. (Eds.), PalygorskiteA Sepiolite: Occurrences, Genesis and Uses. Elsevier, New York, pp. 253–287. Brigatti, M.F., Galań, E., Theng, B.K.G., 2006. Structures and mineralogy of clay minerals. In: Bergaya, F., Theng, B.K.G., Lagaly, G. (Eds.), Handbook of Clay Science. Developments in Clay Science, vol. 1. Elsevier, Amsterdam, pp. 19–86. Engle, J.P., 1994. OTC advisory: antidiarrheal products. Am. Drug. 8, 48–50. Esteban-Cubillo, A., Pina-Zapardiel, R., Moya, J.S., Barba, M.F., Pecharromań, C., 2008. The role of magnesium on the stability of crystalline sepiolite structure. J. Eur. Ceram. Soc. 28, 1763–1768. Galań, E., 1996. Properties and applications of palygorskite-sepiolite clays. Clay Miner. 31, 443–453. Haden Jr., W.L., 1963. Attapulgite: properties and uses. In: 10th National Conference on Clays and Clay Minerals, pp. 284–290, NRC-NAS Monograph No. 12. IARC, 1997. Silica. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 68, 24 pp. Jimeńez-Lo´pez, A., Lo´pez-Gonzalez, J.D., Ramirez-Saenz, A., Rodrı´guez-Reinoso, F., Valenzuela-Calahorro, C., Zurita-Herrera, L., 1978. Evolution of surface area in a sepiolite as a function of acid and heat treatments. Clay Miner. 13, 375.

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Krekeler, M.P.S., Guggenheim, S., 2008. Defects in microstructure in palygorskite–sepiolite minerals: a transmission electron microscopy (TEM) study. Appl. Clay Sci. 39, 98–105. Murray, H.H., 2005. Current industrial applications of clays (Abstract). In: Proceedings of the 13th International Clay Conference, Tokyo, Japan, p. 55. Murray, H.H., 2007. Applied Clay Mineralogy: Occurrences, Processing and Applications of Kaolins, Bentonites, Palygorskitesepiolite, and Common Clays. Developments in Clay Science, vol. 2. Elsevier, Amsterdam, 188 pp. Oulton, T.D., 1963. Commercial production and uses of attapulgite clay products. Georgia Miner. Newsl. XVI (1–2), 26–28. ¨ zdemir, M., Kipcak, I., 2004. Dissolution kinetics of sepiolite in hydrochloric acid and nitric O acid. Clays Clay Miner. 52, 714–720. Robertson, R., 1957. Sepiolite: a versatile raw material. Chem. Ind. 16, 1492–1495. ´ lvarez, A., 1994. Assessment of the health effects of mineral dusts. The sepiolite Santareń, J., A case. Ind. Miner. 1–12, April. Vicente-Rodrı´guez, M.A., Suarez, M., Banãres-Munõz, M.A., Lopez-Gonzalez, J.D., 1996. Comparative FT-IR study of the removal of octahedral cations and structural modifications during acid treatment of several silicates. Spectrochim. Acta A 52, 1685–1694. Vidal, 1996. Dictionnaire Vidal, 72nd Ed., Paris, Editions du Vidal, pp. 7, 673, 1041.

Chapter 13

Pharmaceutical and Cosmetic Uses of Fibrous Clays Alberto Lo´pez-Galindo*, Ce´sar Viseras*{, Carola Aguzzi{ and Pilar Cerezo{ *Instituto Andaluz de Ciencias de la Tierra, IACT, CSIC—Univ. Granada, Avda. Palmeras, 4. 18100 Armilla, Granada, Spain { Departamento de Farmacia y Tecnologıá Farmaceútica, Facultad de Farmacia, Universidad de Granada, 18071 Granada, Spain

1. INTRODUCTION Clays are substances found throughout the Earth’s crust. Given their ubiquity and special characteristics, man has used some of them since very ancient times for their therapeutic effects, in particular, talcum, kaolinite, smectites and fibrous clays (see reviews by Carretero, 2002; Carretero et al., 2006; Cornejo, 1990; Galań et al., 1985; Lo´pez-Galindo and Viseras, 2004 and references therein). The main pharmacopoeias (British Pharmacopoeia (2009); EP 6.0, 2008; USP 32—NF 27, 2009) and the best recognized treatises on pharmaceutical excipients and cosmetic manuals (Braun, 1994; Rowe et al., 2006; Sweetman, 2007; Wenninger et al., 2000) all contain references to these phyllosilicates, and the journal Applied Clay Science recently published a monographic edition dealing with various matters concerning clays and health (Carretero et al., 2007). The applications of clays are favoured by their colloidal size and crystalline structure. The specific function they have in any formulation depends on both their physical properties (particle size and shape, specific surface area, texture, colour and brightness) and their chemical features (surface chemistry and charge). Phyllosilicates can be used in their natural state or after treatment by various physical or chemical processes designed to enhance a particular property. For use in pharmaceutical products and cosmetics, clays must fulfil a number of requirements regarding their chemical properties (stability, chemical purity and inertia, i.e. not reacting with any of the other components in the formulation or the conditioning materials), physical properties (texture, water content, particle size), toxicological properties (atoxicity, microbiological Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00013-X # 2011 Elsevier B.V. All rights reserved.

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safety and purity) and organoleptic properties (colour and taste, affecting patient’s acceptance), all of which are generally met by fibrous clays. These clay minerals occur as odourless and tasteless, fine, white-coloured powders free from grittiness. The tests included in the various pharmacopoeias do not usually distinguish between the ultimate use to be made of a product, and clays used for pharmaceutical (treatment) or cosmetic (care and beauty) purposes are usually taken as one. However, their intended usage should be specified, as it determines both technical aspects of their preparation and questions regarding code of practice and legal matters. In fact, there are numerous directives, reports and regulations and norms on clays for pharmacological or cosmetic use (see Lo´pez-Galindo et al., 2007). Specialists in fields such as geology, mineralogy, geochemistry, pharmacy or medicine should collaborate on the correct characterization of the physical and chemical properties of clays used in formulations and correlate them with their biological effects. Among the phyllosilicates used in pharmacy, sepiolite and palygorskite, often known as attapulgite, particularly in trade circles, present special characteristics (antidiarrhoeic or antacid, for instance) that make them ideal for use as excipients in pharmaceutical technology and dermopharmacy, and as substances with suitable pharmacological activity (Table 1). The elongated shape and small particle size of these two minerals give them unique colloidal properties, with a large surface area and high porosity when naturally or thermally activated, and special resistance to high concentrations of electrolytes. When used as excipients, these minerals facilitate the administration of the active agents, improve their efficiency and ensure stability until the expiry date for use by the patient. Due to their biological activity, fibrous clays are also used as active agents in formulations requiring absorbent, sterilizing, anti-inflammatory or detergent products. They are therefore found in forms of administration that are solid (pills, capsules, granules and powders), liquid (suspensions and emulsions) and semi-solid (ointments, balms, creams, etc.), either for topical application or for oral administration.

2. MINERALOGY, CHEMISTRY AND HABIT Unlike other clay minerals, sepiolite and palygorskite have a fibrous morphology as the result of the 180 inversion occurring in every 6 or 4 silicon tetrahedra, respectively, creating a structure of chains aligned parallel to the a axis, each of which has a 2:1 type structure. This particular three-dimensional ˚ (palygorskite) and ordering also creates open channels with 3.7 6.4 A ˚ 3.7 10–6 A (sepiolite) cross sections containing both ‘zeolitic’ and crystallization water. The fibrous crystalline habit and the presence of zeolitic channels are the main reasons why sepiolite and palygorskite have such notable rheological properties as well as adsorbent properties. These characteristics

TABLE 1 Fibrous Clay Minerals Used in Health Care Preparations.

Mineral Name

Empirical Formula

Palygorskite

(Mg, Al, Fe)5 (Si, Al)8 O20 (OH)2 (OH2)4 (H2O)4

Sepiolite

Mg8 Si12 O30 (OH)4 (OH2)4 (H2O)8

General Chemical Denomination, and CAS and EINECS Numbers Hydrated magnesium aluminium silicate

Pharmaceutical Name Attapulgite Activated attapulgite

Definitions Purified native hydrated magnesium aluminium silicate

12174-11-7; 12174-286; 1337-76-4; 3718950-7; 302-243-0

Colloidalactivated attapulgite

Heat-treated magnesium aluminium silicate

Hydrated magnesium aluminium silicate

Magnesium trisilicate

MgO (20%) and SiO2 (45%) with warying proportion of water/ Mg2Si3O8H2O with MgO (29%) and SiO2 (65%)

18307-23-8; 1319-21-7; 12639-43-9; 14987-043; 15501-74-3; 3936587-2

Other Usual Brand Name

Commercial Products for Health Care

Attapulgite, Activated Attapulgite, Attaclay, Attacote, Attagel, Attapulgus clay, ttasorb, Diluex, Permagel, Pharmasorb-colloidal, Zeogel

AttaclayÒ, AttagelÒ, etc.

Magnosil, Meerschaum, Parasepiolite, Petimin, Silicic acid, Hydrated magnesium salt, Sea foam, Milcon, Hexal, Pangel, Pansil, Quincite

Alenic, AlkaÒ, RecipÒ, etc.

Pharmasorb colloidalÒ

302


make them clearly different from the other phyllosilicates and give them significant economic value. Sepiolite and palygorskite of industrial interest usually are found in sedimentary rocks and are very well represented in some Tertiary continental basins, especially in Spain, USA, Turkey and Senegal. They are often associated with carbonates (calcite and/or dolomite), quartz, mica and smectites. Accessory amounts of amorphous silica, feldspars, kaolinite, gypsum, zeolites, apatite and halite are also found. Layers containing sepiolite can be up to 95 wt% pure, whereas it is unusual to find layers with more than 75% palygorskite. The chemical composition of sepiolite is usually stoichiometric, while palygorskite composition varies widely in the octahedral layer, with very frequent substitution of Mg by Al and Fe, leading Galan and Carretero (1999) to suggest that this mineral is intermediate between di- and trioctahedral phyllosilicates. It is likewise frequent to find small amounts of CaO (up to 2.3 wt%), K2O (up to 0.8 wt%) and Na2O (up to 0.7 wt%). Trace element contents also differ between the two minerals and are usually lower for sepiolite (total content less than 100 ppm in sepiolite and around 500 ppm in palygorskite, Torres-Ruiz et al., 1994). Post and Crawdord (2006) recently showed that in some cases, there can be hazardous amounts of As in palygorskite and As, Pb and Cd in sepiolite. Textural analyses have shown that sepiolite and palygorskite fibres vary considerably in shape and size from one deposit to another and even among samples from the same deposit (Nolan et al., 1991; Torres-Ruı´z et al., 1992). Normally, the elongated particles vary in length from about 1 to 10 mm and are approximately 0.01 mm in width. The most common morphology in sepiolite is that of fibrous aggregates as straw bundles or piles. Average fibre length ranges from 1 to 2 mm, with only a very small proportion (< 1%) of > 5 mm fibres (Bellmann et al., 1997). However, palygorskite crystals form planar or ball-like aggregates, less than 5 mm long, although they can occasionally reach up to 100 mm in length. Although in samples of hydrothermal origin, it is common to find fibre lengths over 20 mm, in sedimentary deposits, which are those usually exploited commercially, 95% of the particles are less than 1.5 mm in length (Lo´pez-Galindo and SańchezNavas, 1989). The air-dried BET-specific surface area of sepiolite is approximately 300 m2/g, and that of palygorskite is 120–180 m2/g, which can increase with heating as the adsorbed and zeolitic water evaporate. Above 300 C, it decreases drastically due to the folding of the crystals and the disappearance of the micropores on the surface. This high specific surface area allows the minerals to absorb over 100% of their own weight of water and liquids of differing polarity. Moreover, the active adsorption centres on the surface (O atoms, water molecules and silanol groups) also allow selective adsorption of different types of molecules. In addition, when the fibre bundles are

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micronized, the resulting particles are easily dispersible in water and other polar liquids, forming a large volume network of interlacing fibres that traps all the fluid, creating highly viscous suspensions.

3. PHARMACEUTICAL AND COSMETIC NOMENCLATURE AND SPECIFICATIONS The various pharmacopoeias, which establish the composition and properties that any particular substance must fulfil for use in pharmaceutical applications, and cosmetics manuals are not uniform in their nomenclature for fibrous minerals, and some ambiguities between mineralogical, chemical and pharmaceutical names can be observed (cf. Lo´pez-Galindo et al., 2007). Specifically, in the British Pharmacopoeia (2009), palygorskite is defined as ‘attapulgite, a purified native hydrated magnesium aluminium silicate, basically consisting in the clay mineral palygorskite’, and ‘activated attapulgite’ is the same product, carefully heated to increase its adsorptive capacity. The USP also includes a monograph for ‘colloidal-activated attapulgite’, and several databases and manuals on the use of ingredients in cosmetic formulations also include ‘attapulgite’ (European Commission, Decision 96/335/EC; Wenninger et al., 2000). In turn, the Food and Drug Administration (FDA, USA) has recently reclassified activated attapulgite from proposed category I to a nonmonographic condition, because of insufficient data on effectiveness for antidiarrhoeal use, and it was not included as a monograph ingredient in the final rule of 2003 (FDA, 2003). The last European Pharmacopoeia (EP 6.0, 2008) has no entry for ‘attapulgite’ either. Sepiolite does not appear under that name in any pharmacopoeia or treatise, appearing invariably as ‘magnesium trisilicate’. Although this product can be prepared artificially, it is stated that it ‘occurs in nature as the mineral sepiolite’ (Anonymous, 1998; Rowe et al., 2006) and main treatises (Sweetman, 2007) or databases consulted on the web used ‘magnesium trisilicate’ and ‘sepiolite’ as synonyms (e.g. the Comparative Toxicogenomics Database or the Canadian Centre for Occupational Health and Safety). In short, while sepiolite and palygorskite are well-known names of minerals, other names continue to be used for exclusively commercial reasons, as is the case of ‘attapulgite’, derived from De Lapparent’s (1936) description of palygorskite at the Attapulgus deposit (Georgia, USA). Consequently, throughout this text, we use those terms used by the various authors in their various scientific articles or treatises. The various pharmacopoeias describe the tests required to characterize all substances listed. In the case of fibrous clays (Table 2), specific tests are mentioned for their correct identification, as well as for control of powder fineness, contents of acid- and water-soluble substances, carbonates, volatile matter, MgO, SiO2, As, Pb, heavy metals, chlorides and sulphates, loss on drying, loss on ignition, microbial contamination, pH and adsorption power.

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TABLE 2 Pharmacopoeial Specifications as Indicated in EP 6th and USP 32 (and Also BP 2009 for Attapulgite). Attapulgite (Palygorskite) USP 32

Magnesium Trisilicate (Sepiolite) BP 2009

Coloidal Act.

Activated

YES

YES

YES

15%

25%

12.5%

EP 6th

USP 32

1.5%

1.5%

Granulometry

Impurities Acid-soluble substances Water-soluble substance

0.5%

Carbonate

YES

YES

Organic volatile impurities

(1)

(1)

Residual solvents

(1)

(1)

Loss on ignition

17–27%

4–12%

15–27%

Loss on drying (105–110 C)

5–17%

4%

17%

Volatile matter (600 C)

7.5–12.5%

3–7.5%

(1)

Water content 17–34%

17–34%


Fineness of powder

Mg

YES

YES

Chapter

SiO2

YES

YES

13

Chemical limitations

2.1–2.37

As

2 ppm

(1)

8 ppm

4 ppm

8 ppm

Pb

10 ppm

(1) 20 ppm

40 ppm

30 ppm

Chloride

500 ppm

550 ppm

Sulphate

0.5%

0.5%

Heavy metals

(1)

(1)

pH

7–9.5

(1)

79.5

Adsorption power

YES

(1)

5–14%

Microbial contamination Technical properties

Acid-absorbing capacity YES: there is a specific test to be done or a mention to this characteristic. (1) It meets the requirements.

YES

100 mL/g

140–160 mL/g


SiO2/MgO

305

306


These specifications often overlap others used for cosmetic products (Nikitakis and McEwen, 1990; Wenninger et al., 2000). As clays can be contaminated during processing, they may be sterilized by heating at over 160 C for not less than 1 h. Besides, they should be stored in airtight containers in cool, dry locations. Usual and common brand names for palygorskite are Actapulgite, Attaclay, Attacote, Attagel, Attapulgite, Activated attapulgite, Attapulgus clay, Attasorb, Diarrest, Diasorb, Diatrol, Diluex, Donnagel, Florex, Gastropulgite, Mucipulgite, Permagel, Pharmasorb regular, Pharmasorb-colloidal, Zeogel. Those for sepiolite are Aid Plus, Hexal, Hydrated magnesium salt, Magnosil, Meerschaum, Milcon, Pangel, Pansil, Parasepiolite, Quincite, Sea foam, Silicic acid, Talcum Plasticum. There are, in addition, several CAS numbers for these two minerals; the most usual are 12174-11-7 for palygorskite, and 18307-23-8, 15501-74-3 and 63800-37-3 for sepiolite.

4. USE AS ACTIVE SUBSTANCES 4.1. Antidiarrhoeals and Antacids Palygorskite and sepiolite can act as adsorbents for toxins, bacteria and even viruses in the intestine. In addition, they act as a protective coating for the stomach and intestine. Purified native palygorskite (colloidal-activated attapulgite) and heat-treated palygorskite (activated attapulgite) are used as active components in the management of acute and chronic diarrhoea (Auzerie, 1999), with recommended manufacturer’s oral daily doses of up to 9 g in adults (Sweetman, 2007), and they are present in several medicinal products and patents (Hu et al., 2006; Sweetman, 2007; Zhang et al., 2004). Palygorskite has high potential for use as an antidiarrhoeal because of its ability to adsorb toxins (Diphtheria toxin) and pathogenic bacteria, and to increase the volume and consistency of the gastrointestinal contents (Fulayyeh et al., 1981). Ogbeide (1987) studied the efficacy of capsules containing palygorskite and homatropine methylbromide in 107 patients between 15 and 65 years of age affected by mild diarrhoea, finding a synergic effect between the actives in reducing the number of depositions and increasing the consistency of intestinal contents. DuPont et al. (1990) compared the activity of palygorskite with other drugs commonly used in the symptomatic treatment of diarrhoea, finding that palygorskite is able to adsorb both toxins and high quantities of water. However, efficacy studies of attapulgite were found to be insufficient by the FDA, and as a result, attapulgite was not included as a monograph ingredient in the April 17, 2003, final rule (FDA, 2003). Despite recent rule changes by the FDA (2003), the majority of the medications at present on the market maintain palygorskite as the active antidiarrhoeal agent, and only one (Kaopectate#) changed its formulation to replace attapulgite by bismuth subsalicylate as the active ingredient in US-marketed products.

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Moreover, in a study carried out in France on the treatment of acute diarrhoea, Uhlen et al. (2004) found that 26% of paediatricians prescribed Mormoiron attapulgite because of their clinical experience with this substance. Attapulgite is an active ingredient in combination with bismuth subcarbonate and kanamycin sulphate in approved animal drug products (FDA, 2008), including both suspensions and veterinary oral tablets (AmforolÒ). The official method for evaluation of the non-specific antidiarrhoeic capacity of an adsorbent is based on its capacity to adsorb methylene blue (‘adsorptive capacity;’ USP 32—NF 27, 2009). The major limitations of this test are associated with the chemical characteristics of the dye and the complex nature of the adsorbents. Water uptake measurement has recently been investigated as a potential complementary test to the methylene blue adsorption assay (Cerezo et al., 2001). They concluded that water uptake capacity and velocity permit a quantitative evaluation of the effectiveness of the studied clay minerals for the non-specific treatment of diarrhoea. Magnesium trisilicate (USP 32–NF 27, 2009; EP 6.0, 2008) possesses antacid properties. When given orally in doses of up to about 2 g (Sweetman, 2007), it reacts with hydrochloric acid in the stomach to form magnesium chloride and silicon oxide. It is often given with aluminium-based antacids to counteract its laxative effect. For example, pH-dependent disintegrating matrices based on magnesium trisilicate and aluminium hydroxide have been recently formulated, using Eudragit E PO as a matrix former and sodium bicarbonate as a disintegrant (and third antacid component), in different proportions (Bajdik et al., 2009). Such formulations are insoluble if the pH of the stomach is less acidic and rapidly disintegrate if necessary (when high levels of acidity are achieved). Jovanovic et al. (1988) examined the effects of binders and disintegrants in wet granulation, as well as those of compression strength on the antacid activity of magnesium trisilicate. They determined a maximum pH value of 5.9 in the artificial gastric juice after 35 min at 8 kN compression and 5% binder concentration. Recently, Iwuagwu and Agidi (2000) examined the disintegration time of magnesium trisilicate tablets, prepared by wet granulation with modified (bleached) and unmodified starches, finding that tablets containing bleached starches disintegrated faster than those containing the unbleached ones. Antacid preparations containing magnesium trisilicate are also available as liquid suspensions. Fritz and Riehl (1989) studied the stabilization (flocculation) of sepiolite aqueous suspensions, finding satisfactory flocculation and improved redispersibility in presence of peptising salts (MgCl2, KOAc, Na tartrate) without cellulose ethers as viscosity-increasing additives. Despite the foregoing, it should be noted that the acid reactivity of magnesium trisilicate powder was reduced by concomitant administration with different drugs, such as digoxin, dexamethasone acetate, diazepam, nitrofurantoin, xytetracycline hydrochloride, ferrous gluconate, ampicillin

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trihydrate and tolbutamide (Al-Gohary, 1990). In particular, all drugs decreased the acid absorption power and buffering capacity of the antacid, while the neutralization effect was reduced.

4.2. Adsorbents and Protectors Muller and Kline (1964) proposed a topical formulation with 20 wt% of Attagel 20 and other components (resorcinol, hexachlorophene and zinc oxide) for the treatment of acne. Palygorskite is also used alone or in combination with other components to adsorb residual active antibiotics, metabolites, bacterial or other toxins and drugs which cause side effects in the gastrointestinal tract (Huguet and Andremont, 2007). Other uses of palygorskite as drug include the treatment of chronic leg ulcers, because of its capacity to adsorb exudates (Fang, 1988) and treatment on inflammatory bowel disease, because of its carbohydrate adsorption capacity (Farmer and Lefkowitz, 2005). Recently, palygorskite has also been proposed for the treatment of renal failure (Zhang et al., 2009). In this case, palygorskite absorbs and fixes various toxins and metabolic products in the alimentary tract and is then discharged by intestinal peristalsis, thus reducing or avoiding bodily injury. Finally, magnesium trisilicate also has a high adsorption capacity and has been proposed as alternative treatment for ciprofloxacin overdose or toxicity (Ofoefule and Okonta, 1999).

5. USE AS EXCIPIENTS Palygorskite and magnesium trisilicate are used worldwide as inactive components in non-parenteral licenced medicines, being regarded as essentially nontoxic, non-irritant materials (Table 3). They are also included in the FDA Inactive Ingredient Guide as excipients for oral solid dosage forms (powders and tablets) (FDA, 2009).

5.1. Solid Dosage Forms Viseras and Lo´pez-Galindo (2000) reported some useful properties (angle of repose, tapped density and Carr index) of fibrous clay powders as excipients for oral solid dosage forms. These clays can fulfil several functional categories when applied as diluents (Mathew et al., 2009), glidants (Gonza´lezPe´rez et al., 2009; Nunes et al., 2007; Yendamuri et al., 2009), disintegrants (Viseras et al., 2001a) and adsorbents (Rowe et al., 2006). Diluents are fillers used to increase the bulk volume of a tablet or capsule. By combining a diluent with the active pharmaceutical ingredients, the final product will have adequate weight and size to assist in production and handling. Glidants (or anti-caking agents) are aimed to improve flow properties of powders,

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TABLE 3 Uses of Fibrous Clay Minerals in Pharmaceutical Products. Pharmacopeial Name

Dosage Form

Administration

Functional Category

Solid

Oral

Adsorbent, glidant, binder and disintegrant

Conventional and modified release tablets

Liquid

Oral/topic

Suspending and anticaking agent

Oral and topical suspensions

Solid/ liquid

Oral

Antidiarrhoeal

Antidiarrhoeal suspension formulations and immediate release tablets

Solid/ liquid

Oral

Antacid

Antacid suspensions

Solid/ liquid

Oral

Antacid

Antacid suspensions and tablets

Examples

Excipients Magnesium trisilicate

Active principles Attapulgite

Magnesium trisilicate

facilitating uniform die filling in tablet manufacture. They are thought to act by lodging in the surface irregularities of solid particles, forming more rounded structures and reducing interparticle friction. Disintegrants are agents added to tablet or capsule formulations to promote the break-up of such dosage forms into smaller fragments in an aqueous medium, thus increasing the available surface area and promoting more rapid release of the drug substance. Palygorskite has been proposed as adsorbent to retain and release cations in solid preparations used in the treatment of infections (Castela et al., 1998) and to prepare antiseptic silver-loaded nanoparticles by sulfhydrilation of the clay surface (Liu and Wang, 2008). It is also used to adsorb liquids in a multiphase composition to assist in producing a solid dosage formulation (Shenoy et al., 2008). Magnesium trisilicate is mainly used as a glidant but can be also used as an adsorbent in pharmaceutical solid dosage forms, for instance, to retain povidone–iodine, an antibacterial drug, thus producing a new pharmaceutical product with satisfactory antibacterial effect for the prevention of infections in hospital wards and buildings (Ohta et al., 1999). Due to their adsorbent

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capacities, both palygorskite and magnesium trisilicate have been proposed as carriers for preventing or reducing surface absorption or for delayed percutaneous delivery of pharmaceutical compounds (Kostyniak and Giese, 2003). In some cases, and depending on the chemical composition of the clay, adsorbent properties can provide protective effects. For example, sepiolite has been considered as a stabilizer for drugs subject to oxidative degradation (Hermosin et al., 1981), whereas palygorskite increased oxidation of hydrocortisone because of its Fe3þ ion content (Cornejo et al., 1980). However, palygorskite has been used as a stabilizer to inhibit hydrolysis, isomerization, elimination, oxidation and/or re-crystallization of reductase inhibitors in oral tablets (Karavas et al., 2008). Because of its hygroscopic nature, the clay swells in the presence of water, providing excellent storage stability caused by the reduction of free water in the composition, protecting drugs (such as statins) against hydrolysis, oxidation and isomerization. Among solid dosage forms, oral tablets are the most commonly used products. Some functional characteristics of oral tablets obtained by direct compression of Spanish palygorskite and sepiolite were studied by Viseras et al. (2000). Recently, the use of magnesium trisilicate in vaginal tablets has been also described (Biradar et al., 2009). Such formulations, based on a cubic phase precursor dry powder obtained by spray drying the clay with glyceryl monooleate (Shah et al., 2006), showed potential patient compliance, compared to conventional vaginal tablets and gels and sustained release of progesterone for 14 h in simulated vaginal fluid. Use of fibrous clays as support for active agents in modified drug delivery systems was recently reviewed by Aguzzi et al. (2007). They concluded that, even if naturally occurring fibrous clay minerals may be suitable to effectively modulate drug release, it is usually necessary to use modified or synthetic clay minerals and/ or polymeric additives to obtain technologically advanced drug products.

5.2. Liquid and Semi-solid Dosage Forms Palygorskite and sepiolite are used as suspending agents in complex mixtures of components to prevent settling and separation in certain liquid medications and to keep the compounds in suspension and uniformly distributed. Dispersed systems have a variety of applications both in pharmaceutical and in cosmetic areas, including liquid (suspensions and emulsions) and semi-solid products (ointments, pastes, creams, gels and rigid foams). In these systems, one discontinuous or internal phase is dispersed in a medium called vehicle or external phase (Russel et al., 1995), and both phases may be solid, liquid or gas. Solid-in-liquid (coarse suspensions and colloidal dispersions) and liquid-in-liquid (emulsions) disperse systems are thermodynamically unstable, and the phases tend to separate with time, requiring accurate rheological and stability controls.

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In aqueous media, fibrous clays form three-dimensional structures (gels) composed of interconnecting fibres. Clay minerals are frequently used to avoid instability in liquid and semi-solid formulations, as it has been recently reviewed by Viseras et al. (2007). Fibrous clay gels retain their stability in the presence of high concentrations of electrolytes, thus making them ideal as thickeners and emulgents in liquid or semi-solid formulations (Eriksson et al., 1990; Fadat et al., 1988; Parkhomenko et al., 1987). Little research has focused on the effects of hydrodynamic factors such as particle size and shape on the final product properties. Viseras et al. (1999) assessed the effects of shear history on the rheology of laminar and fibrous clay mineral dispersions, concluding that the degree of dispersion and the structural changes resulting from differences in particle shape significantly affect the rheological properties of the systems. A linear relation was found between mixing energy and apparent viscosity of the laminar clay minerals, while apparent viscosity was related to mixing power (a measure of the energy used during suspension preparation) for fibrous clays. A subsequent study examined the filtration behaviour of some Spanish fibrous clay–water dispersions, the results of which were compatible with the rheological properties of the systems (Viseras et al., 2001b). Clay-gels exhibit intermediate characteristics between liquid and solid systems and have interesting rheological behaviour, including dilatant or, more frequently, pseudoplastic flows, yield point, thixotropy and high viscosities (Mewis and Macosko, 1994). In particular, thixotropy is an important feature for pharmaceutical products, as apparent viscosity decreases as shear rate rises and then increases again on standing, requiring a certain time for the structure to rebuild. Clay minerals are therefore used in liquid and semi-solid dispersed systems both as stabilizers (suspending and anti-caking agents) and to adjust the rheological patterns of the preparations. Magnesium trisilicate is described as a suspending and anti-caking agent (Rowe et al., 2006), useful to prevent drastic changes in dispersion properties, retarding sedimentation or producing flocculated systems easily re-suspended by mild agitation. Palygorskite can be used to prepare stable aqueous suspensions of sparingly soluble to insoluble drugs, most preferably from about 0.2 wt% to about 0.6 wt% (Namburi et al., 2008). It is also applied in semi-solids to increase skin adhesion and the pharmaceutical effects of water and alcohol-based suspensions and ointments for the treatment of chilblain and frostbite (Shifang et al., 2008a–c). As rheological modifier, magnesium trisilicate is used in suspensions to increase the viscosity of the external phase, reducing the rate of sedimentation of the dispersed particles (Rowe et al., 2006). Fibrous clays can also be used as emulsifying agents in emulsions due to their ability to be wetted by the two liquid phases located at the liquid–liquid interface and to act as a physical barrier to prevent coalescence of the liquid droplets (Viseras et al., 2007). Palygorskite is used as emulgent at around 2–5% (w/v) (Wenninger et al., 2000). The clay is first suspended in water

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under intense agitation, and the resulting smooth gel is used to incorporate the remaining components and emulsify with the oil phase.

6. DRUG INTERACTIONS Absorption of many oral drugs is reduced by co-administration of clay-based gastrointestinal agents (antidiarrhoeals, antacids) or by the presence of clay stabilizers in liquid formulations, so that the interval of administration between these preparations and other active substances should be as long as possible. In particular, absorption of promazine decreased when the drug was administered in association with antidiarrhoeal mixtures containing palygorskite (Sorby, 1965; Sorby and Liu, 1966). Since these studies, many authors have reported a decrease in bioavailability of several drugs due to co-administration with magnesium trisilicate and palygorskite (Babhair and Tariq, 1983; Daabis et al., 1976; Gokhale and Bhalla, 1981; Khalil, 1974; McElnay et al., 1982a,b; Said and Abdullah, 1981). Research interest in clay adsorption of active substances continued and Thona and Lieb (1983) reported strong adsorption properties of magnesium trisilicate and palygorskite (among other agents) for various drugs. Similarly, it was found that some antibiotics (Khalil et al., 1984) and anti-gout agents (Naggar, 1985) are adsorbed by palygorskite and magnesium trisilicate. Qawas et al. (1986) studied the extent of adsorption of chlorhexidine acetate in palygorskite, kaolin and other solids commonly used in the formulation of suspensions for oral or topical administration, and the effects of adsorption on the bactericidal activity of the active ingredient. Cimetidine and propranolol were also studied (Moustafa et al., 1986), and a decrease in bioavailability of the drugs was revealed. Khalil et al. (1987) investigated human bioavailability of riboflavin when given before or in association with palygorskite. In both cases, the observed reduction in urinary excretion of riboflavin was attributed to the retention of the drug by the adsorbent. Other drugs exhibiting reduced efficacy were hyoscine (Ozdemir et al., 1986), quinidine sulphate (Moustafa et al., 1987a), propranolol hydrochloride (Al-Gohary et al., 1987), phenothiazines (Moustafa et al., 1987b), mebeverine hydrochloride (Al-Gohary, 1991; Mohamed et al., 1989), folic acid (Iwuagwu and Jideonwo, 1990), paracetamol and cloroquine (Iwuagwu and Aloko, 1992), erythromycin (Arayne and Sultana, 1993) proguanil (Onyeji and Babalola, 1993), cimetidine and ranitidine (Vatier et al., 1994). Finally, a remarkable inhibiting action of magnesium trisilicate on some antibiotics (Aggag et al., 2004; Arayne et al., 2005; Hussain et al., 2006; Okor et al., 2004) and chlorpheniramine maleate (Tella and Owalude, 2007), an antihistamine agent, has recently been demonstrated, suggesting that the concomitant administration of the drugs and antacid formulations containing clays should be discouraged. On the contrary, in the case of the hypoglycemic agents,

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glibenclamide and gliclazide, bioavailability of the drug was enhanced by magnesium trisilicate (Arayne et al., 2003, 2004). As a result of these studies, it was realized that the effects of such interactions in the concomitant administration of clay minerals with active substances might not be purely negative but could also be used to achieve technological and biopharmaceutical benefits. This was the starting point in the use of clays in modified drug delivery systems, which is a recent development with great potential (Aguzzi et al., 2007).

7. USE OF FIBROUS CLAYS IN COSMETICS Palygorskite and sepiolite are frequently used in different cosmetic products to give the skin opaqueness, eliminate shine and cover up imperfections. Because of their sorption capacities, they are employed as deodorants in powders and creams (Ueda and Hamayoshi, 1992); in dry shampoos to eliminate scalp grease; and in bath powders and baby powders to absorb humidity and odours, avoiding skin irritation. Palygorskite is also used with other components in toothpastes, because of its ability to adsorb and eliminate odours in the mouth, preventing gingivitis and gingival bleeding (Changbing et al., 2008a–f; Jian et al., 2008a–f). Palygorskite is also used in skin moisturizing facial mask powders (Shengying and Qinghua, 2006). Some depilatory creams for humans and animals use this clay as the main component because of its strong adhesive properties, thus remaining well adhered to the surface of skin, increasing the contact area between the depilatory cream and the hair or fur and enhancing the effectiveness of the depilatory cream (Fan et al., 2009a,b). Several Chinese patents propose the use of palygorskite with other components in shampoo formulations. These include green tea (Jian et al., 2009a), providing multiple water-miscible nutrients for the hair, maintaining the balance of grease secretion and penetrating deeper into the hair stem hydrating the hair and making it silky; angelica (Qinghua et al., 2009a), capable of adsorbing sweat, dust and microorganisms; aloe (Jian et al., 2009b); black sesame (Qinghua et al., 2009b); laminaria Japonica (Qinghua et al., 2009c) and other Chinese medicines (Jian et al., 2009c–e; Qinghua et al., 2009d). In some cases, the palygorskite-formulated shampoos are not only for cleaning purposes but also to protect against hair-related disorders such as alopecia (Qinghua et al., 2009e) or dandruff (Qinghua et al., 2009f), improving blood circulation in the scalp, increasing elasticity and tenacity of the hair, inhibiting sebum extravasations, dispelling scurf and preventing itchy scalp. Palygorskite-based shampoos can also be formulated to modify hair colour (Qinghua et al., 2009f), promoting blood circulation and removing blood stasis, activating hair follicle cell viability, inhibiting sebum exudation and enhancing the metabolic capability of hair to make it black and shiny. Palygorskite has been also formulated with other

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components to obtain anti-parasite products, including pulicidal powders (Shifang et al., 2008d), pediculicide ointments (Shifang et al., 2008e) and alcohol-based louisicide (Shifang et al., 2008f,g). Palygorskite is used with several traditional Chinese active agents (angelica root, salvia root, liquorice, mint, lonicera flower, carthamus flower, wild chrysanthemum flower and others) to prepare face masks and mud bath for external application to clean the skin and obtain various cosmetic effects (Qinghua, 2006a–c). Other suggested applications include the use of sepiolite complexes as ultraviolet radiation filters in sun creams (Hoyo del et al., 1998). The interaction products showed improved protection capacity especially in the so-called C range (290–190 nm). It has also been proposed in depilatory creams, with similar actions to those described for palygorskite (Tindal and Mangassi, 2008).

8. HEALTH RISKS OF FIBROUS CLAYS Contact with or inhalation of mineral dust in high concentrations can have harmful effects on human health, as many studies have examined in depth (for instance, Derbyshire, 2005; Elmore, 2003; Guthrie, 1992), the most common associated illnesses being asbestosis and silicosis. Generally speaking, the effects of the manipulation and processing of fibrous clays are not different from other non-fibrous dust (Carretero et al., 2006) and commercial sepiolite is regarded as non-toxic (Santareń and Alvarez, 1994). Consequently, sepiolite and palygorskite should be handled in a well-ventilated environment and dust generation should be minimized. Like most other dusty materials, they can cause mechanical irritation to the eyes, and mucous membrane and respiratory irritation after inhalation. Normal precautions such as eye protection, gloves and a dust mask are recommended. In any case, the permissible exposure limits for respirable (5 mg/m3) and inhalable (10 mg/m3) sepiolite or palygorskite dust are far above the minimal quantities reached by normal manipulation of medicines or creams by the end customer. Given the variability of geological context in which fibrous clay deposits are found, with different types and proportions of associated minerals, as well as differing grain sizes (Bellmann et al., 1997; Nolan et al., 1991), insistence should be made on the importance of carrying out specific tests to control the compositional purity of the clay samples, which is often not very high, the crystallinity of the variety of silica present and the particle size of the fibrous clays. Several studies have been made on the pathogenicity of inhaled fibrous clays (Anonymous, 2003; Baris et al., 1980; Governa et al., 1995; McConnochie et al., 1993; Ross et al., 1993; Santareń and Alvarez, 1994), although the results are contradictory. Nonetheless, no pharmacopoeia suggests any specific test or requisite for control of the grain size of these clays. Studies carried out on humans exposed to sepiolite or palygorskite seem to suggest that

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such exposure does not increase the risk of pulmonary disease, with no evidence of pleural plaques or mesothelioma (Governa et al., 1995; McConnochie et al., 1993), so that contact with these minerals does not appear to represent any risk. As mentioned above, the most common grain sizes of sepiolite and palygorskite (Viseras and Lo´pez-Galindo, 1999) are significantly smaller than what is generally accepted as defining a particle as a fibre, that is, to be over 5 mm in length and with a > 3:1 ration of length to diameter. Some studies give even more exact figures, reporting that intrapleural tumours increase with inhalation of particles of 0.25 mm in diameter and > 8 mm in length (Stanton et al., 1981), that lung cancer can be caused by particles < 0.8 mm in diameter and lengths of 10–100 mm, and that mesothelioma is encouraged by particles < 0.1 mm in diameter and lengths of 5–10 mm (Harrington, 1981; Lippmann, 1988). In general, experimental studies on animals suggest that carcinogenicity is dependent on the proportion of long fibres (> 5 mm) in the samples (Bellmann et al., 1997; Be’gin et al., 1987; Jaurand et al., 1987; Stanton et al., 1981; Wagner et al., 1987). The International Agency for Research on Cancer (IARC) indicates that in humans, there is inadequate evidence for the carcinogenicity of palygorskite, and that in experimental animals, there is sufficient evidence for the carcinogenicity of long palygorskite (> 5 mm) fibres and inadequate evidence for short palygorskite (< 5 mm) fibres. Consequently, IARC (1997a) classifies long palygorskite fibres as Group 2B (possibly carcinogenic to humans) and short palygorskite fibres as Group 3 (they cannot be classified as to their carcinogenicity to humans). Concerning sepiolite, there is inadequate evidence in humans of the carcinogenicity of this clay and limited evidence in experimental animals of long sepiolite (> 5 mm) fibres. So, sepiolite is also included in Group 3 (IARC (1997b)). Quite frequently, particularly in the case of palygorskite, fibrous clays are accompanied by quartz and other less crystalline or amorphous varieties of silica. Crystalline silica should be controlled and avoided as far as possible, as it is classified by the IARC as a product with sufficient evidence of carcinogenicity in laboratory animals and limited evidence in humans (Group 1, IARC (1997c)). In case of a relatively high quartz content, the product requires labelling and other forms of warning in the safety information. Amorphous silica is not classifiable as carcinogenic to humans (Group 3). Apart from As and Pb, whose contents must expressly be controlled following the indications of the various pharmacopoeias, there are others that are potentially dangerous, such as Cd, Hg, Te, Tl, Sb and Se. In view of the results obtained in various clays on the mobility of hazardous chemical elements (Mascolo et al., 1999, 2004; Tateo et al., 2001), it is recommended that both the total content and the mobility and bioavailability of potentially toxic substances in the products should be established by appropriate methodologies (biological membrane absorption test) when clays are used for medical and cosmetic purposes (Adamis and Williams, 2005).

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Finally, in some cases, abuse of antacids containing magnesium trisilicate has been associated with the formation of renal calculi (Matlaga et al., 2003). Caution in the administration of magnesium-based antacids should be used with patients suffering impaired renal function, when concentrations of magnesium above 50 meq can produce hypermagnesaemia.

9. CONCLUDING REMARKS Fibrous clays are frequently used in the preparation of various pharmaceutical and cosmetic products on the basis of their particular physical and chemical properties. Apart from their traditional applications, new uses have been developed in recent years as vehicles for systems with modified release of active agents. The development of new applications will require the collaboration of specialists in different areas, including mineralogists, pharmacists, chemists, biologists and doctors.

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Chapter 14

The Effects of Palygorskite on Chemical and Physico-Chemical Properties of Soils Alexander Neaman*,{ and Arieh Singer{ *Facultad de Agronomıá, Pontificia Universidad Cato´lica de Valparaı´so, Quillota, Chile { Centro Regional de Estudios en Alimentos Saludables, CREAS, Regioń de Valparaı´so, Chile { Seagram Center for Soil and Water Sciences, Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot, Israel

1. INTRODUCTION Palygorskite is a clay mineral characterized by a microfibrous morphology, low surface charge, high magnesium content, and high specific surface area (Singer, 2002). Palygorskite-containing soils occur almost exclusively in arid and semi-arid areas of the world. Palygorskite-containing soils can be cultivated profitably only by using irrigation, where management practices have to take account on the effects of water on soil properties. Singer (2002) reviewed the basic mineral characteristics of palygorskite, as well as the means of its identification, and discussed its origin, as well as its environmental significance. The author emphasized, however, that very little is known about how palygorskite affects soil properties when it constitutes a significant component of the soil clay fraction. This constitutes an impediment for the proper management of palygorskite-containing soils under irrigated agriculture. Following the above-mentioned review, some studies have been performed and progress has been made in understanding the effects of palygorskite on chemical and physico-chemical properties of soils. The present chapter summarizes these recent studies on how palygorskite affects soil properties when it constitutes a significant component of the soil clay fraction. It also discusses future research required in this area.


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2. MAGNESIUM CHEMISTRY OF PALYGORSKITECONTAINING SOILS Palygorskite has been shown to be rather unstable in humid areas of the world. According to Paquet and Millot (1973), palygorskite weathers into smectite when the mean annual rainfall exceeds 300 mm. According to Singer and Norrish (1974), palygorskite is stable only at relatively high Si and Mg activities and alkaline pH. Palygorskite is the most magnesium-rich among the common clay minerals (Singer, 2002; Weaver and Pollard, 1973). Although sepiolite contains more Mg than palygorskite, sepiolite in soils has been reported from only a few locations (Singer, 2002). Based on acid leaching of palygorskite, Mg appears to be preferentially released into solution over Fe and Al (Corma et al., 1987, 1990; Singer, 1977). The aggressive environment created by acids, however, can hardly serve as a model for the soil medium in which palygorskite occurs. Neaman and Singer (2000a) conducted batch experiments with dilute salt solutions under neutral conditions in the presence of cationic resin to study the kinetics of palygorskite dissolution. The presence of the cationic resin was intended to prevent equilibrium being attained, and dilute salt solutions were considered to be a better approximation to soil solution than deionized water. The rates of mineral dissolution differed significantly among five standard1 palygorskite samples of different origins with different chemical compositions. The surface area-normalized rates of Mg release were in the range from 2.0 10 15 to 1.6 10 14 mol Mg/(m2 s), using clay surface areas determined by EGME (ethylene glycol monoethyl ether) adsorption. When clay surface areas determined by nitrogen absorption (BET, Brunauer–Emmet–Teller method) were used for normalization, the rates of Mg release were in the range from 1.0 10 14 to 5.5 10 14 mol Mg/ (m2 s). The differences in release rate of Mg among the palygorskite samples were due to differences in chemical composition. The release rate of Mg increased with Mg content in the clays (Figure 1). A question arises on what surface area is important for the reaction kinetics of palygorskite dissolution and thus for the rate normalization. The BET method determines only external surfaces of palygorskite because N2 molecules do not penetrate significantly into the intracrystalline channels. However, EGME molecules can partially penetrate into the tunnels and channels in a manner similar to their penetration between smectite layers. The surface area of palygorskite obtained by EGME adsorption, however, is still much lower than the theoretical value (Neaman and Singer, 2000a).

1. Herein and below, ‘standard palygorskite (or smectite or montmorillonite or kaolinite or illite) clays’ are defined as clays obtained from deposits, in which palygorskite (or smectite or montmorillonite or kaolinite or illite, respectively) is the major mineral phase and differs from ‘soil clays’, which contain a mixture of minerals.

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A

327

The Effects of Palygorskite

2.0

Mg release rate (mol Mg/(m2 s) ´ 10−14)

Surface area determined by EGME 1.5

1.0 Mg-rich montm.

0.5

0.0 2

4

6

8

10

8

10

Mg content in the clay, % B

6.0

Mg release rate (mol Mg/(m2 s) ´ 10−14)

Surface area determined by BET

4.0 Mg-rich montm. 2.0

0.0 2

4

6 Mg content in the clay, %

FIGURE 1 Release rate of Mg from palygorskite (black squares) and Mg-rich montmorillonite (open squares). Rates are expressed on the basis of the clay surface areas determined by (A) ethylene glycol monoethyl ether (EGME) adsorption and (B) nitrogen absorption (BET, Brunauer– Emmet–Teller equation). After Neaman and Singer (2000a).

With regards to the dissolution mechanism of palygorskite, Gonzalez et al. (1989) suggested that dissolution starts at the openings of the channels and propagates inside the fibres. Corma et al. (1987), however, argued that palygorskite dissolution takes place mostly at the external surfaces with practically no dissolution inside the channels. However, these arguments were based on indirect evidences. We are unaware of a study on direct observation of the dissolution mechanism of palygorskite using modern techniques, such as atomic force microscopy in the study of Bickmore et al. (2001) on the dissolution mechanism of smectites. Thus, it is difficult to decide on what surface area is important for the reaction kinetics of palygorskite dissolution and thus for the rate normalization.

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Since smectite is always present in the clay fraction of palygorskitecontaining soils and may also release Mg into soil solution, Neaman and Singer (2000a) examined the dissolution of Mg-rich montmorillonite as well. The release rate of Mg from this montmorillonite was of 1.9 10 15 mol Mg/ (m2 s) when normalized by the EGME surface area and of 1.5 10 14 mol Mg/(m2 s) when normalized by the BET surface area. Thus, Mg release rate from the Mg-rich montmorillonite was lower than that from palygorskites (Figure 1). Again, a question arises on what surface area is important for the reaction kinetics of smectite dissolution and thus for the rate normalization. The BET surface area of smectite represents external surface only (Tournassat et al., 2003) while the EGME surface area represents both external and interlayer surfaces (Rather-Zohar et al., 1983). However, direct observation of the dissolution mechanism of smectites using atomic force microscopy (Bickmore et al., 2001) indicates that smectites dissolve mainly at the edges while the basal (001) surfaces remain unreactive. Therefore, it may be more appropriate to normalize the dissolution rate of smectite to the edge surface area, which is represented neither by the BET nor by the EGME surface areas and needs to be determined by other methods (Tournassat et al., 2003). Thus, it is difficult to compare accurately the release rate of Mg from the Mg-rich montmorillonite with that from palygorskites. Another question that arises is whether the Mg release rate from standard palygorskite clays is applicable for that from palygorskite-containing soil clays. To elucidate this question, Neaman and Singer (2000b) determined Mg release from some palygorskite–smectite–kaolinite-containing soil clays from which carbonates, iron oxides, and organic matter had been removed. Palygorskite was suggested to be the only significant source of Mg released from the soil clays studied. The release rate of Mg from soil clays was in the range from 2.2 10 15 to 7.5 10 15 mol Mg/(m2 s) when normalized by the EGME surface area. In other words, it was similar to that from standard palygorskites. Thus, when palygorskite-containing soils are involved in irrigation practices, release of Mg will occur from palygorskite into soil solution. Magnesium as an exchangeable cation, in turn, is known to decrease aggregate stability and to enhance the dispersivity of the soil clay fractions (Dontsova and Norton, 2002; Keren, 1991; Shainberg et al., 1988; Zhang and Norton, 2002), which is not desirable for soil management under irrigation. However, magnesium as a dissolved cation is known to reduce phosphorus (P) fixation on calcite, increase desorption of P sorbed on calcite, and inhibit formation of Ca–phosphates (Shariatmadari and Mermut, 1999). The authors suggested that the inhibitory effect of Mg on the sorption of P by calcite results mainly from its interference with the formation of Ca–phosphates complexes and, therefore, the precipitation of Ca–phosphate.

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Shariatmadari and Mermut (1999) also reported that desorption of P from sepiolite–calcite mixture far exceeded that from montmorillonite–calcite mixture (Figure 2). The authors attributed it to a high concentration of Mg in the solution due to Mg release from sepiolite. Although sepiolite contains more Mg than palygorskite, sepiolite in soils has been reported from only a few locations (Singer, 2002). The presence of palygorskite as a slow-release source of Mg can improve P availability in calcareous soils. However, if dolomite would be present in the soil, presence of palygorskite would be of less importance with regard to Mg release into soil solution. This is because the release rate of Mg from dolomite [5 10 8 mol Mg/(m2 s), at pH 7, Chou et al., 1989] exceeds greatly that from palygorskite.

3. POINT OF ZERO CHARGE OF THE PALYGORSKITE SURFACE Point of zero charge (PZC) is the measure of relative affinity of Hþ and OH for amphoteric mineral surfaces. The PZC is the pH at which surface negative charge equals surface positive charge. At the PZC, not only cation and anion exchange capacities are equal in magnitude, but also the ability of the surface to adsorb ions of either charge is at a minimum (McBride, 1994). Also, aggregation of soil particles may also be sensitive to the PZC. Colloidal particles tend to disperse when possess relatively high surface charge, either positive or negative. Palygorskite carries a negative charge on the basal surface of the fibres resulting from isomorphous substitution (Singer, 2002). The degree of the isomorphous substitution is low, so the magnitude of the permanent surface 4

Desorbed P (cmol kg−1)

Sep–CaCO3 3

Mont–CaCO3

2

1

0 0

1

2

3

4

5

6

−1)

Equilibrium P concentration (mg L

FIGURE 2 Desorption of P from sepiolite–calcite and montmorillonite–calcite mixtures. After Shariatmadari and Mermut (1999).

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negative charge and the cation exchange capacity of palygorskite are quite low. Because this charge is intrinsic to the layer structure, its sign and magnitude are independent of pH. Structurally, palygorskite consists of alternation of blocks and tunnels that grow along the length of the fibre. Each structural block is composed of a central octahedral sheet sandwiched between two discontinuous tetrahedral sheets of silica. Due to the discontinuity of the silica sheets, silanol groups (SiOH) are present on the surface of the fibre. These groups are located at the edges of the tunnels acceding to the external surface of the silicate. Silanol groups are formed as a result of broken SiOSi bonds at external surfaces balancing their residual charge by accepting either protons or hydroxyl groups to form SiOH groups (Galań, 1996). The silanol groups at external surfaces of palygorskite can be protonated and deprotonated under acid and alkaline pH conditions, respectively. At the PZC, the surface is essentially uncharged, while at pH values above and below the PZC, it will be negatively and positively charged, respectively. The plots of electrophoretic mobility versus pH at different concentrations of electrolyte intersect at pH 4.1 (Figure 3). This intersect represents the PZC of the palygorskite surface. The change of electrophoretic mobility per pH unit is greater under alkaline than that under acid conditions. The magnitude of the positive charge under acid conditions is relatively low. As discussed below, these charge characteristics of palygorskite surface have important implications of particle arrangement in palygorskite suspensions and its rheological properties.

1.0 PZC Electrophoretic mobility (10–4 cm2 V–1 sec–1)

0.0 0

2

4

6

−1.0

8

10

Electrolyte concentration DW 0.001 N NaCl

−2.0

0.01 N NaCl −3.0 −4.0 pH

FIGURE 3 Relationship between pH and electrophoretic mobility at different electrolyte concentrations for Na-palygorskite from the Negev. The intersect at pH 4.1 represents the point of zero charge (PZC) of the palygorskite surface. After Neaman and Singer (2000c).

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4. RHEOLOGY OF STANDARD PALYGORSKITE SUSPENSIONS Hydraulic conductivity—the ability of soil to conduct water within its volume— is an important factor in soil management under irrigation. Hydraulic conductivity is not a property of the soil alone but rather depends on both the soil pore geometry (permeability) and the fluidity of the permeating phase (Hillel, 1998). When the electrolyte concentration in the percolating solution in the soil is below the clay flocculation value (FV), clay dispersion occurs and flow of solution in the soil changes to flow of clay suspension. Under such conditions, hydraulic conductivity of the soil decreases sharply (Keren and Ben-Hur, 2003; Pupisky and Shainberg, 1979). This decrease in soil hydraulic conductivity is related to the change in the attributes of both the soil and the permeating phase. On one hand, some of the dispersed clay particles can be trapped into the pores and thus change the soil pore geometry (Keren and Ben-Hur, 2003; Lado et al., 2007). On the other hand, clay particles increase significantly the viscosity of the flowing suspension (Keren, 1988; Shainberg and Otoh, 1968) and, therefore, decrease its fluidity (which is the reciprocal of viscosity). Rheology is a science of flow of matter. Knowledge of factors affecting rheological properties of soil clay suspensions is, therefore, essential for understanding of mechanisms of water and solid-particle transfer through the soil profile. The rheological characteristics of clay suspensions can be used to evaluate particle–particle interactions. When particle–particle interactions occur, causing non-Newtonian flow, the suspension viscosity undergoes changes with flow velocity. Although there have been many studies on the rheological behaviour of suspensions of standard clays, studies on the rheology of suspensions of mixed clays and soil clays remain limited. The rheological properties of mixed clay and soil clay suspensions are, therefore, not yet fully understood. Neaman and Singer (2000c) investigated the rheology of aqueous suspensions of six standard palygorskites of different origins with different particle morphologies. Suspensions of 3% (w/v) Na-saturated palygorskites at pH 7 exhibit non-Newtonian flow that is characterized by a change in viscosity with changes in shearing rate (Figure 4). Specifically, there was a progressive decline in viscosity as shear rate increased that is characteristic of pseudoplastic behaviour (van Olphen, 1977). Pseudoplastic flow of colloid suspensions can be described by the Bingham model (Gu¨ven, 1992). According to this model, the slope of the linear part of the flow curve is referred to as ‘plastic viscosity’, and the intercept of the linear portion of the curve with the stress axis is referred to as ‘Bingham yield’. For all studied palygorskite samples, the flow curves become linear at shearing rates 250–1000 s 1. Another rheological characteristic is ‘apparent viscosity’ that is defined as shear stress/shear rate ratio at any shear rate. Here, apparent viscosity was obtained at a shear rate of 1000 s 1.

332


14 Sample location

Shear stress (Pa)

12

Georgia Florida

10

Negev Yucatan

8

Mt. Flinders 6

Mt. Grainger

4 2 0 0

200

400 600 Shear rate, s–1

800

1000

FIGURE 4 Flow curves of 3% (w/v) suspensions of Na-palygorskites at pH 7. After Neaman and Singer (2000c).

The differences in flow curves between different samples (Figure 4) are related to the ellipticity of individual palygorskite fibres. The ellipticity of fibres is defined as L/W ratio where L is the length and W is the width of the fibres. Plastic viscosity, Bingham yield value, and apparent viscosity of palygorskite suspensions are linearly related to the ellipticity of the fibres (Figure 5). The rheological characteristics of the Georgia and Florida palygorskite suspensions, however, are much higher than would be predicted from the ellipticity, due to the presence of smectite impurities. Ellipsoids are oriented during the flow with respect to the flow direction (Gu¨ven, 1992). The amount of dissipated energy required for orientation, and hence, the viscosity of the fluid, increases with the ellipticity of the particles. As discussed below, individual fibres of palygorskite in suspension are associated to each other by faces. The intimacy of association, and hence, Bingham yield value, increases with fibre length due to an increase in the area of the contact. Neaman and Singer (2000c) concluded that the models developed to explain the rheological behaviour of platy clay minerals did not always account for the behaviour of palygorskite, because of differences in particle morphology and surface structure. These differences are exemplified below. The particle arrangement in palygorskite suspensions at low pH values deviates from that in kaolinite and montmorillonite suspensions. The concept of faceto-face, edge-to-face and edge-to-edge type associations in suspension of platy clay minerals (such as montmorillonite and kaolinite) was first proposed by Hofmann and Hausdorf (1945) and developed further by van Olphen (1977). Electrostatic attractive forces between the positively charged edges and negatively charged faces result in edge-to-face particle associations in kaolinite suspensions at pH values below the PZC (Rand and Melton, 1975, 1977).

Chapter

14

Plastic viscosity (mPa s)

A

333


5 Georgia

4

Florida

3

y = 0.052x + 0.7472 R 2 = 0.9296

2 1 0 0

B 10 9 8 7 6 5 4 3 2 1 0

5

10

15

20

30

35

40

45

Bingham yield value (Pa)

Georgia Florida

y = 0.1489x – 1.6733 R 2 = 0.9622

0

5

10

15

20

C 14 Apparent viscosity (mPa s)

25

25

30

35

40

45

Georgia

12 Florida

10 8 y = 0.1986x – 0.9246 R 2 = 0.9723

6 4 2 0 0

5

10

15

20

25

30

35

40

45

Ellipticity (L/W) of the fibers FIGURE 5 Effect of ellipticity of fibres on plastic viscosity (A), Bingham yield value (B) and apparent viscosity (C) of 3% (w/v) suspensions of Na-palygorskites at pH 7. After Neaman and Singer (2000c).

In montmorillonite suspensions, in turn, edge-to-edge association is the primary mode of particle interactions at low pH values (Keren, 1988, 1989). In contrast, domains of parallel-oriented fibres were arranged by face-to-face contacts (Figure 6A and B) in palygorskite suspension at pH 3, a value that is below the PZC of the mineral surface. This particle arrangement may be due to low magnitude of net electrostatic charge of the surface at pH 3 (Figure 3) that allows the van der

334


C

A

1 µm

B

1 µm

1 µm

D

1 µm

FIGURE 6 Scanning electron micrographs of Na-palygorskite from the Negev showing differences in arrangement of fibres in suspension between low and high pH values. (A) and (B) pH 3 at different magnifications, (C) and (D) pH 10 at different magnifications. After Neaman and Singer (2000c).

Waals attraction to predominate over electrostatic repulsion. Thus, at low pH values, the models of particle arrangement in the suspension of platy clays do not account for the behaviour of palygorskite suspensions because of differences in surface structure and particle morphology. At high pH values, however, the particle arrangement in palygorskite suspension was similar to that in montmorillonite and kaolinite suspensions. The net electrostatic charge of palygorskite surface is high at pH 10 (a value that is well above the PZC of the mineral surface). Under these conditions, the single palygorskite fibres repel each other and no domains are formed in the suspension (Figure 6C and D), similar to kaolinite and montmorillonite suspensions.

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Palygorskite has a wide range of industrial applications, one of which is its usage as drilling muds. Palygorskite muds have the advantage over other clays (such as smectite) in being less sensitive to salts, remaining relatively constant even at high electrolyte concentrations (Galań, 1996). In agreement with this industrial experience, plastic viscosity and Bingham yield value of palygorskite suspensions were only slightly affected by electrolyte addition in the pH range below 7. The same characteristics, however, were significantly influenced by electrolyte addition at pH > 9 (Figure 7). At pH 7, palygorskite fibres form micro-aggregates because the magnitude of the negative surface charge is low and van der Waals attraction predominates over electrostatic repulsion. The addition of electrolytes

A


4

Electrolyte concentration

3

DW 0.001 N NaCl 0.01 N NaCl 0.1 N NaCl

2

1

0 0 B

2

4

6

8

10

12

5


4 Electrolyte concentration 3 DW 0.001 N NaCl 0.01 N NaCl 0.1 N NaCl

2 1 0 0

2

4

6

8

10

12

−1 pH FIGURE 7 The effect of electrolyte addition on plastic viscosity (A) and Bingham yield value (B) of 3% (w/v) suspensions at different pH values for Na-palygorskite from the Negev. DW = distilled water. After Neaman and Singer (2000c).

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increases the plastic viscosity and the Bingham yield value by compressing the electrical double layers, thus lowering the energy barrier to van der Waals attraction. The influence of electrolyte addition is slight, because the initial magnitude of the negative surface charge is low and electrostatic repulsion is weak (Figure 3). At pH 9, the fibres of palygorskite tend to repel each other since the magnitude of the negative surface charge is high. As a result, individual fibres of palygorskite can move independently under flow. The addition of electrolyte to palygorskite suspensions at pH 9 significantly increases the plastic viscosity and Bingham yield value due to coagulation of the system.

5. RHEOLOGY OF PALYGORSKITE-CONTAINING SOIL CLAY SUSPENSIONS A question arises if models of particle–particle interactions proposed for standard palygorskite clays are applicable for palygorskite-containing soil clays. Since smectite is always present in the clay fraction of palygorskitecontaining soils, Neaman and Singer (2000d) studied the rheology of mixed palygorskite–montmorillonite suspensions. Addition of montmorillonite to palygorskite suspensions did affect the rheological behaviour of the system. The degree of interaction between palygorskite and montmorillonite particles depended on the montmorillonite concentration in the mixture (Figure 8). Small (up to 10%) montmorillonite additions increased the rheological parameters (plastic viscosity and Bingham yield value). Increased additions (10–20%) of montmorillonite, however, decreased the rheological parameters. At even larger montmorillonite additions, in the range of 20–40%, the suspensions showed nearly Newtonian flow, with a plastic viscosity equal to the initial value of pure palygorskite suspension. Thus, at montmorillonite concentrations of 20–40% in the mixture, the interactions between palygorskite and montmorillonite particles were negligible, possibly because palygorskite and montmorillonite prevent each other from forming a three-dimensional network structure in the suspension. Additions of montmorillonite over 40% increase sharply the rheological parameters of the suspensions. The rheology of some palygorskite–smectite–kaolinite-containing soil clays also was studied (Neaman, 2000). Although soil clays differ considerably from each other in clay particle morphology and predominant clay mineral composition, no significant differences were found in the rheological behaviour of the suspensions among the clays. All suspensions exhibited Newtonian flow even at high (10% w/v) suspension concentrations suggesting that the number of linkages between clay particles was negligible. These results suggest that a three-dimensional network structure does not form in suspensions of soil clays, possibly because different clay minerals prevent each other in doing it, analogous to palygorskite–montmorillonite mixtures. The presence of primary minerals (such as quartz and feldspar) in

Chapter

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10


9 Sample location

8 7

Mt. Grainger Mt. Flinders Yucatan

6 5 4 3 2 1 0 0


B

20

40

60

80

100

3 2.5 Sample location 2 Mt. Grainger Mt. Flinders Yucatan

1.5 1 0.5 0 0

20

40

60

80

100

Percentage of montmorillonite in the mixture FIGURE 8 Effect of montmorillonite concentration on plastic viscosity (A) and Bingham yield value (B) of mixed suspensions of Na-saturated palygorskite and montmorillonite at a total clay concentration of 3% (w/v) and pH 7. After Neaman and Singer (2000d).

the clay fraction of the soil may also play some role in preventing the network structure formation in the suspension. Zhao et al. (1991) reported that the particle arrangement in three suspensions of soil clays, in which kaolinite, montmorillonite and illite were predominant clay minerals, was similar to that in suspensions of standard kaolinite, montmorillonite and illite, respectively. Above-mentioned results of Neaman (2000) show, however, that the rheological behaviour of soil clay suspensions is complex and cannot be predicted from that of suspensions of standard clays that appear in the soil clay fraction. In the particular case of the studied soils, the flow of clay suspensions in the soil profile is not expected to affect significantly the hydraulic conductivity of the soils, as the clay suspensions have low viscosity and exhibit Newtonian flow even at high (10% w/v) solid concentrations in the suspension.

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6. FLOCCULATION OF PALYGORSKITE-CONTAINING CLAYS Flocculation of clays has received much attention because of its effect on the physical behaviour of soil. FV is the minimum electrolyte concentration necessary to flocculate a given colloid dispersion in a given time (van Olphen, 1977). Neaman and Singer (1999) investigated flocculation of standard palygorskites, palygorskite–montmorillonite mixtures and palygorskite-containing soil clays. The flocculation of the clays was determined visually by settling after 24 h of standing in a series of test tubes (Figure 9A). The FV of Na-saturated palygorskites at near neutral pH was found to be significantly lower than that of Na-saturated montmorillonite from Wyoming. The FVs of four standard palygorskites of different origins were in the range from 0.2 to 2.5 cmolc/L NaCl, and the FV of Na-montmorillonite was 13.3 cmolc/L NaCl at near A Palygorskite, Florida (PF1-1), 0.066% (w/v) suspensions 1

2

3

4

5

6

B Palygorskite, Florida (PF1-1), 0.666% (w/v) suspensions 1

2

3

4

5

6

FIGURE 9 Flocculation test shows the Florida palygorskite suspensions after 24 h of standing and effect of clay concentration on settling of clay particles. Clay concentration: (A) 0.066% and (B) 0.666%. NaCl concentrations: (1) 0, (2) 0.3, (3) 1.7, (4) 3.3, (5) 6.7, (6) 8.3 cmolc/L. After Neaman and Singer (1999).

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neutral pH values. The low FV of palygorskite is related to its low degree of isomorphic substitution and to its low negative surface charge (Figure 3). FVs of palygorskite–montmorillonite mixtures at near neutral pH values increased with montmorillonite percentage in the mixture. At a specific concentration of montmorillonite in the mixture (in the range of 40–60%), the FV of the system attains the FV of pure montmorillonite and does not change with further montmorillonite addition. FVs of three palygorskite– smectite–kaolinite-containing soil clays (from which carbonates, iron oxides and organic matter had been removed) at near neutral pH values were the same as FV of standard montmorillonite. Studies by Goldberg and Glaubig (1987) for kaolinite–montmorillonite mixtures as well as the above-mentioned results of Neaman and Singer (1999) indicate that smectite has a dispersive effect on both kaolinite and palygorskite. Thus, presence of palygorskite in the clay fraction of soils that also contain smectite does not influence the FV of the soil clay. The industrial experience of less sensitivity to salts of palygorskite drilling muds in comparison to smectite muds (Galań, 1996) suggests that the FV of palygorskite is higher than that of smectite. Above-mentioned data indicate, however, that the FV of palygorskite at near neutral pH is significantly lower than that of montmorillonite. This apparent contradiction can be explained by the difference in the sedimentation rates of palygorskite particles at different clay concentrations. Only very diluted suspensions (< 0.1%) can be used for FV measurements of palygorskite using flocculation series tests. Settling in the suspension of 0.066% was observed after 24 h of standing, but this did not occur in the 0.666% suspension (Figure 9). Likewise, Dixon and Golden (1990) described the problem of dewatering and sedimentation of palygorskite–smectite suspensions that are waste products in phosphate mining areas in Florida. The sedimentation of these clays proceeds at a very slow pace even in saline environments. Two parts of the flocculation process can be distinguished: (1) association between the individual particles and formation of flocculi (coagulation) and (2) settling of flocculi formed, under gravity. Although coagulation also occurs in concentrated (> 0.1%) palygorskite suspension with salt addition, the flocculi formed cannot settle down under gravity (Figure 9B) due, most probably, to formation of a three-dimensional network structure or ‘scaffolding structure’ throughout the mass of the suspension (van Olphen, 1977). In industrial applications, concentrated palygorskite suspensions are used. Electrolyte addition to such suspensions causes coagulation of the fibres that, in turn, increases viscosity of the suspension. At pH < 7, the influence of electrolyte addition on the viscosity of palygorskite suspensions is slight (Figure 7), because the initial magnitude of the surface charge is low and electrostatic repulsion is weak (Figure 3).

340


Thus, low FVs of dilute palygorskite suspensions and low rheological susceptibility of concentrated palygorskite suspensions to salts known from industry are not contradictory. This apparent contradiction can be explained by the difference in the sedimentation rates of palygorskite particles at different clay concentrations.

7. EFFECT OF PALYGORSKITE ON DISAGGREGATION AND COLLOID MIGRATION IN SOILS The forces leading to particle disruption can be of physical and/or chemical nature. The resistance of aggregates to mechanical stress depends on the intensity of the forces between the particles. Mechanical action will release soil components that are aggregated by very loose bonds only or not at all. Chemical dispersion, however, will disrupt firm bonds. For chemical dispersion to be effective, the exchangeable sodium percentage (ESP) value should exceed 5, and the electrolyte concentration of the solution should be lower than the FV of the clays (Sumner, 1993). Disaggregation is defined as the separation of an individual particle from an aggregate and its lack of re-aggregation within a short time-interval. Disaggregation, accordingly, is a response to a predominantly physical disruptive process and differs from dispersion, which arises predominantly from chemical factors (Bu¨hmann et al., 1996). Neaman et al. (1999) examined the influence of clay mineralogy on disaggregation in some palygorskite–smectite–kaolinite-containing soils of the Jordan and Bet-She’an Valleys, Israel. The soils used in that study had low ESP values (< 5). The disaggregation potential of different minerals in the soil clay fraction was investigated by determining the differences in the mineral composition between the bulk and the disaggregated clay fractions. By shaking the soil with distilled water, calcite, dolomite, feldspar and palygorskite were disaggregated preferentially. Palygorskite was found to be the most strongly disaggregated mineral among the phyllosilicates that appear in the soil clay fraction. The differences in the disaggregation potential between various clay minerals can be explained in terms of the degree of clay aggregation in soils and the intensity of the binding forces between different clays. Smectite and kaolinite, when saturated with Ca, are known to form aggregates (Banin and Lahav, 1968; Dixon, 1977), which are stable to mechanical action, at least at ESP < 5. Scanning electron microscopy observations indicate that palygorskite fibres did not associate into aggregates in soils and suspensions, even when saturated with Ca ions (Figure 10). As a consequence, palygorskite was easily disaggregated following mechanical stress. Observations in the above-mentioned study of Neaman et al. (1999) can be applied to rain-fed agriculture or irrigation with high quality water, which has a low sodium adsorption ratio (SAR). In most cases, however, water used for

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B

1mm

1mm

FIGURE 10 Scanning electron micrographs showing the aggregation of palygorskite in suspension of Ca-saturated soil clays at pH 8. (A) Bulk clay fraction, (B) clay fraction released by a disaggregation test. Fibrous particle up to 1.5 mm long and 0.25 mm wide can be identified. The average length and diameter of palygorskite fibres are known to be in the range of 1–2 and 0.1–0.25 mm, respectively. The fibrous particles observed can be, therefore, defined as individual palygorskite fibres. After Neaman et al. (1999).

irrigation in arid areas has relatively high SAR. For this reason, Neaman et al. (2000) have investigated the migration of fine particle within columns of Na-saturated soil–sand mixtures (containing 10% or 20% of soil). Again, palygorskite was found to be the most mobile mineral among the phyllosilicates. The observation that palygorskite has the strongest disaggregation potential and the highest ability to migrate within the soil among the phyllosilicates suggests that palygorskite particles are likely to move preferentially over smectite and kaolinite downwards in the soil profile, and eventually to clog soil pores. Khademi and Mermut (1999) also reported the eluviation of palygorskite from topsoil and its entrapment by the pedogenic carbonate in the subsurface horizons. Disaggregation and migration of palygorskite from the surface-soil during rainfall and/or irrigation may have an effect on the degree of erosion as well.

8. PALYGORSKITE-CEMENTED CRUSTS (PALYCRETES) Duripan (Soil Survey Staff, 1999) is among the most common surface phenomena that affected ancient and modern landscapes. Essentially, duripans are composed of course-grained mineral material cemented together by matrix materials such as carbonates, silicates or iron oxides. Common duripans in dry

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areas are calcretes and silcretes. Duripans, in which the cementing material was dominantly composed of palygorskite, have been described recently by Meyer and Pena dos Reis (1985), Wright et al. (1993), Rodas et al. (1994) and Stahr et al. (2000). These duripans were tentatively named palycretes. Stahr et al. (2000) suggested that palycretes were formed under a semi-arid, seasonal climate, where strong evaporative processes were active. The authors reported that palygorskite has been eliminated from modern soils developed on the palycrete, indicating that conditions favourable for palygorskite stability had been mainly confined in time when the palycrete had been formed. Palycretes act the way other duripans do in negatively effecting agricultural cultivation practices. Where palycretes occur close to the surface, growth of trees is severely impeded. Palycretes also reduce water permeability of the soils, creating conditions of water-logging that necessitate drainage installation.

9. MECHANISMS OF METAL SORPTION ON PALYGORSKITE Metal contamination of soils has become a worldwide concern because of its potential effects on plant growth and human health. Sorption and desorption of metals on/from soil constituents are of particular interest because these processes determine the mobility and bioavailability of metals in the environment. Sorption is a general term used when the retention mechanism at a surface is unknown (Sparks, 2003). Adsorption and surface precipitation are examples of sorption. In order to elucidate the mechanism involved in the sorption process of Cd on palygorskite, Shirvani et al. (2006a) determined the amounts of magnesium and proton released to the equilibrium solution during the sorption process.

7.6 y = −0.0202x + 7.4812 R2 = 0.958

Equilibrium pH

7.5

7.4

7.3

7.2 0

2

4

6

8

10

Cd sorbed (µmol g−1) FIGURE 11 Effect of Cd sorption on the equilibrium pH of the palygorskite suspension. After Shirvani et al. (2006a).

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22 y = 0.9819x + 11.315 R 2 = 0.941

Mg released (µ moles g−1)

20 18 16 14 12 10 0

2

4 6 Cd sorbed (µ moles g−1)

8

10

FIGURE 12 Effect of Cd sorption on the Mg release from palygorskite. After Shirvani et al. (2006a).

The increase observed in the amounts of Mg2þ and Hþ released with the increase in the amount of Cd sorbed (Figures 11 and 12) suggests two possible sorption mechanisms. The first mechanism is replacement of Mg2þ at the edges of the octahedral layer by Cd2þ. The second mechanism is inner-sphere adsorption of Cd2þ by the functional groups of the mineral and subsequent release of Hþ. However, the authors stated that surface precipitation is another possible mechanism of Cd sorption on palygorskite. In order to elucidate further the mechanism of Cd sorption on palygorskite, Shirvani et al. (2006b) studied isotherms of Cd sorption–desorption on/ from palygorskite. The sorption and desorption reactions did not provide the same isotherms (Figure 13). The authors suggested that inner-sphere adsorption and nucleation might have caused the observed hysteresis. In adsorption, isolated site binding is the dominant sorption process. Nucleation refers to a metal loading on the surface greater than in adsorption. Further increase in metal loading results in surface precipitation (Sparks, 2003). The observed sorption irreversibility (Figure 13) is generally the result of incomplete desorption experiment due to the slow kinetics involved. Shirvani et al. (2006b) suggested that the extent of desorption irreversibility largely depends on the mechanisms involved in the sorption process. The sorption irreversibility observed by the authors is of particular interest because desorption of the sorbed metal ions from the solid phase actually controls the availability and leachability of these ions in soils.

344


12

Cd sorbed (µ mol g−1)

10 8 6 4 2 0 0

10

20

30

40

Cd in solution (µM) FIGURE 13 Cadmium sorption isotherms (♦) and desorption isotherms at three initial Cd loads of 40% (D), 70% (□), and 100% (◊) of the mineral maximum sorption capacity. Lines are Freundlich model predictions of sorption and desorption. After Shirvani et al. (2006b).

10. USE OF PALYGORSKITE FOR METAL IMMOBILIZATION IN SOILS A number of techniques have been developed to remediate metal-contaminated soils. In situ immobilization (or fixation) of metals can be defined as a reduction of the metal concentration in the soil solution by adding an amendment to the soil, leaving the soil porous structure intact (Oste et al., 2001). Thus, the metals are not removed from the soil but are transformed to a less soluble form. Reduction of solubility, in turn, reduces the metal mobility in the soil and its availability to plants and soil organisms. ´ lvarez-Ayuso and Garcıá-Sańchez (2003) evaluated the effectiveness A of palygorskite to reduce the mobility of Pb, Cu, Zn and Cd in a soil contaminated by mining activities. The authors kept palygorskite-amended soils and an unamended soil at 70–80% of their water-holding capacity for 4 weeks. Then, they determined the water-soluble and the NH4NO3extractable metal fractions in these soils. Palygorskite showed an immobilizing effect on the metals in the soils. At the palygorskite dose of 4%, the water-soluble Pb, Cu and Cd concentrations turn out to be almost completely immobilized while that of Zn decreased 60.4% with respect to the unamended soil (Figure 14). At this same palygorskite dose, the NH4NO3-extractable concentrations decreased to 91.7% for Pb, 77.0% for Cu, 76.4% for Zn and 47.5% for Cd. When lower palygorskite

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The Effects of Palygorskite Water 100

98.1 100

100

89.3

100

100 100 100

83.3

% Immobilized

80 60.4

60 42.2

40 15.6

20 0

Pb

Cu

Zn

Cd

NH4NO3 100

91.7 81.3 73.1

% Immobilized

80

77.0

76.4

67.8

65.3

60.3

60

47.5 41.8 36.0

40

22.3

20 0

Pb

Cu 1%

Zn 2%

Cd

4%

FIGURE 14 Effect of palygorskite addition to a metal-contaminated soil on the metal extract´ lvarez-Ayuso ability. The percentage of retention is with respect to an unamended soil. After A and Garcıá-Sańchez (2003).

doses were applied (1% and 2%), the metal concentrations were also greatly ´ lvarez-Ayuso and reduced (Figure 14). The column studies performed by A Garcıá-Sańchez (2003) also showed a high reduction in the metal leaching (50% for Pb, 59% for Cu, 52% for Zn and 66% for Cd) when a palygorskite was applied at dose of 4%. Thus, palygorskite appeared as an effective amendment to immobilize metals in contaminated soils in this laboratory experiment.

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11. RESEARCH NEEDS Although some progress has been made recently in understanding the effects of palygorskite on chemical and physico-chemical properties of soils, much research remains to be done. Performing some additional studies on disaggregation potential and migration ability of palygorskite under conditions close to those in the field (e.g. soil monoliths with undisturbed structure, using a rain simulator) is strongly desirable. Scanning electron microscopy observations on possible changes in aggregation of palygorskite in soils following intensive irrigation might be useful. Studies of the rheological behaviour of soil clays with different mineralogical composition, and palygorskite-containing soil clays in particular, are strongly encouraged. These studies are expected to be beneficial for improving the understanding of the behaviour of palygorskite in a porous soil environment under irrigated agriculture. The sorption of various organic molecules has been studied on monomineralic and homoionic palygorskite specimens (e.g. Shariatmadari et al., 1999 and references therein), and important information on the sorptive properties of palygorskite has been obtained from these studies (Singer, 2002 and references therein). Likewise, palygorskite can adsorb metal irreversibly and appeared as an effective amendment to immobilize metals in contaminated soils in a laboratory experiment. Further studies would be required to confirm these findings under field conditions. Also, little information is available on how the sorptive behaviour of palygorskite translates into soil properties, when palygorskite constitutes a significant component of the soil clay fraction. Therefore, there is a need to study the fate of various contaminants (both organic and inorganic) in palygorskite-containing soils.

REFERENCES ´ lvarez-Ayuso, A., Garcıá-Sańchez, A., 2003. Palygorskite as a feasible amendment to stabilize A heavy metal polluted soils. Environ. Pollut. 125, 337–344. Banin, A., Lahav, N., 1968. Particle size and optical properties of montmorillonite in suspension. Isr. J. Chem. 6, 235–250. Bickmore, B.R., Bosbach, D., Hochella, M.F., Charlet, L., Rufe, E., 2001. In situ atomic force microscopy study of hectorite and nontronite dissolution: implications for phyllosilicate edge surface structures and dissolution mechanisms. Am. Mineral. 86, 411–423. Bu¨hmann, C., Rapp, I., Laker, M.C., 1996. Differences in mineral ratios between disaggregated and original clay fractions in some South African soils as affected by amendments. Aust. J. Soil Res. 34, 909–923. Chou, L., Garrels, R.M., Wollast, R., 1989. Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals. Chem. Geol. 78, 269–282. Corma, A., Mifsud, A., Sanz, E., 1987. Influence of the chemical composition and textural characteristics of palygorskite on the acid leaching of octahedral cations. Clay Miner. 22, 225–232. Corma, A., Mifsud, A., Sanz, E., 1990. Kinetics of the acid leaching of palygorskite: influence of the octahedral sheet composition. Clay Miner. 25, 197–205.

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Sparks, D.L., 2003. Environmental Soil Chemistry, second ed. Academic Press, Amsterdam, 352 pp. Stahr, K., Ku¨hn, J., Trommler, J., Papenfuss, K., Zarei, M., Singer, A., 2000. Palygorskite-cemented crusts (palycretes) in Southern Portugal. Aust. J. Soil Res. 38, 169–188. Sumner, M.E., 1993. Sodic soils: new perspectives. Aust. J. Soil Res. 31, 683–750. Tournassat, C., Neaman, A., Villie´ras, F., Bosbach, D., Charlet, L., 2003. Nanomorphology of montmorillonite particles: estimation of the clay edge sorption site density by low-pressure gas adsorption and AFM observations. Am. Mineral. 88, 1989–1995. van Olphen, H., 1977. An Introduction to Clay Colloid Chemistry, second ed. John Wiley and Sons, New York, NY, 318 pp. Weaver, C.E., Pollard, L.D., 1973. The Chemistry of Clay Minerals. Developments in Sedimentology, vol. 15. Elsevier, Amsterdam, Netherlands, 213 pp. Wright, M.J., Milnes, A.R., Chittleborough, D.J., 1993. Neo-formation of palygorskite in duripans of the Australian arid zone. In: Abstracts of the 10th International Clay Conference, Adelaide, Australia, 55. Zhang, X.C., Norton, L.D., 2002. Effect of exchangeable Mg on saturated hydraulic conductivity, disaggregation and clay dispersion of disturbed soils. J. Hydrol. 260, 194–205. Zhao, H., Low, P.F., Bradford, J.M., 1991. Effects of pH and electrolyte concentration on particle interaction in three homoionic sodium soil clay suspensions. Soil Sci. 151, 196–207.

Chapter 15

Adsorption of Surfactants, Dyes and Cationic Herbicides on Sepiolite and Palygorskite: Modifications, Applications and Modelling Uri Shuali*, Shlomo Nir* and Giora Rytwo{{ *

Department of Soil Science, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel { Department of Environmental Sciences, Faculty of Sciences and Technology, Tel-Hai Academic College, Upper Galilee, Israel { MIGAL, Galilee Technology Center, Kyriat Shmona, Israel

1. INTRODUCTION In 1984 was published the book ‘Palygorskite—Sepiolite: Occurrences, Genesis and Uses’ (Singer and Galan, 1984). While working on an update version of the book, Prof. Arieh Singer passed away prematurely. This chapter is dedicated to commemorate his contribution to the sepiolite/ palygorskite research and his legacy. The current chapter focuses on the adsorption of organic molecules, such as surfactants, and the modelling of the adsorption of cationic and neutral surfactants by these clays.

2. SURFACE-RELATED PHYSICO-CHEMICAL PROPERTIES 2.1. Formulae and Chemical Analyses It was suggested that the ideal structural formulae of sepiolite are represented by two models: Si12Mg9O30(OH)6(OH2)46H2O (Nagy and Bradley, 1955) Si12Mg8O30(OH)4(OH2)48H2O (Brauner and Preisinger, 1956) Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00015-3 # 2011 Elsevier B.V. All rights reserved.

351

352


More recently, Santaren et al. (1990) suggested a formula based on the Brauner and Preisinger (1956) model: Si12O30Mg8(OH,F)4(H2O)4 8H2O. The ideal structural formula of palygorskite is represented by: Si8Mg5O20(OH)4(OH2)44H2O (Bradley, 1940) These clays are characterized by a porous structure, resulting from repeated inversions of the silicate layer, in which the SiOSi tetrahedrons serve as bridging groups between the alternating ribbons of the alumino–Mg silicates. The SiO bond in the bridging SiOSi group has a double-bond character (Yariv, 1986). The two minerals differ in the frequency of this inversion sepiolite having wider channels than palygorskite (0.37 1.06 vs. 0.37 0.64 nm), respectively (Singer, 1989); Kitayama and Michishita (1981) found in sepiolite pore dimensions of 0.56 1.1 nm. Representative data of chemical analysis are given in Table 1. Recently, Garcia-Romero and Suarez (2010) investigated the chemical composition of the two clays using analytical electron microscopy (AEM). In contrast to the accepted claim that a compositional gap exists between them, in which sepiolite occupies the most magnesic and trioctahedral extreme and palygorskite occupies the most aluminic–magnesic and dioctahedral extreme, their findings show that all intermediate compositions may exist between the two pure extremes.

2.2. Crystallography The cell-unit crystallographic data for these clays are summarized in Table 2. Sepiolite is characterized by a strong X-ray reflection in the 110 plane at ˚ ; the ˚ and a back reflection towards the b axis at d ¼ 27 A d ¼ 12 A ˚ corresponding values for palygorskite are 10.5 and 18 A. The information concerning the X-ray spectra of the two clays was gathered and reported by Caillere and Henin (1972) and Bailey (1980).

2.3. Surface Area and Porosity Based on the models for sepiolite and palygorskite, the calculated specific surface areas are 900 and 815 m2/g, respectively (Serna and van Scoyoc, 1979). The experimental values are significantly smaller than the theoretical ones; they depend on the nature of the test agents and the mathematical model for calculations (Del Rey et al., 1985; Fernandez-Alvarez, 1978), and on the degree of dehydration of the clays (Fernandez-Alvarez, 1978; Jimenez-Lopez et al., 1978). Representative values of measured surface areas are: Sepiolite: 230–320 m2/g. (Dandy, 2006; Radojevic et al., 2002; RuizHitzky and Fripiat, 1976; Serratosa, 1978). Palygorskite: 125–195 m2/g. (Barrer et al., 1954; McCarter et al. 1950).

Chapter

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353

Adsorption of Surfactants

TABLE 1 Chemical Analyses (%) of Sepiolite and Palygorskite. Palygorskite, Quincy, Florida

Sepiolite, Vallecas, Spain Galan and Castillo (1984)

Campelo et al. (1987)

Shuali (1991)

Nathan (1969a)

Nathan (1969b)

Shuali (1991)

SiO2

63.10

62.0

55.26

48.61

55.35

59.38

Al2O3

1.08

1.7

1.52

7.71

8.91

8.02

MgO

23.08

23.9

23.08

8.88

10.62

8.40

Fe2O3

0.27

0.5

1.06

3.06

3.64

3.46

FeO

n.d

n.d

n.d

0.16

–

n.d

TiO2

–

n.d

–

0.40

–

0.22

MnO

–

n.d

–

0.02

0.02

–

CaO

0.49

0.5

0.27

0.81

0.95

1.41

Na2O

0.09

0.3

0.1

n.d

0.07

15 A (b) Structural porosity resulting from the repeated inversions of the silicate layer: The two minerals differ in the frequency of this inversion, sepiolite having wider channels than palygorskite (0.37 1.06 vs. 0.37 0.64 nm, ˚. respectively; Singer, 1989). Their effective diameters are < 15 A

354


TABLE 2 Cell-Unit Crystallographic Data.

Sepiolite

Palygorskite

a

b

c or c sinb

Nagy and Bradley (1955)

5.30

27.00

13.40

A/2m

Brauner and Preisinger (1956)

5.28

26.80

13.40

Pnan

Brindley (1959)

5.25

26.96

13.50

–

Zvyagin et al. (1963)

5.24

27.20

13.40

Pnan

Bailey (1980)

5.28

26.95

13.37

Pnan

Galan (quoted by Jones and Galan, 1988)

5.23

26.77

13.43

Pnan

Bradley (1940)

5.20

18.00

12.90

95.83

12.75

Zvyagin et al. (1963) Christ et al. (1969)

5.22 5.24 5.24

Bailey (1980)

5.20

18.06 17.87 17.83 17.90

12.72 12.78 12.70

Space Group

b

90

P2/a

Pn

P2/a

95.78 90.96 107

A2/m

TABLE 3 Specific Surface Areas (m2/g) at Various Dehydration Temperatures. T ( C)

25

Sepiolite

141.5 9.3 213.4 15.8 269.0 21.9

Palygorskite 92.3 6.9

135

250

350

275.3 18.7

220.4 17.3 127.4 6.1

139.4 7.1

158.3 8.3

141.6 7.8

550

700

900

117.2 5.6

108.3 5.6

67.4 5.1

60.8 4.9

T ( C)

450

Sepiolite

176.0 9.9 152.0 8.7

Palygorskite 81.4 6.3

180

74.5 5.2

Although several authors considered that the internal surface areas of the structural pores are not accessible to branched molecules, it has been shown that intra-pore adsorption of small- and short-branched molecules like pyridine and trimethylpyridine (Shuali, 1991; Shuali et al., 1989) or acetone (Kuang et al., 2006) does occur. Ovarlez et al. (2009) showed that dye molecules as large and hydrophobic as indigo can be incorporated inside sepiolite thus changing dramatically the thermal stability of sepiolite and preventing nanoclay folding.

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Based on these findings, Yariv and Cross (2002) suggested a new terminology for the structural porosity which differs between channels and pores: the term ‘pores’ is attributed to vacancies crossing along the clay crystal (boarded by the inverted TOT units and open at the edges), whereas the term ‘channel’ is attributed to vacancies at the edges/broken parts of the clay crystal.

2.4. Adsorption Sites and Ion Exchange Capacity The silanol groups (SiOH) present at external surfaces of the tetrahedral sheet are usually accessible to organic species, acting as neutral adsorption sites (designated N). The ‘N sites’ content of Vallecas–Vicalvaro sepiolite is estimated to be in the order of 0.60 mmol/g (Ruiz-Hitzky, 1974; Ruiz-Hitzky and Fripiat, 1976; Rytwo et al., 1998). Isomorphic substitutions, such as Al3þ for Si4þ, are responsible for the charged adsorption site (P sites) and the cation exchangeability. The reported values for the cation-exchange capacity of palygorskite or sepiolite vary between 0.03 mmol/g (palygorskite) and 0.1– 0.15 mmol/g sepiolite. According to Grim (1968) it is difficult to determine exact values for palygorskite because of existence of montmorillonite impurities. Galan (1987) reported for sepiolite (Vallecas, pangel) CEC of 0.095 meqiv./g clay. Shuali (1991) reported the values 0.061 and 0.076 meqiv./g. (ammonium acetate method) for sepiolite (Vallecas) and palygorskite (Quincy Florida), respectively. Lemic et al. (2005), who studied the modification of sepiolite with various quaternary amines, found that their adsorption capacities, calculated by fitting experimental data to the Langmuir–Freundlich equation, were 324%, 278% and 258% of the CEC for stearylbenzyldimethylammonium (SBDMA; octadecylbenzyldimethylammonium), distearyldimethylammonium (DDA; dioctadecyldimethyl) and hexadecyltrimethylammonium (HDTMA; cetyltrimethylammonium), respectively.

2.5. Thermal Analysis The thermal behaviour and stability of sepiolite and palygorskite were intensively investigated, and results were summarized in several surveys (for instance, Fenoll Hach-ali and Martin Vivaldi, 1968; Singer, 1989). During the heating process, the following changes occur: (a) Loss of adsorbed, bound and zeolitic water. The desorption of bound water occurs in two stages, in which the first one is reversible, and at its end, the clays pass a structural folding (into sepiolite- or palygorskite-anhydride) (b) Dehydroxylation of the clays (c) Collapse of the lattice and transformation into meta-clay state (d) Crystallization into new phases

356


The first three stages are endothermic while the last is exothermic. Shuali (1991), using DTA-TG-DTG-MS and DSC techniques, found in the DTA curve of sepiolite (Vallecas, Spain) four endothermic peaks which were associated with water loss: (a) (b) (c) (d)

135 C 315 and 360 C (two small peaks) in air or 345 C in nitrogen atmosphere 515 C and 825 C

The last endothermic peak was followed by an immediate exothermic peak at 825 C. For palygorskite (Quincy, Florida), he reported the existence of three endothermic peaks which were associated with water loss: (a): 150 C; (b): 265 and 285 C in air and nitrogen atmospheres, respectively; (c): 465 C.

2.6. Infrared Spectroscopy IR spectral data of sepiolites from different sites were reported by van der Marel and Beutelspacher (1976): the characteristic peaks are 503, 534, 784, 1025, 1074, 1200, 3245 and 3620 cm 1. The characteristic peaks for palygorskites are 482, 513, 648, 987, 1037, 1190, 3265, 3540 and 3620 cm 1. Zeolitic water gives rise to absorption bands at 1670, 3250, and 3420 (sepiolites) and 1660, 3290 and 3400 cm 1 (palygorskites). Absorption peaks for bound water in sepiolites or palygorskites are at 3568 and 3625 cm 1 and at 3550 and 3585 cm 1, respectively (Hayashi et al., 1969; Mendelovici, 1973; Prost, 1973, 1975; Serna et al., 1975, 1976, 1977; Serna and van Scoyoc, 1979). Prost and Serna et al. explain that the appearance of two stretching bands in the spectra of bound water is due to the fact that the two water molecules, which are bound to the octahedral cation exposed to the pores, are not identical. The clayey properties of sepiolite and palygorskite (relatively high surface area, existence of the ‘N’- and ‘P sites’ and the thermal stability) together with, acidic nature and molecular-sieving ability due to the structural pores/ channels, enable their use as excellent substrates for adsorption of neutral organic molecules and organic cations (Alvarez, 1984; Shuali, 1991).

3. SURFACE MODIFICATIONS OF SEPIOLITE BY SURFACTANTS—LITERATURE SURVEY The interactions between clay minerals and surfactants (e.g., quaternary amines, organic dyes, silanes), which change the chemical nature of the clayey matter, have been widely investigated and reviewed during the past two decades (e.g., Yariv and Cross, 2002). Sepiolite and palygorskite, which

Chapter

15


357

have large surface areas, zeolitic-like porous structure and a moderate cationexchange capacity (0.03–0.15 meqiv./g clay), have been found suitable for such treatments. A representative list of studies dealing with the modifications of these two clays, and their utilization in slow-release pesticide-formulations or remediation of polluted water, is given in Tables 4 and 5. The formation of organically modified sepiolite or palygorskite is carried out by applying the surfactants as monomers, micelles or liposomes, using the same techniques employed for the treatment of other clays, (e.g., Arrma¨ zcan et al., 2007; O ¨ zdemir et al., 2007; gan et al., 2003; Lemic et al., 2005; O Rytwo et al., 1998; Sanchez-Martin et al., 2006; Yildiz and Gu¨r, 2007). Rytwo et al. (2000, 2002) found that the adsorption on sepiolite of divalent organic cations (e.g., paraquat, diquat, methyl green) was between 100% and 140% of the CEC, and adsorption of monovalent organic cations (Rytwo et al., 1998) reached more than 400% of the CEC. They proposed that in the case of the divalent organic cations there is almost no interaction with the neutral sites. In contrast monovalent organic cations are mainly bound to the ‘N sites’. Arrmagan et al. (2003) who investigated the adsorption of negatively charged azo dyes on natural sepiolite and hexadecyltrimethylammonim (HDTMA) modified sepiolite, found that the adsorption on the natural clay occurred through electrostatic attraction of the dye molecules onto oppositely charged sites while the adsorption onto the modified clay is governed initially by electrostatic attraction onto the head groups of the already adsorbed quaternary amine. Modified sepiolite yields adsorption capacities of 169, 120 and 108 mg/g for ‘reactive yellow’, ‘reactive black’ and ‘reactive red’, respectively.1 The molecular weights of these dyes were not provided and therefore it is difficult to express the adsorbed amounts in units such as moles per gram for the estimation of the degree of coverage; yet, the adsorbed amount can imply adsorption above the ion exchange capacity (IEC) even if taking values of 800 Da. Characterization of the modified clays was carried out by means of XRD (Dogan et al., 2008; Tartaglione et al., 2008), SEM (Tartaglione et al., 2008) HRTEM (Tartaglione et al., 2008), zeta potential measurements (Alkan et al., 2005a,b; Demirbas et al., 2007; Dogan et al., 2008; Sabah et al., 2007) FTIR (Alkan et al. 2005b) and thermal analysis (Dogan et al., 2008; Lemic et al., 2005).

3.1. Cationic Surfactants Examination of the adsorption of cationic surfactants revealed that the molecules are sorbed in multilayers, with the first layer formed by cation exchange between magnesium and surfactant cations, whereas the other layers are 1. These azo dyes (manufactured by Everlight Chemical Industrial Corporation, Taiwan) are known to contain anionic sulphonate groups.

358

TABLE 4 Sepiolite Modification by Surfactants. Surface-treatment agents

References

Remarks

Dodecylammonium (DDA, DA)

Akçay and Yurdakoç (2000)

Removal of phenoxyalkanoic acid herbicides

Dodecylethyldimethylammonium (DDEDMA)

¨ zcan et al. (2005) O

Adsorption of nitrate ions

¨ zcan (2008) ¨ zcan and O O

Adsorption of Yellow 99

Go¨k et al. (2008)

Adsorption of naphthalene

Cationic surfactants

Dodecyltrimethylammonium (DDTMA)

Hexadecyltrimethylammonium (HDTMA)

Rodriguez-Cruz et al. (2008)

Sorption of fungicides (penconazole and metalaxyl); significance of the long-chain organic cation structure

Sabah and Celik (2002a)

Adsorption mechanism

Arrmagan et al. (2003)

Adsorption of negatively charged azo dyes

Li et al. (2003)

Removal of anionic contaminants Adsorption mechanism

Hexadecyltrimethyl ammonium (HDTMA)

Lemic et al. (2005)

Adsorption capacity and isotherm

Yildiz and Gu¨r (2007)

Adsorption of phenol and chlorophenols

Dihexadecyldimethylammonium (DHDDMA)




Hexadecylpyridine (HDPy)

Sabah and Celik (2002a)

Adsorption of pesticides, isotherms

Lemic et al. (2005)


Rodriguez-Cruz et al. (2007, 2008)


del Hoyo et al. (2008)

Physico-chemical study

Lemic et al. (2005)


¨ zdemir et al. (2007) O

Effect of adsorption parameters



Sanchez-Martin et al. (2008)

Studying the effect of clay structure on adsorption capacity

¨ zdemir et al. (2007) O

Effect of adsorption parameters



Rytwo et al. (1998), Shariatmadari et al. (1999)

Calculations estimate the contribution of different adsorption sites capacity


Effect of clay structure on adsorption capacity

Rytwo et al. (1998), Shariatmadari et al. (1999)

Calculations estimate the contribution of neutral adsorption sites

Anionic surfactants Sodium dodecylsulfate (SDS)

Sodium dodecylbenzenesulfonate (SDBS)


Sanchez-Martin et al. (2003, 2006)

15

Dioctadecyldimethylammonium (DODDMA)

Chapter

Octadecyltrimethylammonium (ODTMA)

Neutral Surfactants Triton 100

15 Crown ether 5 (15 C-5)

Continued

359

360

TABLE 4 Sepiolite Modification by Surfactants.—Cont’d Surface-treatment agents

References

Remarks

Rytwo et al. (1998)

Modelling

Shariatmadari et al. (1999)


Eren et al. (2010)

Adsorption, kinetics, thermodynamics

Rytwo et al. (1998)



Modelling, contribution of different adsorption sites

Methyl green (MG)

Rytwo et al. (2000, 2002)

Adsorption, modelling

Thioflavin-T (TFT)

Casal et al. (2001)

Photo and thermal stabilization of pesticides (trifluralin, TFT).

Dimethyloctadecylchlorosilane (DMODCS)

Alkan et al. (2005b)

Modification, FTIR, z potential

Dimethyldichlorosilane (DMDCS)



3-aminopropyltriethoxysilane



Demirbas et al. (2007)

Modification, adsorption of ions, electrokinetics

Turan et al. (2008)

Characterisation by FTIR, XRD, thermal analysis

Tartaglione et al. (2008)

Thermal and morphological characterisations, (TGA, SEM, HRTEM)

Cationic Dyes Crystal violet (CV)

Methylene blue (MB)

Triethoxy-3-(2-imidazolin-1-yl) propylsilane


Silanes

Surface-treatment Agents

Chapter

TABLE 5 Modifications of Palygorskite with Surfactants.

Tetramethylamine (TMA)

Chang et al. (2009)

Anion–cation premodification by quaternary amines and SDS, sorption of p-nitrophenol

Hexadecylpyridine (HDPy)


Sorption of fungicides (penconazole and metalaxyl); significance of the structure of long-chain organic cation

Hexadecyltrimethylammonium (HDTMA)

Li et al. (2003)

Removal of anionic contaminants

Chen and Zhao (2009)

Removal of congo red

Chang et al. (2009)

Anion–cation modified, sorption of p-nitrophenol


Sorption of fungicides (penconazole and metalaxyl); significance of the structure of long-chain organic cation

Chang et al. (2009)

Anion–cation modified, sorption of p-nitrophenol

Sanchez-Martin et al. (2003, 2006, 2008)

Sorption of pesticides, isotherms

Rodriguez-Cruz et al. (2007, 2008)

Sorption and retention of pesticides

Huang et al. (2008)

Selective adsorption of tannin from flavonoids


Estimates of the contribution of different adsorption sites


Studying the effect of clay structure on adsorption capacity


Studying the contribution of different adsorption sites, modelling

Cationic surfactants

Dihexadecyldimethylammonium (DHDDMA)

Octadecytrimethylammonium (ODTMA)


Remarks

15

References

Neutral Surfactants Triton X

15 Crown ether 5 (15 C-5)

361

Continued

362

TABLE 5 Modifications of Palygorskite with Surfactants.—Cont’d Surface-treatment Agents

References

Remarks


Influence of clay structure and surfactant nature



Al-Futaisi et al. (2007)

Adsorption isotherms, kinetics



Al-Futaisi et al. (2007), Shariatmadari et al. (1999)

Adsorption isotherms, kinetics

Chen and Zhao (2009)

Removal of anionic dye, adsorption kinetics, isothems

Anionic Surfactants Sodium dodecylsulfate (SDS) Cationic Dyes Crystal violet (CV)

Anionic Dyes Congo red


Methylene blue (MB)

Chapter

15


363

formed by hydrophobic bonding between surfactant molecules [Lemic et al. (2005) who studied the modification of sepiolite by octadecyltrimethylammonium (applied as monomers or as micelle species), by dioctadecyldimethylammonium and by hexadecyltrimethylammonium]. The large amount of released magnesium ions, compared to the CEC of sepiolite, was attributed to some dissolution of the minerals present. A kinetic study of the adsorption process showed that the adsorption process reached equilibrium in less than 1 h. The thermal behaviour of the modified sepiolite provided evidence for the multilayer adsorption. Similar behaviour was reported by Sabah and Celik (2002a).

3.2. Anionic Surfactants Adsorption isotherms of anionic surfactants exhibit three regions having different slopes. The first region is characterized by the complexation of the anionic surfactants with Mg2þ ions at the octahedral sheet or hydrogen bonding between the oxygen groups of anionic head groups of surfactant and Hþ of the bound or zeolitic water. The second region is characterized by the release of Mg2þ ions and their precipitation; the third region marks both the ¨ zdemir beginning of a plateau and micellar dissolution of the precipitate. (O et al. (2007) studied the modification of sepiolite by sodium dodecylsulphate (SDS) and sodium dodecylbenzenesulfonate (SDBS)).

3.3. Neutral Surfactants Adsorption of neutral surfactants (Triton X-100) on sepiolite occurred both on the surface and in the structural channels (del Hoyo et al., 2008 XRD, FTIR and TA techniques). Changes observed in the wave numbers of the OH vibration modes of the clay mineral indicated interactions of surfactant with the silicate through the functional groups of organic compound and the water coordinated to the exchangeable cations of the clay minerals, by ion-dipole or hydrogen bonding. Absorption bands corresponding to CH2 stretching and bending modes of hydrocarbon chain groups of Triton X-100 exhibit shifts to higher wave numbers. These displacements were related by del Hoyo et al. (2008) to a reorganization of the organic molecules when interaction with the adsorbent was established.

3.4. Pesticide Formulations The efficiency of palygorskite and sepiolite, modified with a cationic surfactant (hexadecyltrimethylammonium, ODTMA), in the adsorption of pesticides (penconazole, linuron, alachlor, atrazine and metalaxyl), was compared to that of kaolinite, montmorillonite, muscovite- or illite- modified by ODTMA. Sanchez-Martin et al. (2006) concluded that the efficiency of the ODTMA–clays

364


for adsorption of pesticides depends on the degree of saturation of adsorption of the organic cation. Depending on the clay content soils saturated with ODTMA provide natural barriers for decreasing the mobility of non-ionic pesticides, depending on the degree of hydrophobicity of the pesticides. The significance of the structure of long-chain organic cation in the sorption of pesticides was also investigated by this group of researchers; they compared the efficiency in sorption of the fungicides penconazole and metalaxyl by the above listed clays, modified by ODTMA, hexadecylpyridinium (HDPy) or the two-chain cationic surfactant dihexadecyldimethylammonium (DHDDMA; the bromide salt is also designated in literature as DDAB; Rodriguez-Cruz et al., 2008). The results obtained showed that the DHDDMA clays increased the sorption of the fungicides, depending on the type of clay (i.e., higher for layered than for non-layered clays), and on the fungicide hydrophobicity (higher for penconazole with Kow ¼ 3.72 than for metalaxyl with Kow ¼ 1.75). Modelling of adsorption was studied, for instance, by Rytwo et al. (1998), Shariatmadari (1998), Shariatmadari et al. (1999), Sabah et al. (2002a,b), ¨ zcan et al. (2007). This topic is treated intensively in the next part of the curO rent review.

4. MODEL EQUATIONS The model presented here was developed by Rytwo et al. (1998) for sepiolite. The same approach was described by Shariatmadari et al. (1999) for both sepiolite and palygorskite. In developing the model for adsorption of cations and neutral molecules to sepiolite the guiding information was that, it has an open structure exhibiting a microfibrous morphology with a high specific surface area (ca. 340 m2/g) and a large micropore volume (around 0.44 cm3/ g) due to the existence of intracrystalline cavities (tunnels). Sepiolite with Si12O30Mg8(OH,F)4(H2O)48H2O as unit cell formula (Brauner and Preisinger, 1956; Santaren et al., 1990) is structurally formed by an alternation of blocks and tunnels that grow up in the fibre direction (c axis). Each structural block is constructed by two tetrahedral silica sheets enclosing a central magnesia sheet similarly to other 2:1 silicates, such as talc, albeit in sepiolite there are discontinuities of the silica sheets that give rise to structural tunnels. Such arrangement determines that some silanol groups (SiOH) are present at the border of each block located at the ‘external surface’ of the silicate (Alrichs et al., 1975). These SiOH groups are usually accessible to organic species, acting as neutral adsorption sites (designated as N). The N sites content for the Vallecas–Vicalvaro (Spain) sepiolites can be estimated at about of 0.60 mmol/g (Ruiz-Hitzky, 1974; Ruiz-Hitzky and Fripiat, 1976). Certain isomorphic substitutions in the tetrahedral sheet, such as Al3þ instead of Si4þ, are responsible for the exchangeable cations that are needed to compensate for the electrical charge and constitute the charged

Chapter

15


365

adsorption sites (P sites). The cationic exchange capacity (CEC) for the considered sepiolites is in the range of 0.10–0.15 molc/kg. The model employed (Rytwo et al., 1998) is an extension of that described by Nir (1986) Margulies et al. (1988) and Rytwo et al. (1995), which accounts for the ability of charged organic monovalent cations to adsorb to neutral binding sites on the silicate layer. The rationale for such adsorption stems from analysing results by Alvarez et al. (1987), where adsorption of the neutral molecule Triton X-100 did not release exchangeable Mg2þ to the solution. Consequently, it was deduced that TX-100 adsorbs to neutral sites denoted by N. Let Xiþ denote a monovalent cation that binds to a singly charged negative site, P, on the surface of the silicate, creating the neutral complex PXi: P þ Xiþ , PXi

ð1Þ

with a binding coefficient, Ki, which satisfies: Ki ¼

½PXi ð½P ½Xiþ ð0Þ Þ

ð2Þ

in which [Xi(0)þ] is the concentration of the cation at the surface. The adsorption of another organic monovalent cation to the neutral complex creates a charged complex P(Xi)2þ: PXi þ Xiþ , PðXiÞþ 2

ð3Þ

with a binding coefficient ¼ Ki

½PðXiÞþ 2 ð½PXi½Xiþ ð0Þ Þ

ð4Þ

Such reaction was essential to explain adsorption at amounts higher than the CEC of montmorillonite, or its charge reversal (Margulies et al., 1988). When several organic monovalent cations interact with the clay, we can also have the formation of mixed complexes, but this consideration was not essential in this case. The calculations employed the relation: Xið0Þ ¼ XiY0 Zi ;

ð5Þ

where Xi(0) is the molar concentration of cation i in its monomeric form close to the mineral layer, Xi is the molar concentration in the equilibrium solution (at infinite distance from the clay), Zie fis the valence of the given ion, Y0 is a 0 Boltzmann factor defined as Y0 ¼ e kT in which the energy is given by the product of the surface potential (’0) by the absolute magnitude of an electronic charge (e). For a negatively charged surface Y0 > 1, and the concentration of the cation at the surface, Xi(0) may be significantly larger than Xi. When charge reversal

366


occurs, the surface potential is positive and Y0 < 1, reducing significantly the concentration of non-adsorbed cations in the double layer region below their equilibrium solution concentration. The excess concentration of cation i in the double layer region above the equilibrium concentration is calculated. The intrinsic binding coefficients, in Equations (2) and (4), were determined for MB (Methylene Blue) and CV (Crystal Violet) in the case of montmorillonite (Rytwo et al., 1995) from adsorption data. The extension of the model for sepiolite is required to consider the reaction N þ Xiþ ¼ NXiþ

ð6Þ

with a binding coefficient, Kn, Kn ¼

½NXiþ ð½N½Xiþ ð0Þ Þ

ð7Þ

Some organic cations (i.e. MB) can form dimers, trimers and even higherorder aggregates in solution (Cenens and Schoonheydt, 1988; Spencer and Sutter, 1979). Expressions for the general distribution of aggregates (Nir et al., 1983) yield that the total concentration of primary molecules in solution Xit, is given by ½Xit ¼

½Xi ð1 Kag½XiÞ2

ð8Þ

in which Kag is the corresponding coefficient (M 1) for aggregation in solution. Aggregation of dye molecules reduces the concentration, Xi, of dye monomers. The adsorption of dimers or higher-order aggregates was ignored, in order to reduce the number of parameters and was not needed for the simulation of the adsorption results. It may be noted that dye aggregation in solution can only have an influence when its total added amounts are above the CEC of the clay, since below the CEC, essentially all the dye is adsorbed (Rytwo et al., 1991, 1995; and results on sepiolite).The total site concentration, PT, equals the sum of concentrations of all sites, free and complexed. In Equation (9) below, the sum on PX0 is the sum of concentrations of all neutral complexes. P P s P PXþ NXþ ¼ P 20 P þ ð9Þ sini P þ PX þ PX2 The Gouy–Chapman equation yields: ekT X 1X s2 ¼ n1 ðY0Zi 1Þ ¼ XiðY0Zi 1Þ 2p G

ð10Þ

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where e is the dielectric constant of the medium, n1 is the number of molecules of each ion per unit volume in the equilibrium solution, and G depends on T, the Avogadro number, and the system of units (Nir, 1984). For the case of 1, 2, 3 and 4 valent cations, and mono and divalent anions, the combination of Equations (9) and (10) gives a polynomial equation for Y0. The solution has been obtained by numerical procedures. In certain limiting cases of no binding, where all ions have the same valency, analytical solutions are also available. The equations shown form a closed set. Thus, the mass balance of ion i may yield the values Xi, if Y0 and P are known. Equations (9) and (10) may yield P, if the different Xi, complexes and Y0 are known. By introducing the values of the binding coefficients, the equations can be solved iteratively by the following procedure: 1. 2. 3. 4. 5.

An initial value for Y0 and P is assumed. The values of Xi are calculated by using Equations (1)–(7). A new value of Y0 is obtained by using Equation (9). A new value is obtained for P, by subtracting from PT the bound sites. Another iteration starts from stage 2.

The iteration steps may be continued until the desired degree of convergence is reached. The determination of the binding coefficients that give the best fit of the calculated adsorbed amounts to the experimental values has been described in Nir et al. (1986), Hirsch et al. (1989), Rytwo et al. (1995, 1996b) and Rytwo et al. (1996a). Only one parameter, Kn was employed, fixing the values of other parameters from the adsorption of CV and MB by montmorillonite. The concentration of neutral binding sites was determined by analysis of the adsorption to sepiolite of two neutral molecules, TX100 (Triton X 100) and 15C5 (15-Crown-5) according to the Langmuir equation.

5. RESULTS OF MODEL APPLICATION Unless specified, we refer to results of Rytwo et al. (1998). In order to minimize molecular aggregation in solution and at the same time reach large adsorbed amounts relative to the CEC of the clay the concentration of the clay was reduced to 0.2% (2g/l), which was still within the limits of sensitivity of the measurements. Indeed, the largest adsorbed amount of TX100 was 0.315 molc/kg, that is, 25% more than the value in Alvarez et al. (1987). The largest adsorbed amount of MB was 0.57 molc/kg clay which was 25% more than in Aznar et al. (1992). The largest adsorbed amount of CV was 0.64 molc/kg, that is, 4.57-fold of the CEC of sepiolite. The study of the adsorption of the neutral molecules to sepiolite was also intended to provide an estimate for the amount of neutral sites, N, which was required for calculating the adsorbed amounts of the organic cations

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MB and CV. In calculations according to Langmuir equation, the experimental values were used to obtain the values of the binding constants, k, by assuming a wide range of N values. The value of N which gave the smallest variation of k-values was chosen. Next, the value of k that gave the best fit to all the experimental values was determined. TX 100 adsorption yielded N ¼ 3.4-fold of the CEC. In comparison, Shariatmadari et al. (1999) deduced that N was about fourfold of the CEC of sepiolite but their assumed value of the CEC was about 0.786-fold of that in Rytwo et al. (1998) which amounts to a similar estimate of N. In both cases, the results imply that the simplest binding model, which assumes no cooperativity in the binding of the neutral molecules to sepiolite, can adequately explain the experimental data. The value of N for palygorskite was about twofold larger than the CEC of palygorskite, which was interpreted to reflect smaller surface area in the latter case, that is, 222 versus 384 m2/g for sepiolite. Interestingly, the adsorbed amounts of MB and CV by sepiolite and palygorskite showed a similar pattern; the adsorbed amounts to palygorskite were about 10–15% smaller than to sepiolite. Shariatmadari et al. (1999) found that no reduction of pH accompanied the adsorption of the neutral molecules, but there was one unit decrease following MB or CV adsorption, indicating some release of Hþ. In both cases, it could be deduced that the neutral adsorption sites become more important as the dye adsorption approaches saturation. In all cases, the model accounted well for the adsorbed amounts. In both cases, FTIR results contributed to the interpretation of the experimental adsorption studies and their modelling, and vice versa. The band representing the external neutral sites of sepiolite, OH vibrations of silanol groups at 3716 cm 1 (Rytwo et al., 1998) or 3720 cm 1 (Shariatmadari et al., 1999) decreased in intensity due to sorption of the neutral molecules as well as MB and CV. The band at about 3680 cm 1 which corresponds to OH vibrations of hydroxyl groups linked to Mg ions located in the interior of sepiolite blocks remained unperturbed by the adsorption of MB or CV. Shariatmadari et al. (1999) observed some perturbance of this band. In conclusion, the organic cations mostly adsorbed to neutral sites in the outer surface, and to sites where large structural defects occur. Rytwo et al. (2002) determined experimentally adsorption of the divalent organic cations paraquat (PQ), diquat (DQ) and methyl green (MG) on sepiolite and performed analysis with the adsorption model described above. The largest amounts of DQ, PQ and MG adsorbed were between 100% and 140% of the CEC of sepiolite. Those amounts were considerably lower than reported for the adsorption of monovalent organic cations. This outcome led to the hypothesis that these divalent organic cations do not interact with the neutral sites of sepiolite. This assumption was confirmed by infrared spectroscopy (IR) measurements, which did not show influence in the peaks arising from the vibrations of external Si OH groups of the clay, when the divalent organic cations were added, unlike changes which were clearly observed

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when monovalent organic cations were added to the mineral. The model could adequately simulate the adsorption of the divalent organic cations DQ and PQ, without considering interaction of those ions with the neutral sites. The model also yielded good fit for the results of competitive adsorption between the monovalent dye MB and DQ. In competitive adsorption experiments, when total cationic charges exceeded the CEC, monovalent organic cations were preferentially adsorbed on the clay at the expense of the divalent cations. A similar effect was observed in other clays, such as montmorillonite. Interestingly, in an earlier study by Rytwo et al. (2000), it was shown that the divalent organic cation, methyl green (MG) undergoes a slow transformation (6 h) to a monovalent cation, carbinol (MGOHþ) upon dilution of its solution (10 mM), or upon addition of a buffer at neutral pH. Adsorption of MG on sepiolite raised the possibility that a certain fraction of MG2þ transformed into the monovalent form during the incubation period. The maximal adsorbed amounts of MG2þ and MGOHþ were 0.09 and 0.30 mol/kg sepiolite, respectively. In passing, we note that recent studies (Rytwo et al., 2009) indicate that at least a small part of the large amount of CV adsorbed on sepiolite undergoes partial degradation to Arnold base, whereas the products of the process remain bound to the clay. Such a process was more extensively observed in Texas vermiculite, but IR measurements indicate that it also occurs partly in sepiolite.

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Chapter 16

Sepiolite and Palygorskite as Sealing Materials for the Geological Storage of Carbon Dioxide Emilio Galań, Patricia Aparicio and Adolfo Miras Departamento Cristalografıá, Mineralogıá y Quı´mica Agrıćola, Facultad de Quı´mica, University of Sevilla, Professor Garcıá Gonza´lez 1. 41012 Seville, Spain

1. INTRODUCTION 1.1. The Geological Storage of Carbon Dioxide Atmospheric concentrations of greenhouse gases (e.g. carbon dioxide, methane and nitrous oxides) have increased significantly as a result of human activity since the pre-industrial era (AD 1000–1750). Fundamentally, carbon dioxide (CO2) has increased from a pre-industrial level of 275–285 ppm to 379 ppm in 2005 (Solomon et al., 2007). This increase has been caused mainly by fossil fuel consumption and, to a lesser extent, concrete production and changes in land use. The increase in average global temperatures since the mid-twentieth century may be ascribed to increased emission of anthropogenic greenhouse gases (Metz et al., 2005). International concern about climate change led to the establishment in 1992 of the United Nations Framework Convention on Climate Change (UNFCCC). The ultimate objective of UNFCCC is the ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that prevents dangerous anthropogenic interference with the climate system’. One mean for reducing net greenhouse gas emissions is ‘Capture and storage of CO2’. Carbon dioxide capture and storage (CCS) include technologies to capture, transport and store CO2. The storage of CO2 may be effected through a number of mechanisms, including ex situ mineral carbonation, oceanic storage, underground injection for enhanced fossil fuel recovery and injection into saline aquifers or other geological reservoirs, an approach known as in situ mineral carbonation (Giammar et al., 2005; Metz et al., 2005; Xu et al., 2005). Developments in Clay Science, Vol. 3. DOI: 10.1016/B978-0-444-53607-5.00016-5 # 2011 Elsevier B.V. All rights reserved.

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The subsurface constitutes the largest reservoir of carbon on Earth. The vast majority of the world’s subsurface carbon is held in the form of coal, oil, gas, organic-rich shales and carbonate rocks. Geological CO2 storage has occurred naturally in the Earth’s upper crust for hundreds of millions of years. Carbon dioxide derived from biological activity, igneous activity and chemical reactions between rocks and fluids accumulates in the natural subsurface environment as carbonate minerals, in solution, or in a gaseous or supercritical state, either as a gas mixture or as pure CO2. Geological storage of anthropogenic CO2, as a greenhouse gas mitigation strategy, was first proposed in the 1970s. No significant research, however, was done until the early 1990s, when the idea gained credibility through the work of individuals and research groups (Bachu et al., 1994; Baes et al., 1980; Gunter et al., 1993; Holloway and Savage, 1993; Kaarstad, 1992; Koide et al., 1992; Korbol and Kaddour, 1994; Marchetti, 1977; van der Meer, 1992). Geological CO2 storage in sedimentary basins may be achieved within a variety of geological settings, the most suitable formations being oil fields, depleted gas fields, deep coal seams and saline formations. To this end, CO2 gas must first be compressed to a dense fluid state known as ‘supercritical’. Depending on pressure and temperature increases with depth, the density of CO2 increases to a depth of 800 m or more. At this point, the injected CO2 will be in a dense supercritical state (Figure 1). According to Hitchen (1996), the geological storage of CO2 through injection into deep reservoirs involves three different processes: (i) hydrodynamic trapping as a gas or supercritical fluid below a cap rock of low permeability; (ii) solubility trapping, through dissolution of CO2 in aqueous solutions; (iii) mineral trapping, through the precipitation of secondary carbonates formed by dissolution of primary silicates and Al silicates upon injection of CO2 into aquifers. Other authors (e.g. Metz et al., 2005) have differentiated between physical and geochemical trapping: (i) physical trapping, comprising both the stratigraphic and the residual trapping. The former occurs below low-permeability seals or cap rocks, whereas the second takes place in saline formations, where fluids migrate very slowly over long distances even in the absence of closed traps; (ii) geochemical trapping, encompassing solubility trapping and mineral trapping. Mineral trapping is especially attractive because CO2 is permanently ‘fixed’ (as stable carbonate minerals) in relatively deep geological formations, preventing its return to the atmosphere.

1.2. CO2 Reactivity and Integrity of the Cap Rock When CO2 is injected in a sedimentary basin, it has a strong tendency to react with rocks.

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Sepiolite and Palygorskite as Sealing Materials for CCS

100 0 20 11 0.5

Depth (km)

3.8 3.2

1

2.8

1.5

2.7

Assuming a geothermal gradient of 25 ⬚C/km from 15 ⬚C at the surface, and hydrostatic pressure

2

2.7 2.5

0

200

600 400 Density of CO2 (kg/m3)

800

1000

FIGURE 1 Variation of CO2 density with depth, assuming hydrostatic pressure and a geothermal gradient of 25 C km–1 from 15 C at the surface (based on data of Angus et al., 1973). Carbon dioxide density increases rapidly to a depth of 800 m when CO2 reaches a supercritical state. The cubes represent the relative volume occupied by CO2. To a depth of 800 m, this volume dramatically decreases with depth. At depths below 1.5 km, the density and volume become nearly constant (Metz et al., 2005).

Much research effort has been focused on the mineral trapping of CO2, through carbonate precipitation. A prerequisite for carbonate precipitation is the availability of aqueous metal cations, derived from non-carbonate minerals, and their ability to combine with dissolved CO2. The dissolution of metalbearing silicate minerals is a very important potential source of these cations. The dissolution rate of such minerals is mainly controlled by the pH and temperature of the medium in contact with the mineral surfaces, whereas the influence of hydrodynamic conditions is nil, at least for surface-controlled processes. Kaszuba et al. (2005) attempted to replicate the active processes in a typical CO2 sequestration site by reacting a mixture of quartz, feldspar, biotite and shale with a CO2-rich NaCl brine at 200 C. Magnesite and siderite (FeCO3) were precipitated validating their potential for mineral trapping. They observed that shale actively participates in coupled dissolution/precipitation processes, indicating the potential of CO2-rich fluids for decreasing rock integrity. Substantial amounts of aqueous Si were released into solution, which could serve as a source for cement in sandstones or Si mineralization

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in veins. More recently, Kohler et al. (2009) have investigated the mineralogical and chemical changes in clays (as possible clay-rich sealing cap rock) after exposure to partial CO2 pressure. The principal alterations were illitization of clay minerals combined with formation of anhydrite, dolomitization and an increase in dissolved CO2 in the porous media. Xu et al. (2005) have examined the effects of injecting carbon dioxide into a common sedimentary basin sequence, a shale-bounded sandstone. The total amount of CO2 trapped in carbonate minerals depends mainly on the composition of the rock. For a representative rock composition, 90 kg/m3 of CO2 can be trapped during 100,000 years, mainly in sandstone. A great deal of CO2 trapping is a consequence of the presence of an adjacent shale unit, providing many of the cations that form trapped secondary carbonates. The interaction of acidic CO2-rich fluids with shale, however, tends to be a two-edged sword. Although it provides the essential metals for trapping CO2 in carbonate minerals, the leaching of these metals may increase shale permeability, favouring the release of CO2 into the atmosphere as Moore et al. (2005) have suggested. In developing reactive transport models for the interaction between CO2rich solutions and cap rock of the Sleipner sequestration site, Gaus et al. (2005) came to a similar conclusion. They found that the porosity and permeability of the cap rock can be either increased or decreased depending on the exact composition of the rock. Marini and Accornero (2009) have identified several drawbacks in the geochemical modelling of reactions occurring during the geological storage of CO2. The main target of the geological storage of CO2 is represented by sedimentary basins where brines are commonly present. Thus, it is necessary to describe specific interactions among solute species at the pertinent salinities, compute correct activity coefficients and extrapolate these interaction parameters to the temperature and pressure conditions of the aquifer of interest. Unfortunately, the computer-stored thermodynamic and kinetic databases, required for describing chemical reactions of CO2 with the aqueous solution and aquifer solid phases, are incomplete (Marini and Accornero, 2009). Consequently, the geochemical evolution of the system, following injection of pressurised CO2, cannot be precisely described using computer-based models. Nevertheless, geochemical modelling is the only available tool to evaluate the long-term reactive effects of geological CO2 storage. This is because the duration of laboratory and field experiments and their duration cannot be extended up to multiples of the life time of human beings. In contrast, computer experiments have no time limitation and are open to future improvements. The sealing rock must be capable of preventing the escape of CO2 that forms when low-permeability rocks are dissolved by local reaction with acidic CO2-rich fluids. The aim of this contribution is to evaluate the potential of sepiolite- and palygorskite-rich rocks to serve as sealing materials in the cap rock for the geological storage of CO2. Because of their structural characteristics, such

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rocks would be capable of trapping CO2 physically. However, the Mg ions present in their structures, and the exchangeable cations, could react with CO2 to form carbonates.

1.3. Characteristics of Palygorskite and Sepiolite Palygorskite and sepiolite contain ribbons with a 2:1 type layer structure, each ribbon being linked to the next inverted SiO4 tetrahedral sheet along a set of Si—O bonds. Thus, tetrahedral apices point in opposite directions in adjacent ribbons. The ribbons are aligned parallel to the X-axis and have an average width along the Y-axis of three linked pyroxene-like single chains in sepiolite and two linked chains in palygorskite. In this framework, rectangular channels run parallel to the X-axis between opposing 2:1 ribbons. As the octahedral sheet is discontinuous at each tetrahedral inversion, oxygen atoms in the octahedra at the edge of the ribbons are coordinated to cations on the ribbon side only, while coordination and charge balance are completed along the channels by protons, coordinated water and a small number of exchangeable cations (Brigatti et al., 2006). For details on the structure, see Chapter 1. Sepiolite and palygorskite can take up extraneous liquids, gases and vapours into their microporous channels. These minerals also contain macropores when individual (unit) particles combine to form aggregates (Galań, 1996) Because of their extensive surface area and porosity, these minerals have found useful applications as absorbents of gaseous toxic compounds, decolourizing agents of oils and solid supports of enzymes and anaerobic bacteria (Galań, 1996).

2. INTERACTION OF SEPIOLITE AND PALYGORSKITE WITH SUPERCRITICAL CO2 2.1. Material Characterization and Methodology Sepiolite (SEP) from Vica´lvaro, Madrid basin (Spain) and palygorskite (PAL) from Theis (Senegal), both commercialized by TOLSA, S.A., were selected for this study. The mineralogical composition of the raw materials was determined by X-ray diffraction (D8 Advance model, Bruker) and the chemical composition performed by X-ray fluorescence (Axios model, Panalytical). The elemental carbon content was measured using an elemental analyser (LECO CHNS 932). The sepiolite sample is composed of sepiolite (75%) together with quartz and calcite. The palygorskite sample contains about 50% palygorskite, together with carbonates (calcite and dolomite), minor amounts of quartz and traces of sepiolite and smectite. The chemical composition of the samples is consistent with their mineralogy (Tables 1 and 2). Elemental carbon contents are 0.45 wt% for sepiolite and 1.91 wt% for palygorskite.

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TABLE 1 Mineralogical Composition (wt%). Sample

Quartz

Calcite

Dolomite

Sepiolite

Palygorskite

Smectite

SEP

16

5

Traces

75

–

–

PAL

6

34

10

20%). Young’s modulus and HDT are also increased using sepiolite as nanofiller in a PA-6 nanocomposite compared with neat nylon-6 due to the reinforcement effects of the high-aspect-ratio nanofillers (Xie et al., 2007; Table 1). In contrast to MMT/PA nanocomposites, sepiolite exhibited the highest level of reinforcement on the Young’s modulus, which may be due to the more efficient interfacial stress transfer. However, PA/palygorskite nanocomposites exhibit reinforcement in thermal and mechanical properties with increasing nanoclay loading and slower relaxation in segments mobility compared with neat polyamides because of the grafting of the polymer chains to the nanoclay (Shen et al., 2006). In the case of the thermal properties, nanocomposites based on sepiolite and palygorskite nanoparticles increase the decomposition and glass transition temperature (Td and Tg) of the polymer matrix which could be due to the good interfacial adhesion between the matrix and silicate fibres or to the restriction of molecular mobility of polymeric segments near the silicate surface (Shen et al., 2006; Wang et al., 2008). Other important application of these materials is the introduction of nanoclays in a polymer in order to improve their fire resistance and avoid the production of corrosive or toxic gases and smoke during combustion allowing to extend their use to most applications (polymers like PP are limited

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Advanced Materials and New Applications

TABLE 1 Thermo-Mechanical Properties of PA-6 Nanocomposites. Nanoclay (%)

D Young Modulus (%)

D HDT (%)

PA-6

0

–

–

Sepiolite YV

7

106

140

Sepiolite Y

10

151

197

Sepiolite T1

10

158

206

Sepiolite Y

10

152

176

0

–

–

6

98

143

Nanomer I.24 TL

8

115

175

PA-6

0

–

–

5

69

68

PA-6 Nanomer I.24 TL

Cloisite

b

a

Nanocor: Nano effect in situ Nylon-6 Nanocomposites. Ying Liang et al. (in situ polym.) http://www.nanocor.com/tech_papers/antec2001.asp. SCP: Cloisite Application Data. http://www.nanoclay.com/testdata.asp#.

a

b

by its high combustibility in automotive, electricity and electronic, construction and other industries) (Laoutid et al., 2009). Nowadays, the most used flame retardants are based on halogeneous compounds in conjunction with antimony trioxide. However, directives and regulations (EU: EN 135501; CDP 89/106/EEC; EN 45545) restrict the use of these halogeneous compound as flame retardant because of the large amount of smoke and toxic gases produced during burning (Marosfoi et al., 2008). Inorganic fillers can be an alternative to substitute these halogeneous compounds. According to the literature, additions of inorganic filler like sepiolite can be used to inhibit or to stop the polymer combustion process because these additives reduce the content of combustible products and modify the thermal conductibility and the viscosity of the resulting material depending on the nature and chemical structure of the polymer. The incorporation of a relatively low quantity of these flame retardant additives acts physically formatting a protective layer during combustion (Lewin, 2006; Zanetti et al., 2002). In this process, the viscosity of the polymer/silicate nanocomposite decreases with increasing temperature and facilitates the migration of the fibrous sepiolita to the surface providing a consistent and durable char (Jang et al., 2005; Song et al., 2007). This char acts as a protective barrier that limits heat transfer into the material, volatilization of combustible degradation products and diffusion of oxygen needed to produce and maintain combustion

408


in the material. However, some industrial applications, like cable insulation, need the addition of other flame retardant additives to achieve the wanted properties (Ca´rdenas et al., 2009). In fact, metal hydroxides (aluminium and magnesium, especially) are the mineral flame retardants most commonly used. However, as a consequence of their high loading level in the polymer, the synergist effect with other additive like sepiolite or MMT is being studied. Sepiolite has been tested in combination with magnesium hydroxide in PP. The combination of these microand nanofillers used together decreases the maximal heat realize rate and reduces the time to ignition compared to the pure PP and the magnesium hydroxide and MMT combination. These effects are attributed to the heat absorption mechanism via water release of metal hydroxide and the formation of the barrier layer due to the sepiolite. In a similar case, fumed silica and sepiolite nanoparticles have shown a synergist effect with a high thermal stability and a high amount of inorganic residues. However, it is well known that the dispersion, chemical structure and geometry have a considerable influence on the flammability (Pastore et al., 2004). In this way, combination of fibrous sepiolite and a layer silicate like MMT have been tested to improve the barrier layer formed during combustion. As a result, the use of these nanoclays increases the time to ignition and decreases heat release rate because the combination of these nanofillers stabilizes the char formed and leads to increased strength of the residue. Moreover, the use of two types of nanoparticles with different morphology enhanced the degree of nanodispersion during processing. This is in agreement with the results obtained in PP where the mixture of sepiolite and MMT provides the largest improvement in the physical stability. The synergistic effect between both clay minerals has also been observed with reactive amine flame retardant (Toldy et al., 2007). Moreover, sepiolite has shown to be an effective flame retardant in bituminous emulsions improving the stability and controlling system flow and consistency at high temperature (100 C) avoiding sagging (Figure 3).

FIGURE 3 Flame resistance test of two bituminous emulsions with and without sepiolite.

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3. SEPIOLITE-BASED FUNCTIONAL MATERIALS: HYBRID NANOMATERIALS Hybrid nanomaterials based on sepiolite and palygorskite are known since several centuries ago, as it is the case of the Maya blue pigment, although its true nature has been studied only in the past years. Diverse aspects on this wonderful pigment are treated in depth in this book, but it must be remarked here that it is in fact a very stable nanostructured hybrid material based on palygorskite and the indigo natural dye (Figure 4) (Berke, 2007; Chiari et al., 2008; Sańchez del Rıó et al., 2006, 2009). Indigo was able to penetrate into the nanosized pores of palygorskite, which is supposed to accede inside the structural tunnels of the clay mineral fused with melted copal (Arnold et al., 2008). In view of advanced applications, some recent examples on dye-sepiolite nanomaterials that are systematically studied on the basis of the modern techniques and scientific knowledge are introduced below. Actually, sepiolite and palygorskite behave as many clay minerals in their ability for assembling organic species to produce organic–inorganic hybrid materials. From the mechanistic point of view (Table 2), the special structural features of these microfibrous silicates determine differences in the clay–organic interactions with respect to those of smectites and other clay minerals (Ruiz-Hitzky et al., 2004). In this section, we introduce an overview of organic–sepiolite hybrid materials with emphasis on our own results to illustrate the versatility and impact in new applications of this type of nanostructured hybrid materials. The assembling of organic species to sepiolite has as major interest in the introduction of

FIGURE 4 Detail of a piece decorated with the ancient Maya blue pigment (from http://www.dailygalaxy.com/my_weblog/2008/03/ancient-mystery.html).

410


TABLE 2 Mechanisms of Clay–Organic Interactions in Charged Layered Silicates (Smectites and Vermiculites) and Sepiolite and Palygorskite Fibrous-Clays. Nature of the Interactions

Characteristics in Charged Layered Silicates

Characteristics in Sepiolite and Palygorskite

Electrostatic

High ion-exchange capacity associated to interlayer cations: penetration of positively charged species into the interlayer region

Low ion-exchange capacity with a nonclear location of charge

Clay–organic interactions through water molecules from the interlayer cations hydration shell

Clay–organic interactions through

Hydrogen bonding and water bridges

Organic cations assembled to external surface and penetration in tunnels discriminated by molecular size

1. Silanol groups on the external surfaces 2. Water molecules (zeolitic and coordinated) at the external surfaces or in the tunnels discriminated by molecular size

Covalent bonding

No applicable

Chemical reactions with silanol groups

Van der Waals forces

Specially applicable to interlayer region

Applicable to external and internal surfaces discriminated by the molecular size of the organic molecule

Other mechanisms – Ion dipole and coordination – Proton transfer – Electron transfer

Governing clay– organic interactions due to the charge surface, nature of interlayer cations and water polarization.

Mainly limited to Mg bonded to coordinated water

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functionality to enlarge the applications of the raw clay. Other alternatives to develop sepiolite functional nanostructured materials refer to the preparation of biohybrids and sepiolite-nanoparticles composite materials.

3.1. Sepiolite–Organic Compound Interactions As indicated in Table 2, several mechanisms can be ascribed to explain the nature of sepiolite and palygorskite interactions with organic species to give hybrid nanostructured materials. Electrostatic interactions are related with a low electrical deficit in the charge balance that can be attributed to the existence of isomorphous substitutions as, for instance, Mg2þ by Al3þ in the octahedral layers, similar to smectites and other layered charged silicates. From these substitutions, sepiolite behaves as a cation exchanger of low cation exchange capacity (CEC), typically in the order of 10–20 mEq/100 g, which is about five times fold lower than smectites. However, this capacity allows the uptake of organic cations being the basis of the development of commercial sepiolite-based organoclays, as well as other hybrid nanomaterials based on sepiolite. The presence of different types of water molecules drives to diverse modes in sepiolite–organic interactions. For instance, coordinated water molecules could be involved in hydrogen bonding (water bridges) or can be replaced by organic molecules giving rise to direct coordination to Mg2þ ions in the border of the structural blocks (channels and tunnels). Surface SiOH groups may also participate in the assembling of organic species through low-energy hydrogen-bonding interactions or by strong covalent bonding when sepiolite interacts with certain reagents as, for instance, organo-chlorosilanes. Organic species can interact with sepiolite mainly at the external surface and in some cases also within the internal surface by penetration into its structural tunnels. In fact, sepiolite and palygorskite act as molecular sieves discriminating the adsorption of species by its molecular size (Barrer and Mackenzie, 1954; Barrer et al., 1954; Ruiz-Hitzky, 2001). The channels on the external surface of sepiolite and palygorskite also contribute to adsorption of organic adsorbates whose molecular size and shape closely match the dimensions of the open channels at the exposed crystal faces (Ruiz-Hitzky et al., 2004). In this way, good separation of components in binary mixtures of n-paraffins (n-pentane) from branched paraffins (isopentane or neopentane) by passing them through palygorskite columns, in agreement to Barrer and co-workers (Barrer and Mackenzie, 1954; Barrer et al., 1954). These authors reported a comparable behaviour in sepiolite but its selectivity is different from that of palygorskite, due to the larger channel width of sepiolite. However, in addition to the appropriate size, it is necessary to have a polar character for the molecules to be able to penetrate into the tunnels, as occurs, for instance, with ammonia, methanol, ethanol, acetone, ethylene glycol and pyridine (Ruiz-Hitzky, 2001; Ruiz-Hitzky et al., 2004). Infrared spectroscopy is a useful tool to confirm the penetration of adsorbed molecules, such as

412


methanol or acetone (Serna and VanScoyoc, 1979), into the tunnels by the shift of absorption bands associated with H2O coordinated to Mg2þ ions. Solid-state NMR techniques also support the penetration of this type of molecules inside the tunnels (Facey et al., 2005; Kuang et al., 2006). Pyridine and pyrrole among other aromatic and heterocyclic compounds are also able to penetrate into the intracrystalline tunnels of sepiolite (Glen et al., 2005; Inagaki et al., 1990; Kuang et al., 2003; Ruiz-Hitzky, 2001). One of the most salient features in the adsorption of pyridine on sepiolite and palygorskite is the remarkable stability of the resulting nanohybrid materials, which is related to the direct coordination of pyridine to the edge Mg2þ sites at the border of the ribbons inside the tunnels (Kuang et al., 2003). In sepiolite, pyridine molecules sequestered in the tunnels become directly coordinated to Mg2þ in the hybrid nanomaterial (Scheme 1) when heated above 140 C, being stable up to 450 C.

3.1.1. Dye-Sepiolite Nanostructured Hybrids The presence of molecules inserted into the sepiolite and palygorskite tunnels can determine a shift in the crystal folding temperature above 300 C. This stability is also applicable to hybrid materials based on dyes included into the tunnels of the silicates, as could be the case of indigo in Maya blue. In this last case, the dye inside the tunnels of palygorskite cannot be extracted with solvents, resists hot concentrated mineral acids and persists upon heating to about 250 C. Analogous stable pigments have been prepared from palygorskite or sepiolite with indigo (Hubbard et al., 2003; van Olphen, 1966). Certain cationic dyes that exhibit molecular dimensions compatible with the geometry of the tunnels are able to penetrate inside these nanopores. This is the case of methylene blue (MB) of 1.7 0.76 0.32 nm3 dimensions that is adsorbed by sepiolite from aqueous solutions showing H type isotherms corresponding to a high affinity of the dye towards the silicate surface (Aznar et al., 1992a; Rytwo et al., 1998). Taking into account the tendency to form molecular aggregates, this dye is impeded to penetrate into the structural tunnels except when it is present in diluted solutions (< 10 3 M) (Ruiz-Hitzky, 2001). The IR modification of bands associated with the coordinated water molecules mainly located inside the tunnels supports the access of MB and other cationic dyes of comparable molecular dimensions such as acridine orange and thioflavine-T (Casal et al., 2001) to those nanopores in sepiolite. H

H O Mg

H

N

O Mg

H

N

– H2O

Mg : N

+ H2O SCHEME 1 Reaction scheme showing direct coordination of pyridine to Mg2þ by replacing coordinated water molecules.

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In the case of acridine orange, the highly specific directional binding of the dye to sepiolite, the molecules being arranged with their long axes parallel to the sepiolite fibre axis, has been shown (Ridler and Jennings, 1980). Cationic dyes are also adsorbed at the external surface of sepiolite giving rise to nanohybrid materials that have been investigated for their possible use in the photostabilization of coadsorbed labile pesticides (Casal et al., 2001), in a similar way to the cationic dye-MMT compounds studied by Margulies et al. (1985). Diverse applications can be derived from the confinement of organic dyes into the structural tunnels of sepiolite and palygorskite. In this way, the same strategy than in Maya blue pigment has been applied to prepare colourful and resistant organo-inorganic hybrid additives compatible with aqueous formulations for high-performance paints based on the entrapment of organic dyes into inorganic solids (Gańdara et al., 2006, 2009). In this way, direct adsorption of methyl red, MB, auramine O and Alcian blue (pyridine variant) dyes on sepiolite gives hybrid materials that are of particular interest as they show improved photostability when compared to the dye alone or containing dye plus sepiolite subsequently added in a post-addition process (Gańdara et al., 2009). These stabilized pigments are also more resistant towards photodegradation than those based on pigments prepared by the ‘bottle-around-a-ship’ procedure, recently applied for the synthesis of dye-containing zeolites (Figure 5) (Gańdara et al., 2006). In addition, the confinement of photoactive species in the sepiolite tunnels is of interest for the potential development of materials for microlaser applications, in the same way than reported for the encapsulation of Rhodamine FIGURE 5 Encapsulated organic dyes acridine orange (left) and crystal violet (right) encapsulated by bottle-around-a-ship synthesis in a SiAl zeolite and MB (left) and methyl red (right) adsorbed in the pores of sepiolite, before (A) and after (B) 10 h of weathering treatment (UV irradiation under saturated moisture) (data from Gańdara et al., 2009).

Zeolitedye A

B

Sepiolitedye

414


dyes in the structural cavities of aluminophosphate (AlPO) single crystals (Bockstette et al., 1998; Braun et al., 2000). From collaboration between TOLSA and the Materials Science Institute of Madrid, a novel procedure for easy detection of thiols or mercaptans (R SH) has been recently patented (Aranda et al., 2006b). The research concerns a sensorial system that by means of changes of colouration allows the detection of the presence of certain thiols, which can be considered as model molecules for unpleasant odours. The denominated Michler’s hydrol is an organic reagent dye of the diphenylcarbinol type that can be adsorbed on the sepiolite micropores generating stable pigments of bluish colour. This indicator system changes its colour in the presence of thiols, which can react with sulphydryl groups (SH) producing a colourless adduct compound as the reaction progresses (Figure 6). The sepiolite acts as a substrate supporting the Michler’s hydrol as well as the reaction medium, facilitating the visual colour changes produced during the reaction. This system can be used to develop optical sensors for the detection of the present molecular agents responsible for unpleasant odours in general, as well as those originating from industrial waste, wastewater treatments, halitosis, animal breeding and domestic animals (Aranda et al., 2006b).

3.1.2. Grafted Organic Derivatives of Sepiolite by Covalent Bonding Organic derivatives of clay minerals are formed by grafting organic groups onto the silicate surfaces through covalent bonds. Sepiolite offers the possibility to act as a privileged substrate for this purpose due to the presence of surface SiOH groups allowing reaction with different compounds. The most

H3C CH3 N

CH–OH

H3C CH3 N

+ [H ]

CH+

H3C CH3 N

R–SH

CH–SR

H2O

N H3C

N

CH3

H3C

blue

CH3

N H3C

CH3

colorless

FIGURE 6 Scheme showing the detection mechanism of thiols by reaction with sepiolite-Michler’s hydrol hybrids, which change colour after reacting with RSH compounds (results based on Aranda et al., 2006b).

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stable hybrid organic–inorganic compounds derived from sepiolite are obtained by reaction with organosilanes containing SiX groups (X ¼ OR, Cl), yielding siloxane bridges (SiOSi) (Equation 1). ½sepiolite surfaceSi OH þ X SiR3 ! ½sepiolite surfaceSi O SiR3 þ XH

ð1Þ

The number of silanol groups necessary to react with the organosilanes reagents could be produced by acid treatment following the so-called cohydrolysis procedure (Ruiz-Hitzky and Van Meerbeek, 1978), which can be applied to sepiolite (Fripiat and Mendelovici, 1968; Ruiz-Hitzky and Fripiat, 1976; Van Meerbeek and Ruiz-Hitzky, 1979; Zapata et al., 1972). Octahedral cations (e.g. Mg2þ) belonging to the octahedral silicate sheet are extracted by acid (e.g. HCl) giving fresh silanol groups that can be involved in further reactions with silanes. In these cases, the synthesis of organic derivatives of sepiolite is characterized by a significant structural and textural alteration of the silicate, although the organic derivatives of sepiolite obtained by cohydrolysis with alkyl- and alkenyl-silanes preserves the microfibrous morphology typical of the parent clay (Ruiz-Hitzky and Fripiat, 1976). As already reported, the grafting of functionalized organosilanes on sepiolite gives different organo-sepiolites of interest in various fields of applications (Ruiz-Hitzky, 1988). In this context, the presence of unsaturated grafted species can be employed in further copolymerization processes giving rise to nanocomposites in which the polymer is covalently bonded to the modified silicate (Ruiz-Hitzky, 1974). Arylsilanes, such as Cl2Si(CH3)(CH2)2Ph grafted on sepiolite, produce very stable phenylethyl derivatives that treated in a further process with chlorosulphonic acid are transformed into arylsulfonic species (Aznar and Ruiz-Hitzky, 1988; Aznar et al., 1992b). These materials show strong acidic character useful for applications as heterogeneous catalysts in alcohol dehydrogenation and in Beckman rearrangement reactions (Gutierrez et al., 1991). Following a similar approach, grafting of organosilanes bearing thiol groups ( SH), as, for instance, 3-mercaptopropyltrimethoxysilane (3-MPTMS), originates modified sepiolites showing ability for the uptake of heavy metals from aqueous solutions. For instance, the resulting derivatives can be used to complex Cd2þ ions via thiol–cation interactions (Celis et al., 2000). Other heavy metals such as osmium can be introduced by grafting on sepiolite, following a procedure involving the addition of OsO4 to unsaturated grafted groups (Barrios-Neira et al., 1974). After controlled hydrogenation (Barrios et al., 1981), the resulting Os-derivatives act as effective oxidation catalysts for the production of free hydrogen from water photodecomposition (Casal et al., 1985).

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Less stable covalently bonded species to sepiolite can be prepared via SiOC links. For instance, 1,2-epoxides have been used for anchoring diverse organic groups to sepiolite either in the vapour phase or in organic solvents (Casal and Ruiz-Hitzky, 1977, 1984) (Equation 2). Si-OH + CH2CH-R O

Si-O-CH-R CH2OH

ð2Þ

In the same way, diazomethane reacts with surface silanol groups of sepiolite and palygorskite (Hermosıń and Cornejo, 1986) through SiOC bonds (Equation 3). Si OH þ CH2 N2 ! Si O CH3 þ N2

ð3Þ

The use of isocyanates (R N¼¼C¼¼O) in reactions with silanols of sepiolite gives rise to organic–inorganic hybrid materials, in which aliphatic or aryl groups remain attached to the mineral surface through silyl urethane bridges (SiOCO NHR) (Equation 4) (Fernańdez-Hernańdez and RuizHitzky, 1979). Si OH þ R N ¼ C ¼ O ! Si O CO NH R

ð4Þ

In this case, the organic groups (R) also remain covalently attached to the sepiolite surface through Si OC bonds, their stability being dependent on the nature of the R substituent (Ruiz-Hitzky et al., 2004). In any event, attachment to the silicate surface through covalent Si OC bonds is of low stability since they are very sensitive to water molecules, the more stable hybrid organic–inorganic compounds being obtained by reaction of the silicates with functionalized organosilanes as indicated above.

3.2. Biohybrids and Biomimetic Materials Based on Sepiolite Bio-nanohybrid materials are an emerging class of organic–inorganic hybrids resulting from the assembling of molecular or polymeric species of biological origin to inorganic substrates through interactions at the nanometric scale (Ruiz-Hitzky et al., 2007). Diverse species such as lipids, carbohydrates, proteins and even fragments or entire biological bodies such as viruses or cells can be combined with different inorganic solids to form biohybrids presenting synergistic characteristics from the integrating components. In Nature, nacre, ivory and bones are typically biohybrids formed by assembling of inorganic solids and biopolymers, produced by living organisms and showing an amazing hierarchical arrangement of their organic and inorganic components from the nanoscale to the macroscopic scale (Darder et al., 2007; Ruiz-Hitzky et al., 2005). By mimicking the exceptional features of natural biohybrids, different research teams have developed the so-called biomimetic materials (Darder et al., 2007; Dujardin and Mann, 2002). Clay minerals, including

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sepiolite, have been used as inorganic moieties in the assembling to biopolymers to prepare bio-nanocomposites (Darder et al., 2008; Ruiz-Hitzky et al., 2005, 2008a, 2009c,e). Lipids such as phosphatidylcholine (PC) are long-chain surfactants of biological origin that have been recently assembled to sepiolite, the resulting biohybrids (bio-organoclays) being characterized by a homogenous coverage of the external surface in mono- or bilayer arrangement (Wicklein et al., 2010a). These biohybrids can be considered as supported lipid membranes that are associated to the sepiolite core by electrostatic bonds resulting from the cation-exchange processes, and also through hydrogen bonding of the lipid head-group moieties with the SiOH groups and the coordinated water molecules located at the channels on the external surface of this silicate. The versatility of bio-organoclays as immobilization hosts for diverse biological species was demonstrated in a mycotoxin retention study demonstrating a superior aflatoxin B1 sequestration capacity as compared to that of commercial alkylammonium organoclays (Wicklein et al., 2008, 2010a). The obtained bio-organoclays have been also tested as support of various enzymatic systems, such as urease, observing that the bioactivity of the enzymes can be effectively maintained for long periods of time (Wicklein et al., 2010b). It must be noticed that these bio-organoclays are ecomaterials less harmful to the environment than the corresponding conventional organoclays based on clay-long chain alkylammonium species. Bio-nanocomposites result from the combination of clay minerals and naturally occurring polymers (biopolymers), having a great scientific and industrial interest, since they are derived from abundant, cheap and ecological sources, and offer biocompatibility and biodegradability, as well as, in some cases, good mechanical and barrier properties. Because of these characteristics, they are receiving a great attention in view to potential applications in areas so diverse than ranges from adjuvants of viral vaccines to reinforced bioplastics for food packaging (Ruiz-Hitzky et al., 2009e). Sepiolite is able to give bio-nanocomposites by assembly with different types of biopolymers. The interaction mechanisms governing the formation of sepiolite-based bionanocomposites can be mainly ascribed to hydrogen bonding between the silanol groups and the water molecules at the external silicate surface with the hydroxyls, amino, carboxylic and other groups belonging to the biopolymers. It can also interact through electrostatic bonds with positively charged polymers, such as chitosan, the negatively charged surface of sepiolite being balanced by the protonated amino groups of the biopolymer (Darder and Ruiz-Hitzky, 2007; Darder et al., 2006). Multilayer coverage of chitosan can take place at high equilibrium concentrations of the biopolymer adsorbed as polymer aggregates. Sepiolite becomes strongly integrated within the biopolymer structure providing nanohybrids that show good mechanical properties, with elasticity modulus superior to those of the components measured separately, in agreement with the characteristic synergistic effect

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reported for structural polymer–clay nanocomposites. In addition to the good mechanical properties, these bio-nanocomposites show the interesting feature of being easily processed as self-supporting films (Darder et al., 2006). These properties show their potential interest in uses as membranes for different processes related to separation of ions and gases, or as components of electrochemical devices. Further, chitosan–sepiolite bio-nanocomposites could be used as substrate for the assembly of more complex materials of biological origin, including nucleic acids and living bodies such as algal cells and viruses. Similar to sepiolite, palygorskite is able to assemble biopolymers following the same approaches. For instance, it can be combined with polysaccharide derivatives, such as starch modified by the grafting of polyacrylamide giving rise to bio-nanohybrids with extraordinary superabsorbent properties (ca. 500 g of water per g of bio-nanocomposite) (Li et al., 2005). However, hydroxyethyl- and hydroxypropyl-cellulose can be associated to this silicate with a strong modification of the rheological behaviour of the clay dispersions (Chang et al., 1991). A complex composite formed by mixing dry biomass powder obtained from Ulva sp. microalgae and sepiolite has been reported as adsorbent of uranium (VI) in aqueous solutions (Donat et al., 2009). These materials cannot be classified as nanocomposites but are interesting as they open the way to new approaches for the uptake of toxic and/or radioactive pollutants by using sepiolite–algae compounding. In sepiolite bio-nanocomposites involving fibrous proteins such as collagen, the interaction results in an exceptional arrangement of the biopolymer oriented in the same direction as the sepiolite fibres (Pe´rez-Castells et al., 1987). Further treatment with glutaraldehyde confers to the composites a strong improvement in mechanical properties, the materials being positively tested for bone repair applied to in vivo assays (Olmo et al., 1996). Strongly related with collagen, gelatine–sepiolite bio-nanocomposites has been prepared recently mainly in view of potential structural applications (Fernandes et al., 2009, 2011). This approach was addressed to develop biocompatible formulations showing at the same time good mechanical properties. In fact, these bio-nanocomposites exhibit an increase of the elastic modulus up to 250% with respect to the starting gelatine matrix. It must be remarked that subtle changes introduced into sepiolite by grafting reactions or by thermal treatments resulted in dramatically different mechanical properties of the bio-nanocomposites, although at the present moment absolute knowledge of the interactions nature between sepiolite and gelatine has not yet established. Other structural protein is zein, an abundant storage biopolymer present as one of the major components in corn. It is a hydrophobic protein with a high content of non-polar amino acids that confers it interesting properties from the point of view of its compatibility with organophilic media. Its combination

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with different clay minerals and organoclays in view to produce biohybrids has been recently investigated and patented by the Ruiz-Hitzky’s group (Alcaˆntara et al., 2008; Ruiz-Hitzky et al., 2009b). Zein is adsorbed from ethanol/water mixtures on organoclays derived from MMT (Closite 30B) and on unmodified sepiolite and palygorskite microfibrous clays (Alcaˆntara et al., 2008). Zein–clay biohybrids can be employed as fillers in biopolymer matrices, resulting in bio-nanocomposites of interest as bioplastics (green nanocomposites) useful among other applications as films for food preservation and packaging (Ruiz-Hitzky et al., 2009b). Interestingly, the use of the zein–clay biohybrids as reinforcing agents in biopolymer matrices gives materials that can be easily processed in different ways, as for instance, films, rigid foams or microbeads. Zein–clay reinforced biopolymers processed as films by casting procedures result in bioplastics showing improved gas and water vapour barrier properties in comparison to the uncharged biopolymers. Current investigation is addressed to process these materials as foams, by means of freeze-drying techniques, or as microbeads for slow-release systems of bioactive molecules (drugs, pesticides, etc.) and other different applications. Due to their biocompatibility and non-toxicity, they can be also interesting materials for biomedical applications in controlled drug delivery and tissue engineering (Ruiz-Hitzky et al., 2010c). Bio-nanocomposites based on sepiolite and different types of water soluble biopolymers can be prepared as hierarchically organized macroporous materials (cellular structure) by means of freeze-drying processes (Figure 7B) (Darder et al., 2007). Bio-nanocomposite foams (Figure 7A) based on microfibrous clay minerals (sepiolite and palygorskite) in combination with diverse polysaccharides (starch, agarose, guar gum, locust bean gum, chitosan, alginate, xanthan, carrageenan) or structural proteins (gelatine, collagen) can exhibit enhanced mechanical properties (Ruiz-Hitzky et al., 2009d). These foams are light- or ultralight-weight materials with density

FIGURE 7 (A) Polysaccharide–sepiolite bio-nanocomposite processed as a foam prepared by the procedure described by Ruiz-Hitzky et al. (2009d) and (B) SEM image showing the macroporous structure of a chitosan–poly(acrylamide)–sepiolite bio-nanocomposite processed by freezedrying (picture from Darder et al., 2007; Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission).

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values, even below 0.05 g/cm3. They show improved mechanical properties than analogous materials based on layered silicates, and interestingly, they behave as fireproof and even, in some cases, as self-extinguishing materials. Therefore, these bio-nanocomposite foams could receive numerous applications in the fields of thermal and acoustical insulation, as well as in the packaging industry. Another remarkable property of these foams is the combination of macroporosity and biocompatibility making them potentially interesting for their use as biomaterials. For instance, chitosan–sepiolite foams can act as support of microalgae such as Chlorella vulgaris and Anabaena sp. PCC7120, which colonize the bulk of the bio-nanocomposite showing long time periods (> 1 month) of cells viability (Darder et al., 2010). However, Ruiz-Hitzky et al. (2009a) reported on the use of sepiolite–xanthan gum polysaccharide bio-nanocomposites for immobilization of viral particles. This is the case of viruses interacting via mucosa, such as the Influenza virus, which is homogenously dispersed on the bio-nanocomposite. The xanthan gum is an anionic polysaccharide that offers binding sites to immobilize the positively charged viral particles through electrostatic interactions (Figure 8). In contrast, unmodified sepiolite is not able to retain viral particles, the virus being agglomerated and segregated in large clusters. Remarkably, these sepiolite-based biohybrids show bioactivity as demonstrated by the formation of specific antibodies and the protection against Influenza virus in experiments carried out in mice. Moreover, these systems can be both intramuscularly and intranasally delivered, inducing mucosal immunity at the site of virus entry and enhancing its effectiveness by about fivefolds compared to vaccines prepared from the virus alone (Ruiz-Hitzky et al., 2009a).

FIGURE 8 TEM image of virus/bionanocomposites based on xanthan–sepiolite biohybrids (image from Ruiz-Hitzky et al., 2009a. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission).

200 nm

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3.3. Sepiolite–Carbon Materials Clay minerals of different topologies, from layered silicates such as MMT or taenolite to porous sepiolite or pillared clays, have been used as porous substrates to produce nanostructured carbonaceous materials using molecular and polymeric organic precursors, such as pyrene, safranine, styrene, propylene, acrylonitrile or sucrose, submitted to controlled thermal treatments that produce highly oriented or disaggregated carbon–clay mineral or carbonaceous materials (Bakandritsos et al., 2004; Bandosz et al., 1996; Darder and Ruiz-Hitzky, 2005; Duclaux et al., 2000; Fernańdez-Saavedra et al., 2004; Kyotani et al., 1988; Sandı´ et al., 1996; Sonobe et al., 1990, 1991; Zhu et al., 1999). The use of sepiolite may result in materials that show microfibrous or nanofibre morphologies depending on the nature of the organic molecule used as source of the carbon, for instance, ethylene and propylene (Sandı´ et al., 1999) or acrylonitrile (Fernańdez-Saavedra et al., 2004), respectively. In the case of acrylonitrile, the monomer accedes to the tunnels of sepiolite where it can be thermally polymerized to PAN. Thermal heating of the nanocomposite to a relatively low temperature (ca. 750 C) under an inert atmosphere transforms the polymer in a graphite-like conducting material (Fernańdez-Saavedra et al., 2004). In situ SAXS studies show the existence of a delay of the sepiolite folding that occurs at temperatures above 530 C instead of 487 C, corroborating the presence of the molecules into the tunnels of the mineral (Fernańdez-Saavedra et al., 2009). After elimination of the silicate substrate, it is possible to recover carbon nanofibres of ca. 1 mm by 20 nm dimensions (Fernańdez-Saavedra et al., 2004). Interestingly, the resulting graphite-like materials show higher electrical conductivity than other carbon-based materials prepared from PAN–zeolites (Enzel et al., 1992) or PAN–MCM41 mesoporous silica (Wu and Bein, 1994), being in the order of 10 6 S/cm for the carbon–sepiolite nanocomposite at room temperature (Aranda et al., 2006a). This behaviour has been interpreted on the basis of the formation of graphene units that show electronic conjugation and the intermolecular condensation of ladder-like carbon structures during the pyrolysis treatment (Fernańdez-Saavedra et al., 2008). Carbon–clay nanocomposites are, in general, prepared as source of nanostructured carbonaceous materials in view to be applied as electrodes of lithium batteries and supercapacitor devices (Aranda, 2007; Aranda et al., 2006a; Sandı´, 2001). It is remarkable that carbonaceous materials derived from sepiolite show a better behaviour as electrode for rechargeable Li-batteries than those prepared from smectite templates (Sandı´ et al., 1999). This is the case, for instance, of templated carbons prepared from PAN–clay nanocomposites in which the efficiency for reversible Liþ insertion was higher for the sepiolite-based nanostructured carbon even though the electrical conductivity of the precursor was higher in the MMT derivative than in the sepiolite one (Aranda, 2007). These nanostructured carbons can be also used as

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electrode of supercapacitors showing specific capacitance values of 22.4 and 50.2 F/g for the carbons templated using MMT and sepiolite clay minerals, respectively. In this case, the higher capacitance value for the nanostructured carbon prepared from sepiolite has been ascribed to a higher active surface associated with a lower content in micropores that are not accessible to the electrolyte molecules (Fernańdez-Saavedra et al., 2008). Comparing carbons prepared from propylene and ethylene templated on sepiolite samples of different textural characteristics, Sandı´ et al. (2003a) observed that porosity of the resulting carbons plays an important role in the diffusivity of lithium ions. Moreover, these carbons apart from application as electrodes of lithium batteries (Sandı´ et al., 2003b) can be also used for hydrogen storage (Sandı´ et al., 2003c). Conducting carbon–MMT nanostructured materials can be also prepared from cheap and abundant sources, such as sucrose, without (Darder and Ruiz-Hitzky, 2005) or in the presence of sulphuric acid (Bakandritsos et al., 2004). The use of sepiolite instead of MMT is especially interesting because, besides the fact that the resulting carbon–sepiolite nanocomposites show good electrical conductivity (around 10 2 S/cm at room temperature) (Go´mezAvile´s et al., 2007), it can be directly used as electrode of rechargeable lithium batteries (Fernańdez-Saavedra et al., 2008; Go´mez-Avile´s et al., 2010). The presence of sepiolite in this conducting nanocomposites offers the possibility to be further functionalized by grafting of organosilanes in an easy way (Figure 9), allowing the preparation of multifunctional materials that simultaneously are provided of conductivity and specific organic functions (Go´mezAvile´s et al., 2007, 2010). In this way, it has been reported the preparation of carbon paste electrodes (CPEs) based on multifunctional carbon–sepiolite materials in which amino and sulphonic groups have been incorporated

X- NH

3

H3N

H3CO Si H3CO H3CO

NH2

X-

OO Si O

[H+]

Carbon–sepiolite nanocomposite

Si OO O

Si O O O

O Si O O

XNH3

XH3N

Functionalized carbon–silicate nanocomposite (conducting solid polyelectrolyte) FIGURE 9 Scheme of the synthetic route employed for the preparation of functional and conducting carbon–sepiolite nanocomposites by grafting of organosilanes. In this case, the grafted silane is amino-propyltriethoxysilane that procures the incorporation of amino-functionalities (based on Go´mez-Avile´s et al., 2007).

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(Go´mez-Avile´s et al., 2007, 2010). The resulting CPEs show a high selectivity due to the discrimination on the size of the monovalent ions (anion or cation) probably due to the special nanoporosity of the nanocomposites (Go´mezAvile´s et al., 2007, 2010). Another way to prepare conducting carbon–clay nanostructured materials refers to the direct assembly of CNTs to sepiolite and palygorskite as recently patented (Ruiz-Hitzky and Fernandes, 2009). The role of fibrous silicate is to procure a coadjuvant effect allowing to keep CNTs in homogeneous colloidal aqueous suspensions that are stable during long periods of time (> 6 months) attributed to a synergistic effect between the two components. One of the main interests of these CNTs–sepiolite systems is to take profit of the presence of both nanofillers to prepare polymer and biopolymer nanocomposites to which procure simultaneous structural reinforcement and tunable electronic conductivity.

3.4. Sepiolite as Support of Nanoparticles Sepiolite has been used as support of nanoparticles of different origin, such as metals, metal oxides, hydroxides and oxyhydroxides. Nowadays, the term nanoparticle is commonly employed but the first reports on the preparation of nanoparticle-sepiolite materials refer to the formation of metal clusters on the surface of sepiolite. The term cluster was coined by Cotton in the early 1960s, as accounted by Mingos (1990), and refers to compounds containing metal–metal bonds. In another definition, a cluster compound contains a group of two or more metal atoms where direct and substantial metal–metal bonding is present (http://en.wikipedia.org/wiki/Cluster_chemistry). In the present section, nanoparticles and clusters will be regarded as two names for the same concept that refers to a group of bonded atoms that are intermediate in size between a molecule and a bulk solid, that is, showing nanometric dimensions. In Table 3 are collected various representative examples of nanoparticle-sepiolite materials prepared in view to applications that include ion exchangers, adsorbents of pollutants, supported catalysts and photocatalysts, antibactericide action or active phases of magnetic sensors. Some of these examples will be described in more depth in the following paragraphs in order to show the variety of strategies applied for the preparation of such nanomaterials and the versatility in their applications. The structural, textural and surface characteristics of sepiolite allow different strategies for the generation of nanoparticles on the surface, chemically bonded or even templated on the pores of the mineral. For instance, Cu nanoparticles have been formed on the surface of sepiolite incorporating first [Cu(NH3)4]2þ species by an ion-exchange reaction. The stabilization of Cuþ species determines interesting properties of these materials in methanol dehydrogenation to methyl formate (Sun Kou et al., 1992). In the same way, [Pd(NH3)4]2þ species can be incorporated to sepiolite by ion exchange

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TABLE 3 Some Examples of Nanoparticles Supported on Sepiolite. Preparation

Behaviours

Applications

Authors

OsO4

Grafting through vinylsilanes

Clusters of homogeneous nanometric dimensions

Photocatalytic decomposition of water

Barrios-Neira et al. (1974) and Casal et al. (1985)

NiOx

Ni acetate thermal decomposition

Nanoparticles of ca. 5 nm

Microwave decomposition of pesticides

Salvador et al. (2002)

TiO2

Ti-isopropoxide/CTAB

Homogeneous nanoparticles of ca. 6 nm

Photocatalytic decomposition of pollutants

Aranda et al. (2008)

SiO2–TiO2

Ti-isopropoxide/TMOS/CTAB


Photocatalytic decomposition of pollutants

Aranda et al. (2008)

[Mg2Al(OH)6 Cl]nH2O

AlCl3; MgCl2; NaOH

Nanoparticles of variable size

Cationic/anionic exchange. Catalysis

Ruiz-Hitzky et al. (2008b)

Fe3O4

Adsorption from ferrofluids in sepiolite and organo-sepiolites

Superparamagnetic behaviour

Adsorption of species, water treatment, magnetic nanofiller, etc.

Ruiz-Hitzky et al. (2010b)

Ni

Precipitation and reduction of Ni nitrate


Hydrogenation of toluene

Zhai et al. (2005)

Cu

Ion exchange with [Cu (NH3)4]2þ

Cuþ predominant species

Methanol dehydrogenation to methyl formate

Sun Kou et al. (1992)

Pd

Ion exchange with [Pd (NH3)4]2þ

Metal clusters from 2 to 7 nm

Suzuki coupling reaction in water

Shimizu et al. (2004)

Soot combustion

Gu¨ngo¨r et al. (2006)

Ag


Nature of Nanoparticles

Chapter

Au, Ag, Fe, Co

Acid treated sepiolite þ metal salt and reduction

3–6 nm nanoparticles

Potential use due to their optical and magnetic properties

Pecharromań et al. (2006)

Cu


6 nm nanoparticles

Antibacterial activity

Esteban-Cubillo et al. (2006b)

Fe2O3

Acid treated sepiolite þ metal salt

Nanoparticles < 15 nm

Humidity sensor

Esteban-Cubillo et al. (2007)

Fe and Fe3O4


Superparamagnetic and ferromagnetic behaviour

Magnetic applications

Esteban-Cubillo et al. (2008a)

Ni


Superparamagnetic and ferromagnetic behaviour

Magnetic applications

Esteban-Cubillo et al. (2010)

Au


3–6 nm nanoparticles

Optical properties

Pecharromań et al. (2009)


Catalytically active AgOx phases

17

Ag nitrate impregnation on sepiolite–Zr–K–O

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and after reduction is detected the presence of metal clusters of 2–7 nm that are active in the Suzuki coupling reaction in water (Shimizu et al., 2004). Alternatively and as described in Section 3.1.2, chemical modification of the external surface of sepiolite with covalently bonded unsaturated organic groups allows the incorporation of metal oxide particles, such as osmiate groups (Barrios-Neira et al., 1974), with activity to participate in the photocatalytic decomposition of water (Casal et al., 1985). In other cases, the so-called impregnation methods in which metal oxyhydroxides are first precipitated on the surface of sepiolite and further transformed in metal or metal oxide nanoparticles have been employed. This is the case, for instance, of nickel oxyhydroxides precipitated from nitrate solutions in which further reduction drives to the formation of Ni nanoparticles of ca. 100 nm that can be active in the hydrogenation of toluene (Zhai et al., 2005). However, it has been shown by XPS that the preparation method strongly influences the nickel–sepiolite catalysts (Anderson et al., 1993). In other cases, it is possible to form NiOx by thermal decomposition of precipitate nickel acetate to generate nanoparticles of the oxide of about 5 nm diameter by thermal heating at 400 C for 2 h. These nanoparticles are very active in pesticide (e.g. lindane) decomposition under MW irradiation (Salvador et al., 2002). To stabilize the support, in certain cases sepiolite is thermally treated at temperatures even close to that of sepiolite dehydroxylation (ca. 840 C) before the impregnation process. This is the procedure used by Watanabe and co-workers (2000) to prepare highly stable Pt/Fe catalysts for hydrocarbon oxidation. The formation of iron oxide particles with diameters of 2–5 nm on the surface of heated sepiolite that also contains Pt particles procures a synergistic action to assure the complete oxidation of hydrocarbons with simultaneous suppression of SO2 oxidation, which could be specially useful for diesel engines. A similar effect has been observed on sepiolite modified with ZrO2 and further impregnated with silver nitrate to stabilize silver nanoparticles for soot combustion (Gu¨ngo¨r et al., 2006). The high external surface of sepiolite makes it very attractive as support of TiO2 nanoparticles for the development of photocatalysts. In this way, Aranda and co-workers (2008) recently reported an innovative route to produce TiO2 anatase nanoparticles of about 4–8 nm on the external surface of sepiolite fibres. TiO2 is generated by controlled hydrolysis of titanium tetraisopropoxide incorporated in a sepiolite previously modified with a cationic surfactant (e.g. cetyltrimethylammonium ions) (Figure 10). This approach is related to that previously reported for the preparation of inorganic–inorganic nanocomposites in which the generation of a silica network in the interlayer region of smectites and vermiculite provokes the delamination of the phyllosilicate (Letaıëf and Ruiz-Hitzky, 2003; Letaıëf et al., 2006). Anatase nanoparticles are stabilized after calcination at 500 C for 1 h in N2 atmosphere and 5 h in air flux in a process in which the organic matter is removed. The incorporation of thiourea to the initial colloidal system allows the stabilization of the

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Si O Mg MgOH SiOH H2O (coord.) H2O (zeol.)

B

CTA CTAB

A

TiO2 nanoparticles

ion-exchange

100 nm

C

Sol–gel Alkoxide D/[O2] Calcination Nanoparticles

Oxide coverage

200 nm

FIGURE 10 (A) Schematic representation of the procedure reported by Aranda and coworkers (2008) to prepare TiO2–sepiolite materials by controlled hydrolysis of titanium alkoxides on the surface of organo-sepiolite. (B) TEM and (C) FE-SEM images showing the presence of the TiO2 nanoparticles covering sepiolite fibres. (Scheme and images reproduced from Aranda et al. (2008) with permission from the American Chemical Society.)

anatase phase at temperatures above 400 C due to the incorporation of S traces into the titanium dioxide crystal lattice during the thermal treatment. The TiO2–sepiolite nanocomposites show high efficiency in the phenol photodegradation that can reach more than 90% of conversion (Aranda et al., 2008). The simultaneous treatment with titanium and silicon alkoxides (e.g. titanium isopropoxide, Ti(PrO)4, and tetrametoxisilane, TMOS) allows the formation of TiO2–SiO2 nanoparticles on the surface of the fibres. As also observed in TiO2–SiO2/smectite and TiO2–SiO2/vermiculite prepared by a related colloidal route (Manova et al., 2010), the activity as photocatalysts of these supported mixed oxides is significantly lower than for the TiO2–clay materials (Aranda et al., 2008). Although this seems a drawback, Jung and Grange (2004) demonstrated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) that the direct assembling of hydroxyl groups of sepiolite and Ti(OH)4 occurs via SiOTi bonds and can be responsible for generation of acidity. Therefore, the presence of protons able to act as Bro¨nsted acid sites in the TiO2–SiO2 mixed oxide nanoparticles generated on the surface of sepiolite can be of interest for use of these materials as acid catalysts. Sepiolite is used to improve the rheological properties during the extrusion process of the ceramic paste used in the preparation of ceramic monolithic catalysts, improving also the porosity and mechanical strength of the heattreated composite (Sua´rez et al., 2005). These monoliths or honeycomb catalysts can be employed as support of TiO2 nanoparticles for use in the

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photocatalytic degradation of pollutants such as trichloroethylene (Sua´rez et al., 2008). In the same way, TiO2 Degussa P25 nanoparticles have incorporated monolithic pieces of macroporous structures assembled from sepiolite and a silica sol using a cryogenic process called ISIA (ice-segregation-induced self-assembly) (Nieto-Sua´rez et al., 2009). Monodispersed metallic nanoparticles and their precursors can be also synthesized following wet chemical route within the framework of sepiolite (Pecharromań et al., 2006). In this case, the preparation method is initiated with an acid attack to an aqueous sepiolite dispersion under high shear mixing. Sepiolite is previously purified and micronized by a wet process to obtain a final product more than 95% content of sepiolite. As a result, magnesium cations are leached from the phyllosilicate matrix, and consequently, silanols are formed into the tetrahedral layer (Hernańdez et al., 1985). In this regard, sepiolite is treated in acid to leach a sufficient amount of magnesium cations without collapsing the structure by the formation of siloxanes (Jimeńez-Lo´pez et al., 1978), and subsequently, a solution of the required metal is added to the suspension. After an elapsed time to homogenize the suspension, the metallic cations are exchanged in the sepiolite matrix. Mg8 Si12 O30 ðOHÞ4 ðH2 OÞ4 8H2 O þ Hþ >Mg8x H2x Si12 O30 ðOHÞ4 ðH2 OÞ4 8H2 O Mg8x H2x Si12 O30 ðOHÞ4 ðH2 OÞ4 8H2 O þ xM2þ >Mg8x Mx Si12 O30 ðOHÞ4 ðH2 OÞ4 8H2 O

The ionic exchange takes place after the addition of NaOH to increase the pH of the solution under high shear mixing. Cationic metals will preferentially occupy the octahedral sites (Corma et al., 1985; Sabah et al., 2002; Vico, 2003) in substitution of the Mg2þ because they have solubility constants smaller than that of magnesium (Ersoy and Celik, 2002). The obtained precursors following this procedure are treated at different temperature depending on the metal in an air or a reductor atmosphere to obtain metallic or metal oxide nanoparticles, respectively. Although the thermal process induces a double dehydration process in natural sepiolite corresponding to the loss of two pairs of water molecules and a structural folding (Preisinger, 1959), in the case of metal-substituted-sepiolite only a single dehydration process can be found by differential thermal analysis. This single thermal step can be assigned to a coupling of dehydration process to the reduction of the metallic cations following the scheme of a Hedvall effect (Hedvall, 1966). After this treatment, nanoparticles appear disperse and their size distributions are remarkably narrow with an average particle size ranging from 3 to 6 nm. The nature and location of the different metallic species within the sepiolite structure (Figure 11) are dependent on the conditions of the wet chemical preparation and, specifically, on the initial acidic treatment of the sepiolite because the amount of the vacancies available to incorporate a foreign cation depends on the magnesium cations leached from sepiolite during the acid treatment (Esteban-Cubillo et al., 2008a). So, a moderate acid treatment

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FIGURE 11 Transmission electron micrographs of nickel (A), copper (B), iron (C), silver (D), cobalt (E) and gold (F) in sepiolite with their corresponding metal particle size distributions (insets). (Pictures from Pecharromań et al., 2006; Copyright John Wiley and Sons. Reproduced with permission.)

(pH ¼ 2 for 1 h) only removes the magnesium cations corresponding to the particle surface, while a stronger acid treatment (pH ¼ 0 for 1 h) is able to leach the Mg2þ cations located at the edges of the octahedral layer both into the channels and at surface (Esteban-Cubillo et al., 2008b). The fraction of magnesium cations leached by acid treatment appears to be crucial because it has been shown that a short treatment is unable to remove enough Mg2þ cations located at distal positions while a more prolonged one removes all the Mg2þ cations from the sepiolite framework to give place to the formation of porous silica (Gonza´lez et al., 1984). In this sense, during the raising of pH

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process, metallic cations are incorporated into the sepiolite magnesium vacancies (Corma et al., 1985; Vico, 2003) forming a variety of stable metalliccontaining sepiolites which are found to be naturally occurring and that are known as ferricsepiolite or xylotile (Preisinger, 1959), nickeliferous sepiolite ´ lvarez, 1984), aluminous sepiolite or manganese sepiolite or falcondoite (A (Semenov, 1969). Once all the magnesium vacancies have been occupied and in case that the aqueous solution would contain an excess of metallic cations, they would precipitate on the sepiolite surface as poorly crystallized metal oxides or oxyhydroxides. The main advantage of this matrix is the large quantity of metallic nanoparticles that can be easily obtained. Additionally, as a consequence of the fact that nanoparticles are supported on silicate microparticles, manipulation becomes easier and health risks, recently reported (Oberdo¨rster, 2004; The Royal Society, 2004), are considerably reduced. Moreover, these nanoparticles have been revealed to be remarkably stable against oxidation because the transformed sepiolite matrix becomes a diffusion barrier for oxygen. It should be noted that unprotected metallic nanoparticles are very sensitive to oxidation, especially iron nanoparticles. The advantage of preparing nanoparticles in a sepiolite matrix is their resistance to oxidation because most of them remain completely embedded into a silica matrix formed by the thermal decomposition of sepiolite. In fact, Esteban-Cubillo et al. (2008a) have found that iron nanoparticles remain non-oxidized even under a thermal treatment of 2 h at 250 C in air. Moreover, Ni nanoparticles present a similar behaviour. In this case, samples which have been stored in air for a period of 1 year only present a 20% reduction of the X-ray diffraction peak corresponding to (111) planes, while its half width increases (Esteban-Cubillo et al., 2010). Metallic nanoparticles present suitable properties to be used for biocide, optical, magnetic applications or humidity control. In case of the biocide properties, it is well known that some kind of metallic nanoparticles, such as silver, copper and zinc, have antibacterial capabilities (Horiguchi, 1980) and they are being used in biomedical (Hu¨tten et al., 2004; Tartaj et al., 2003) and biological (Sondi and Salopek-Sondi, 2004) applications. Several research about the use of these nanostructured materials as bactericides are being developed because these metal nanoparticles show a higher efficiency against microorganisms than conventional materials due to their larger specific surface, which allows the same biocide activity reducing the dose of metal and improving its biocompatibility. Moreover, there is a special interest to substitute the traditional organics bactericides with their limited applications (low heat resistant, high decomposability and short life) by new bactericides based on inorganic materials in order to increase the effectiveness decreasing their cost (Li et al., 2002). Other reason to develop new bactericide agent is that microorganisms are developing an important resistance to multiple antibiotics.

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In fact, the inorganic biocides market ($10 billions) increases at an average annual rate of 6% (Esteban-Tejeda et al., 2009a). This kind of biocides has a large spectrum of applications in different fields such as medical instruments, medical implants, water treatment, food processing, public areas and agriculture. In this regard, copper and silver monodispersed nanoparticles (< 20 nm) supported on a sepiolite matrix have been tested as antibacterial agent versus two different microorganisms, a Gram-positive bacterium (Micrococcus luteus) and a Gram-negative bacterium (Escherichia coli), following the Flash Shake Method (ASTM E 2149-01). Antimicrobial test for antimicrobial activity (Table 4) has revealed that copper and silver nanoparticles supported in sepiolite are an excellent antibacterial agent on Gram-positive and Gram-negative bacteria (Esteban-Tejeda et al., 2009a,b). As it can be seen from Table 4, a concentration of 0.036 wt % of silver nanoparticles and 0.054 wt% of copper reduces significantly the number of Gram-positive and Gram-negative colonies. The high effectiveness of these nanoparticles in sepiolite could be due to the combination of a large specific surface of metallic nanoparticles along the sepiolite fibres and to the intrinsic large specific surface area of sepiolite (ca. 100 m2/g). Sepiolite acts as an effective scaffold, where copper and silver nanoparticles are perfectly monodispersed avoiding agglomeration of nanoparticles and acting as an accessible surface enhancer of these nanoparticles. Copper nanoparticles have been compared with Triclosan (Levy et al., 1999; McMurray et al., 1998), a commercial broad-spectrum antibacterial/ antimicrobial agent. As a result, copper supported on sepiolite shows a strong bactericide activity similar to Triclosan (Esteban-Cubillo et al., 2006b). According to the literature (Chang et al., 2007; Lan et al., 2007; Page et al., 2007), the mechanism followed by these metallic nanoparticles against the E. coli and M. luteus microorganism is based on the amount of metal cation leached to the solution (Feng et al., 2000). In this regard, sepiolite allows a controlled lixiviation and a safe disinfestation. Moreover, copper, silver and gold particles in a nanometric size display light absorption maxima in UV–vis spectrum corresponding to surface TABLE 4 Antibacterial Activity of Copper and Silver Nanoparticles Supported on Sepiolite. Microorganism

Log Reduction (24 h)

Log Reduction (48 h)

nano-Cu-Sep

nano-Ag-Sep

nano-Cu-Sep

nano-Ag-Sep

Micrococcus luteus

7

5.65

7.5

8.02

Escherichia coli

6.5

>10

7

>10

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plasmon resonances. These phenomena are associated to large increases of the local electromagnetic field (Flytzanis et al., 1991; Hache et al., 1986; Shalaev, 1996) and only take place when metallic nanoparticles have a particle size smaller than 10 nm and they are perfectly monodispersed, so that the size is much smaller than the incident wavelength in order to avoid light scattering. These metallic nanoparticles have been embedded in a sepiolite matrix to obtain monodispersed particles along the sepiolite fibres with sizes smaller than 10 nm and intensive colours associated to the resonance of the surface plasmon (Pecharromań et al., 2009). In fact, these metallic nanoparticles show vivid colours in contrast to most of the metallic colloids that are dark (Freestone et al., 2007; Maxwell-Garnett, 1904; Pe´rez-Arantegui et al., 2001). The UV–vis spectra of Ag, Au and Cu nanoparticles embedded in sepiolite have been reported in the literature. These metallic nanoparticles have shown the characteristic maxima absorption indicating their nanometric size and homogeneous dispersion. The position of these absorption peaks gives information about the local environment of the metallic nanoparticles, size, shape and concentration of nanoparticles (Bohren and Huffman, 1983; Pecharroman et al., 2006; Ung et al., 2001). In this regard, optical measurements such as permittivity values allow to confirm that metallic nanoparticles are embedded into a low-porosity matrix and the position and shape of the light absorption maxima indicate a narrow size distribution of nanoparticles in sepiolite with a spherical shape. Concerning the magnetic properties of the nanoparticles, there is a huge interest to synthesize stable and monodispersed nanoparticles of Fe, Ni and Co due to their potential applications. These magnetic nanoparticles can be used in a large variety of applications such as high-density recording media, contrast enhancement in magnetic resonance imaging, hyperthermia, magnetic carriers for drugs targeting, separation and selection, catalysis, magnetic refrigeration systems, magnetic and magnetooptical sensors or catalysis. Magnetic behaviour of metallic particles such as iron, cobalt, nickel and metal oxide as magnetite or maghemite embedded or supported in sepiolite and palygorskite have been characterized in the literature because the great interest that this kind of nanoparticles present as a consequence of their specific properties associated to their finite size effects, size distributions and interparticle interactions. In fact, sepiolite and palygorskite have a huge potential to modify the magnetic behaviour of these nanoparticles associated to size, shape and their distributions because these clay minerals allow the control of the location of the metallic cations during the synthesis process. Particle size, stability and magnetic behaviour of the metallic nanoparticles have revealed to be strongly dependent on the location of the nanoparticles in the matrix. By controlling the acid treatment, it is possible to incorporate metallic cations into the octahedral site of the structural channels or to deposit them onto the sepiolite surface (Esteban-Cubillo et al., 2008a). Thus, metal or metal oxide nanoparticles can be

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obtained with a superparamagnetic behaviour when they have a size smaller than 10 nm and are embedded in the sepiolite structure or as single domain when they are larger and deposited on the surface of this clay minerals. According to experimental results, nanoparticles obtained after a weak acid attack have a large and broad particle size distribution laying on the surface of sepiolite fibres while those obtained in a strong acid attack are monodispersed, very small and embedded along the fibres. These embedded nanoparticles show a single-magnetic-domain behaviour in case of iron and cobalt metallic nanoparticles and a superparamagnetic behaviour in nickel nanoparticles at room temperature. Additionally, it is possible to obtain different metallic species by modifying the thermal treatment of sepiolite. Therefore, magnetic oxides such as magnetite or maghemite have been obtained with a superparamagnetic behaviour at room temperature. Moreover, sepiolite matrix presents another important advantage: the embedded nanoparticles have been revealed to be remarkably stable against oxidation because the transformed sepiolite matrix becomes a diffusion barrier for oxygen. The chemical stability of the metallic nanoparticles in the collapsed matrix also has a strong influence in their magnetic properties. In fact, metallic nanoparticles such as Ni, Fe and Co are susceptible to be degraded by oxidative or corrosive environments due to their large specific surface area. Intensive research has been carried out to stabilize these metallic nanoparticles by coating, core–shell coupling or nanocomposites based on metallic particles embedded in a silica matrix. As an example of their high reactivity, a simple exposure of non-passivated Ni nanoparticles to air is enough for their complete oxidation (Park et al., 2005). Contrarily, these metallic nanoparticles embedded in sepiolite have shown to be strongly resistant to oxidation even at high temperatures. In this regard, Ni and Fe metallic nanoparticles remained unaltered even after a thermal treatment of 2 h at 250 C in air. This large range of stability would allow the use of this magnetic material for applications in extreme conditions (high temperatures and oxidizing environments). According to the magnetic properties described before, the opportunity to modify the magnetic behaviour of the nanoparticles in sepiolite during the synthesis process and the stability of these to remain non-oxidized opens the possibility to use these nanocomposites in different applications where magnetic corrosion resistant nanoparticles are required, as it is in the case of ferrofluids. More sophisticated inorganic particles, such as those of layered double hydroxides (LDHs), have been recently prepared on fibres of sepiolite (Ruiz-Hitzky et al., 2008b). Firstly, the use of sepiolite as support of the LDH particles procures an increase in the silicate specific surface area. Moreover, the formation of the supported LDH particles has the interest to combine two solids with opposite ion-exchange capacity originating a new material with synergistic properties towards the simultaneous ion exchange and adsorption of cations and anions. The assembly of both particles occurs via

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interaction of hydroxyl groups of the LDH solid and the silanol groups of sepiolite as corroborated by NMR studies (Ruiz-Hitzky et al., 2008b). The potential interest of this type of materials includes their use as adsorbents of gases and pollutants in aqueous media, ion exchangers or support of catalysts among others. Other synthetic routes imply the incorporation of already formed nanoparticles by using an adequate carrier. This is the case of the treatment of sepiolite or organo-sepiolites, for instance, exchanged with cetyltrimethylammonium, with a ferrofluid that contains Fe3O4 nanoparticles of ca. 10 nm recently patented by Ruiz-Hitzky and co-workers (2010b). The resulting modified sepiolite samples show superparamagnetic behaviour maintaining the sorption properties of the silicate. In this way, they can be used for adsorption of different species in water, for instance, the MB dye, with the possibility of being easily recovered from it with the help of an external magnetic field (Ruiz-Hitzky et al., 2010b).

3.4.1. Functionalized Materials by Assembling Metallic Nanoparticles to Sepiolite Nanoparticles embedded in sepiolite allow to disperse them in an inorganic matrix in order to obtain diverse functionalized materials. These assembled nanoparticles have been used to functionalize matrices like glaze or glass, but can be also used to obtain nanocomposites based on sintered or compacted nanoparticles in sepiolite. The use of nanoparticles has been limited in a ceramic or glass matrix up to now due to the problems associated with the nanometric character of the particles like their tendency to aggregate or the oxidation of the metallic particles but the principal problem is the necessity to manufacture large quantities. Sepiolite allows to solve these problems because it is possible to obtain large quantities of nanoparticles in this matrix according to the process described before, avoiding the agglomeration of these particles. For ceramics, sepiolite has a composition compatible with the glaze. In fact, nanoparticles embedded in a sepiolite matrix have been used to develop new multifunctional nanostructured glazes (Jaquotot et al., 2009). These developed glazes possess a nanostructure which is responsible for different properties like metallic aspect, hydrophobic response, bactericide and fungicide properties, and self-cleaning characteristics. The obtained glazes with nanoparticles in sepiolite display a nanostructure along the layer deposited on the tile where the metallic cations are located. Two principal mechanisms take place to obtain the different properties, a spinodal decomposition and diffusion process in the glaze due to the major interface energy between solid–liquid than solid–gas. As a consequence, metallic nanoparticles rise to the glaze surface where redox reactions produce different effects and a superficial cellular structure with a nanotexture like lotto flower

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is formed. Glazes with self-cleaning surfaces are obtained because of this nanotexture. Other advantage due to these nanoparticles embedded in sepiolite is that nanoparticles stay monodispersed in a glaze even after heating them at high temperatures. In this context, it is possible to stabilize nanoparticles like silver or copper to give antibacterial or antifungal properties and obtain new aesthetic effects in the glaze. According to the optical properties described, materials based on silver, copper and gold metallic nanoparticles in a sepiolite matrix display intensive colours as a result of the surface plasmon resonance. These metallic nanoparticles in sepiolite can be used as inorganic pigments. The efficiency of the supported nanoparticles allows the use of the nanoparticles technology to obtain multifunctional glazes and powered the innovations into the tile product with added value, reducing the environmental impact of the metal and metal oxide nanoparticles due to the low amount required and because these nanoparticles are embedded in a sepiolite matrix. The surface plasmon resonance observed in silver, copper and gold nanoparticles embedded in sepiolite and in glasses doped with them opens the possibility to use these materials in electromagnetic field amplifier (SERS and SEIRA) devices or in optoelectronic applications (optical storage devices), where this kind of nanoparticles is being used due to its very large values of the third order non-linear susceptibility effects. Moreover, other interesting aesthetical effects have being obtained because of the nanometric size of the particles. These nanoparticles in sepiolite are being used to develop new glazes with innovative characteristics for the tile industry. As a consequence of the dispersion and protection of the nanoparticles in sepiolite, this is the first case where nanoparticles are used in the ceramic sector following conventional ceramic process according to Jaquotot et al. (2009). In this sense, new glazes show a similar behaviour to the standard glazes. Different aesthetical effects such as colours, coloured glazes with intensive shine and reflection, metallic shine aspects or matt finish have been obtained by changing the amount of nanoparticles in sepiolite and the thickness of the glaze layer. Moreover, these are the first glazes with metallic aspects based only on inorganic materials. However, soda lime glass powder containing silver and copper metallic nanoparticles has been obtained following a ‘bottom-up’ route starting from Ag-sepiolite and Cu-sepiolite as the source of metal. Doped glasses obtained following the procedure carried out by Esteban-Tejeda et al. (2009a,b) show a narrow size distribution and a homogeneous dispersion of metallic nanoparticles. Similar to the glaze, the nanostructured character of the silver particles is preserved even after a high-temperature thermal treatment when glasses are sintered at 800 C. Metal nanoparticles have a strong tendency to aggregate when they are embedded into a soda lime glass. However, the tendency of silver and copper nanoparticles supported on sepiolite to coarse is limited because the starting

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metal nanoparticles are embedded into sepiolite fibres so that sepiolite matrix forms a higher-viscosity phase (silica rich) in its neighbourhood during the melting process which avoids this aggregation. In this sense, the nanometric size and dispersion of the metal nanoparticles were confirmed because doped glasses display a surface plasmon resonant in the UV–vis spectrum corresponding to silver and copper nanoparticles. Antibacterial and antifungal effect of both glasses coating silver and copper nanoparticles were tested according to the Flash Shake Method (ASTM E 2149-01) against three different microorganisms: E. coli JM 110 (Gram-negative bacteria), M. luteus (Gram-positive bacteria) and Issatchenkia orientalis (yeast). As a result, the presence of nano-Ag-doped glass at a concentration of 0.036 wt% of silver reduces the number of colonies of E. coli, M. luteus and I. orientalis more than five orders of magnitude. In the case of glass doped with copper nanoparticles, it is necessary to increase the concentration of copper nanoparticles from 0.036 to 0.054 wt% to achieve an antibacterial and antifungal activity similar to that of the glass doped with silver nanoparticles (Table 5). According to the literature, the mechanism for the silver against the E. coli and M. luteus is well known and it depends on the amount of metal cations leached in solution while the mechanism to I. orientalis varies (Droby et al., 1997; Holmes et al., 1991; Tian et al., 2002). The role of the glass matrix is extremely relevant because it allows a quick acting of the metal nanoparticles to obtain a safe and fast disinfection. So, doped glasses (Figure 12) increase the antibacterial activity with respect to Ag or Cu sepiolite powder while in case of the yeast I. orientalis, these glasses show a higher antifungal activity. The different biocide and fungicide activities between the doped glasses and nano-Ag- and nano-Cu-sepiolite can be explained considering the lixiviation of calcium from the glass matrix which by itself inhibits the growth of

TABLE 5 Antibacterial and Antifungal Activity of Nanoparticles Supported on Sepiolite. Microorganism

Log Reduction (24 h) Glass Doped Nano-Cu–Sep 0.036 wt%

0.054 wt%

M. luteus

3

5

E. coli

1

I. orientalis

0.5

Glass Doped Nano-Ag–Sep 0.036 wt%

Log Reduction (48 h) Glass Doped Nano-Cu–Sep

Glass Doped Nano-Ag–Sep 0.036 wt%

0.036 wt%

0.054 wt%

>10

3.5

>10

6.95

> 10

>10

8.5

>10

>10

3.5

5.88

0.7

5

5.38

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A2

A1

n-Cu glass

n-Cu glass t = 30 min

0.6 mm B

t = 30 min

0.2 mm

n-Cu glass

t = 90 min n-Cu

0.2 mm FIGURE 12 Transmission electron micrographs of a glass doped with copper nanoparticles in sepiolite and E. coli after (A) 30 min and (B) 90 min of biocide test (from Esteban-Tejeda et al., 2009a; reproduced with permission of IOPScience).

microorganisms. In this sense, the combined effect of metal nanoparticles with the Ca2þ lixiviated from the glass may present a synergistic effect with very high biocide efficiency as it has been previously reported in the literature with different chemical agents. In summary, nanoparticles embedded in sepiolite and glasses doped with this material facilitate the nanoparticle handling reducing health risks and present a high antibacterial and antifungal activity. Its high efficiency is due to the large specific surface, which is inversely proportional to their reduced particle size. The other advantages of these materials are their low toxicity, chemical stability, long-lasting action period and thermal resistance versus organic antibacterial agents which allow these materials to be used in different applications such as paint or plaster (Lee et al., 2003) for coating hospital equipment, as well as in fittings for public places, public transport, toys and kitchen, school and hospital utensils. Moreover, nanocomposites based on sepiolite as matrix have also been obtained after different treatment such as spark plasma sintering (SPS) or uniaxial pressure in order to obtain a compacted material. As it has been seen before, gold nanoparticles on sepiolite display an intensive colour because of the surface plasmon resonance. As a consequence

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of its reduced size (< 10 nm), spherical shape and concentration, this nanostructured material may present high values of the non-linear dielectric susceptibility (Ballesteros et al., 1999; West et al., 2003). However, in order to measure these optical properties, it is necessary that nanoparticles are dispersed in a transparent and dielectric matrix because powdered samples cannot be done. One way to obtain gold nanocomposites with optical transparency and non-linear optical response consists in sintering gold nanoparticles embedded in sepiolite by SPS at low temperature. SPS process allows to prepare, to handle, to conform and to sinter nanostructured composites avoiding the coalescence phenomena of these nanoparticles. Moreover, the possibility to reduce the sintering temperature allows us to preserve the nanometer size character of gold particles. Compacted discs of gold/clay nanocomposites have been obtained by SPS at 815 C and 50 MPa using gold nanoparticles embedded in sepiolite (8 wt%) as raw material. These cylindrical dense compacted pellets (Figure 13) with a density of 2.5 0.2 g/cm3 and a hardness of 2.0 0.2 GPa presented a Bordeaux-red colour indicating the presence of gold nanoparticles non-agglomerated. According to Pecharromań et al. (2009), the shape and symmetry of the peak obtained in the visible absorption spectra associated with the surface plasmon resonance indicate that no coarsening of the gold particles seems to be produced during sintering. In fact, sepiolite preserves the nanostructured character of the metallic nanoparticles even after the sintering process. Following this sintering process, it has been possible to obtain high values of the effective non-linear susceptibility (w3 ¼ 2 10 10 esu) in samples with a Au content of 8 wt%. This value compare satisfactory with systems in which the nanocomposite is obtained directly from nanoparticles (Del Coso, 2004). However, the concentration used (8 wt%) is quite far from the percolation threshold, where optimal values of w3 have been obtained (Okada et al., 2004). It can be remarked that this nanocomposite based on gold nanoparticles

FIGURE 13 Pellet of gold nanoparticles in sepiolite obtained by spark plasma sintering (from Pecharromań et al., 2009; reproduced with kind permission of Springer Science and Business Media).

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embedded in sepiolite can be applied to different metallic nanoparticles such as copper, silver or even palladium, nickel, iron and several others. Compacted material from nanoparticles in sepiolite has also been obtained by a simple pressure method to take advantage of the unusual properties of nanoparticles. In this case, iron oxide nanoparticles in sepiolite were selected as a consequence of their interesting properties like humidity sensors. Nowadays, the knowledge of relative humidity (RH) value is of crucial importance in a large amount of applications. In this regard, humidity sensors are being widely used in different industrial applications such as production of paper, fibres, materials for electronics, fabrication of precision instruments as well as for the end-user market in air conditioners, dryers, dehumidificators, MW ovens or electrical household medical equipment such as apparatus for assisted respiration, sterilizers, incubators. These sensors are used in agriculture systems to programmed irrigation for water saving and control of the hothouse cultivation and for safety and environmental control systems. Porous ceramics have been widely investigated as humidity sensors because of their structures with open pores which tend to favour water and gases adsorption and condensation. Humidity sensor based on electrical properties is largely related to the porosity and the pore size distribution of the open pores. Therefore, a porous matrix with a high specific surface area like sepiolite and a nanostructured material improves the sensibility of the different sensors. Metal oxide nanoparticles such as iron oxide, zinc oxide, aluminium oxide, titanium oxide or tin oxide are normally used as active phases of humidity sensor devices. A special role is played by iron oxides such as a-Fe2O3 and Fe3O4, as a consequence of their functional properties. These types of nanoparticles are normally used in combination with other oxides having different oxidation states or doped with different cations (Neri et al., 2001, 2003; Zucco et al., 2002). Electronic and/or ionic conductivity of these oxides can change significantly with grain size and with its microstructure. In fact, the variation of the RH response drastically enhance when a metallic oxide is prepared as nanoparticles. However, in order to avoid agglomeration problems, the nanometer-size phase must be dispersed into a porous matrix. In this way, sepiolite is one of the optimum candidates to be the host for oxide nanoparticles. This silica matrix allows us to obtain a compacted disc of iron oxide nanocomposites by pressure and heat, avoiding the agglomeration of the nanoparticles (Esteban-Cubillo et al., 2007). According to this, iron oxide nanoparticles (hematite) supported on sepiolite were compacted as pellets by uniaxial pressure and heated at 520 C for 1 h to measure their response as humidity sensor. In this case, two pellets with an iron oxide nanoparticle content in sepiolite of 10.73 and 46.76 wt% were obtained. These pellets with their corresponding interdigitated metallic electrodes were found to be very appropriate to operate as humidity sensors

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FIGURE 14 Humidity sensor obtained with different content of iron oxide nanoparticles in sepiolita with integrated electrodes (from Esteban-Cubillo et al., 2007; reproduced with permission of Elsevier).

(Figure 14) over a wide RH range. These humidity sensors presented a strong decrease of their resistance over an RH range between 5% and 98%, depending on the iron oxide nanoparticles content. The response of these sensors can vary with the content of iron oxide in sepiolite because the water condensation in the microporosity of the sepiolite pores is governed by hematite additions and the hematite content can be adjusted so that the resistance decrease happens in the desired RH range in function of the application. Sensor with a hematite content of 10.73 and 46.76 wt% shows a huge resistance variations between 5–55% and 5–20% of RH, respectively, and a very fast response time even after 1 min of measurement. The response of these sensors varying the RH from 0% to 80% by alternating cycles with different steps during 2.5 days was so fast that it just needed 3 min to detect a variation and no hysteresis processes were presented. The fact that a-Fe2O3 particles have nanometre size and appear monodisperse plays an important role in the ability of the adsorbents as humidity controller due to their large specific surface area. According to these results, sepiolite retains water vapour in air at high RH and reacts by large variations in the amount adsorbed, whereas this variation occurs for iron oxide nanoparticles at low-medium RH, as a function of the iron oxide concentration. In this way, a mixture of different iron oxide concentrations on sepiolite produces differences in the variation of the resistance and widens the range of the application of sepiolite as humidity device.

4. CONCLUSIONS As pointed out by Robertson (1957) several decades ago, sepiolite and palygorskite are versatile raw materials deserving many diverse applications. Both silicates exhibit unique textural and structural characteristics and therefore exclusive properties of interest in application fields as diverse as thickener for paints or support of pesticides.

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The incoming era of nanotechnology is affecting all the materials that surrounded us. Clays and clay minerals are not out of this situation and sepiolite and palygorskite are more and more regarded as nanomaterials. In this way, their future uses are very promising for advanced applications as diverse as nanofillers in polymer–clay nanocomposites, support of nanoparticles for sensor devices and high-performing catalysts or to build clay-biological interfaces for tissue engineering, new adjuvants for vaccines and bioreactor devices.

ACKNOWLEDGEMENTS This work was partially supported by the CICYT, Spain (MAT2009-09960), and the CSIC, Spain (PIF08-018).

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Chapter 18

The Maya Blue Pigment Manuel Sańchez del Rıó*, Antonio Domeńech{, Marıá Teresa Domeńech-Carbo´{, Marıá Luisa Va´zquez de Agredos Pascual#, Mercedes Sua´rez} and Emilia Garcıá-Romerok,} *

European Synchrotron Radiation Facility, BP220, Grenoble Cedex, France Departament de Quı´mica Analı´tica, Universitat de Vale`ncia, Dr. Moliner, 50, Burjassot, Vale`ncia, Spain { Departament de Conservacio´ i Restauracio´ de Bens Culturals, Institut de Conservacio´ del Patrimoni, Universitat Politećnica de Vale`ncia, Camı´ de Vera 14, Vale`ncia, Spain # Departament de Histo`ria de l’Art, Universitat de Vale`ncia, Passeig al Mar, Vale`ncia, Spain } ´ Area de Cristalografıá y Mineralogıá, Departamento de Geologıá, Universidad de Salamanca, 37008 Salamanca, Spain k Instituto de Geociencias (UCM-CSIC), Ciudad Universitaria, 28003 Madrid, Spain } Departamento de Cristalografıá y Mineralogıá, Universidad Complutense de Madrid, Facultad de Geologıá, 28003 Madrid, Spain {

1. HISTORY OF MAYA BLUE Maya blue (MB) is a pigment that was extensively used in ancient times in Mesoamerica, covering today Mexico, Guatemala, Nicaragua and Belize. This ‘standard’ blue was invented during the first Millennium AD in the Maya area. The MB use extended to nearly all Mesoamerican cultures. Examples of its use are in Figures 1–3. Merwin (1931) described a blue paint found on remains of a Maya wall painting at the Temple of the Warriors in Chicheń Itza´. He recognized the blue colour as a new pigment (Gettens and Stout, 1942). Early workers described MB as an unknown material because of its resistance to chemical and thermal treatments: “. . . the color is not discharged by boiling nitric acid nor by heating much below redness. The conclusion seems justified that this is an inorganic color.” Although Merwin stressed one of its most peculiar characteristics, its resistance to acids, he thought it resembled a clay mineral. Gettens and Stout (1942), coined the ‘Maya blue’ name. Gettens (1962) analysed several samples of MB and recognized their similarity with the blue described by Merwin, which differs from any blue used elsewhere. He gave physical and chemical properties and reviewed earlier studies that “suggested that it might be derived from, or related to, silicates


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FIGURE 1 Fragment of the Mural of the Battle (photo: Christophe Morisset). This fragment shows a warrior of high rank, with one of them vanquished. The murals in Cacaxtla have details typical from Teotihuacan, while the figures possess characteristics of the end of the Mayan Classic period. Although these murals have been dated very early (AD 650–750), the fact that the MB pigment is used indicate that it should come after the ninth or tenth century AD. (Reyes-Valerio, 1993).

of the chlorite group which are colored blue or greenish-blue by ferrous iron”. However, they did not identify MB as any known material. X-ray data showed that MB gave a distinct pattern, in agreement with samples of palygorskite from Attapulgus (Georgia, USA). Thus, MB was confirmed as an apparent mixture of palygorskite with other clays, but the mixture did not explain the colouration, as palygorskite is white. Experiments mixing palygorskite with indigo reduced with sodium hydrosulfite produced a blue pigment that was not resistant to nitric acid. Gettens (1955) described samples not only from the Maya area but also from other areas in Mexico and Central Mexico, Veracruz and Oaxaca. Gettens stressed the importance of MB: (i) tracing Maya trade routes, (ii) historical aspects of preparation and (iii) that MB may have been continued in native communities until now. Gettens also remarked the presence of sepiolite in some samples and suggested “that Maya blue is an artificial blue, and that more than one kind of clay could be used in its preparation”. Shepard (1962; Shepard and Gottlieb, 1962) suggested that a blue organic colourant was present in MB, in addition to palygorskite. The optical properties and the behaviour of the pigment when heated may indicate an organic complex. In addition, the intensity of the colour is independent of the grain

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FIGURE 2 A Warrior figurine from the Jaina Island (photo: Thomas Aleto). It is remarkable the good conservation of the MB pigment as compared to other colours that are degraded.

size. Grains with low colouration may indicate an incomplete penetration of the colourant. When MB is heated to 300–400 C, it turns to grey and then white which may be related to degradation of organic substances. Although failing to identify the organic complex, Shepard argued: “I fully agree with Gettens that it cannot be a dye coating the surface of the clay particles. But there is another relation between the clay and organic matter complex—the so-called clay organic complex—that should be considered”. Tae Young Lee, from Gettens team, first identified indigo in MB. See Shepard and Pollock (1971) and Torres et al. (1976) for additional details. van Olphen (1966) studied the synthesis of indigo–clay complexes. He found that to achieve stability to acids, the pigment must be heated at moderate temperatures. van Olphen, heating to 75 C for several days or at 105–150 C for shorter periods, produced stable pigments using fibrous clays (Figure 4) with a tunnel structure (palygorskite and sepiolite) and failed to obtain stable pigments using clays neither with plate-like structures (e.g. kaolinite, nontronite) nor with mordenite (a zeolite with a cage-type structure). van Olphen noted that “the indigo molecules are undoubtedly too big to enter the channels of attapulgite or sepiolite, and the relatively small adsorption capacity of the minerals for indigo suggests that the dye is indeed adsorbed

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Developments in Palygorskite-Sepiolite Research FIGURE 3 Colonial wall decoration at the Oaxtepec Convent (Morelos, Mexico, sixteenth century) in black and blue (MB) tonalities (photo: Manuel Sańchez del Rıó).

on only the external surfaces of the particles. . .. Yet the precise mechanism of the stabilization of the complexes by heating is not clear; if one assumes that the heated indigo–attapulgite complex is indeed the synthetic equivalent of Maya blue, the solution of this puzzle has created a new one”. This new puzzle has not yet been solved after more than 40 years. van Olphen did not argue that the Mayas prepared the pigment in the same way, just that a pigment with the same characteristics can be synthesized in a laboratory. Kleber et al. (1967) studied archaeological samples and considering fixating indigo onto palygorskite. They created a resistant pigment by mixing palygorskite with a solution of leucoindigo and then heating at 150 C for 20 h. They also prepared MB by mixing indigo powder (0.5–50 wt%) with palygorskite subsequent heating (120 or/and 150 C for 5, 20 or 25 h) and removing the excess of indigo by sublimation at 130 and 190 C at 1 Torr. The pigments heated at 190 C were resistant to boiling nitric acid but not for those heated at 120 C, in partial contradiction to results of van Olphen. Kebler et al. unambiguously detected indigo only in synthetic samples with > 15% indigo. With infrared absorption spectroscopy, indigo was detected where samples were prepared with > 3% indigo. The indigo-characteristic lines at 1485, 1460, 1380 and 1305 þ 1290 cm 1 were detected in seven archaeological samples. Kebler et al. suggested that the irreversible fixation of indigo is related to loss of zeolitic H2O at 150–200 C (hygroscopic H2O is lost near 100 C and structural OH2 at 375–425 C). They suggested that based on steric considerations, the penetration even of indigo molecules in the tunnels is possible.

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Arnold and Bohor (1975) published a seminal work on materials used by the Mayas to fabricate MB. More recently, Littmann (1980) disagreed with earlier theories, suggesting the importance of montmorillonite. Littman studied experimentally indigo–palygorskite complexes, some using indigo from Indigofera tinctoria, and some samples were resistant to nitric acid. He suggested the importance of the heat treatment to stabilize the pigments and suggested complex mechanisms that the Maya may have used in preparation of MB. Torres (1988; Torres et al., 1976) has provided overviews of the geographic and chronologic distributions of MB, as well as possible fabrication techniques. In 1990, Tagle et al. (1990) reported the presence of MB in Cuban colonial wall paintings (1750–1860). In 1993, Reyes-Valerio (1993) suggested a fabrication technique that may have been used by the Mayas to produce the pigment. Jose-Yacaman et al. (1996) published an scanning electron microscopy (SEM) study that suggested the presence of superlattices and nanoparticles, which were later rejected. They are discussed below.

2. EXPERIMENTAL TECHNIQUES 2.1. Diffraction Studies Diffraction studies provide direct information on the crystal structure of mineral phase in MB. High resolution diffraction may be sensitive to clay’s changes in the hydration state and lattice parameters due to the interaction with indigo. Solid state indigo is crystallized (Susse et al., 1988) but losses its crystallinity when combined with palygorskite in MB. Residual peaks of indigo can be found in MB (Arnold, 1971; Kleber et al., 1967; Sańchez del Rıó et al., 2006a, 2009a) but may only indicate an excess of indigo. The palygorskite crystal structure was refined using synchrotron powder diffraction (Chiari et al., 2003) and neutron diffraction (Giustetto et al., 2006). Chiari et al. (2003) and Giustetto et al. (2006) examined synthetic MB and concluded that the indigo molecules enter in the tunnels of the orthorombic or monoclinic palygorskite. They suggested that both indigo and zeolitic H2O coexist in the tunnels. Indigo resides in the middle of the tunnel with the centre of the molecule in the centre of a triple cell. The disposition of the zeolitic H2O is more disordered in MB than in palygorskite. The affinity of indigo is greater for orthorhombic palygorskite (30% occupancy) than for monoclinic (19%). Sańchez del Rıó et al. (2006a) studied the effects of acids on MB. The results (see Figure 5) looked to pigment decolouration and destruction of the clay structure, as revealed by X-ray diffraction. They showed that palygorskite- and sepiolite-based pigments do not decolour when immersed in concentrated nitric or hydrochloric acids at room temperature for 5–15 min. However, sepiolite pigments are more degraded than palygorskite pigments, with the former destroyed with long treatments (several hours to a few days)

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Developments in Palygorskite-Sepiolite Research FIGURE 4 TEM images for palygorskite crystals. (A) Pristine palygorskite from Sacalum (Yucatań, Mexico); (B) MB sample from the archaeological site of Mayapań (Yucatań, Mexico), post-classic period. TEM examination reveals fibrous crystals of palygorskite. In MB samples the fibres exhibit a corrugate surface, associated to the evacuation of water during the preparation of the pigment (Domeńech et al., 2006, 2007a,b)

A

50 nm

B

50 nm

in concentrated acids at room temperature. No changes are observed by palygorskite pigments after long (greater than 1 week) acid submersion at room temperature.

2.2. Infrared Spectroscopies Fourier transform infrared (FTIR) spectroscopy was used for identifying the organic component in MB. Kleber et al. (1967) identified a few indigo bands in archaeological samples spectra. Indigo and palygorskite modifications under thermal treatment were used to try to unveil the nature of the clay– colourant interactions (Giustetto et al., 2005; Leona et al., 2004; Sańchez del Rıó et al., 2009a; Figure 6). The mid-infrared (MIR) spectrum of MB, like in palygorskite, can be analysed in three parts: (1) A zone with wave numbers to 1200 cm 1, which corresponds to the vibrations of the tetrahedral (SiOSi in 950–1250 cm 1) and octahedral (AlAlOH at 913 cm 1, AlFeOH at 865 cm 1 and MgMgOH at 650 cm 1) sheets. (2) The 1200–2000 cm 1 range, indigo bands minimally overlap with palygorskite bands. Bands in the range of 1290–1485 cm 1 zone were used

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140

120 Untreated palygorskite (scaled by 1/6)

Intensity (counts/s)

100 HNO3 90 °C 5 h 80 HCl 90 °C 5 h 60 HNO3 90 °C 30 h 40 HCl boiling 30 h 20

0

10

20

30

40

2q (deg) FIGURE 5 XRD spectra of palygorskite pigments, where it can be appreciated the destruction of the palygorskite structure for several strong acid attacks: (from top to bottom) untreated palygorskite, 1% indigo–palygorskite pigment treated with 7N HNO3 for 1.5 h at 90 C, the same for 6N HCl, 3% indigo–palygorskite pigment treated with 6N HNO3 for 30 h at 90 C and 3% indigo– palygorskite pigment treated with 6N HCl for 30 h in ebullition. The diffractograms are shifted vertically for clarity. (Sańchez del Rıó et al., 2009b).

by Kleber et al. (1967) to identify MB. A broadband around 1652 cm 1 is related to the zeolitic H2O (also producing two broadbands in the 3200–3400 cm 1) and is always present in MB, indicating that fully H2O dehydration never occurs. In Figure 6, the ATR–FTIR spectra of indigo, MB and indigo–montmorillonite (a non-stable clay–indigo mixture) are compared between 1400 and 1600 cm 1. The absorption bands are essentially the same for indigo and the montmorillonite mixture. In contrast, the number of indigo bands increases in MB with palygorskite. The complex, multiple-band spectra of MB suggest the presence of different topological isomers of indigo and, possibly, of other indigoid molecules attached to different sites in the palygorskite matrix. The 1650–1750 cm 1 bands are attributed to the superposition of the d(H2O) mode of structural OH2 associated with indigo and n(C¼¼O) antisymmetric stretching mode of palygorskite-associated indigo and dehydroindigo molecules (Domeńech et al., 2006, 2007b,c). (3) The zone with wave numbers higher than 3500 cm 1 includes the vibrations of the hydroxyl groups of palygorskite linked to cations: AlMg OH at 3540 cm 1, AlFeOH at 3580 cm 1 and

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Developments in Palygorskite-Sepiolite Research FIGURE 6 ATR–FTIR spectra in the 1400–1600 cm 1 region of indigo, indigo– montmorillonite and indigo– palygorskite (MB). The richness of structures in the MB spectra is a proof of the complexity of the indigo–palygorskite interactions. (Domeńech et al., 2009c).

100

80

Indigo

60

%T

100

90

Indigo + montmorillonite

100

90 Indigo + palygorskite 1600

1500 cm-1

1400

AlAlOH at 3613 cm 1 (Sua´rez and Garcıá-Romero, 2006). The latter is very sharp and was proposed as an indicator for palygorskite in archaeological MB (Sańchez del Rıó et al., 2008). The 3613 cm 1 band of palygorskite (AlAl OH) shifts to higher wave numbers ( 10 cm 1) with increasing temperature in palygorskite and palygorskite–indigo compounds (Sańchez del Rıó et al., 2009a). This shift is explained by the dehydration process, although Manciu et al. (2007) and Polette-Niewold et al. (2007) assigned these changes to interaction of the indigo with Al. Near-infrared (NIR) spectroscopy has been used to investigate palygorskitecontaining clays (Chryssikos et al., 2009; Gionis et al., 2006, 2007) because the anharmonic coefficients of the stretching modes for the structural OH groups are typically smaller than those of H2O. Thus, NIR offers a good separation of the OH and H2O modes compared to MIR. This separation is enhanced using second derivative analysis that eliminates broad features and background effects. When a palygorskite–indigo mixture is heated to produce MB, the surface silanols dehydrate and then fully rehydrate when returning to room temperature. This effect is analogous to the case of palygorskite, thus manifesting that indigo does not obstruct the clay surface to rehydrate (Sańchez del Rıó et al., 2009a).

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Therefore, MB is not a surface compound, contrary to the proposal by Chiari et al. (2008), Fuentes et al. (2008), Manciu et al. (2007) and Polette-Niewold et al. (2007). Moreover, the NIR technique is sufficiently sensitive to detect minimum penetration of H2O in the MB tunnels.

2.3. Raman Raman spectroscopy can be used to correlate bands in archaeological pigments with those of well-characterized references (Vandenabeele et al., 2005) to identify MB. In addition, Raman spectroscopy is used to study the chemical interaction occurring during heat treatment to obtain acid resistant pigments (MB). An advantage of Raman over FTIR is that palygorskite is not seen in Raman unless NIR frequencies are used (McKeown et al., 2002). Raman spectra of MB were first published, using 442 and 632.8 nm excitations (Grimaldi, 2000) and 1064 nm (Andreev et al., 2001). Witke et al. (2003) compared the spectrum (excited with 514.5 nm) of indigo to a spectrum from a Maya clay bead and reported, in addition to the bands of indigo, additional bands at 1017, 1128 and 1380 cm 1. Also, the intensity increase for bands at 1253, 1593 and 1633 cm 1, which occur because of activation of Raman-inactive modes at 1299, 1592 and 1627 cm 1. This activation may be a consequence of perturbations in the planarity of the indigo molecule resulting by the interaction with the clay. Differences between the Raman spectra of indigo and MB using a 785 nm laser (Leona et al., 2004) suggested (i) modifications affecting the charge distribution of the indigo during fixation on palygorskite, (ii) disappearance of possible vibrational coupling owing to inter-molecular indigo– indigo interactions and (iii) different hydrogen bonding conditions between C¼¼O and NH in pure indigo and in the indigo–palygorskite complex. Giustetto et al. (2005) measured synthetic and archaeological MB with 325 nm excitation. They found the Raman spectra of MB coalescenced indigo bands at 1576 and 1586 cm 1 into a single peak, the disappearance of indigo bands at 1189 and 1370 cm 1 and the shift of indigo bands at 1315 and 1703 to 1325 and 1688 cm 1. Calculations (Giustetto et al., 2005) predicted the shift in vibrational frequencies when going from isolated indigo to indigo interacting with two formula units of the clay, displaying a moderate shift (13 cm 1) in the C¼¼O stretch at 1692 (coupled with C¼¼C stretch) and important shifts ( 26 cm 1) in some vibrations including the NH group. Sańchez del Rıó et al. (2006c) found that most features present in the spectrum of MB, which are not present in indigo, are not exclusive to MB and they cannot relate to resistance to chemicals (Figure 7). Montmorillonite and kaolinite produce similar spectral features in Raman spectra to palygorskite– and sepiolite–indigo spectra. Ab initio vibrational calculations that allow the assignment of bands of the indigo spectrum also indicate that a single loss of planarity (a few degrees rotation of half molecule around the C¼¼C axis) is not sufficient to explain the acid resistance observed in indigo–clay mixtures.

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1680

1633

1583

1490

1380

1253

1017

400

1128


Raman intensity/Arbitr.units

(5) 300 (4)

200

(3)

(2)

100

(1) 0 400

600

800

1000 1200 1400 Wavenumber (cm–1)

1600

1800

FIGURE 7 Experimental Raman spectra (excitation laser at 532 nm, green light) of indigo (1%) mixed with different clays (99%). From bottom to top: (1) synthetic indigo, (2) palygorskite and indigo, (3) sepiolite and indigo, (4) montmorillonite and indigo and (5) kaolinite and indigo. The Raman spectrum of the indigo changes when indigo interacts with clays. Vertical dash-dotted lines mark the position of the changes from Witke et al. (2003), (Sańchez del Rıó et al., 2006c).

Recently, Manciu et al. (2007) described the peaks at 425 and 1395 cm 1, accompanied by an intensity increase of a peak at 606 cm 1 and concomitant decrease in intensity of peaks at 635 and 1701 cm 1 with an increase in heating time for synthetic MB specimens prepared with 6–16% indigo and palygorskite. The peaks at 425 and 606 cm 1 were assigned to band owing to Al N and AlO bonding, respectively. These attributions should be taken with caution. The peak at 606 cm 1 has been also observed in the indigo spectrum, whereas the 425 cm 1 peak may correspond to a Raman-inactive vibration mode of symmetry Au (Tomkinson et al., 2009) activated by planarity perturbation. Sańchez del Rıó et al. (2009a) studied changes in the Raman spectra (excitation at 785 nm) of a indigo (1%) palygorskite adduct during thermal treatment to produce MB. Raman spectra showed changes in the temperature interval of 70–130 C owing to vibrations involving the double bonds C¼¼C and C¼¼O. In the same temperature interval, XRD showed a reduction of ˚ owing to loss of zeolitic H2O. This suggests the a cell parameter by 0.16 A that the modification of the indigo chromophore correlates with a reduction in the tunnel width in palygorskite.

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Raman spectra support the suggestion by Domeńech et al. (2006) that dehydroindigo, an oxidized form of indigo, occurs with dye in MB. The enhancement of the Raman peak at ca. 1635 cm 1 observed in MB by Sańchez del Rıó et al. (2006c) and Witke et al. (2003) can be attributed to the activation of the Raman-active vibration mode at 1627 cm 1 as a consequence of perturbations in the planarity of the molecule (Witke et al., 2003), or to the presence of imino units (C¼¼N) whose stretching mode appears typically at 1639 cm 1. Recently, Tomkinson et al. (2009) analysed the vibrational spectrum of indigo and found that the mid-frequency bands corresponding to the g(NH) and g(CH) modes are better indicators for the dye because they are sensitive to local bonding environment. Because solid indigo possesses intramolecular hydrogen bond between carbonyl and NH groups, attachment of indigo to the clay matrix in MB should break the intramolecular hydrogen bond system of indigo. Accordingly, the Raman band at 750 cm 1 will disappear as a result of the replacement of indigo by dehydroindigo (Tomkinson et al., (2009); Domeńech et al., 2011a), in agreement with the observations reported by several authors (Giustetto et al., 2005; Manciu et al., 2007; Sańchez del Rıó et al., 2006c).

2.4. Optical Spectroscopies Reinen et al. (2004) studied by UV–vis spectroscopy complexes of palygorskite– indigo and palygorskite plus other organic colourants and considered changes in colour after heating. They observed an irreversible shift of the absorption bands when heating the palygorskite–indigo mixture, with colour shifts to green–blue hues. These results indicate an interaction between palygorskite and indigo during the heating process. They also showed that MB-like stable pigments with different colours can be synthesized using indigoderivates. Leona et al. (2004) compared the UV–vis spectra of indigo and MB, and other studies (Fuentes et al., 2008; Tilocca and Fois, 2009) compared quantum chemistry calculations and optical spectroscopy. The differences in the spectral features of MB as compared with reference compounds (indigo and dehydroindigo) are attributed to: (i) additional absorbing compounds (Rondao et al., 2010), (ii) different palygorskite–dye associations (i.e. different topological dye isomers) and (iii) the appearance of dye–dye associations (Domeńech et al., 2009a).

2.5. Voltammetry The voltammetry of microparticles considers the voltammetric response of a micro- or nano-solid non-conducting sample on a surface of an inert electrode (usually graphite) immersed in a suitable electrolyte (Grygar et al., 2003; Scholz and Meyer, 1998; Scholz et al., 2005). Domeńech et al. (2006, 2007b,c, 2009a,b) obtained the electrochemical response of solid indigo and MB samples attached to graphite electrodes in contact with aqueous electrolytes. Figure 8 compares the voltammetric response of two archaeological

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MB samples in contact with aqueous sodium acetate buffer. Here, two indigocharacteristic peaks at potentials of þ450 and 300 mV vs. AgCl/Ag are recorded. These peaks are assigned, respectively, to the oxidation of indigo in dehydroindigo, and the reduction of indigo to leucoindigo via two-proton, two-electron processes (Figure 9). The variation of peak potentials and peak current ratios for the above peaks on the timescale of the electrochemical experiment (potential scan rate, square wave frequency) differs significantly from indigo to MB. Additionally, MB specimens frequently show peak splitting. The analysis of these voltammetric parameters indicates that indigo and dehydrodingo are strongly attached to the palygorskite whereas temperature-variable experiments indicate that enthalpy and entropy values associated in the process of dye attachment to the clay matrix vary (Domeńech et al., 2006). Chemometric analysis of MB samples from Campeche and Yucatań archaeological sites permits a classification of such samples into different ‘electrochemical types’ and suggests that various preparation procedures were used by the ancient Maya people (Domeńech et al., 2007a, 2009d).

2.6. Nuclear Magnetic Resonance Hubbard et al. (2003) used 29Si CP/MAS to determine that indigo is not located in tunnels of sepiolite in contrast to what happens to smaller organic molecules, such as acetone or pyridine. This is in contrast to recent works (Dejoie, 2009; Ovarlez et al., 2009; Giustetto, 2010). A dramatic change is observed in the 13 C CP/MAS spectra for pure indigo compared to indigo crushed with sepiolite, suggesting the alteration of indigo. The altered indigo coexists with unaffected FIGURE 8 Square wave voltammograms for MB samples from (A) Kuluba´ (Yucatań, Me´xico), (B) Dzibilnocac (Campeche, Me´xico), both dated in the Classical Period, attached to paraffin-impregnated graphite electrodes immersed into 0.50 M aqueous sodium acetate buffer at pH 4.75. Potential scan initiated at 0.75 V in the positive direction. Potential step increment 4 mV; square wave amplitude 25 mV; frequency 5 Hz. Peak I corresponds to the oxidation of indigo to dehydroindigo, while peak II corresponds to the reduction of indigo to leucoindigo.

A II I

B

II I

1 µA

1.0

0.6 0.2 –0.2 Potential (V vs. AgCl/Ag)

–0.6

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O H N

Indigo N H

O

–2H+, –2e–

Oxidation

Reduction

+2H+, +2e–

O

OH H N

N

N

N H

O

OH

Dehydroindigo

Leucoindigo

FIGURE 9 Scheme for the electrochemical oxidation of indigo to dehydroindigo and the electrochemical reduction of indigo to leucoindigo.

pure indigo in the unheated mixture but is the only molecule present in heated sepiolite with indigo (20 wt%) after rinsing with nitric acid. They suggested a model where the carbonyl and amino groups of the indigo are anchored by hydrogen bond interactions with edge silanol groups of the clay (not with the internal surfaces of the tunnels), thus blocking tunnel entrances. 1H MAS NMR data on indigo, palygorskite and newly synthesized MB were published by Giustetto et al. (2005). New features in the NMR spectra (at 13.0 and 17.8 ppm) of MB and not present in the spectra of the reactants suggested the presence of hydrogen bonds (NH O and C¼¼O HO, respectively). The 1H13C CP MAS, 27Al, 29Si NMR spectra for palygorskite and indigo plus palygorskite synthetic specimens were studied by Domeńech et al. (2009a). The 27Al MAS NMR spectra have an intense peak close to 4 ppm, accompanied by a weak peak near 60 ppm. The 4 ppm peak is assigned to sixfold coordinated aluminium to oxygen atoms, and the 60 ppm is attributed to a small amount of fourfold coordinated aluminium. The close similarity between the spectra of palygorskite and MB suggests that there are no significant modifications in the environment of the Al3þ centres upon attachment of indigo to the palygorskite matrix. These results suggest that aluminium is structurally stable and that direct interaction with the dye does not exist. In contrast, 29Si NMR spectra for palygorskite and MB are significantly different. The 29Si NMR signals are shifted to stronger fields and became broader as a consequence of dye incorporation in palygorskite. The observed features suggest an increase in the electronic density close to (SiO4)4 tetrahedra.

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2.7. Computer Modelling Chiari et al. (2003) calculated energies and forces as well as structure optimization concluding (i) the adsorption of indigo on dehydrated (hydrated) palygorskite is exothermic (endothermic), (ii) the displacement of water if slightly favoured for the orthorhombic polymorph as compared to the monoclinic, and the absorption energy for indigo is similar and (iii) the C¼¼O of indigo form hydrogen bond with structural OH2. They concluded that indigo is stabilized by irreversible encapsulation into the tunnels by high temperature diffusion, after the zeolitic H2O is lost. Fois et al. (2003) modelling also suggested that indigo molecules are trapped into the tunnels by a strong hydrogen bonds, stressing the role of the tight fit of the sorbate in the tunnels because only related guest–host interactions are not sufficient to explain pigment stability. Giustetto et al. (2005) calculations, also including vibrational frequencies, also supports this model, suggesting that C¼¼O H bonds, the dye and the clay exist, but not NH O bonds. Tilocca and Fois (2009) calculated electronic excitation spectra of the carbonyl group in indigo and dehydroindigo interacting with several clay sites, and concluded that the direct interaction indigo–cation is more important than the H bond indigo–OH2, being the Al interaction responsible for the change in colouration. Indigo and dehydroindigo may coexist, but dehydroindigo better matches the experiment. Chianelli et al. (2005) and Polette-Niewold et al. (2007) regarded that MB is a ‘surface compound’ compatible with their DFT calculations and ab initio model of the visible MB spectrum. In this model, the indigo bonds to the aluminium surface defects in the palygorskite, which holds metallic impurity ions at a silicon site. Fuentes et al. (2008) used the same structural model with Al replacing Si in palygorskite for reproducing the features of the MB visible spectrum and explaining the unusual stability of MB. The impact of quantum chemistry computations in the search for a reasonable structural and interaction model in MB is at present limited by the complexity of palygorskite. The models used for palygorskite are quite simplified: for example, the use of a fully magnesian (Fois et al., 2003) or magnesiumless (Fernańdez et al., 1999) palygorskites. The suggested interaction of indigo with Al3þ rather than with Mg2þ (Tilocca and Fois, 2009) is opposite to the idea that the external octahedral sites are occupied by Mg2þ (Chryssikos et al., 2009; Sua´rez and Garcıá-Romero, 2006). Also, the fundamental role assigned to tetrahedral Al3þ (Fuentes et al., 2008; Polette-Niewold et al., 2007) does not seem consistent if one considers that almost 100% of Al is octahedrally coordinated. Moreover, the simulation of palygorskite with tunnels empty from zeolitic H2O contradicts the experimental evidence that only a mild heating (less than 100 C for 20–30 min; Reyes-Valerio, 1993), unable to evacuate the whole zeolitic H2O (as revealed by FTIR), is sufficient to stabilize the complex.

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3. THE SYNTHESES, PROPERTIES AND NATURE OF MB 3.1. The Synthesis of MB We distinguish between two kinds of synthesis procedures of MB: methods used by the Mayas and other Mesoamerican people with natural indigo, and the syntheses performed with synthetic indigo, under controlled conditions. Laboratory synthesis (van Olphen, 1966) used three techniques: (i) soaking palygorskite with indoxylacetate; (ii) vat dyeing, where leucoindigo is soaked with palygorskite in a water suspension with prolonged stirring and (iii) crushing indigo with powdered palygorskite. Mixtures are then heat treated from a few minutes (Reyes-Valerio, 1993) to hours or days (van Olphen, 1966) for different temperature ranges [90–100 C (Reyes-Valerio, 1993), 190 C (Kleber et al., 1967) or even 250–300 C (van Olphen, 1966)]. Specimens prepared from indigo and sepiolite (Hubbard et al., 2003; Ovarlez et al., 2009; Sańchez del Rıó et al., 2006a; van Olphen, 1966) and in colloidal suspensions (Yasarawan and van Duijneveldt, 2008) were reported. Variations of these modern recipes were proposed in Littmann (1982). The properties of MB are influenced by parameters during the synthesis, such as pH, temperature, duration of the heating process, indigo concentration and morphology of the clay (after a more or less intensive crushing). Post-treatments are applied to remove the excess of reactant: washing in acetone, acid washing, Soxhlet extraction, etc. Ageing or light resistance test has been employed. Although palygorskite is not available in synthetic grade, MB-like pigments using sol–gel or crystallization of the mineral phase with indigo are being studied.

3.2. The Chemical Resistance of the Pigment MB is remarkably stable to chemical attacks, as noted by Merwin and Gettens (Gettens, 1962): many workers used this stability as ‘quality control’ for wellprepared MB. ‘Gettens’ tests’ (Gettens, 1962; Littmann, 1980) consist of using concentrated reactants (nitric, hydrochloric, sulfuric acids, aqua regia, sodium hydroxide) at room temperature during 18 h, and then heating to test the chemical stability. With few exceptions, the tests are neither quantified by concentration, temperature or duration of the attack, nor are they compared to the indigo and clays contained in MB. Results are derived from a visual inspection of the loss of pigment colour, not correlated to the molecular or crystallographic structures changes in the indigo and clay (chemical change is not always accompanied by a change in colour). Sańchez del Rıó et al. (2006a) studied palygorskite–indigo and sepiolite–indigo pigments by applying acid attacks of different intensity (varying concentration, duration and temperature). They showed the relationship of colour change and crystallographic structure of the synthetic MB pigment. Total destruction is possible at room temperature for sepiolite pigments but requires intense heating (e.g. boiling in hydrochloric acid for 30 h) for palygorskite pigments.

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Chemical stability between palygorskite–indigo and sepiolite–indigo pigments together with the common use of palygorskite in archaeological MB suggest that the name ‘Maya blue’ should only be applied to the palygorskite–indigo pigment (Arnold et al., 2008; Sańchez del Rıó et al., 2006a). Noneless, sepiolite–indigo interactions are sufficiently close to those of palygorskite–indigo to suggest that indigo molecules attach to the clay matrix in a similar way for palygorskite or sepiolite when forming MB-like materials (Domeńech et al., 2009a). However, the fact that indigo links to sepiolite only at one side of its molecule also contribute to the weakness of sepiolite-based pigments with respect to MB (Giustetto, 2010).

3.3. The Hue of MB MB has a characteristic hue, a pale blue, with tonalities that may change from greenish to turquoise. The colour is bright and thus attracts considerable attention by archaeologists and researchers. The first hypothesis attributed the colour to an unknown blue mineral of the palygorskite group. Then, the presence of indigo in the MB was demonstrated, although Jose-Yacaman et al. (1996) reported iron metal and iron oxide nanoparticles in MB. They suggested (see also Fernańdez et al., 1999; Polette et al., 2002) that Mie-type light dispersion in nanoparticles should produce the characteristic hue of MB. This hypothesis was later discarded (e.g. Chiari et al., 2003; Fois et al., 2003; Giustetto et al., 2005; Hubbard et al., 2003; Ovarlez et al., 2006; Reinen et al., 2004; Sańchez del Rıó et al., 2004, 2005) and it is now agreed that the MB colouration is caused by bathochromic shift of the indigo absorption bands resulting from the dye having an inorganic support. Colour may also be related to the presence of secondary indigoid products, namely leucoindigo (Domeńech et al., 2007b; Vandenabeele et al., 2005), and indirubin in several MB samples (Domeńech et al., 2007b). Recent results from Domeńech et al. (2011b) suggest that the ancient Mayas could have prepared yellow pigments with dye plus palygorskite association similar to MB containing isatin, dehydroindigo and/or ochre and possibly other minor organic compounds. In the research of new MB-inspired pigments, Giustetto et al (2011) made a stable red pigment encapsulating methyl red in palygorskite, and Dejoie et al. (2010) demonstrated that indigo stabilizes when it is encapsulated in Silicalite. The peculiar hue of MB and its variability are understood comparing the visible spectra of MB specimens with indigo and dehydroindigo. In Figure 10, the visible spectrum of MB from Mayapań (Yucatań, Mexico) shows two maxima at 425 and 570 nm. Indigo in ethanolic solution yields an absorption band in the visible region at lmax ¼ 605 nm, whereas dehydroindigo produces a band at lmax ¼ 440 nm. The spectrum of MB contains the superposition of indigo and dehydroindigo absorption, both affected by a bathochromic shift attributable to the interaction of the dyes with palygorskite. The variability in the colour of MB specimens probably results from the different dehydroindigo/indigo ratio in the palygorskite, which is controlled by the heating

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FIGURE 10 Visible spectra for indigo (diamonds), dehydroindigo (squares) and for a Maya blue sample (triangles) from Acanceh (Yucatań, Mexico), late classic period. (Domeńech et al., 2009d).

70 60

Abs. u.a.

50 40 30 20 10 0 350

450

550 650 Wavelength (nm)

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temperature (Domeńech et al., 2006, 2007a). The formation of dehydroindigo by aerobic oxidation of indigo (Domeńech et al., 2006, 2007b) is favoured with increasing temperature. The hue of archaeological MB varies geographically and along the history. The blue in the Aztec zone is darker than in the Maya zone. Moreover, the green pigment used in the Maya zone, like in Bonampak paints, contains palygorskite.

3.4. Structural Aspects: The Attachment of Indigo to the Clay van Olphen (1966) suggested a structural mechanism of complex formation, by indigo molecules attaching to surface channels of the clay. Kleber et al. (1967) suggested that indigo penetrates (partially or deeper) into the tunnels in the clay structure. Papers often support Kleber et al. (e.g. Chiari et al., 2003; Fois et al., 2003; Giustetto et al., 2005, 2006; Tilocca and Fois, 2009) or van Olphen (e.g. Chianelli et al., 2005; Chiari et al., 2008; Fuentes et al., 2008; Manciu et al., 2007; Polette-Niewold et al., 2007). Hubbard et al. (2003) proposed that indigo attaches to the entrance of the tunnels blocking the entrance. Vibrational and diffraction data (Sańchez del Rıó et al., 2009a) support this model. The three possibilities, penetration of the molecule, covering the nanotunnels and attaching to the surface, may coexist depending on preparation. However, what is the cause of MB chemical stability? Palygorskite and sepiolite may differ in the attachment of indigo. Pigments made with these two clays present different resistance to acids (Sańchez del Rıó et al., 2006a), and they have different tunnel sizes. The dimensions of the sepiolite tunnel, which are larger than in palygorskite, may allow for greater diffusion. Palygorskite has tunnels of nearly the same width as the

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indigo molecule. Steric arguments are compatible with the idea that indigo penetrates in sepiolite (Dejoie, 2009; Giustetto and Wahyudi (2011); Ovarlez et al., 2009) but blocks the entrances of the tunnels in palygorskite (Dejoie, 2009; Hubbard et al., 2003; Sańchez del Rıó et al., 2009a).

3.5. The Nature of the Palygorskite–Indigo Association The proposed models for the interaction that anchors indigo to palygorskite include (i) formation of hydrogen bonds between the C¼¼O and N H groups with edge silanol units of the clay (Hubbard et al., 2003), (ii) formation of hydrogen bonds between indigo molecules and structural OH2 groups (Chiari et al., 2003; Giustetto et al., 2005), (iii) hydrogen bond formation between indigo carbonyls and structural OH2 (Fois et al., 2003), (iv) direct bonding between the clay octahedral cations and the dye molecules without H2O nor OH2 (Chiari et al., 2008; Tilocca and Fois, 2009), (v) specific bonding to Al substituted Si sites in tetrahedral centres (Chianelli et al., 2005; Fuentes et al., 2008; Polette-Niewold et al., 2007). The possibility of significant Van der Waals interactions was introduced by Fois et al. (2003) and further considered by Domeńech et al. (2007c) There are serious arguments against a direct dye–Al3þ interaction in MB. The first is the possibility of forming stable aluminium-free MB-type pigments with sepiolite. Additionally, 27Al MAS NMR data suggest that the coordination of Al3þ in palygorskite is not affected by indigo (Domeńech et al., 2009a). Consistent with NMR data, NIR data on AlAlOH stretching band at ca. 3620 cm 1 in MB probes do not find evidence of Al–indigo interaction (Sańchez del Rıó et al., 2009a). In contrast, the 29Si NMR spectrum of MB differs significantly from that of palygorskite. The 29Si NMR spectrum suggests that electronic density increases near to (SiO4)4 tetrahedra, thus indicating adsorption of indigo near silicon atoms. This dye–silicon interaction may occur through surface SiOH groups of clay and p bonds of dye. This particular bonding does not seem crucial for inducing chemical stability to MB, and it may also be present in indigo attached to laminar clays, explaining some similar features in their Raman spectra compared to MB (Sańchez del Rıó et al., 2006c). MB can be regarded as a hybrid organic–inorganic material with polyfunctional characteristics where different topological isomers of indigoid molecules coexist attached to palygorskite (Domeńech et al., 2009a,c).

4. MB RESEARCH IN RELATION WITH THE ARCHAEOLOGICAL AND HISTORICAL CONTEXTS 4.1. Historic Relevance of Indigo Indigo is a natural blue dye formed by indigotin (3H-indol-3-one, 2-(1,3-dihydro3-oxo-2H-indol-2-ylidene)-1,2-dihydro), a quasi-planar molecule of approximate ˚ , containing a slightly elongated central CC bond and dimensions 4.8 12 A two elongated CO bonds.

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The technique of dyeing with indigo was discovered and extensively applied independently by several cultures using plants from the Indigofera family: Indigofera tinctoria in India, Indigofera indigotica or tein-cheing in China, Polygonum tinctorium in the Far East. In Europe, woad (Isatis tinctoria) was cultured from the twelfth to the seventeenth century, having an important economic, political and cultural role (Harry, 1930). In the late seventeenth century, the woad industry in Europe initiated the decline because woad plant produces less indigo than the Indigofera plants, which were available at lower price from India and America. The synthesis of indigo from isatin was developed by Baeyer in 1870 (Baeyer and Emmerling, 1870) but a more economical method was devised by Heumann in 1890 (Heumann, 1890a,b). Commercialization of synthetic indigo displaced indigo production from plants in the late 1890s (McKee and Zanger, 1991). The recipes used by the ancient Mesoamericans for dyeing with plants (Hernańdez, 1959) are related to water solutions. The Mesoamericans obtained indigo from a maceration of leaves of plants (mainly Indigofera suffruticosa) generically termed an˜il (in Spanish) or xiuquitlitl (in Nahuatl, the Aztec language), followed by a prolonged aeration/stirring process termed batido (Domeńech et al., 2007c, 2009c). Indigo was traded in Mesoamerica during the conquest, and great quantities of indigo were sent to Spain (Sarabia Viejo, 1994). Indigo was prepared in pre-Columbian times for being used as a dye, but there are no references of its trade for the fabrication of MB pigment. Possibly, a similar preparation with the addition of clays was used by the ancient Mayas for the production of MB (Reyes-Valerio, 1993). Electrochemical monitoring of indigo during preparation by traditional procedures suggests that the hydrolysis of the indigo precursors may involve two competing reaction pathways, via intermediate formation of leucoindigo or isatin (Domeńech et al., 2007b). For Indigofera leaves and a suspension of palygorskite, leucoindigo should be favoured with the use of an alkaline quick-lime suspension, because this product, contrary to indigo, is slightly soluble in alkaline media (Domeńech et al., 2009d).

4.2. Palygorskite in Contemporary and Ancient Mesoamerica The use of MB disappeared after the Spanish conquest, and there is no written record describing its fabrication and use. It has been argued that MB continued to be used in contemporary communities. Moreover, some aspects of the use and significance of MB may have been transmitted orally. The link between palygorskite and Maya culture was first established by the ethnographic work of Arnold in Yucatań (Arnold, 1967, 1971). Palygorskite was mined, used and traded among the contemporary Yucatec Maya in Ticul and Sacalum (Arnold, 1967, 1971; Arnold and Bohor, 1975; Folan, 1969) for pottery temper and for medicinal purposes. Two probable ancient mines for the mineral were suggested, one at the cenote in Sacalum (Arnold, 1967;

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Arnold and Bohor, 1975; Folan, 1969) and a second at Yo’ Sah Kab near Ticul (Arnold, 2005). Palygorskite occurs elsewhere in the Yucatań peninsula (Arnold et al., 2007; Sańchez del Rıó et al., 2009b), but not beyond, except in samples from Peteń (Guatemala; Arnold et al., 2007). Arnold et al. (2007) described two hypothesis for palygorskite provenance in MB: (i) The Shepard/Arnold/Bohor hypothesis, where MB was widely traded from the Yucatań Peninsula because of its widespread use on pottery intended for household rituals. Sacalum and Yo’ Sah Kab could be the sources of palygorskite. (ii) The Littmann hypothesis: MB was derived from local palygorskite deposits and the synthesis technique moved rather than the pigment itself. Sańchez del Rıó et al. (2009b) found palygorskite of great purity in several sites 40 km around Uxmal, suggesting that palygorskite is a frequent mineral in the area as discussed in Arnold et al. (2007) and Littmann (1982). Palygorskites within this area have similar crystallographic characteristics, but variation in trace element concentrations may be used to obtain chemical fingerprints of the different sources (Arnold et al., 2007). Krekeler and Kearns (2009) found palygorskite ca. 200 km from Uxmal in the south-eastern Yucatań Peninsula. Arnold collected samples with the cultural characteristics of palygorskite, suggesting a source of palygorskite in Guatemala. Cecil (2010) determined that pigment from Ixlu (El Peten, Guatemala) has the MB structure, but suggesting a local manufacture from clays in central Peten. Although pigments found in Guatemala are MB, no XRD studies on the pristine clays have been done. Thus, the occurrence of palygorskite in Guatemala has not been fully demonstrated yet.

4.3. Sepiolite and MB Sepiolite has occasionally been reported in Yucatań (Isphording and Wilson, 1974; Stinnesbeck et al., 2004), but not in surface outcrops. Recent studies using Yucatecan palygorskites (Chiari et al., 2003; Pablo-Galań, 1996; Sańchez del Rıó et al., 2009b) did not identify sepiolite. However, sepiolite may occur with palygorskite in archaeological MB (Gettens, 1962; Shepard and Gottlieb, 1962). Shepard (1962) found a related mineral in Aztec MB, probably sepiolite. Ortega Avile´s (2003) found palygorskite together with sepiolite in a sample from The Great Temple in Tenochtitlan. Sepiolite is only found in archaeological samples from the MexicoTenochtitlan area, corresponding to the Aztec Empire, in agreement with Shepard (Shepard and Gottlieb, 1962): “It is noteworthy that sepiolite has not yet been found in any Yucatecan or Mayan sample.”

4.4. The Production and Use of MB in Ancient Times Indigo may have been used as an intermediate ingredient from Indigofera leaves, then mixed with palygorskite to obtain MB. Alternatively, MB may

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have been processed directly with palygorskite and Indigofera leaves (ReyesValerio, 1993), a recipe proposed from interpretations of historical documents. Reyes-Valerio demonstrated that a pigment with the characteristics of MB can be obtained by macerating Indigofera leaves in a clay–water solution, and this method may have been used by the Maya. Cabrera Garrido (1969) suggested other possible preparations: (i) direct dyeing of palygorskite using the colourant from the Indigofera plant, (ii) heating a mixture of palygorskite and indigo, (iii) heating indigo with water vapour for fixation on palygorskite, (iv) using a vat-dyeing technique, (v) burning Indigofera leaves and mixing the ashes with other ingredients, (vi) mixing indigo with palygorskite and heating to adulterate indigo and (vii) ritual ceremonies by burning ingredients like Copal. Copal is a resin widely used in Prehispanic times. Cabrera detected copal in a sample from Tlatelolco and determined that fabrication involved heating, which may have been related with rituals like burning Copal. This idea was recently developed by Arnold et al. (2008). Domeńech et al. (2007a) compared the electrochemical response, composition (from SEM/EDX) and UV–vis spectral properties of samples from 12 archaeological sites of Yucatań and Campeche. The electrochemical responses provide dehydroindigo/indigo ratios, and mineralogy and textural properties of the samples can be correlated with spectral and compositional signatures. Samples are categorized into ‘electrochemical types’ that suggest (i) different procedures for the preparation of MB were used; (ii) these procedures evolved from not using thermal treatment, to moderate and more intensive heating; (iii) ochre and other pigments were possibly used during the crushing/thermal treatment process; (iv) the variation of MB types along time corresponds to an evolution of fabrication technique, where new processes were incorporated progressively showing a ramified scheme as a function of time. The fabrication of MB and use of colours, as well as all the arts, and nearly all social life in Mesoamerican civilization were under strict religious control (Reyes-Valerio, 1993). The artist that created codices, sculptures, temples and pyramids was educated in the different institutional centres. The possibility that MB production was done in a ritual context was developed by Cabrera Garrido (1969) and Arnold et al. (2008). It is unclear how artists managed to obtain the different tonalities and hues. Optical microscopy examination of cross sections of paint layers shows each MB layer as an apparently homogeneous blue or greenish-blue layer. According to Domeńech et al. (2006, 2007a, 2009c), the modulation of the hue of the pigment from dark blue to more or less greenish-blue could be obtained by varying the dehydroindigo/indigo ratio upon varying the temperature treatment during the preparation of the pigment. In several samples, yellow aggregates were added to MB pigment to obtain a more greenish colour (Domeńech et al., 2007a, 2009d). The Mg concentration in a mural sample

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from Cacaxtla with two blue hues indicates the probable mixing of the pigment with other products (lime water, i.e. calcite; Sańchez del Rıó et al., 2004). Another control of the tone is the depth of the pigment layer (one or several layers). Layers of different colours are often visible in mural paints (Reyes-Valerio, 1993). However, multilayers of the same pigment cannot be excluded, but they are more difficult to detect because there are no compositional differences.

4.5. Trade and Distribution and of MB in Ancient Times Gettens (1962) gave several reasons to the study of MB: (i) “Maya blue might have been an important item of trade among the preconquest Maya”, (ii) “the source, preparation and use of Maya blue are of importance to the history of technology”, (iii) “it is not yet certain that the Maya blue disappeared after the conquest”. In addition, if MB is to be used as evidence that artefacts are true antiquities, then the history of MB must be known. If MB was traded (Littmann, 1980), then MB “is archaeologically significant not only in demonstrating trade or cultural contacts, but also in determining whether these contacts consisted of trade in a material substance or the transmission of a technique. The difference represents the distinction between discovery and invention”. There is crystallographic evidence that MB from Central Mexico used a palygorskite that differs from the Yucatecan palygorskite (Sańchez del Rıó et al., 2009b). This is based on the fact that Yucatecan palygorskite is nearequal mixture of monoclinic and orthorhombic palygorskite (Chiari et al., 2003; Sańchez del Rıó et al., 2009b). In contrast, diffractograms from archaeological MB from Central Mexico indicate mostly orthorhombic palygorskite (Sańchez del Rıó et al., 2009b). In addition, MB from Central Mexico may contain sepiolite. Interestingly, Zaachila blue, a variety of MB found in a tomb explored in 1962 by the archaeologist R. Gallegos in Oaxaca, produces a diffractogram (Christ et al., 1969) in good agreement of palygorskite used in Aztec MB (Sańchez del Rıó et al., 2008). From these data, it appears that the Aztecs exploited a palygorskite mine different from those of Yucatań (Sańchez del Rıó et al., 2009b).

4.6. Chronology and Distribution of MB The mural paintings of Bonampak (Classic period, eighth century) are considered the oldest accurately dated artworks containing MB (Reyes-Valerio, 1993). MB is abundant in archaeological finds from the beginning of Maya Late Classic (ca. sixth century) in the Usumancinta, Puuc and Peten regions, as well as in Guatemala (Cecil, 2010). Outside the Maya area, MB has been found in Mexico in El Tajin (Veracruz, Totonac culture), Cacaxtla (Tlaxcala, Olmec-Xicalanca culture, see Figure 1), Tamuin (San Luis Potosı´, Huastec culture), Zaachila (Oaxaca) and is very abundant in the Aztec area. However, MB

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has not been found in important sites like Teotihuacan, Monte Alban or Xochicalco, indicating the late arrival in the Mexican plateau (Reyes-Valerio, 1993). The age of MB has been revised recently, with a possible earlier use in Yaxchilan (Torres et al., 1976) or possibly earlier, in different locations in the archaeological site of Calakmul, belonging to the Late Preclassic (Domeńech et al., 2006; Va´zquez de Agredos et al., 2011) and Early Classic (Garcıá Moreno et al., 2008). MB was used in Colonial times for decorating convents and churches (Reyes-Valerio, 1993), at least until the end of the sixteenth century (see Figure 3). Noteworthy, occurrences include Actopan (Hidalgo), Tecamachalco (Puebla), Epazoyucan (Hidalgo), San Pedro Tezontepec (Hidalgo) and Jiutepec (Morelos). Comparative studies of Precolumbian MB and Colonial MB (Sańchez del Rıó et al., 2006b) may be of importance to determine how pigment technology was transferred during the Conquest. Reyes-Valerio showed that Spanish monks used Indian artists for the decoration of chapels. Juan Gersoń, an Indian artist, made numerous paintings in Tecamachalco (Puebla; Reyes-Valerio, 2000) in 1562. Because MB pigment was used frequently during the first century of the Conquest in religious decorations of the three important Monastical orders (Augustinians, Dominicans and Franciscans), the monks probably could know how to fabricate MB. Possibly, the technology was controlled by a few Indian artists, but the number of convents and the total painted surface (> 400 m2) suggested that the technology had been acquired by the religious hierarchy. A variety of MB, known as La Havana blue, decorated civil buildings in Colonial Cuba (Tagle et al., 1990) during the seventeenth and eighteenth centuries. MB was last used in Mexican colonial mural paints around 1580. It is unclear if the Cuban MB came from Mexico, or if it was fabricated in situ. MB probably came from Mexico, because of the small distance that separates Yucatań from Cuba, and because palygorskite is unknown in Cuba. X-ray data of Cuban blue (Tagle et al., 1990) are identical to Yucatecan palygorskite (Sańchez del Rıó et al., 2009b). The time of first import to Cuba is unknown, and how MB evolved from religious to civil decorations is uncertain. Because MB must have coexisted in Mexico and Cuba suggests that either it was used in Mexico after the end of the sixteenth century, or it was imported to Cuba earlier. It is probable that Cuba obtained the recipe in colonial times, where it is known that slave markets and trade exist between Cuba and Yucatań.

4.7. Symbology of MB Colours were essential in the Prehispanic world. However, the colour symbology is different for the Maya than for the cultures in the valley of Mexico (Reyes-Valerio, 1993). The Maya associated the blue to several meanings, among them, the sacrifice. Based on Landa (1566), this is noted by Thomson (Gan and Thompson, 1931) “Before being sacrificed the victim was stripped

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of his clothing and painted with a blue unguent, blue being the sacrificial color”, and lately by Gettens (1962). Blue is associated also with water and rain, and therefore related to several deities, for example, god of rain for both Mayas (Chaac) and Aztecs (Tlaloc). Colours are also related to the Cardinal Points by the peoples of Central Mexico, and blue is sometimes related to West (Reyes-Valerio, 1993).

ACKNOWLEDGEMENTS MSR dedicates this work to the memory of Constantino Reyes-Valerio (1922– 2006), historian, chemist, photographer, who made great contributions to the study and popularization of MB. His guidance and dedication were essential for the accomplishment of several of the works cited here. S. Guggenheim is acknowledged for a pertinent review and useful editing. Financial support by CICYT (project CGL2009-10764) is acknowledged.

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Chapter

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Subject Index

Note: The letters ‘f ’ and ‘t’ following the locators refer to figures and tables respectively.

A Acid EDTA, 358t PLA, 403–404 treatment, 284, 285t, 428, 429f Active organomineralization, 221, 222 Adana basin, 185–186. See also Misis-Adana basin Adsorption, surfactant fibrous clay pharmaceutical uses for, 308 model application results, 367–369 model equations, 364–367 palygorskite surface modifications, 361t sepiolite surface modifications, 356–364 surface-related properties, 351–356 Advanced materials conclusions about, 440–441 hybrid nanomaterials biohybrids and biomimetic, 416–420 carbon, 421–423 clay-organic interactions mechanisms, 409–411, 410t dry-sepiolite nanostructured, 412–414 functionalized, 434–440 grafted organic derivatives by covalent bonding, 414–416 MB pigment, 409, 409f, 412–413 overview, 409–440, 410t sepiolite support of nanoparticles, 423–440 sepiolite-organic compound interactions, 411–416 nanoclays, 395–408 fire retardancy properties, 405–408, 408f mechanical properties, 405–408, 407t organomodified, 395–396 thermal properties, 405–408, 407t thermoplastic and thermosetting based nanocomposites, 396–405 nanocomposites, 396–405 background about, 396 gold pellet, 438f preparation methods, 397

in situ polymerization, 397 overview, 393–395 structural blocks and, 393–395, 394f thermoplastic and thermosetting polymersclay nanocomposites, 396–405 Agglomeration, 285 Agriculture, 274, 290–291 Allou Kagne deposit, 76–77 Aluminous precursors, 135 Amargosa, 78–79 applications, 90 deposit, 89–90 mineral content of, 89 origin, 90 overview about, 157 Amargosa saponite chemical composition, 269–272, 269t dispersal properties, 271–272 functional properties, 269–272, 270t geographic settings, 265–268, 267f geologic settings, 265–268 markets, applications, 272–275 overview, 265 sorptive properties, 271 Amargosa sepiolite chemical composition, 268f, 269–272, 269t dispersal properties, 271–272 fibrous length, 266–267, 268f, 269–270 functional properties, 269–272, 270t geographic settings, 265–268, 267f geologic settings, 265–268 grades, 266–267, 268f health and, 276 markets, applications, 272–275 overview, 265 sorptive properties, 270–271 Amboseli chemical analysis, 141t deposit, 157 3-aminopropyltriethoxysilane, 358t Andhra Pradesh Siddiqui, 141t Animal feed, 274, 290, 291 Anionic dyes, 361t

483

484 Anionic surfactants, 358t, 361t, 363 Antacids, 306–308 health and, 316 palygorskite and, 309t sepiolite and, 307, 309t Antibacterial applications, 431, 431t Antibacterial effect, in glass, 436, 436t Antibacterial glazes, 435 Antidiarrhoeals, 306–308, 309t Antifungal effect, in glass, 436, 436t Antifungal glazes, 435 Applications absorbent grade Chinese clays, 259 absorption characteristics, 289–290 chemically inert, 289 high porosity, 289 large surface area, 289 mechanical strength, 289 non-combustible and non-flammable, 289 Amargosa. 90, See also Amargosa saponite and Amargosa sepiolite antibacterial, 431, 431t biomedical, 430 Chinese palygorskite clay, 256, 259 conclusions about, 297 fifth generation products, 295–296 first generation products, 287–290 absorption and, 289–290 animal feeds, 290 chemical reaction resistance and, 288 filtration and, 288–289 pet litters, 288 phytosanitary carriers, 290 processing, 287–288 soil evaporation retardation, 290 fourth generation products, 293–295 CIMSILÒ, 293, 295f mortar, 293–295 synergistic combinations, 293 gel grade Chinese clays, 256, 259 health and safety issues, 296–297 humidity sensors, 439 overview, 86, 281–282, 282f, 283t physico-chemical features, 282–286 agglomeration, 285 defects, 284 needles, 286 rheological properties, 286 stability, 285 thermal/acid treatment effects, 284, 285t unique properties, 282, 283t

Subject Index processing flow sheet and, 286, 287f properties enhancements, 286–296 second generation products, 290–292 agriculture, 290–291 animal feeds, 291 ceramic tile adhesives, 292 concrete, 291–292 construction, 291 examples, 290 mortar, 292 mycotoxin absorption, 291 rubber, 290 suspension capacity enhancement, 291 toxic waste, 292 water treatment, 292 water-based compositions, 291 six generations of, 288t sixth generation products, 295–296 South Georgia-North Florida deposit, 87, 89 Theis, 94 third generation products, 293 examples, 293 organophilic thixotropic additive, 293, 294f waste treatment, 293 Turkey and, 194 Ukraine and, 96 Vallecas-Vica´lvaro-Yunclillos, 91 Argentina, 78 As¸ag˘i Pinarbas¸i, 179f, 181 Attapulgite, 6–7. See also Palygorskite nomenclature and, 85, 303 Australia, 96 Authigenic clays, 125, 126f

B Basalt cycle, 245, 245f as source material, 247–253 Basalt pebble weathering zones, Chinese palygorskite clay resources, 251, 251f chemical analysis, 252, 252t core, 251, 251f, 252t, 253 magnesium and, 253 middle zone, 251, 251f, 252t, 253 rim, 251, 251f, 252t, 253 titanium depletion and, 252, 260t Batallones deposit, 148–150 deposit chemical analysis, 141t lithological units, 148, 150f sepiolite unit in, 148–150, 151f

485

Subject Index Batteries, 421–422 BCM. See Biologically controlled mineralization Bedrock LAs and, 164–165, 164f ultramafic, 73–74 Bercimuel deposit, 91f, 92–93, 152 deposit chemical analysis, 91f, 92–93, 141t BET-nitrogen, 380, 383t BIM. See Biologically induced mineralization Bingham model, 331 Biogenic origin of sepiolite biomineralization process, 221–223 dolomite-sepiolite Miocene biomorphs and, 223–228, 223f hypothesis, 232–235, 234f mineral and biological similarities, 219, 220f mineral formation recognition, 228–232 biomass mineralization, 228–229, 229f geochemical evidence, 231–232 from microfibrils to sepiolite, 230–231, 230f, 231f, 234f overview, 219–223 Phanerozoic minerals and, 220 Biohybrids and biomimetic materials, based on sepiolite, 416–420 bio-organoclays, 417 biopolymers, 417–418 collagen and, 418 foams, 419–420, 419f gelatine and, 418 Nature and, 416–417 xanthan gum and, 420, 420f zein and, 418–419 Biologically controlled mineralization (BCM), 221 Biologically induced mineralization (BIM), 221 Biomass mineralization, 228–229, 229f Biomedical applications, 430 Biomimetic materials, based on sepiolite, 416–420 bio-organoclays, 417 biopolymers, 417–418 collagen and, 418 foams, 419–420, 419f gelatine and, 418 Nature and, 416–417 xanthan gum and, 420, 420f zein and, 418–419 Biomineralization process, 221–223 active and passive, 221, 222

carbonates and, 222–223 macromolecules and, 221–222 microbial mats and, 222 Biomorphs, dolomite-sepiolite Miocene, 223–228, 223f Bio-organoclays, 417 Biopolymers, 417–418 Bleaching, 258–259, 258f Blocks structural, 393–395, 394f tunnels and, 330 Bottle-around-a-ship procedure, 413–414, 413f

C Cadmium, 342–343, 342f, 343f, 344f Calatayud basin. See Mara Calatayud Tertiary lacustrine basin, 73 Campo de Calatrava, 153–154 Çanakkale region, 186 Cap rock reactivity and integrity, 376–379 Carbon, hybrid nanomaterials, 421–423 batteries and supercapacitors and, 421–422 CPEs and, 422–423 graphite-like, 421 organosilanes grafting and, 422–423, 422f Carbon dioxide (CO2) aqueous, 134 atmospheric concentrations, 375 biomineralization process and, 222–223 supercritical compression of, 376, 377f Carbon dioxide capture and storage (CCS) cap rock reactivity and integrity, 376–379 concluding remarks about, 390 geochemical trapping, 376 geological, 375–376 injection processes, 376 modeling, 389–390 naturally occurring, 375–376 overview regarding, 375–379 palygorskite and sepiolite characteristics concerning, 379 physical trapping, 376 research background, 376 in sedimentary basins, 376 supercritical CO2/palygorskite and sepiolite interaction study, 379–388 supercritical compression and, 376, 377f technologies, 375 Carbon paste electrodes (CPEs), 422–423 Carbonatic chemical facies, 164f, 165 Carcinogenicity, 315 Cat litter, 272

486 Cationic dyes, 358t, 361t Cationic surfactants, 357–363, 358t, 361t C ¸ ayırag˘zı magnesite deposit, 184–185 CCS. See Carbon dioxide capture and storage Ceramic tile adhesives, 292 Ceramics antibacterial or antifungal glazes and, 435 humidity sensors and, 439 multifunctional glazes and, 435 nanoparticles assembled to sepiolite in, 434 particle size and, 435 self-cleaning glazes and, 434–435 Channel, 355. See also Open channel defects structural blocks and, 393–394, 394f Chemistry absorption characteristics regarding, 289 Amargosa saponite, 269–272, 269t Amargosa sepiolite, 268f, 269–272, 269t Amboseli and, 141t Andhra Pradesh regarding, 141t basalt pebble weathering zones, 252, 252t Batallones and, 141t Bercimuel and, 91f, 92–93, 141t carbonatic chemical facies, LAs, 164f, 165 central Atlantic Cretaceous, 109–110 chemical reaction resistance and, 288 Chinese palygorskite clay resources, 247–251, 252, 252t clayey chemical facies, LAs, 164f, 165–166 continental environments and, 137, 141t continuous composition and, 51, 51f, 52f cosmetic uses of fibrous clays and, 300–303, 315 deviations in purity of, 35–36 environmental influences and, 81 Eskisehir and, 141t Guanshan basalt pebble weathering zones, 252, 252t deposit chemical analysis, 141t depositional environment data interpretation, 247–251 health and, 315 impurities and, 33–34 of indigo, 470 intermediate minerals structural arrangements, 51–55, 53f, 55f introduction, 33–36 magnesium and, 328, 329f Mara and, 141t marine sediments, 109–110 MB and, 467–468

Subject Index open questions regarding, 55 overview, 351–352, 353t of palygorskite, 44–49, 44t, 45f, 46f, 302 palygorskite and sepiolite compositional gap, 41f, 49–51, 51f, 52f pharmaceutical uses and, 300–303, 315 phosphorus and, 328, 329f physico-chemical features, 282–286 polysomatic clusters and, 54–55, 55f polysomatic continuous series and, 53–54, 53f pure composition calculations, 35–36 of sepiolite, 36–43, 37t, 38f, 39f, 41f, 132, 302 Spain and, 72 surface-related properties and, 351–352, 353t Turkey, 187, 189f Chert-sepiolite interface, 224–225, 226f China, 95, 158. See also Guanshan Chinese palygorskite clay resources applications, 256, 259 basalt pebble weathering zones, 251, 251f conclusions about, 260 depositional environment, 247–255, 254f geology, 243–246 regional geotectonic and volcanic setting, 244–246, 245f Guiwu formation, 242f, 244 Huaguoshan formation, 242f, 243 locations of, 239–240, 240f mineralogy, 246, 246t overview, 239–241, 240f previous work regarding, 241–243 processing, 256–259 absorbent grades, 257–259 gel grade, 256 stratigraphic column of Guanshan, 242f Zhangshanji formation, 242f, 243 CIMSILÒ, 293, 295f Clayey chemical facies, 164f, 165–166 Clay-organic interactions, 409–411, 410t Clays, fibrous drug interactions of, 312–313 as excipients, 300 health risks, 314–316 Iran and, 215 overview about, 299–300 requirements of, 299–300 used in health care, 301t

487

Subject Index Climate. See also Carbon dioxide capture and storage environmental influences and, 81 Iran and, 203–204, 204f UNFCCC and, 375 CO2. See Carbon dioxide; Carbon dioxide capture and storage Cohydrolysis procedure, 414–415 Collagen, 418 Colloid migration, 340–341, 341f Common palygorskite, 47–48 Computer modeling, of Maya blue, 466 Concrete, 291–292 Congo red, 361t Construction, 291 Continental environments Amargosa, 157 Amboseli deposit, 157 arid regions and, 127 El Bur deposit, 158 favorable conditions and, 128–129, 128t final remarks, 159–166 on LAs, 163–166 on sedimentary environments, 159–162 genetic conditions, 130–136 characteristics and occurrences, 137–158 lacustrine environments, 130–131 palygorskite deposits summary, 138t palygorskite formation evidence, 135–136 selected deposit chemical analyses, 137, 141t sepiolite deposits summary, 139t sepiolite formation evidence, 131–135 geotectonic settings and, 125–126, 127f Guanshan deposit, 158 neoformation vs. transformation discrimination and, 131t other deposits in, 157–158 overview, 125–130 pH and, 129 research history and, 101–102 sedimentary environments, 159–162 soils and, 127 Spain, 137–154, 142f Batallones deposit, 148–150 other deposits, 151–154 Tagus basin, 137–150 Vallecas-Vica´lvaro-Yundlillos deposits, 145–148 Turkey, 154–157 Eskisehir deposit, 74–75, 74f, 95, 155–157 geological setting, 154–155, 154f Ventzia basin deposit, 158

weathering environments and, 127 XRD patterns, 140f Controlled hydrolysis, 426–427, 427f Cosmetic uses of fibrous clays, 313–314 as excipients, 300 health risks and, 314–316 liquid and semi-solid dosage forms, 310 markets and, 274 mineralogy and chemistry related to, 300–303 nomenclature, 303–306 overview about, 299–300 requirements regarding, 299–300 specifications, 303–306, 304t Covalent bonding, 410t, 414–416 CPEs. See Carbon paste electrodes Cretaceous, central Atlantic, marine sediments clay mineral distribution, 108–109, 109f conclusions about, 111–112 DSDP site 105, 105f, 107, 108f DSDP site 370, 105–106, 105f, 106f DSDP site 417, 105f, 106–107, 107f geochemistry, 109–110 occurrence, 105–107 palaeoceanography and palaeogeography, 110–111, 111f Crystal violet (CV), 358t, 361t Crystalline silica, 315 Crystallography, 352, 354t CV. See Crystal violet

D

DDA. See Dodecylammonium DDEDMA. See Dodecylethyldimethylammonium DDTMA. See Dodecyltrimethylammonium Decolorizing, 274 Deep-Sea Drilling Project (DSDP) central Atlantic Cretaceous, 104–112 clay occurrence, 105–107 site 105, 105f, 107, 108f site 370, 105–106, 105f, 106f site 417, 105f, 106–107, 107f clay mineral distribution and, 108–109, 109f Defects open channel cause of, 26–27 H2O content and, 27 microtexture and, 26–28, 27f sorption and, 28 yofortierite, 26, 27f physico-chemical features and, 284 planar, 28–29 Denizli sector, 154f, 155

488 Denizli-Bozkurt area, 182 Depositional environment, Guanshan, 247–255, 254f chemical data interpretation, 247–251 Huangnishan and, 247, 250t lithologic column profile, 253–254, 254f model of formation, 254, 255f oxides, 247, 248t source material as basalt, 247–253 Deposits. See also specific deposit Australia, 96 China, 95, 158 conclusions about, 97 Greece, 95–96, 158 Guatemala, 96 India, 96 Kenya, 157 locations overview, 86, 86f other occurrences, 96–97 other worldwide, 95–97 overview, 85–86 palygorskite deposits summary, 138t Russia, 97 Senegal, 93–94 sepiolite deposits summary, 139t Somalia, 96, 158 Spain, 90–93, 145–150, 151–154 Turkey, 94–95, 155–157 Ukraine, 96 United States, 87–90 Detrital clays, 125, 126f Detrital facies, 164f, 165 DHDDMA. See Dihexadecyldimethylammonium Diffraction studies, on Maya blue, 457–458, 459f Dihexadecyldimethylammonium (DHDDMA), 358t, 361t Diluents, 308–309 Dimethyldichlorosilane (DMDCS), 358t Dimethyloctadecylchlorosilane (DMODCS), 358t Dioctadecyldimethylammonium (DODDMA), 358t Direct precipitation, 136 Disaggregation, 340–341, 341f Disintegrants, 308–309 Dispersal properties, Amargosa saponite, 271–272 sepiolite, 271–272 Dissolution-precipitation transformation, 136 Divalent organic cations, 357 DMDCS. See Dimethyldichlorosilane

Subject Index DMODCS. See Dimethyloctadecylchlorosilane DODDMA. See Dioctadecyldimethylammonium Dodecylammonium (DDA), 358t Dodecylethyldimethylammonium (DDEDMA), 358t Dodecyltrimethylammonium (DDTMA), 358t Dolomite, 69 dolomite-sepiolite Miocene biomorphs, 223–228, 223f chert-sepiolite interface, 224–225, 226f gypsum-dolomite interface, 224, 225f microorganism recognition, 224–228, 225f, 227f sepiolite-dolomite marl, 225–226, 227f Morocco and, 76 Dolomitic facies, 164 Doped glass, 436–437, 437f Double Lakes, 79, 80f Drilling. See also Deep-Sea Drilling Project Amargosa Drug interactions, of fibrous clays, 312–313 absorption inhibition, 312 Dry-sepiolite nanostructured hybrids, 412–414 DSDP. See Deep-Sea Drilling Project Duero basin, 151–152. See also Bercimuel Duripans, 341–342

E Economy, Turkey and, 194 EDTA. See Ethylene diamine tetra acetic acid El Bur, 78, 158 Electrolyte flocculation and, 335f, 339 Iran southern soils and, 210 rheology and, 335–336, 335f Electrostatic interactions, 410t, 411 Elmadag˘ area, 185 Emulsifying agents, 311–312 Environmental influences in Argentina, 78 climate and, 81 closed basin clay distribution and, 69–71, 71f favorable, 128–129, 128t in Kenya, 75 in Morocco, 76 overview about, 69–71 pH and, 129 physico-chemical parameters and, 81 in Senegal, 76–77 for sepiolite production, 69–71

489

Subject Index Environmental influences (cont.) sesquioxide-free hydrous magnesium silicates and, 69, 70t soils, 127 in Somalia, 78 source material and, 80 in South Africa, 77 in Spain, 71–73, 72f summarized, 79–81 in Tunisia, 76 in Turkey, 73–75, 74f in United States, 78–79 weathering, 127 Eocene marine sediments, 112–118 clay distribution, 115–116, 115f conclusions about, 117–118 Gulf of Guinea, 112–113, 112f palaeoenvironment, 117 Sargasso Sea, 113–115, 114f Turkey marine occurrences, 177f, 178 Epoxy, 404–405 Eskisehir chemical analysis, 141t deposit, 74–75, 74f, 95, 155–157 lacustrine basin, 74–75, 74f loughlinite and, 156 meerschaum and, 95, 156 mineral content, 95 Eskisehir-Konya sector, 154, 154f Ethylene diamine tetra acetic acid (EDTA), 358t Excipients, fibrous clay pharmaceutical uses as, 300, 308–312, 309t liquid and semi-solid dosage forms, 309t, 310–312 solid dosage forms, 308–310, 309t Extrusion process, 427–428

F Falcondoite formation conditions, 16–17 as group member, 6–7, 6t Fars Province, 210, 213 Fibres ellipticity, 332, 333f Fibrous clays. See Clays, fibrous; Cosmetic uses of fibrous clays; Pharmaceutical uses of fibrous clays Fibrous length, Amargosa sepiolite, 266–267, 268f, 269–270 15 Crown ether 5, 358t, 361t

Fifth generation products, 295–296 Filtration and, 288–289 Fire retardancy properties, 405–408, 408f First generation products, 287–290 Flocculation, 338–340 Flocculation value (FV), 338 Flooding events, 148, 149f Florida. See South Georgia-North Florida Flow curves, 331, 332f Foams, 419–420, 419f Fourth generation products, 293–295 Fuller’s earth, 85 Functional properties, Amargosa, 269–272, 270t FV. See Flocculation value

G Gallation Volcanic Belt, 75 Gel grade, Chinese clay processing, 256 applications, 256, 259 roller press, 256 steps of, 256, 257f Gelatine, 418 Genesis, Turkey and, 187–194, 190f Genetic conditions, continental environments, 130–136 characteristics and occurrences, 137–158 lacustrine environments, 130–131 palygorskite deposits summary, 138t palygorskite formation evidence, 135–136 selected deposit chemical analyses, 137, 141t sepiolite deposits summary, 139t sepiolite formation evidence, 131–135 Genetic relations, 16–18 Geochemistry. See Chemistry Georgia. See South Georgia-North Florida Geotectonic settings, 125–126, 127f, 244–246, 245f Glass aggregation and, 435–436 antibacterial or antifungal effect in, 436, 436t doped, 436–437, 437f soda lime glass powder, 435 Glazes antibacterial or antifungal, 435 multifunctional, 435 nanoparticles assembled to sepiolite in, 434 particle size and, 435 self-cleaning, 434–435 Glidants, 308–309 Gold nanocomposite pellet, 438, 438f

490 Gondwana break-up, 202–203 Grafted organic derivatives of sepiolite, by covalent bonding, 414–416 cohydrolysis procedure, 414–415 organosilanes and, 415 Graphite-like hybrid nanomaterials, 421 Great Kavir Basin, 207–208, 210f, 211f Greece deposits, 95–96, 158 Ventzia basin deposit, 158 Guanshan basalt pebble weathering zones, 251, 251f deposit, 158 deposit chemical analysis, 141t depositional environment, 247–255, 254f geology, 243–246 Guiwu formation, 242f, 244 Huaguoshan formation, 242f, 243 mineralogy, 246, 246t previous work about, 241–243 stratigraphic column of, 242f Zhangshanji formation, 242f, 243 Guatemala, 96 Guiwu formation, 242f, 244 Gulf of Guinea clay distribution and, 115–116, 115f Eocene, 112–113, 112f Gulf Trough-Apalachicola embayment, 87–88, 87f Gypsiferous soils, 213 Gypsum-dolomite interface, 224, 225f

H

H2O. See Water HDPy. See Hexadecylpyridine HDTMA. See Hexadecyltrimethylammonium Health. See also Cosmetic uses of fibrous clays; Pharmaceutical uses of fibrous clays carcinogenicity and, 315 chemical elements and, 315 crystalline silica and, 315 fibrous clays and, 301t, 314–316 issues, 296–297 mineral dust and, 314–315 risks of fibrous clays, 314–316 sepiolite and, 276 Heating process changes, 355–356 Hekimhan region, 75, 177, 177f Herbicide, 275 Hexadecylpyridine (HDPy), 361t Hexadecyltrimethylammonium (HDTMA), 358t, 361t

Subject Index Hirsizdere magnesite deposit, 74 Hole stabilization, 273 Hormite, 6–7 Huaguoshan formation, 242f, 243 Huangnishan, 247, 250t Humidity, 325–326 Humidity sensors, 439 Hydraulic conductivity, 331 Hydrodynamic trapping, 376 Hydrogen bonding, 410t, 411 Hydrothermal sepiolite occurrences, Turkey, 184–185 serpentinite-hosted, 184–185 volcanic-hosted, 184 Hydroxide (OH), 15–16 Hypothesis, passive organomineralization of sepiolite, 232–235, 234f

I Ice-segregation-induced self assembly (ISIA), 427–428 Impregnation methods, 426 India, 96 Indigo chemistry of, 470 to clay attachment, 469–470 historical relevance of, 470–471 palygorskite association with, 470 sources of, 471 trade, 471 Infrared (IR) Maya blue spectroscopies, 458–461, 458f, 460f spectroscopy of surface-related properties, 356 Injection processes, 376 Intermediate minerals compositional ranges of, 50, 52f continuous composition and, 51, 51f, 52f open questions regarding, 55 polysomatic clusters and, 54–55, 55f polysomatic continuous series and, 53–54, 53f structural arrangements, 51–55, 53f, 55f Intersilite, 5–6 formation conditions, 16–17 group comparison with, 6–7, 6t single-crystal X-ray studies of, 8–12 Ion exchange capacity, 355 Ion-exchange reaction, 423–426 iPP. See Isotactic PP IR. See Infrared

491

Subject Index Iran central soils and sediments, 205–208 chronostratigraphic distribution and, 215 climate, 203–204, 204f conclusions about, 214–215 fibrous clay and, 215 future research directions, 215 Gondwana break-up, 202–203 Neo-Tethys, 202–203 north-eastern soils, 213 overview, 201–202 palaeogeography, 202–203, 203f palaeogeology, 202–203 southern soils and parent rocks, 208–213 Tethyan region, 202–203 western soils, 204–205 Zagros orogenic phase, 202–203 Iron, humidity sensors and, 439, 440f Isfahan Province, 206–207, 209f ISIA. See Ice-segregation-induced self assembly Isomorphous substitution, 329–330 Isotactic PP (iPP), 399

J Jbel Rhassoul, 76

K Kalifersite, 5–6 formation conditions, 16–17 as group member, 6–7, 6t single-crystal X-ray studies of, 12–13 Kangal sub-basin, 182–183, 183f Kaolinite, 213t, 336–337 Karapinar area, 179f, 181 Kenya Amboseli deposit, 157 environmental influences in, 75 Kepeztepe area, 180 Kerolite, 69, 70t Amargosa saponite/sepiolite and, 272 genetic relationship, 130, 131 Kenya and, 75 kerolite-stevensite deposit, 143f, 157, 167 precipitation, 157, 161 sepiolite, 134 stability diagram, 132–133, 133f syngenetic formation, 161f Kizildag˘, 183–184, 183f Kizildere, 186 Kleber, R, 456 Konya basin, 155

L Lacustrine environments, 130–131 Calatayud Tertiary lacustrine basin, 73 LAs and, 163 Turkey, 178–184 LAs. See Lithological associations Layered double hydroxides (LDHs), 433–434 Light absorption maxima, 431–432 Liquid and semi-solid dosage forms cosmetic uses, 310 emulsifying agents and, 311–312 gels, 311 pharmaceutical uses, 309t, 310–312 Lithologic column profile, Guanshan, 253–254, 254f Lithological associations (LAs) bedrock, 164–165, 164f carbonatic chemical facies, 164f, 165 clayey chemical facies, 164f, 165–166 detrital facies, 164f, 165 dolomitic facies and, 164 lacustrine environments and, 163 palustrine conditions and, 163 palustrine-lacustrine wetland and, 163 sedimentary environments and, 163–166 six types of, 164f swamp environments and, 163 Llano Estacado, 79 Loot Desert, 207, 208f Loughlinite Eskisehir and, 156 formation conditions, 16–17 as group member, 6–7, 6t

M Madrid basin, 72 dolomite-sepiolite Miocene biomorphs in, 223–228, 223f geological setting, 134, 140, 143f Magnesian Unit and, 142–144 during Neogene, 146f palygorskite in, 145 Spain and, 72, 134, 140, 143f Tagus basin and, 134, 140, 143f Magnesic palygorskite, 47–48 Magnesium chemistry and, 325–329, 329f Chinese palygorskite clay resources and, 253 palygorskite effects on soil, 325–329

492 Magnesium (cont.) sources in sedimentary environments, 159–160, 160t stability relations regarding, 69, 70t Magnesium trisilicate. See Sepiolite Magnetism, 432, 433 NMR and, 464–465 superparamagnetic behavior, 397–398, 433 Mara, 72f, 73, 91f deposit chemical analysis, 141t deposits, 92, 153 Marine occurrences, Turkey, 177–178 Eocene to Miocene, 177f, 178 Palaeocene, 177–178, 177f Upper Cretaceous, 177, 177f Marine sediments, Palygorskite clays in central Atlantic Cretaceous clay mineral distribution, 108–109, 109f conclusions about, 111–112 DSDP site 105, 105f, 107, 108f DSDP site 370, 105–106, 105f, 106f DSDP site 417, 105f, 106–107, 107f geochemistry, 109–110 occurrence, 105–107 palaeoceanography and palaeogeography, 110–111, 111f Eocene, 112–118 clay distribution, 115–116, 115f conclusions about, 117–118 Gulf of Guinea, 112–113, 112f palaeoenvironment, 117 Sargasso Sea, 113–115, 114f overview, 101 research history, 101–104 research perspectives, 118 significance of, 109 Markets Amargosa saponite, 272–275 Amargosa sepiolite, 272–275 Marl, sepiolite-dolomite, 225–226, 227f Mashhad, 213 Maya blue (MB) chemical resistance, 467–468 chronology regarding, 474–475 dry-sepiolite nanostructured hybrids and, 412–413 experimental techniques, 457–466 history of, 453–457, 454f, 455f, 456f hue of, 468–469, 469f hybrid nanomaterials and, 409, 409f, 412–413 indigo to clay attachment, 469–470

Subject Index palygorskite-indigo association, 470 research ancient production and use, 472–474 ancient trade and distribution, 474 archaeological and historical context regarding, 470–476 structural aspects, 469–470 symbology, 475–476 synthesis, 467–470 MB. See Maya blue; Methylene blue Meerschaum, 75, 95, 156 Meigs-Attapulgus-Quincy District. See South Georgia-North Florida Mersin area, 186 Mesoamerica, 471–472 Metal immobilization, 344–345, 345f Metal sorption, mechanisms on palygorskite, 342–343 cadmium, 342–343, 342f, 343f, 344f irreversibility and, 343, 344f Methylene blue (MB), 358t, 361t. See also Maya blue Methylene green (MG), 358t Mezgi ridge, marine occurrences, 177–178, 177f MG. See Methylene green Michler’s hydrol, 414, 414f Microbial mats, 222 Microfibrils, 230–231, 230f, 231f, 234f Microtexture introduction to, 3 OCDs and, 26–28, 27f palygorskite to smectite transformation and, 22–25, 26f planar defects and, 28–29 polysome-width disorder and, 22–25, 23f related topics, 22–29 stacking errors and, 28–29 Mid-infrared (MIR), 458–460, 460f Mihaliççik, 180 Mineral dust, 314–315 Mineral trapping, 376. See also Carbon dioxide capture and storage Mining, processing flow sheet, 286, 287f Miocene, 28, 88f, 90, 95, 101, 113, 137, 138t, 139t, 140, 142, 145, 149f, 153, 156, 158, 177–184, 186, 195t, 196t, 202, 205, 206, 208, 209f, 219, 220f, 242t, 243–244 dolomite-sepiolite biomorphs and, 223–228, 223f peri-marine, 79 Turkey marine occurrences, 177f, 178 MIR. See Mid-infrared

493

Subject Index Misis-Adana basin, 186 Monovalent organic cations, 357 Montmorillonite, 336, 337f palygorskite transformation to, 25 palygorskite-containing soil clay suspensions and, 337 Morocco, 76 Mortar, 292, 293–295 Mycotoxin absorption, 291

N Namaqualand area, 77 Nanoclays, 395–408 fire retardancy properties, 405–408, 408f mechanical properties, 405–408, 407t organomodified, 395–396 thermal properties, 405–408, 407t thermoplastic based nanocomposites, 396–405 thermosetting based nanocomposites, 396–405 Nanocomposites, 396–405 background about, 396 gold pellet, 438f preparation methods, 397 in situ polymerization, 397 thermoplastic and thermosetting polymers-clay, 396–405 Nanoparticles, sepiolite support of, 423–440 acid treatment and, 428, 429f antibacterial applications, 431, 431t biomedical applications, 430 clusters and, 423 controlled hydrolysis and, 426–427, 427f extrusion process and, 427–428 functionalized materials from, 434–440 impregnation methods, 426 ion-exchange reaction and, 423–426 ISIA process and, 427–428 LDHs and, 433–434 light absorption maxima and, 431–432 magnetism and, 432, 433 superparamagnetic behavior, 397–398, 433 overview of, 423, 424t oxidation and, 430, 433 wet chemical route and, 428 Near-infrared (NIR), 460–461 Needles, 286 Neoformation, 125, 131t Neogene lacustrine basin, 75 Madrid basin during, 146f

Neo-Tethys, 202–203 Neutral surfactants and, 358t, 361t, 363 NIR. See Near-infrared Nitrogen adsorption isotherms, 380–382, 383f NMR. See Nuclear magnetic resonance Nomenclature history regarding, 85 overview of, 6–7, 6t pharmaceutical and cosmetic, 303–306 Nuclear magnetic resonance (NMR), 464–465

O

OCDs. See Open channel defects Octadecyltrimethylammonium (ODTMA), 358t, 361t, 363–364 Octahedral cations, 189f Octahedral sheets, 33, 35f Octahedral-tetrahedral misfit, 18–21, 19f ODTMA. See Octadecyltrimethylammonium OH. See Hydroxide OH2. See Water Open channel defects (OCDs) cause of, 26–27 H2O content and, 27 microtexture and, 26–28, 27f sorption and, 28 yofortierite, 26, 27f Ophiolitic units, 175, 176f Optical spectroscopies, 463 Optical transparency, using SPS, 438 Organomineralization active and passive, 221, 222 passive hypothesis of, 232–235, 234f Organomodified nanoclays, 395–396 Organophilic thixotropic additive, 293, 294f Organosilanes, 415, 422–423, 422f Orogenic area, Zagros, 212 Orogenic phase, Zagros, 202–203 Owl Cigar Company, 87

P

PA-6. See Polyamide-6 Paar reactor experiments and, 380, 382f, 382t Paint, 273–274 Palaeoceanography, 103–104 Palaeocene, 177–178, 177f Palustrine conditions, 163 Palustrine-lacustrine wetland, 163 Palycretes, 341–342 Palygorskite. See also Chinese palygorskite clay resources; Marine sediments, Palygorskite clays in; Soils, palygorskite effects on; specific subject

494 Palygorskite. See also Chinese palygorskite clay resources (cont.) antacids and, 309t antidiarrhoeals and, 306–307, 309t applications overview, 281–282, 283t brand names, 306 in central Iranian soils, 205–208 chemistry of, 44–49, 44t, 45f, 46f, 302 common, 47–48 compositional ranges, 44, 44t, 50, 52f crystallography, 352, 354t essential features, 4–5 formation conditions, 16, 17t formation environments, 125–126 formation evidence, 135–136 aluminous precursors, 135 direct precipitation, 136 dissolution-precipitation transformation, 136 smectite precursor, 135–136 H2O, OH2, and OH positions in, 15–16 ideal, 44, 47–48, 352 Kenya and, 75 Madrid basin and, 145 magnesic, 47–48 in Mesoamerica, 471–472 minerals associated with, 69, 70t to montmorillonite transformation, 25 Morocco and, 76 overview about, 239 oxides in, 39f, 41f, 44, 45f, 46f properties, 282–286, 283t pure composition of, 35–36 Rietveld refinements of, 7–8, 9t sedimentary environments and, 159–160, 159f, 160f, 160t Senegal and, 76–77 sepiolite compared to, 284 sepiolite compositional gap with, 41f, 49–51, 51f, 52f to smectite transformation, 22–25, 26f soils associated with, 127 Somalia and, 78 South Africa and, 77 Spain and, 71–73, 72f species and nomenclature related to, 6–7, 6t, 303 specifications, 303–306, 304t structural characteristics, 4–6, 5f substitutions in, 14 surface area of, 302–303 synthesis conditions, 17–18

Subject Index tetrahedral and octahedral sheets of, 33, 35f textural analyses on, 302 trace elements and, 49, 302 Tunisia and, 76 varieties, 239 water in, 49 weathering environments and, 127 in western Iranian soils, 204–205 Palysepioles, 6–7 PAN. See Poly(acrylonitrile) PANI. See Polyaniline Passive organomineralization, 221, 222 of sepiolite hypothesis, 232–235, 234f PBS. See Poly(butylene succinate) PBT. See Poly(butylene terephthalate) PCL. See Poly(e-caprolactone) PE. See Polyethylene Pedogenic carbonate, 205, 207f Pedogenic palygorskite occurrences, Turkey, 185–187 Peri-marine Miocene, 79 Pesticide, 275, 363–364 PET. See Poly(ethylene terephthalate) Pet litter, 272 pH electrophoretic mobility vs., 330, 330f environmental influences and, 129 palygorskite suspension values of, 334, 334f sepiolite chemical ratios and, 132 Phanerozoic minerals, 220 Pharmaceutical uses of fibrous clays as active substances, 306–308 adsorbents and protectors, 308 antidiarrhoeals and antacids, 306–308 drug interactions, 312–313 absorption inhibition, 312 as excipients, 300, 308–312, 309t liquid and semi-solid dosage forms, 309t, 310–312 solid dosage forms, 308–310, 309t health risks and, 314–316 mineralogy and chemistry related to, 300–303 nomenclature, 303–306 overview about, 299–300, 301t requirements regarding, 299–300 specifications, 303–306, 304t Phosphorus chemistry and, 328, 329f magnesium chemistry and, 328, 329f Phyllosilicates, ideal 2:1, 4–6, 5f Physical trapping, 376

495

Subject Index Phytosanitary carriers, 290 PLA. See Poly(lactic acid) Planar defects, 28–29 Platy clay minerals, 332 PMMA. See Poly(methyl methacrylate) Point of zero charge (PZC), palygorskite soil effects and, 329–330 blocks and tunnels, 330 electrophoretic mobility vs. pH, 330, 330f isomorphous substitution, 329–330 Polat area, 180–181 Poly(acrylonitrile) (PAN), 402 Poly(butylene succinate) (PBS), 403 Poly(butylene terephthalate) (PBT), 400 Poly(e-caprolactone) (PCL), 403 Poly(ethylene terephthalate) (PET), 400–401 Poly(lactic acid) (PLA), 403–404 Poly(methyl methacrylate) (PMMA), 401 Polyacrylamide, 401 Polyamide-6 (PA-6), 398–399, 399f Polyaniline (PANI), 402 Polyethylene (PE), 400 Polypropylene (PP), 399–400 Polysome formation open questions regarding, 55 polysomatic clusters and, 54–55, 55f polysomatic continuous series and, 53–54, 53f structure and, 21–22 Polysome-width disorder microtexture and, 22–25, 23f TEM of, 22–24, 23f Polyurethane (PU), 405 Polyvinyl alcohol (PVA), 401–402 Polyvinylpyrrolidone (PVP), 402 Pores, 355 Porosity, 289 external, 353 pores and channels and, 355 spectrum, 352–355 structural, 353, 355 PP. See Polypropylene Processing flow sheet, 286, 287f Processing of Chinese clays, 256–259 absorbent grades, 257–259 gel grade, 256, 259 Production, world, 85–86 Protectors, 308 PU. See Polyurethane PVA. See Polyvinyl alcohol PVP. See Polyvinylpyrrolidone PZC. See Point of zero charge

Q Quaternary ammonium salt, 395–396

R Rafsanjan area, 205 Raite, 5–6 formation conditions, 16–17 as group member, 6–7, 6t rotation angle in, 20 single-crystal X-ray studies of, 8, 11t, 12f Raman spectroscopy, 461–463, 462f Rheological properties, 286 Rheology defined, 331 palygorskite-containing soil clay suspensions, 336–337 kaolinite-montmorillonite-illite and, 337 montmorillonite and, 336, 337f smectite-kaolinite and, 336–337 standard palygorskite suspensions, 331–336 Rietveld refinements, 7–8, 9t Roller press, 256 Rotation angle, 20 Rubber, 290 Russia, 97

S Safety issues, 296–297 Salafchegan Plain, 205 Salinity, 132–133, 132f, 134 Salt lakes, 21 Saponite, 69 Saponite, Amargosa, See Amargosa saponite Sargasso Sea clay distribution and, 115–116, 115f Eocene, 113–115, 114f Sarvestan intermontane basin, 210 SDBS. See Sodium dodecylbenzenesulfonate SDS. See Sodium dodecylsulfate Sealant, 275 Sea-MudÒ, 273 Seawater evaporation, 131–132 Second generation products, 290–292 Secondary carbonates formation, 386, 388t Sedimentary environments calcareous, 211–212, 212f CCS and, 376 continental, 159–162 LAs and, 163–166 magnesium and silica sources in, 159–160, 160t

496 Sedimentary environments (cont.) palygorskite formation in, 159–160, 159f, 160f, 160t sepiolite formation in, 161, 161f, 162f Self-cleaning glazes, 434–435 Semi-solid and liquid dosage forms cosmetic uses, 310 emulsifying agents and, 311–312 gels, 311 pharmaceutical uses, 309t, 310–312 Senegal deposits, 93–94 environmental influences in, 76–77 formation history, 93 Theis deposit, 93–94 Sepiolite. See also Amargosa sepiolite; Biogenic origin of sepiolite; Biomorphs, dolomite-sepiolite Miocene; Drysepiolite nanostructured hybrids; Grafted organic derivatives of sepiolite; Hydrothermal sepiolite occurrences; specific subject antacids and, 307, 309t applications overview, 281–282, 282f, 283t brand names, 306 chemistry of, 36–43, 37t, 38f, 39f, 41f, 302 closed basin clay distribution and, 69–71, 71f compositional ranges, 37, 37t, 50, 52f crystallography, 352, 354t effective production environment for, 69–71 essential features, 4–5 formation conditions, 16, 17t formation environments, 125–126 formation evidence, 131–135 aqueous CO2, 134 chemical ratios and pH, 132 salinity, 132–133, 132f, 134 seawater evaporation, 131–132 stability diagrams, 132–133, 132f, 133f H2O, OH2, and OH positions in, 15–16 ideal, 35f, 36, 37, 351 Kenya and, 75 MB and, 472 minerals associated with, 69, 70t Morocco and, 76 palygorskite compared to, 284 palygorskite compositional gap with, 41f, 49–51, 51f, 52f properties, 282–286, 283t pure composition of, 35–36 Rietveld refinements of, 7–8, 9t

Subject Index sedimentary environments and, 161, 161f, 162f Senegal and, 76–77 Somalia and, 78 South Africa and, 77 Spain and, 71–73, 72f species and nomenclature related to, 6–7, 6t, 303 specifications, 303–306, 304t structural characteristics, 4–6, 5f substitutions in, 14 surface area of, 302–303 synthesis conditions, 17–18 tetrahedral and octahedral sheets of, 33, 35f textural analyses on, 302 trace elements and, 302 Tunisia and, 76 water in, 43 weathering environments and, 127 Sepiolite support of nanoparticles, 423–440 acid treatment and, 428, 429f antibacterial applications, 431, 431t biomedical applications, 430 clusters and, 423 controlled hydrolysis and, 426–427, 427f extrusion process and, 427–428 functionalized materials from, 434–440 impregnation methods, 426 ion-exchange reaction and, 423–426 ISIA process and, 427–428 LDHs and, 433–434 light absorption maxima and, 431–432 magnetism and, 432, 433 superparamagnetic behavior, 397–398, 433 overview of, 423, 424t oxidation and, 430, 433 wet chemical route and, 428 Sepiolite-organic compound interactions, 410t, 411–416, 412 dry-sepiolite nanostructured hybrids, 412–414, 413f, 414f grafted organic derivatives by covalent bonding, 414–416 hybrid nanomaterials and, 410t, 411–416, 412, 413f, 414f Serinpınar-Acipayam basin, 182 Serpentinite-hosted occurrences, 184–185 Sesquioxide-free hydrous magnesium silicates, 69, 70t

Subject Index Silanes, 358t, 395–396 Silica, 159–160, 160t Single-crystal X-ray studies of intersilite, 8–12 of kalifersite, 12–13 of raite, 8, 11t, 12f structure revealed by, 8, 11t, 12f of tuperssuatsiaite, 8, 11t Sivas sector, 154f, 155 Sivrihisar, 178 Sixth generation products, 295–296 Smectite flocculation and, 339 magnesium chemistry and, 328 palygorskite transformation to, 22–25, 26f, 129 palygorskite-containing soil clay suspensions and, 336–337 precursor, 135–136 in southern Iran, 213t Soda lime glass powder, 435 Sodium dodecylbenzenesulfonate (SDBS), 358t Sodium dodecylsulfate (SDS), 358t, 361t Soils continental environments and, 127 evaporation retardation, 290 Iran central, 205–208 Iran north-eastern, 213 Iran southern, 208–213 Iran western, 204–205 Soils, palygorskite effects on disaggregation and colloid migration, 340–341, 341f flocculation, 338–340 magnesium chemistry and, 325–329 metal immobilization, 344–345, 345f metal sorption, 342–343, 344f overview, 325 palycretes and, 341–342 PZC and, 329–330 blocks and tunnels, 330 electrophoretic mobility vs. pH, 330, 330f isomorphous substitution, 329–330 research needs, 346 soil clay suspensions rheology, 336–337 kaolinite-montmorillonite-illite and, 337 montmorillonite and, 336, 337f smectite-kaolinite and, 336–337 standard suspensions rheology, 331–336

497 Solid dosage forms diluents, glidants and disintegrants and, 308–309 pharmaceutical uses, 308–310, 309t Solubility trapping, 376 Soluble ions, 382–385, 384t Somalia deposits, 96, 158 El Bur deposit, 158 environmental influences in, 78 Sorption, OCDs and, 28 South Africa environmental influences in, 77 Namaqualand area in, 77 South Georgia-North Florida applications, 87, 89 deposit, 87–89 Gulf Trough-Apalachicola embayment in, 87–88, 87f history, 87 mineral content of, 89, 89f mineralogy compared to Guashan, 246, 246t stratigraphic section, 87–88, 88f Spain, 137–154, 142f Batallones deposit, 148–150 Bercimuel deposit, 91f, 92–93, 152 Calatayud Tertiary lacustrine basin and, 73 Campo de Calatrava, 153–154 chemical parameters and, 72 deposits, 90–93, 145–150, 151–154 Duero basin, 151–152 environmental influences in, 71–73, 72f Madrid basin and, 72, 134, 140, 143f Mara, 72f, 73, 91f, 92, 153 other deposits, 151–154 other settings in, 73 overview about, 71–72 palygorskite and, 71–73, 72f production quantities, 85–86 Tagus basin, 137–150 Torrejoń deposit, 91f, 93, 152–153, 153f Vallecas-Vica´lvaro-Yunclillos deposits, 90–91, 91f, 145–148 Spark plasma sintering (SPS), 437 gold nanocomposite pellet using, 438, 438f optical transparency using, 438 Species overview, 6–7, 6t SPS. See Spark plasma sintering Stability, 69, 70t, 285 diagrams, 132–133, 132f, 133f Stacking errors, 28–29 microtexture and, 28–29

498 Stacking varieties, 13 Stevensite, 69, 70t Amargosa saponite/sepiolite and, 272 formation, 25 Morocco, 76 genetic relationships, 130 kerolite/stevensite deposits, 89, 91, 143f, 157, 165, 272 salinity, 132, 132f, 134, syngenetic formation, 161f Stratigraphy, 87–88, 88f, 215, 242f, 243–244 Structural blocks, 393–395, 394f Structure characteristics, 4–6, 5f discussion, 18–22 genetic and synthesis relations, 16–18 H2O, OH2, and OH positions, 15–16 introduction to, 3 parameters described, 18 polysome formation, 21–22 related topics, 4–18 Rietveld refinements revealing, 7–8, 9t salt lakes and, 21 single-crystal X-ray studies revealing, 8, 11t, 12f species and nomenclature related to, 6–7, 6t stacking varieties, 13 substitutions and, 14 tetrahedral-octahedral misfit and, 18–21, 19f vacancy content and, 21 Stucco, 275 Supercapacitors, 421–422 Supercritical compression, 376, 377f Superparamagnetic behavior, 397–398, 433 Surface area absorption characteristics and, 289 Amargosa sepiolite and, 275 BET-nitrogen, 380, 383t magnesium chemistry and, 326–327, 328 measurements, 352–355, 354t of palygorskite and sepiolite, 302–303 Surface-related properties, 351–356 Suspensions. See also Rheology capacity enhancement, 291 Swamp environments, 163 Symbology, Maya blue, 475–476 Synthesis MB, 467–470 palygorskite conditions of, 17–18 sepiolite, 131, 132 structure and, 16–18

Subject Index

T

Tagus basin, 137–150. See also VallecasVica´lvaro-Yunclillos geological setting, 137–145 Madrid basin within, 134, 140, 143f Tanzania, 157 TEM. See Transmission electron microscopy Tethyan region, 202–203 Tetrahedral sheets, 33, 35f Tetrahedral-octahedral misfit, 18–21, 19f Tetramethylamine (TMA), 361t TFT. See Thioflavin-T Theis applications, 94 deposit, 93–94 mineral content, 93–94, 94f acid treatment analysis of endothermic peaks, 356 heating process changes, 355–356 of surface related properties, 355–356 Thermal properties, of nanoclays, 405–408, 407t Thermal/acid treatment effects, 284, 285t Thermoplastic and thermosetting based nanocomposites, 396–405 Thermosetting polymer-clay nanocomposites, 396–405 Thioflavin-T (TFT), 358t Third generation products, 293 Titanium depletion, 252, 260t TMA. See Tetramethylamine Torrejoń deposit, 93, 152–153, 153f deposit chemical analysis, 141t Toxic waste, 292 Trade ancient, 474 indigo, 471 Transmission electron microscopy (TEM) challenges of, 22–24 of palygorskite to smectite transformation, 24–25, 26f polysome-width disorder and, 22–24, 23f Trapping potentials, 380–388 Triethoxy-3-(2-imidazoline-1-yl) propylsilane, 358t Trioctahedral smectite (TS), 16, 17t Triton X, 361t Triton x 100, 358t TS. See Trioctahedral smectite Tunisia, 76

499

Subject Index Tunnels blocks and, 330 structural blocks and, 393, 394–395, 394f Tuperssuatsiaite, 5–6 formation conditions, 16–17 as group member, 6–7, 6t rotation angle in, 20 single-crystal X-ray studies of, 8, 11t Turkey Adana basin, 185–186 applications from, 194 As¸ag˘ı Pınarbas¸ı, 179f, 181 C ¸ anakkale region, 186 C ¸ ayirazi magnesite deposit, 184–185 Denizli sector, 154f, 155 Denizli-Bozkurt area, 182 deposits, 94–97 economy and, 194 Elmadag area, 185 environmental influences in, 73–75, 74f Eskisehir deposit, 74–75, 74f, 95, 155–157 Eskisehir-Konya sector, 154, 154f Gallation Volcanic Belt in, 75 genesis and, 187–194, 190f geochemistry, 187, 189f geological setting, 154–155, 154f geology and mineralogy, 175–187 Hekimhan region of, 75 Hirsizdere magnesite deposit in, 74 hydrothermal sepiolite occurrences, 184–185 serpentinite-hosted, 184–185 volcanic-hosted, 184 Kangal sub-basin, 182–183, 183f Karapinar area, 179f, 181 Kepeztepe area, 180 Kizilda, 183–184, 183f Kizildere, 186 Konya basin, 155 lacustrine occurrences, 178–184 marine occurrences, 177–178 Eocene to Miocene, 177f, 178 Palaeocene, 177–178, 177f Upper Cretaceous, 177, 177f Mersin area, 186 Mihaliççik, 180 Neogene lacustrine basin in, 75 ophiolitic units in, 175, 176f overview, 175 pedogenic palygorskite occurrences, 185–187 Polatli area, 180–181 SE Eskisehir, 178, 179f

Serinpinar-Acipayam basin, 182 Sivas sector, 154f, 155 Sivrihisar, 178 summary of occurrences in, 194–197, 195t SW Eskisehir, 179, 179f three sectors of, 94 ¨ çkuyular, 178–179 U ultramafic bedrock in, 73–74 Uakgo¨l plain, 184 Yo¨ru¨kakçayir Village, 179–180

U

¨ çkuyular, 178–179 U Ukraine applications and, 96 deposits, 96 mineral content, 96 Ultramafic bedrock, 73–74 United Nations Framework Convention on Climate Change (UNFCCC), 375 United States Amargosa deposit in, 89–90 Double Lakes in, 79, 80f environmental influences in, 78–79 Llano Estacado in, 79 peri-marine Miocene and, 79 South Georgia-North Florida deposit, 87–89 Upper Cretaceous, 177, 177f Uakgo¨l plain, 184

V Vacancy content, 21 Vallecas-Vica´lvaro-Yunclillos applications, 91 composition of, 90, 92f chemical analysis, 141t deposits, 90–91, 91f, 145–148 flooding events and, 148, 149f formation, 90 lithological sections from, 145, 147f Neogene and, 146f reserves, 90 Van der Waals forces, 410t Ventzia basin, 158 Vertisols, 213 Vica´lvaro. See Vallecas-Vica´lvaro-Yunclillos Volcanic Belt, Gallation, 75 Volcanic setting, Guanshan, 244–246, 245f Volcanic-hosted occurrences, 184 Voltammetry, of Maya blue, 463–464, 464f, 465f

500

Subject Index

W Water, 15–16 bridges, 410t OCDs and, 27 treatment, 275, 292 Water-based compositions, 291 Weathering environments, 127 Weathering zones, basalt pebble, Chinese palygorskite clay resources, 251–260, 251f

X Xanthan gum, 420, 420f X-ray diffraction (XRD) patterns. See also Single-crystal X-ray studies of continental samples, 140f South Georgia-North Florida, 89, 89f supercritical CO2/palygorskite and sepiolite interaction study palygorskite patters in, 385, 387f, 388f

palygorskite peak fitting results in, 385, 387f, 387t sepiolite patterns, 382–385, 385f, 386f sepiolite peak fitting results in, 382–385, 386f, 386t

Y Yofortierite formation conditions, 16–17 as group member, 6–7, 6t OCDs, 26, 27f Yo¨ru¨kakçayir Village, 179–180 Yunclillos. See Vallecas-Vica´lvaro-Yunclillos Yunclillos-Cabanãs de la Sagra area, 147f, 148

Z Zagros orogenic area, 212 orogenic phase, 202–203 Zein, 418–419 Zhangshanji formation, 242f, 243

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