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DEVELOPMENTS IN SEDIMENTOLOGY 31

ELECTRON MICROGRAPHS OF CLAY MINERALS

FURTHER TITLES IN THIS SERIES VOLUMES I , 2, 3, 5, 8 and 9 are out of print 4 F.G. TICKELL T H E TECHNIQUES OF SEDIMENTARY MINERALOGY 6 L. VAN DER P L A S T H E IDENTIFICATION OF DETRITAL FELDSPARS I S. DZULYNSKI and E.K. W A L T O N SEDIMENTARY FEATURES OF FLYSCH A N D GREYWACKES 10 P.McL.D. DUFF, A . H A L L A M and E.K. W A L T O N CYCLIC SEDIMENTATION I 1 C.C. REEVES Jr. INTRODUCTION TO PALEOLIMNOLOGY 12 R.G.C. BATHURST CARBONATE SEDIMENTS A N D T H E I R DIAGENESIS 13 A.A. MANTEN SILURIAN REEFS OF G O T L A N D 14 K. W . GLENNIE DESERT SEDIMENTARY ENVIRONMENTS 15 C.E. W E A V E R and L.D. POLLARD T H E CHEMISTRY OF CLAY MINERALS 16 H.H. RIEKE 111 and G.V. CHILINGARIAN COMPACTION OF ARGILLACEOUS SEDIMENTS 17 M.D. PICARD and L.R. HIGH Jr. SEDIMENTARY STRUCTURES OF EPHEMERAL STREAMS 18 G.V. CHILINGARIAN and K.H. W O L F COMPACTION OF COARSE-GRAINED SEDIMENTS 19 W . S C H W A R Z A C H E R SEDIMENTATION MODELS A N D QUANTITATIVE STRATIGRAPHY 20 M.R. W A L T E R , Editor STROM ATOLITES 21 B. VELDE CLAYS AND CLAY MINERALS IN NATURAL A N D SYNTHETIC SYSTEMS 22 C.E. W E A V E R and K.C. BECK MIOCENE OF T H E SOUTHEASTERN UNITED STATES 23 B.C. HEEZEN, Editor INFLUENCE OF ABYSSAL CIRCULATION O N SEDIMENTARY ACCUMULATIONS IN SPACE A N D T I M E 24 R.E. GRIM and N . GUVEN BENTONITES 25A G. LARSEN and G.V. CHILINGARIAN, Edirors DIAGENESIS IN SEDIMENTS A N D SEDIMENTARY ROCKS 26 T. SUDO and S. S H I M O D A , Editors CLAYS AND CLAY MINERALS OF JAPAN 21 M.M. M O R T L A N D and V.C. F A R M E R INTERNATIONAL CLAY CONFERENCE 1978 28 A . NISSENBAUM, Editor HYPERSALINE BRINES A N D EVAPORITIC ENVIRONMENTS 29 P. T U R N E R CONTINENTAL R E D BEDS 30 J.R.L. ALLEN SEDIMENTARY STRUCTURES A N D T H E I R PHYSICAL BASIS

DEVELOPMENTS IN SEDIMENTOLOGY 31

ELECTRON MICROGRAPHS OF CLAY MINERALS TOSHIO SUDO Emeritus Professor, Tokyo University of Education, Tokyo 1.53, Japan

SUSUMU SHIMODA Assistant Projessor, Institute of' Geoscience, University of' Tsukuba, Ibaragi Prej: 300 -31, Japan

HARUO YOTSUMOTO Assistant Director, ScientiJic Instrument Project JEOL Ltd., Tokyo 196, Japan

SABURO AITA Assistant Manager, JEOL Ltd., Tokyo 196, Japan

1981

KODANSHA LTD. Tokyo

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam-Oxford-New York

Copyright @ 1981 by Kodansha Ltd. All rights reserved No part of this book may be reproduced in any form, by photostat, microfilm, retrieval system, or any other means, without the written permission of Kodansha Ltd. (except in the case of brief quotation for criticism or review)

ISBN 0-44-99751-2 0-44-41238-7

(Vol. 31) (Series)

Library of Congress Cataloging i n Publicalion D a l a

Main entry under t i t l e : Electron micrographs of c l a y minerals. (Developments i n sedimentology ; 31) Bibliography: p. Includes index. 1. C l a y minerals. 2. Electron microscopy. I. sUd6, Toshio, 19U11. Series. 1980 549l.6 80-24451 QE389.625.Ek3 ISBN 0-444 -99751-2 (Elsevie r )

Copublished by KODANSHA LTD. 12-21 Otowa 2-chome, Bunkyo-ku, Tokyo 112 and

ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat, P.O. Box 21 1, lo00 AE Amsterdam, The Netherlands ELSEVIER NORTH-HOLLAND, INC. 52 Vanderbilt Avenue, New York, N.Y. 10017 Printed in Japan

Contributors

Numbers in parentheses indicate the Chapter Toshio SUDO, Emeritus Professor of Tokyo University of Education 20-7, Miyasaka 3-chome, Setagaya-ku, Tokyo 156, Japan. (1) Susumu SHIMODA, Assistant Professor of Geoscence, Tsukuba University, Sakura, Niiharigun, Ibaragi Prefecture, 300-3 1, Japan. (3) Haruo YOTSUMOTO, Assistant Director, Scientific Instrument Division, JEOL LTD. 1418 Nakagami, Akishima Tokyo 196, Japan. (2) Saburo AITA, Assistant Manager, Application Lab., 1st Technical Dept., Scientific Instrument Division, JEOL LTD. 1418 Nakagami, Akishima, Tokyo 196, Japan. (2)

V

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Preface

Clays and clay minerals show variability, diversity and complexity in their structural, chemical and physical properties. A variety of instruments and experimental techniques has been employed in clay-mineral research. A wider scope of research has been opened by investigators in various branches of science and technology due to their increasing interest in such materials. Electron microscopy is one of the important methods for studying clays and clay minerals. Although a previous contribution of electron microscopy was to clarify the morphology of fine clay particles, the outstanding recent development of improved instruments and techniques has provided more detailed information on the crystal-morphologicaland structural properties. About ten years ago, we discussed the possibility of making a picture-atlas of clays and clay minerals, based on selected samples as well as instruments and techniques in Japan. However, with the continuous improvement of instruments and techniques which has followed ever since, we have been obliged to spend much time in realizing our hope. The present volume provides a picture-atlas of electron micrographs of clays and clay minerals, and is also intended to serve as a lively textbook and technical reference book having multiple functions. (1) Chapter 1, as the introduction to the book, gives a brief statement on what clays and clay minerals are, and also outlines their crystal structures giving special reference to the variability in order-disorder configurations which has been revealed so far by X-ray and electronoptical analyses. As in the past, detailed research on clay-mineral structures will emerge increasingly as one of the significant objects of progressive electron microscopy, since this technique permits studies on finely divided individual crystallites in contrast to X-ray analysis which indicates diffraction effects of a macroscopic single crystal or powder speciemens as a whole. It is hoped that Chapter 1 will provide a useful introduction for students of advanced mineralogy and crystallography who are interested in electron microscopy. (2) Chapter 2 gives detailed descriptions particularly concerning sample preparation and experimental techniques which are used practically for taking highly resolved diffraction images. The topics include lattice images, structure images, preparation of micro-grids, ultrathin sectioning, and selected-area diffraction. It is hoped that this Chapter will provide a useful technical guide to the electron microscopy of clay minerals. (3) Chapter 3 contains the electron micrographs and diffraction pictures, together with an explanatory text describing the localities, origins and modes of occurrence of the samples used. Most pictures were taken with the instrumentation, sample preparation and experimental techniques described in Chapter 2. Most of the samples were selected from the collections at our laboratories. It is hoped that Chapter 3 will serve as a useful data source for all those interested in electron microscopy and in clay mineralogy and geology. (4) Although the present volume was prepared essentially for clay studies, some space has vii

viii PREFACE

also been allotted to interesting pictures of minerals related to clays, e.g. zeolites, and mineral particles in dusts. In certain cases, the samples of clay minerals in our collections were inadequate or unsatisfactory. Various colleagues therefore kindly provided photographs of their valuable samples, often with great enthusiasm, so helping to make this publication possible. In this connection, our principal thanks are due to: Professor H. Hayashi, Akita University Professor S. Honda, Akita University Professor S. Kakitani, Hiroshima University Dr. N. Kohyama (National Institute of Industrial Health) Dr. K. Tazaki (Okayama University) Dr. T. Matsuda, Okayama University Dr. T. Nishiyama, Toyo University Professor N. Ueda, Kyoto University Professor K. Yada, Tohoku University. Particular thanks are due to Messrs. E. Tashiro, K. Ohbori, I. Ohta and A. J. Smith of Kodansha for their editorial and linguistic assistance in the preparation of the final manuscript. T. SuDo S. SHIMODA H. YOTSUMOTO S. AITA

Contents

List of Contributors Preface vii

v

Chapter 1 Outline of the Crystal Morphology and Structure of Clay Minerals 1 A. Definition and General Properties of Clays and Clay Minerals B. Structure of Crystalline Clay Minerals: completely regular models I. Layer-structures 5 11. Layer-ribbon-structures 17 C. Structure of Crystalline Clay Minerals : random fashions 18 I. Layer-structures 18 11. Layer-ribbon-structures 25 D. Interstratification and Intergrowth 26 26 I. Interstratified (or mixed-layer) structures 11. Sepiolite-palygorskite intergrowths 29 E. Non-Crystalline and Poorly Crystalline Clay Minerals 29 F. Electron-Optical Investigations 30 References 34

Chapter 2

1

2

Photographic and Specimen Preparation Techniques in Electron Microscopy

A. General Considerations 38 B. Photographic Techniques 38 I. Selection of magnification 38 11. Dispersed powders 39 111. Replica specimens 40 IV. Lattice images 40 V. Multi-beam lattice images (structure images) C. Electron Diffraction 48 I. Introduction 48 11. The scan-micro method 53 111. The Geiss method 53 IV. Field limiting method 56 56 V. Angular resolution (aperture angle) D. Specimen Preparation 56 56 I. Supporting the specimen 57 11. Hydrophilic treatment of carbon film 111. Microgrids 58 62 IV. Powder specimen dispersion method V. Replica techniques 62 VI. Ultrathin sectioning 63 E. Conclusion 67 References 68 I

\

ix

44

37

x

CONTENTS

Chapter 3

Electron Micrographs of t h e Principal Clays a n d Clay Minerals and O t h e r Related Mineral Species 69

Brief Guide to the Clays and Clay Minerals Appearing in the Photographs I. Toseki 70 11. Roseki 70 111. Kuroko 71 IV. Green tuff 72 V. Loam 73 73 VI. Note of the Mineral Names used in this Chapter B. Electron Microphraphs of Clays and Clay Minerals 74 I. Kaolinite-serpentine group-Kaolinite sub-group 74 11. Kaolinite-serpentine group-Serpentine sub-group 77 111. Pyrophyllite and talc 79 IV. Mica clay group 80 V. Chlorite group 82 VI. Vermiculite group 83 VII. Smectite group 84 VIII. Interstratified minerals 86 IX. Sepiolite and palygorskite 90 X. Zeolites 90 91 XI. Other clays and clay minerals References 94 A.

Electron Micrographs Index 201

96

69

Chapter 1

Outline of the Crystal Morphology and Structure of Clay Minerals

A. Definition and general properties of clays and clay minerals B. Structure of crystalline clay minerals: completely regular models I. Layer-structures 1. Layer types 2. Groups 2.1. General considerations 2.2. Interlayer materials 2.3. Vermiculite 2.4. Smectite 2.5. Halloysite (10 A) 2.6. Thickness (height) of the unit structure 3. Sub-groups 4. Polytypes 4.1. Mica 4.2. Chlorite 4.3. Minerals of the 1: 1 layer-type 11. Layer-ribbon-structures C. Structure of crystalline clay minerals: random fashions I. Layer-structures 1. Curvature of the layers 2. Stacking disorder 3. Distortion within individual sheets or layers 4. Order-disorder relation in isomorphous substitution 5. Distribution of vacant sites 6. Diffuse scattering by X-rays and electrons 11. Layer-ribbon-structures D. Interstratification and intergrowth I. Interstratified (or mixed-layer) structures 11. Sepiolite-palygorskite intergrowths E. Noncrystalline and poorly crystalline clay minerals F. Electron-optical investigations References

A. Definition and General Properties of Clays and Clay Minerals The definition of clays is diverse. Variations exist among different fields even though fundamentally there is much in common among the visualized materials. The definitions so far used may be grouped into three kinds as follows. (I) Clays have been defined on the basis of an assembly of certain specific characters such as plasticity, small particle size, hardening on firing, and chemical constitution (i.e. as consisting 1

2

CRYSTAL MORPHOLOGY A N D STRUCTURE OF CLAY MINERALS

largely of silica, alumina and water). This definition is applied in a broad sense. Detailed discussions have been given by Mackenzie (1963). (2) Regarding particle size as of primary importance, the term “clay” has been used for material of specific particle size (the clay fraction), the particles of which have less than a specific equivalent spherical diameter (e.s.d). Many inconsistencies exist regarding the upper limit of e.s.d for the clay fraction, both among different fields and among different workers in the same field. The range is 1-10 microns, although 2 microns seems to be commonly used in the field of clay or soil mineralogy. (3) The definition of clays has also been given in terms of the relative proportion of the clay fraction in rocks or soils to the fractions of sand and silt. However, inconsistencies still exist in different fields concerning the clay-area designated on the triangular diagram with the component proportions of sand and silt. Based on our knowledge of clays to date, the following definition can be given. Clay is a material having the ability to normally demonstrate marked plasticity when wet, and, in general, the properties such as adsorption, hydration, solvation, ion-exchange and hardening when dried and on firing. Clay consists principally of fine-grained inorganic materials involving hydrous phyllossilicates usually with principal chemical components such as SO,, Alz03, Fez03,FeO, MnO,MgO, CaO, K,O, Na,O and H,O, and/or non-crystalline materials largely composed of chemical components such as SO,, A1,0,, Fez03, FeO, MnO, and HzO as usual. These materials are termed clay minerals. Indeed, based on the properties of their fine-grained particles, they represent source materials which may contribute to the properties of clays mentioned above. A classification of clay minerals is given in Table 1.1. The clay minerals are divided into crystalline and non-crystallineminerals, although the degree of “crystallinity” is diverse as seen from the need to use such terms as “disordered crystals” or “poorly crystalline minerals.” The clay minerals on the crystalline side are involved in phyllosilicates having layer-structures or layer-ribbonstructures. The basic configuration of the layer-structure can be described in terms of planes, sheets, layers, interlayers, and the unit-structure (recommendations of AIPEA Nomenclature Committee). A single plane of atoms or ions (or water molecules (H,O)) is the minimum unit. A sheet is an articulated combination of planes, and a layer is an articulated combination of sheets. Layers may be separated from one another by an interlayer. The total layer-plus-interlayerassembly is called the unit-structure. The thickness of the unit-structure can be known from the basal spacing, d(001), calculated from a series of basal reflections indexed as (OOl), (002). . . on X-ray powder patterns (cf. 2.6). Layer-structures can be divided into two layer-types, 2: 1 and 1 : 1, based on the population of the sheets within the layer. Chlorite is involved in the 2: 1 layer-type. Each type can be divided into groups based on the magnitudes of the interlayer-charges. The group is divided into two sub-groups, trioctahedral and dioctahedral, based on the population of atoms in the octahedral sheet within the layer. Further subdivision into species can be made based on either chemical properties (i.e. composition) or structural properties (i.e. stacking fashion of the layers- polytypes).

B. Structure of Crystalline Clay Minerals : completely regular models Recent advances in crystal structure analysis have indicated that disordered features are commonly revealed to various extents in crystallineclay-mineral structures, so that completely ordered structures really represent no more than ideal images. However, in order to understand materialized structures fully, it is helpful to examine clay-mineral structures from the viewpoint of completely ordered models.

TABLE 1.1 Classification scheme for clay minerals (I) Crystalline minerals (A) Layer-structure

Layertype

Sub-group Tri. : Trioctahedral Di. : Dioctahedral

Group (Charge per formula unit) Pyrophyllite-talc ( x

-

0)

Species

Tri.

Talc Mg3Si4010(OH)z Minnesotaite (Fe, Mg)3Si4010(OH)z

Di.

Pyrophyllite A1zSi4010(OH)z

Di .

Montmorillonite

Eo+.33(A~I.~7Mgo.~3)Si4010(OH)z .nHzO -nHzO Beidellite E~.33A1z(Si3.~7AIo.33)Olo(OH)z Nontronite E8.33Fe:+(Si3.s7Alo.33)Olo(OH)z .nHzO Volkonskoite E0+.33(AI,Fe3 ,Cr)z(Si, Al)4O,o(OH)z.nHzO +

2: 1

Vermiculite (0.6

< x < 0.9)

Mica ( x

-

Chlorite ( x

1)

-

~

variable)

~

~

Tri.

Vermiculite Eo+.86(Mg,Fez+,Fe3+,AI)&i, AI)40~o(OH)z.nH20

Di.

Vermiculite Eo+.~Alz(Si, ~)4010(oH)z 'nHz0

Tri.

Phlogopite KMg3(Si3Al)Olo(OWZ Biotite K(Mg, Fez+,Fe3+,Mn)3(Si3Al)Olo(OH)z Lepidolite K(A1, Li)&3i, A1)4010(OH)z Zinnwaldite K(AI, Li, Fez+,Fe3+)&3i,Al)4010(OH)2

Di.

Muscovite KA12(Si3Al)Olo(OH)z Paragonite NaAlz(Si3AI)0 o(OH)z Al-mica clay mineral K,(AI, Mg),(Si, A1)401~(OH)~.nH& ( x < 1) Fe-mica clay minerals Celadonite KMgFe3+Si4010(OH)z Glauconite K(R: ~33RZoX7)(Si3.67Al~.33)ol~(o~~

~~~~~~~~

~

Tri.

Mg-chlorite clinochlore (Mg5Al)(Si,Al)Ol,(OH))8 Fe-chlorite chamosite (Fe: AI)(Si,Al)O o(OH)8 Nichlorite nimite (Ni5AI)(Si3Al)OIo(OH)8 Mn-chlorite pennantite (Mn: +AI)(Si,Al)O ,(OH)8 Oxidized chlorite (Mg6_,-,Fe~'-,Fe:'AIx) ~Si4-xAlx~Olo+ ,(OW8- I +

(

(continued)

TABLE 1.1-(continued) ~

Layertype

Brittle mica ( x

-

2)

Di.-Tri.

Al-chlorite Sudoite (Mg3- $1, + x ~ ~ ~ ~ 4 - x A ~ x ) O l o ~ O H ~ , Cookeite ( A ~ , L I ) ( S ~ ~ A ~ ) ~ ~ ~ ( ~ H ) ~

Di.

Al-chlorite Donbassite Al, + x,3(Si4-.A1,)0

,(OH),

Tri .

Clintonite Ca(Mg,Al),(Si, A1)4010(OH)2

Di.

Margarite CaA12(Si2A12)010(OH)2

Surite

Di.

Kaolinite-serpentine (x 0)

Tri.*

Mg-serpentine Antigorite, Lizardite, Chrysotile Mg3Si20s(OH), Mg-Al-serpent ine Amesite (Mg2A1)(Si AI)O,(OH), Fe-serpentine Cronstedite (Fe: Fe3+)(Si Fe3+)O,(OH), Berthierine (Fez+, Mg),.3(Fe3+, A1)0.7(Si1.4Alo.6)O~(OH)4 Greenalite (Fe:.~,Fe~.~)Si20s(OH>4 Ni-serpentine Garnierite (?)

1: 1

+

Di.**

Kaolinite, Dickite, Nacrite Al2Si2OS(OH),

(B) Layer-ribbon-st ruct ure

(1I) Noncrystalline and poorly crystalline minerals Allophane Hisingerite Penwithite Imogolite

1

N

2 Si02.A1203.nH20

x SiOz .A1203. n H 2 0 x

1

S i 0 2 . M n0.nH z 0 1.5 SiOZ.Al2O3.2 3 H 2 0 ((OH)~A~ZO~S~OH)

N

N

Notes: (1) The table is intended primarily to give a classification scheme for clay minerals, but it also involves “macrocrystalline analogues of clay minerals.” The compilation of this Table follows the Recommendations of AIPEA Nomenclature Committee in regard to (a) the classification scheme of layer-structures, (b) specific phyllosilicate names such as chlorites, celadonite and glauconite (cf. Bailey, 1980). (2) For common minerals, the formulea given are general approximate ones, and in some groups they correspond to end-members. E+ indicates exchangeable cations represented by monovalent cations. For rare minerals empirical formulae are given (e.g. Brindley, 1961). (3) For minerals with layer-structures, formula units are shown. (4) For celadonite, tetrahedral A1 (or Fe3+): 0-0.2 atoms per formula unit. d(060) < 1.510A. For glauconite, Fe3+>Al and Mg>Fe2+ (unless altered). Tetrahedral Al (or Fe3+): usually greater than 0.2 atoms per formula unit. Trivalent octahedral cation (R3+): correspondingly greater than 1.2 atoms. d(060) > 1.510 A. ( 5 ) The formulae of sepiolite and palygorskite are shown on a half-unite11 basis: that of sepiolite is for the Brauner-Preisinger model, and that of palygorskite for the Bradley model. (6) The formula of imogolite is that derived by Cradwick et al. (1972) from the crystal structure model. *Serpentine sub-group or “serpentine minerals”. **Kaolinite sub-group or “Kaolin minerals”

Structure of Crystalline Clay Minerals: completely regular models

5

I. LAYER-STRUCTURES

1. Layer types In accordance with the ion-ratio of Si: 0, silicon favours a co-ordination number of 4, so that it is surrounded by 4 oxygens. The co-ordination polyhedron is a regular tetrahedron. Silicon is situated at the centre of the tetrahedron (the tetrahedral site) as a tetrahedral cation. A continuous tetrahedral sheet is formed when the tetrahedra are linked by sharing oxygens at 3 corners of each (the basal oxygens), with the oxygen at the fourth corner (the apical oxygen) projecting to the same side (Fig. 1.1). It is possible for ions such as aluminium, iron, and beryllium to be located at the tetrahedral site substituting for silicon. Both the basal oxygen plane and the apical oxygen plane form regular hexagonal nets referred to the a-axis involving 3 equivalent axes (the a,, a2, and a3 axes), and the b-axis (with 3 equivalent axes, b,, b2, and b,) perpendicular to the a-axis. (OH)-ions are located at the centre of each hexagon of the apical oxygen net. The unit-length of the a-axis (a,) is related to that of the b-axis (b,) according to the equation: b, = a,,/% Two tetrahedral sheets may be joined in parallel with their apical oxygen planes contacting each other in close-packed junction. A continuous octahedral sheet is then visualized between these tetrahedral sheets. This is shown schematically in Fig. 1.2(A) and the normal projection onto the basal oxygen plane of the tetrahedral sheets is illustrated in Fig. 1.2(B). Each of the octahedral sites is surrounded by 6 anions. Four out of the 6 anions are apical oxygens, and the remaining 2 are (OH)-ions. The octahedral sites are occupied by cations such as aluminium, magnesium, manganese, and iron which prefer to take a co-ordination number of 6. An assembly of two tetrahedral sheets and one octahedral sheet is called the 2 : 1 layer, and clay minerals having the 2: 1 layer are said to belong to the 2: 1 layer-type. A single hexagonal net of (OH)-ions may be joined to the apical oxygen net of a tetrahedral sheet in close-packed junction. A continuous octahedral sheet is then formed between the (OH)plane and the tetrahedral sheet (Fig. 1.3(A) (B)). Each of the octahedral sites is surrounded by 6 anions. Four out of the 6 are (OH)-ions and the remaining 2 are apical oxygens. An assembly of one tetrahedral sheet and one octahedral sheet is called the 1 : 1 layer, and clay minerals having the 1: 1 layer are said to belong to the 1 : 1 layer-type. The projected figures shown in Figs. 2(B) and 3(B) are referred to a rectangular cell having its edges a, and b, in the relation b, = a,,& and in these figures, the stagger between two tetrahedral sheets, or between the (OH)-plane and the tetrahedral sheet, is shown parallel to the a,-axis.

+a,

Fig. 1.1

Tetrahedral sheet in the layer-structure. Oxygens and tetrahedral cations are omitted. the center of the apical oxygen hexagonal ring.

(OH) located at

6

CRYSTAL MORPHOLOGY AND STRUCTURE OF CLAY MINERALS t

4

I

(B) Fig. 1.2 2: 1 layer. (A) Schematic view. T: Tetrahedral sheet. 0: Octahedral sheet. .(OH). (B) Normal projection onto the ab plane. Octahedral cation. 0 (OH) located at the centre ofthe apical oxygen hexagonal ring. Oxygens and tetrahedral cations are omitted. The rectangular cell (ABCD) has [its cell edges a,, and bo in the relation bo = uoz/J.The hexagon (abcdef) is the apical oxygen net ofthe lower tetrahedral sheet.

b

(A)

(B)

Fig. 1.3 1 : 1 layer. (A) Schematicveiw. For the notations of “T” and “0”,see Fig. 1. 2. 0 (OH). (B) Normal projection onto the ub plane. Octahedral cation. 0 (OH) located at the centre of the apical oxygen hexagonal ring. 0 (OH) of the (OH)-plane. 0Basal oxygens in the adjacent 1 : 1 layer above. For the notation of the rectangular cell (ABCD), and hexagon (abcdef), see Fig. 1.2.

A mirror plane can be observed. However, the orientations shown in these figures are in fact arbitrary, since the stagger could be equally well directed along the 3 equivalent axes (ul, u2, and ug).In any case, these figures may be termed normal projections onto the ub plane, or the (001)plane, or the basal plane. The principal clay minerals belonging to the 2 : 1 layer-type include pyrophyllite, talc, smectite, vermiculite and chlorite. The 1 : 1 layer-type includes minerals such as kaolinite and serpentine. Both the 2: 1 and 1 : 1 layers may be simply called silicate layers.

2. Groups 2.1. General considerations The magnitude of the charge density on the layer surface (layer-charge) can be taken as a major criterion in the subdivision of layer-structures into several groups. It is expressed, for example, as the charge per formula unit (Table 1.1).


7

The layer-charge is null in clay minerals such as kaolinite antigorite, pyrophyllite and talc, all of which are simple in chemical composition without isomorphous substitutions. In other clay minerals with layer-structures, the layer-charge is demonstrated as a net negative electric charge derived from the layer resulting from isomorphous substitutions when cations with higher valencies are replaced by those with lower valencies, such as when Si4+ is replaced by A13+, or A13+ is replaced by Mg2+. It is possible that the excess negative charge is satisfied by other substitutions within individual layers, but is commonly satisfied by the positive charges of interlayer materials such as cations (in micas and brittle micas), exchangeable cations (in vermiculite and smectite), and hydroxide sheets (in chlorite). In micas, two layers are stacked in such a way to produce interlayer sites having a co-ordination number of 12 for each (in the completely regular form), where larger cations such as sodium or potassium are preferably accommodated. 2.2. Interlayer materials (a) Chemical bonds prevailing in the interlayer region Generally speaking, the nature of the chemical bond between the layer and interlayer material is diverse compared to that prevailing within individual layers. In the interlayer, besides the ionic bond, more complicated chemical bonds are apparent. These include (a) the oxygen-oxygen interaction in talc and pyrophyllite, (b) oxygen-hydroxyl interaction (hydrogen bonding) in kaolinite, antigorite and chlorite, (c) water molecule-oxygen-hydroxyl interaction in halloysite (10 A), and (d) exchangeable cation-water molecule-oxygen interaction in smectite and vermiculite. (b) Variability of interlayer materials depending on physical and chemical pre-treatments In general, the interlayer materials can be modified by physical and chemical treatments more easily than the layer materials themselves. Such modifications include (a) the dehydration of the interlayer water of smectite, vermiculite and halloysite (10 A) on heating, (b) the dehydroxylation of the interlayer hydroxide sheet of chlorite on heating, (c) the adsorption and ion-exchange of smectite, vermiculite and halloysite (10 A) by pre-treatments with inorganic salt solutions or organic compounds, and (d) the leaching of the interlayer cations of mica by pre-treatment with acids. The water molecules of smectite and vermiculite begin to dehydrate at around 100°C and those of halloysite (10 A) at about 50°C. Rehydration occurs in smectite and vermiculite after heating in certain ranges and successively wetting. Particularly in smectite, the interlayer water content shows a sensitive variation in response to relative room humidity. Many kinds of organic molecules can also be adsorbed in the interlayer regions of smectite, vermiculite and halloysite (10 A). It should be noted that the processes of hydration, dehydration and adsorption do not result in abrupt disintegration of the structure as a whole, but result in the initiation of stepwise variations in the thickness of the interlayer region, i. e. the thickness (height) of the unit-structure. This interesting phenomenon is related to the fact that absorption of liquid water molecules or adsorption of organic molecules may take place in the interlayer region more or less abruptly yielding planes or sheets with a definite conguration of these molecules and lying parallel to the basal oxygen plane. 2.3. Vermiculite The vermiculite commonly found in nature is of the Mg-type having exchangeable magnesium ions. Macropscoic flakes are not uncommon. As proved by structure analysis (Walker, 1956), water molecules are arranged regularly on a plane (two-dimensional network structure) tied on the basal plane. In the fully hydrated state showing a unit-structure height of ca. 14.8 A, a double plane of water molecules in which the molecules occupy all the available sites, are interleaved between the interlayer region, and magnesium ions are located midway between these planes. Release of water molecules upon heating results in contractions from the 14.8 A in response to reduction of the double plane to a single plane (ca. 11.6 A), and further to a dehydrated phase

8

CRYSTAL MORPHOLOGY AND STRUCTURE OF CLAY MINERALS

(ca. 9 A).During this dehydration process, complicated gradual contractions occur in response to the partial release of water molecules from individual sheets or planes, resulting in distortion of the water molecule-cation configuration. In any case, these dehydration states can be confirmed by obtaining stepwise values which contract from the 14.8 A. It has been found that the kind of exchangeable cations affects (a) the dehydration behaviour and (b) the basal spacings at different relative humidities or in a state of saturation with water. Expansion of the basal spacing in the state of saturation with water is limited in vermiculite as compared to smectite. Mg-vermiculite with a spacing of about 14.4A is found at room humidity and may expand to about 14.8 A when saturated with water. Expansion of the basal spacing by the uptake of organic molecules such as glycerol or ethylene glycol, is affected by the kind of exchangeable cations. It has been found that the 14.4A-Mg-vermiculite does not expand beyond about 14.5 A. Compared to smectite, vermiculite with higher layer-charges tends to fix potassium or ammonium ions in the interlayer resulting in contraction of the basal spacing to about 10 A. This is probably due to the fact that the structure of vermiculite is higher in three-dimensional regularity as compared to smectite, so that these ions are fixed in the hexagonal hole of the basal oxygen net as in the case of micas. 2.4. Smectite It is considered probable that the configuration of water molecules in the interlayer region of smectite is similar to that of vermiculite. In general smectite exhibits a strong hydration ability compared to that of other minerals with layer-structures. The water content in smectite tends to increase with increasing relative humidities and further so in a state of saturation with water. This increase results in increase of volume (swelling) in water, and also in an expansion of the basal spacing. Stepwise expansion of the basal spacing is particularly demonstrated in the initial hydration state. It is considered that the hydration of smectite tends to take place through the formation of successivelystacking planes of water molecules, but additional water tends to adopt the nature of liquid water. The kind of exchangeable cations may affect (a) the extent of development of non-liquid water planes, (b) the degree of expansion of the basal spacing, and (c) the dehydration behaviour. The naturally occurring montmorillonites usually have mixed-exchangeable cations such as K, Na, and Ca. They tend to show a basal spacing of about 15.4 8, under moderate humidities: this corresponds to a double plane of water molecules per interlayer. Montmorillonites having exchangeable alkali cations tend to show a basal spacing of about 12.4 A under moderate humidities: this corresponds to a single plane of water molecules per interlayer. In a state of saturation with water, Mg-montmorillonite normally expands to about 18 A or more, while the expansion of Ca-montmorillonite tends to be limited up to about 18 A. Na-montmorillonite tends to swell markedly until the individual layers dossociate completely as the water content is increased (Norrish, 1954). In any case, the degree of swelling and the expansion of the basal reflection in smectite are larger than that of vermiculite. Uptake of ethylene glycol or glycerol results in an expansion to about 17.0 A, which is clearly large compared to Mg-vermiculite. It is not easy to explain the formation of non-liquid water planes. This may result from the composite effect of (a) the character of the exchangeable cations as regards their arrangement and hydration ability, and (b) the interaction between the water molecules and basal oxygens which will act as a “template.” The formation of non-liquid water planes may be facilitated in smectite because of the fine particle size, disordered structures, and the interlayer region involving weak bonds expressed by the small magnitude of the layer-charges. 2.5. HaIIoysite (10 A) Halloysite (10 A) has been regarded as essentially free from exchangeable interlayer cations, but


9

it has a single plane of water molecules per unit-structure. Dehydration is initiated at about 50°C, resulting in a transformation from halloysite (10 A) to halloysite (7 A). It has been reported that the interlayer water lost at room temperature is not ordinarily regained (Grim, 1953). Halloysite (10 A) expands to about 1 1.O A through the uptake of single plane of highly polar organic molecules. Minerals such as smectite, vermiculite, and halloysite (10 A) have been termed expandable minerals. It should be noted, however, that kaolinite which has been regarded as non-expandable, can be expanded by pre-treatment with certain kinds of organic materials such as potassium acetate (Wada, 1961), urea (Weiss, 1961), and hydrazine (Weiss, et al., 1963). The interlayer materials, especially of the common expandable minerals, show a variability against behavioural tests. However, limited behavioural tests may in some cases fail to differentiate between different groups, particularly between smectite and vermiculite. The results of careful tests can nevertheless give a valid differentiation between different groups of such minerals. 2.6. Thickness (height) of the unit-structure In Fig. 1.4, the unit-structures of the principal clay mineral groups are shown schematically. The thickness of the unit-structures is much more variable as compared to the parameters a, and bo and also to the thickness of the silicate layers. In materialized sense, the parameters a, and bo of all these groups are around 5 A and 9 A respectively forming pseudo-hexagonal cells for which bo is equal or nearly equal to a,J3. But the parameters are closely related to lattice distortions and chemical composition, then precisely determined values are useful to test the crystallo-chemical properties related to isomorphous substitution of clay minerals of each group. The thickness of the silicate layers is around 6.6 A in the 2: 1 layer and around 4.5 A in the 1 : 1 layer The thickness of the unit-structures is useful to differentiate the principal clay mineral groups; the thickness of each expandable mineral in this figure, is given by a common value obtained under room temperature and moderate relative humidity. The unit-cell may be defined as as parallelopiped bordered with the priodicities of three crystallographic axes, i.e. a,, bo, and c,. The thickness of the unit-cell is not necessarily the same as that of the unit-structure. One-layer structure is described with the unit-cell having the same thickness as the unit-structure. Two-, three- . . . layer-structures (multiple layer-structures) are described by the unit-cells which are

I

2 : 1-layer-type

I

c l : l-layer-typeA

Fig. 1.4 Diagrammatic representation of the layer-structures, mainly to compare the relative thickness of the unit-structures. Octahedral cations. o Exchangeable cations. w: Planes of water molecules. (OH) located at the centre of the apical oxygen hexagonal net, or in the interlayer hydroxide sheet of chlorite, or in the (OH)-plane of 1 : 1 layer-type minerals. K: Potassium ion. (a) Pyrophyllite-talc. (b) Mica. (c) Smectite. (d) Vermiculite. (e) Chlorite. (f) Kaolinite-serpentine (g) Halloysite (10 A).

10


.

twice, thrice, . . as thick as those of the unit-structures. In regular structures, the stacking fashion of layers may define the periodicity of c-axis (co) and the direction of the axis as indicated by u and fl angles. Examples are : one-layer structure of kaolinite and serpentine, two-layer structures of dickite and vermiculite, and six-layer serpentine. The unit-cell data are illustrated as follows : co(4 Y bO(4 U P a,@) 7.14 91.8" 104.5" 90" (Brindley and 5.14 8.95 Kaolinite -105" Robinson, 1946) 91"40' 1W40' 90" (Zvyagin, 1960) 7.25 5.13 8.89 96"48' 14.42 90" 5.15 (Newnham and 8.95 Dickite Brindley, 1956) 90" lOO"20' 90" (Zvyagin, 1967) 8.89 14.6 5.14 Nacrite 90" 97"07' 90" (Shirozu and 28.89 9.225 5.349 Vermiculite Bailey, 1966) 90" (Brindley, 1961) 7.31 9.20 5.31 One-layer ortho-serpentine (lizardite) 90" (Brindley, 1961) 43.6 9.22 5.32 Six-layer ortho-serpentine Basal reflections on X-ray powder patterns can be tentatively indexed as (OOl), (002) . . . , and a value for d(001) termed commonly a basal spacing can be obtained. The indices, however, are not necessarily true except one-layer structure. In a multiple (n)-layer structure they are larger by a factor n. Chemical formulas can be given on the basis of the unit-cell, or unit-structure (i.e. the cell bordered with the priodicities of a,, b, and the unit-structure thickness (height), or the half of the cell based on the unit-structure. The unit-structure contains one or more chemical formula units (Z). 3. Sub-groups Clay minerals with layer-structures can be divided into two sub-groups on the basis of the octahedral population. The cell bordered with a,, b,, and unit-structure height contains 4 tetrahedral sites for each tetrahedral sheet, and 6 octahedral sites for each octahedral sheet. The formulae based on the cell can be divided into two types as regards the octahedral occupancy which takes one of the two values, 6 or 4. The origin of this difference may be understood by comparing the formula of talc with that of pyrophyllite, or that of antigorite with kaolinite. All of these minerals have relatively simple chemical compositions, and silicate layers which are electrically neutral. In these minerals, the total of the octahededral cation-charges is 12. The octahedral sites are fully occupied by divalent cations (e.g. magnesium ions in talc and antigorite), while 4 out of 6 sites may be occupied by trivalent cations, so that the remaining 2 sites are vacant (e.g. aluminium ions in pyrophyllite and kaolinite). The octahedral occupancies which take one or the other of the two values, 6 and 4, may serve to subdivide clay minerals with layer silicates in terms of so-called trioctahedral and dioctahedral sub-groups. These terms refer to the ideal number of octahedral cations per formula unit, that is, per half of the cell based on the unit-structure. Chlorite is somewhat complicated in regard to its octahedral nature. There are two octahedral sheets, one in the interlayer region and the other within the silicate layer, and these are occupied by both divalent and trivalent cations in various ratios. However, in the ideal sense, each of these two octahedral sheets may be defined as having either a trioctahedral or a dioctahedral nature. Chlorite may be thus divided into 4 sub-groups:di.-di. (simply dioctahedral), di.-tri., tri.-di., and tri.-tri. (simply trioctahedral), where the first term refers to the octahedral sheet in the silicate layer, and the second term to that in the interlayer region.


11

4. Polytypes

The term “polymorphism” which has long been used in mineralogy texts, is defined the phenomenon whereby a chemical compound may exist as two or more kinds of phases (polymorphs) with different crystal structures. Polymorphic phase transition has been grouped into several types on the basis of structural changes, such as the reconstructive, displacement, order-disorder types, etc. The term “polytypism” has also come into use. This is conventionally defined as a one-dimensional form of polymorphism, restricted to layer-like structures. In the structures of different polytypes (or the polytypic modifications) of a compound, structural differences result from different stacking fashion of similar layers or sheets. Recently, it has been found that small differences in composition and structure are not uncommon among the different polytypes of a compound. It seems reasonable to consider that the bonding requirements for different stacking sequences may distort the symmetery of the layer- or sheet-unit. As a result, the definition of polytypism has recently been modified to permit minor deviations (so far unspecified as to magnitude) in overall composition and structure (recommendation of a Joint Committee on Nomenclature between the International Union of Crystallography (IUC) and the International Mineralogical Association (IMA)). Among clay minerals, kaolinite, dickite and nacrite are known to represent polymorphs, and are now termed polytypes. The finding of many polytypes of single compounds has led to the conclusion that polytypes should not be given individual mineral names. Instead, they are to be designated by a single mineral name followed by several structural symbols that may give information such as the observed symmetery, periodicity of layers, and interchange of axes. Various systems have been proposed, but no agreement has yet been reached on a universal a

f \

+a2

(a) L

\

\

_ _ _ _ -J- -

II

I

\\\

L------J

C

I+al d

I+al f

a f

-------\

(b) \

\ \\\

.I L- - - ---/ C

(A)

/ //

d (B)

Fig. 1.5 Octahedral cation sets (I or 11) (A) Set-I. (B) Set-11. (a) 2: I layer. (b) 1 : 1 layer. Hexagons: apical oxygen hexagonal ring or (OH) hexagonal ring. The lower apical oxygen ring (denoted abcdef) is drawn with dashed lines. Octahedral cation. 0 (OH) in the (OH)-plane of the 1 : 1 layer. Oxygens are omitted.

12


system. The system of symbols proposed by Ramsdell (1947) for Sic polytypes has been widely used for other compounds, including clay minerals with layer-structures. It comprises a number which may indicate the number of layers in the repeat along the normal or quasi-normal to the layers, and a capital letter to indicate the crystal system. Subscripts 1,2,3, . . . are used to distinguish different polytypes of the same periodicity and symmetry, e.g. 2M,, 2M,. This notation does not specify actual details of the mode of stacking. However, the symbols have been correlated with specific, simple stacking sequences for the micas by Smith and Yoder (1956) and for the 1 : 1 type layer silicates by Bailey (1969). The unique system was devised by Zvyagin (1967) for 1 : 1 type layer silicates, micas and chlorites. Many polytypes are known in clay minerals with layer-structures. The description of the polytypes is based on the mode of stacking between the sheets or layers, which may be designated by relative displacements or rotations. The displacement is defined by specific directions and amounts, where direction is referred to the plus or minus directions of the crystallographic axes, and amount is expressed by specific fractions of the periodicities of the axes. As a result, a particular stacking fashion can be expressed by a vector, i.e. a stacking vector. The angle between successive stacking vectors is termed the stacking angle as measured in a counterclockwise direction in projection on (001). In any case, for the sake of convenience, the relative displacement or rotation may be expressed firstly by referring to one of the a-axis fixed in an initial layer. The a,-axis is set in the direction from south to north (using map convention) through the projected centre of the tetrahedral ring below, and its plus direction is to the south. With reference to the fixed axes, the nature of the orientation between two tetrahedral sheets or between the (OH)-plane and the tetrahedral sheet will next be considered in relation to the distribution of the octahedral cations. In both the 2: 1 and 1 : 1 types, the positions of the octahedral set can be grouped into two sets (I and 11) as shown in Fig. 1.5. It is evident that set I can be transformed to set I1 by rotation of the layer by +60° or 180".In the case of the 2: 1 type, cation set I requires a shift of the upper tetrahedral sheet along the minus direction of the a-axis by an amount of a0/3 on (001); this will be referred to simply hereafter as a shift of -a0/3. Cation set I1 requires a shift of + a0/3. Such staggers may visualize octahedral sheets which slant in opposite directions for the two cases in [OlO]projection (not to be confused with the direction of the shift between two tetrahedral sheets). Similarly, in the 1: 1 type, a hydroxyl plane must fit on the occupied I or TI set to form octahedra which slant in opposite directions. It should be noticed that the I1 octahedral set requres a shift, -ao/3, of the upper layer relative to the layer below. Concerning the octahedral occupancy of the dioctahedral sub-group, the locations of the vacant sites can be grouped into two. One is on the mirror plane of each 2: 1 or 1 : 1 layer, and the other is located at one of the positions related by the mirror image (Fig. 1.6). A structure version may be given by the piling up of sheets or layers according to bonding requirements. Polytypes of a compound may be derived according to various possible stacking a

r-----7 f

Fig. 1.6 Octahedral vacancy. o Vacant site. 0 Occupied site. (A) Vacancy-1 (vacant site lies in dioctahedral monoclinic micas on the mirror plane of each 2: 1 layer (B) Vacancy-2 (vacant site lies at one or the other of a pair of equivalent octahedral sites related to each other by this symmetry element). For hexagon (abcdef), see Figs. 1.2 or 1.5.


13

fashions in response to the bonding requirements. The resultant symmetry of the polytype does not necessarily coincide with the symmetery of the initial layer. The stacking fashion defines the periodicity of the c-axis (c,) and its direction, i.e. the u- and 8-angles. Polytypes having one-layer structure are called one-layer polytypes, and multiple-layer polytypes are similarly defined. In the one-layer polytype, the orientation of the one-layer unit needs be identical in each successive layer; thereby, it needs to consider only one orientation of the initial layer. However, in multilayer structures, the orientation of the layer need not be identical, that is, all possible orientations of the repeating layer must be considered, and shifts within the layer itself also need to be specified. 4.1. Micas Mica polytypes are derived on the basis of an a0/3stagger in the octahedral sheet in each silicate layer combined with the ideally hexagonal symmetry of the basal oxygen network (Fig. 1.7). The relative positions of adjacent layers are defined by the stagger. The structures may be summarized as follows (Smith and Yoder, 1956). (a) 0" interlayer stacking angle corresponding to successive - a , shifts (1 M), and 180" stacking angle corresponding to alternate - a , and + a , shifts (20): these are the only polytypes that involve shifts along just one axis.

C2/m

a

5.3i

3T

6H c 60i

P3,12

LJV Fig. 1.7 Polytypes of micas (Smith and Yoder, 1956). Full-line vectors represent the direction and amount of the stacking in one layer. Broken line vectors show the stackingin the next layer. Rectangular or rhombus shapes give the base of the resultant unitcell of each polytype. This m a y or may not coincide with the fixed cell-base used for the initial layer.

14


(b) Continuous alternation of k 120“ stacking angles (2MJ and continuous alternation of

+60” stacking angles (2M,): in these, shifts are considered along two axes. (c) Continuous sequence of 120” or of 240°C stacking angles (3T) or continuous sequences of 60” or of 300” stacking angles (6H): in these, all three a-axes are used for shift directions and each of these polytypes gives two mirror-image structures. 4.2. Chlorite In polytypes of one-layer structure (Bailey and Brown, 1962), it is necessary to consider only one octahedral cation set in the initial silicate layer, either I or I1 (Fig. 1.5, Fig. 1.8(A)). The interlayer hydroxide sheet is designated by “I” if it has the same (parallel) slant as the octahedral sheet in the silicate layer. It is designated by “IJ” if its slant is opposite (antiparallel). The position of the interlayer hydroxide sheet is designated by “a” if the interlayer cations project onto the tetrahedral cations below, and by “b” if they do not (Fig. 1.8(B), Fig. 1.9). Thus 4 layer-types (Ia, Ib, Ira, and IIb) arise. The possible positions of the next silicate layer relative to the interlayer hydroxide sheet are considered in 6 ways. These are defined by the six possible positions of the centre of the tetrahedral ring relative to the hydroxyl surface below. (OH)-ions in the upper plane of the interlayer hydroxide sheet are selected so as to form a hexagon centered on the mirror plane of the initial silicate layer (Fig. 1.8(C)). The hexagon is divided into 6 triangles, and the centres of the

a3

4-- - - - ?\

2 3 2 (4

2) 2 6

a

6

7

a

Fig. 1.8 Stacking fashion of the Ia-2 polytype of the one-layer structure of chlorite (after Bailey and Brown, 1962). (A) (001) projection. The orientation of the initial layer is indicated by the stagger along the mirror plane in the minus direction of the al-axis. The black dots are octahedral cations. (B) The stacking fashion of the interlayer hydroxide sheet onto the initial layer. The black dots are interlayer cations. (C) The (OH) anions in the upper (OH)-plane ofthe interlayer octahedral sheet are selected so as to make a hexagon centered on the N-S mirror plane of the initial layer. The hexagon is composed of 6 regular triangles. (D) The 6 possible manners ofpositioning ofthe next layer relative to the interlayer below are defined by the centre of a hexagonal ring in the overlying tetrahedral sheet projected onto one of the centres of these triangles numbered , 0, @, In this polytype, the center of the tetrahedral sheet is projected onto the center of the triangle numbered @ as indicated by the black dot. The polytype notation is given by Ia-2. The numbers, 1,2, 3, . , indicating the hexagons mean the levels illustrated in Fig. 1.4.

. ..

..


15

Ib

Ib

Ib

f

-a

Ia ,8=97'

R @ 1 I

Ib 8=97'

Ib p=90'

IIb

9 I

. .

IIb

I

@ I

=aE\ JIb

.. a

IIa ,8=97'

IIa ,8=90'

d

IIb ,8=97'

Fig. 1.9 Schematic diagrams of six stacking structures of chlorite in [OlO] projection (Shirozu and Bailey, 1965). Tetrahedral cation. 0 Octahedral cations in the 2: 1 layer or the interlayer hydroxide sheet. Dashed lines indicate the vertical superposition of cationcation or cation-(OH). Oxygens are omitted.

triangles are numbered 0, 0, . . .@, as shown in Fig. 1.8(C). A hydrogen bond may be formed between the hydroxyl in the upper plane of the interlayer sheet and the basal oxygen in the lowermost plane of the overlying silicate layer if a hexagonal ring in the overlying tetrahedral sheet projects onto one of the 6 numbered sites (Fig. 1.8(D)). Among 24 polytypes, 12 unique ones can selected, because certain structures are equivalent to others after 180" rotation about the b-axis or because of an enantiomorphic relation. Fig. 1.8 gives the projection of the structure of Ia-2 onto the basal plane. Fig. 1.9 shows schematic views of six stacking structures of chlorite in [OlO] projection (Shirozu and Bailey, 1965). In two-layer structures (Lister and Bailey, 1967), individual one-layer units in the two-layer structure are no longer equivalent. In general, the variables which need to be considered for building up individual layers in the two-layer structure, comprise 6 ways for building up the layer, 4 stacking fashions for the interlayer hydroxide sheet, and 6 positions for the next layer. This is illustrated schematically in Fig. 1.10. Numerous theoretically possible two-layer polytypes are obtained. However, equivalencies of structure reduce the number of polytypes. All of the

16


a.0,0.0,0.@ I a , I b , IIa, IIb

Fig. 1.10 Stacking fashions requiring considerationin the derivationof the two-layer structures of chlorite (after Lister and Bailey, 1967).

unique polytypes can be oriented so that the displacement within the initial layer is along the minus direction of the a,-axis. 4.3. Minerals of the I: 1 layer-type (a) Trioctahedral sub-group The interlayer bonding requirement of the 0.-(OH) pairing, which results in the formation of hydrogen bond, is seen in various stacking fashions as follows (Bailey, 1969). (1) No shift of successive layers results from an exact superposition or relative stacking angles of f60" or 180". (lT, e.g. lizardite and antigorite, and 2H,). In the 1T polytype, the octahedral cation set is the same in each layer, whereas in 2H, the set alternates regularly in successive layers. (2) Assuming the I1 octahedral cation set in the initial layer, the shift of the second layer is designated by -a0/3, and succeeding possible interlayer shifts are considered to take place in 6 directions, f a , , fa,, and *a3 relative to this initial shift. The polytypes are grouped into three sets with regard to the number of axes along which shifts occur. 1M and 20r have shifts along one axis, 2M, and 2M2 along two axes, and 3T and 6H along three axes. The polytypes 3T and 6H have two mirror image structures for each. (3) Interlayer shifts of +bo/3 along any of the b-axes (2T, 2H2, 3R, and 6R). (b) Dioctahedral sub-group The structures of kaolinite would be assigned to the 1M type if they were trioctahedral. However, in well crystallized kaolinite the vacant octahedral site is actually located at the same position in each layer and either one or the other of the mirror images results in the creation of triclinic symmetry in the structure. In dickite, the vacant site alternates regularly between one and the other of the mirror images. As a result, this mineral has a monoclinic two-layer structure. The structure of nacrite (Hendricks, 1939) has been described as a primitive cell which is two-layer monoclinic by Bailey (1963) 'and Zvyagin (1 967), though the


17

cell-settings proposed by these authors are different from each other. Kaolinite is one-layer polytype and dickite and nacrite are two-layer polytypes. 11. LAYER-RIBBON-STRUCTURES

In the tetrahedral sheet, the fourth corners do not point in the same direction: the tetrahedra in the sheet are arranged in systematic inversion (Fig. 1. I 1). Such tetrahedral sheets are piled up in parallel so as to create octahedral sites between them. The octahedral sites do not then form a continuous octahedral sheet but octahedral ribbons alternating in parallel with channels. This structure may appropriately be called layer-ribbon-structure. The structure resembles that of an inosilicate. However, this resemblance is somewhat illusory, since the silicate chain units in the layer-ribbon-structure are joined by sharing oxygens (Fig. 1.12) (Pedro, 1967). The channels accommodate water molecules accompanying exchangeable cations, the water being called channel water. Part of the channel water existing near the walls of the octahedral ribbons is relatively strongly bound to the magnesium ions exposed on the walls, and is conventionally termed “bound water” (expressed by OH,), while the remaining water is termed “zeolitic water” (H20).

(B)

-b

0-0-0-0

D‘

0- 0-0- 0

c

l 8 . 0 A F

Fig. 1.1 1 Layer-ribbon-structures. Normal projections onto (001) of the unitcells (left figures) and their schematic views (right figures). (A) Sepiolite (Brauner-Preisinger model). (B) Palygorskite (Bradley model). Octahedral cation. (OH). @ Bound water (OH,). 0 “Zeolitic water” (H,O). Tetrahedral oxygens and cations are omitted.

(B)

‘OH,- b

(C)

OCa

-b

Fig. 1.12 Comparative schematic diagrams of (A) a layer-structure (talc or pyrophyllite), (B) a layer-ribbonstructure (palygorskite), and (C) a chain-structure of an inosilicate (tremolite) (Pedro, 1967). The structure of palygorskite follows the Bradley model (1940).

18


The c-axis is set in parallel to the elongation of the octahedral ribbon, and minerals having this structure show a fibrous morphology: sepiolite and palygorskite are well known. As models of the structure of sepiolite, Nagy and Bradley (1955) proposed a monoclinic cell, and Brauner and Preisinger (1956) proposed an orthorhombic cell. Subsequent studies have usually supported the Brauner-Preisinger model (Fig. 1.1 l(a)). A model for the structure of palygorskite was proposed by Bradley (1940) (Fig. I . 1l(b)). The detailed structures and polytypes of sepiolite and palygorskite have not yet been fully established.

C. Structure of Crystalline Clay Minerals : random fashions Advanced studies on the crystal structures of clay minerals have revealed that disordered fashions are universally found in crystalline clay minerals to various extents. As mentioned above, the completely regular structure is no more than an ideal image. I. LAYER-STRUCTURES 1.

Curvature of the layers

There is abundant evidence for tubular shapes in halloysite and chrysotile, and for a wavy pattern of layers in antigorite. Simple calculations using a value of 1.60 A for the Si-0 distance in silicates (Smith, 1954), inform us that the b-dimension of a tetrahedral sheet occupied only by Si is larger than that of an octahedral sheet occupied only by Al, where the A1-0 distance is taken as 1.89 A, the sum of the ionic radii. Further, the b-dimension of the tetrahedral sheet is smaller than that of the octahedral sheet occupied by Mg. Bates et al. (1950) first reported a tubular morphology for halloysite based on electron microscopic studies, and pointed out that the morphology could result from an adjustment of the misfit between tetrahedral and octahedral dimensions. Bates (1959) subsequently developed a similar argument for the particular morphologies of the magnesian minerals. A1 substituting for Si would increase the tetrahedral dimension, and A1 substituting for Mg would decrease the octahedral dimension. This double substitution would modify both sheets so as to yield a better fit. Roy and Roy (1954) syntheseized an Al-serpentine having the composition(Mg2~,A1,~,)(Si, .5A10.5) 0,(OH)4, and showed that it occurred as euhedral platy crystals. Since 1951, the present author and his collaborators have extended the study of halloysite to that occurring as spherules (for a recent summary, see Sudo et al. (1977)). The shapes can be termed onion-like, cabbage-like, as well as chestnut-shaped spheres. Such particles are considered to be a product in the process of crystallization of allophane into halloysite. The overall process can be visualized as follows. Fine particles of allophane coagulate into spherulitic particles, which show relatively uneven surfaces at the first step, and grade into spherules. The spherules come to be crystallized into halloysite (10 A) shell-by-shell. On advancing crystallization, fine fibres tend to project outwards, often appearing like a “horn.” This shape probably results from the peeling of a thin crystallized layer from the central spherule, and then partial rolling of the layer into a tubular particle. The horn-shaped particle appears to have been rolled into a tubular form to the greatest extent at its top, while its bottom is still spread over the surface of the central spherulitic body. On further advancing crystallization, fibrous particles are also found as isolated particles distributed in the interspaces between the spherules. Such isolated particles commonly show a parttubular form with undulating edges, but at the final stage ‘they are found to be well-defined tubular particles of typical halloysite (10 A). In this sense, the spherule has been called an allophane-halloysite spherule; however, emphasizing the halloysite crystallization process, it may be

Structure of Crystalline Clay Minerals: random fashions

19

termed “spherulitic halloysite.” Honjo and Mihama (1954) first performed a single crystal electron diffraction study on halloysite (“Hongkong kaolin”) and pointed out that the majority of the crystals has the b[01] axis as the tube axis. These authors indexed one diffraction pattern having the b-axis as the tube axis on the basis of a monoclinic cell: a, = 5.14 A, bo = 8.93 A, co = 14.7 A, p = 104”.This indicates a two-layer structure. The requirement of this structure is based on the clear reflections along 021 lines falling midway between the 001 reflections; the basal reflections must be indexed as 002, 004, 006, . . . Honjo and his co-workers indicated that this material had some threedimensional order which would be higher than in halloysite previously considered. It should also be noted that these authors first showed a fine structure of the (hkO) reflections from a cylindrical lattice in halloysite. Chukhrov and Zvyagin (1966) also determined a two-layer structure of halloysite (ao= 5.14 A, bo = 8.90 A, co = 14.7 A, p=96”) on the basis of X-ray photographs and electron texture patterns. These authors, on the grounds of the two-layer structure being different from those of dickite and nacrite, and of tubular forms, stressed that halloysite is a distinct species not as only an end-member of the kaolinite sub-group. It is considered that these structural data are all for dehydrated forms because the specimens had been dehydrated at the high vacuum of the electron microscope. However, recently Kohyama et af. (1978) analysed the structure of a hydrated form (halloysite (10 A)) by means of a conventional electron microscope equipped with an environmental cell, and pointed out that the two-layer structure is still valid in the hydrated from. Yada (1967), employing high resolution electron microscopy, first revealed the fine textures appearing in cross sections of chrysotile fibers. Each fiber consists of spirally rolled layers occurring as multistart spirals, as indicated by several independent layers rolling up together. The central areas of the fibers seem to be empty. In some patterns, edge dislocations are visible corresponding to the start of an additional layer. Two kinds of electron-optical fringe systems are known: one involves the radial fringe corresponding to the (020) plane and the other involves the circumferential one of the (001) plane. Antigorite has a lath or platy shape. X-ray and electron diffraction analyses have demonstrated a long spacing of about 43 A, which is considered to be defined by the periodicity of the corrugated structures analysed crystallographically by Kunze (1956) (Fig. 1.13). The majority of clay minerals with layer-structures reveal a platy morphology. It is necessary to determine how the misfit in them has been adjusted. This question will be discussed in section C-1-3 of this chapter. Disordered structures of clay minerals have been interpreted from two viewpoints: one principally concerns stacking disorder, and the other concerns the distortion within individual sheets or layers. 2. Stacking disorder

It is now well known that clay minerals with layer-structures are universally found to have

Fig. 1.13 Schematic view of the structure of antigorite as viewed along the 6-axis (after Kunze, 1956).

20


layers stacking randomly to various extents. Stacking disorder is not limited only to clay minerals. For example, it has been noticed in graphite, hexagonal cobalt, potassium iodide, etc. and it has long been a matter of interest not only in clay crystallography but also in the wider field of X-ray crystallography. The nature of the stacking disorder is related to the two-dimensional structure, in other words, a one-dimensionally disordered crystal. Theoretical treatment of two-dimensional X-ray diffraction was first undertaken by Laue (1932), and later by various authors such as Warren (1941), Brindley and Mtring (1951), Wilson (1949, a, b) and Jagodzinski (1949). The rotation photograph about the c*-axis shows a distribution of diffuse streaks along the direction of the axis. The reciprocal pattern corresponding to the diffuse streak may be given by a continuous line of intensity parallel to the c*-axis. The distribution of the intensity functions along this line is seen to be diffuse to various extents according to the degree of randomness and also the thickness of the flaky particles. The line is also usually diffused in the a*b* plane according to the size and shape of the crystal layers. In this sense, the line may be termed a reciprocal lattice “rod.” In the case of a randomly oriented powder, the rods are oriented in all directions. The total X-ray powder intensity at any value of 8 is obtained by integrating the intensity function for the rod on the surface of intersection with the powder sphere with a radius d* = (2 sin @/A. The distribution of the intensity on the reflection sphere appears as an asymmetrical profile with a steep slope on the low angle side and a gentle slope on the high angle side (two-dimensional diffraction band). Hendricks (1940) studied the disorder structures of clay minerals such as mica, pyrophyllite, talc, vermiculite, cronstedtite, etc., and pointed out that, as a stacking disorder, random displacements of successive layers by multiples of b0/3 in the b-direction were frequently found in many forms of the principal clay minerals. This type of disorder is called “b-axis disorder,” i.e. a stacking disorder related to one axis. Actually, the displacements of 0 and +bo/3 need to be considered. If the displacements occur with equal probability, the reflections with k = 3 are unaffected (Wilson, 1949a), and the reflections with k # 3 occur essentially as two-dimensional hk-bands. Anion or cation planes consisting of layer structures have ion arrangements in lines parallel to the b-axis and at intervals of multiples of bo/3 except the basal oxygen planes and the Si-planes. The 0-OH paring between adjacent layers of 1: 1 type minerals and also chlorite. is not altered by the displacements of nbo/3 which would be expected to occur rather easily with no marked energy changes. A structure consisting of layers stacked in parallel and in a completely random fashion has been referred to as a “turbostratic structure” (Biscoe and Warren, 1942). The term denotes structures that give X-ray diffraction patterns consisting of (001) reflections accompanying two-dimensional bands, as is usually seen in those of halloysite and smectite. Turbostratic structures of random rotations are seen in smectite (Mtring and Oberlin, 1967, 1971). However, some papers have indicated the existence of local ordering in smectite. For example, Nakahira (1952) suggested that the form of the tailed X-ray diffraction bands can be explained by local ordering with random displacements of nb,/3. The electron-microscopic distinction of halloysite from kaolinite has been made on the basis of the tubular morphology. X-ray and/or electron-diffraction analyses have proved that some tubular kaolin minerals have a higher degree of crystalline order than that appropriate for usual halloysite. In order to reconcile these apparent contradictory results, several suggestions have been given : (a) Partial dehydration might improve the crystallinity of halloysite (Zvyagin, 1954). (b) Brindley (1961) stated that extreme care must be taken to see if a sample is a mixture of halloysite and kaolinite, and tubes are very noticeable in electron micrographs and they might also be selected preferentially in the course of sample preparation. Brindley and Comer (1956) indicated that the X-ray pattern of a synthetic 1 : 1 mixture of kaolinite and dehydrated halloysite is hardly distinguishable from that of a pure kaolinite. Brindley, Souza Santos et al. (1963, 1964), on the basis


21

of X-ray and electron-micrographic studies of synthetic mixtures, pointed out that some samples of so-called tubular kaolin minerals are mixtures of kaolinite plates and halloysite tubes. (c) Brindley and Souza Santos (1966) reported that the crystal morphology of both kaolinite and halloysite is variable to a great extent involving platy, part-tubular and tubular forms, and pointed out a discordance between morphology and X-ray properties.

3. Distortion within individual sheets or layers The results of crystal structure analyses of "macro-crystalline analogues of clay minerals" are of great interest and aid work on the structural aspects of clay mineralogy. It is worthy of note that all attempts at refined structure analysis have indicated considerable distortions within individual layers from the previously accepted regular structures. The existence of layer distortion was indicated in earlier studies on dickite (Newnham and Brindley, 1956), vermiculite (Mathieson and Walker, 1954), chlorite (Steinfink, 1958), brittle micas (TakCuchi and Sadanaga, 1959, TakCuchi, 1965), and mica (2M,) (Radoslovich, 1960) and pyrophyllite (Rayner and Brown, 1965). The discussion was extended by Radoslovich (1961, 1962, 1963) and Radoslovich and Norrish (1962), who established a means of providing a general model for the main distortions and general principles concerning layer-structures of silicates and played an important role in extending subsequent active research by many workers (e.g. Donnay er al., 1964). The disordered structures, as a first step, can be visualized as distortions of tetrahedral and octahedral sheets. The basal oxygen network in the tetrahedral sheet, which is considered to take an ideal form with hexagonal symmetry, may be distorted to a ditrigonal symmetry by rotation of the basal oxygen triads about the c*-axis alternately clockwise and anticlockwise through an angle, LY, which varies from a few degrees to near the theoretical maximum of 30" (Fig. I . 14). The octahedra are flattened in the direction normal to the octahedral sheet. Such flattening may be derive from shortening of the edges shared between adjacent octahedra in order to prevent repulsion of cations in adjacent octahedra. The flattening results in a lengthening of the edges that lie in the ab-plane and increases the bond angle which is defined as 55'44' in the ideal octahedron, probably due to difficulty in changing the octahedral cation to the oxygen bond length. Tetrahedral rotation to decrease the lateral extension of the tetrahedral sheet serves to release the misfit between tetrahedral and octahedral sheets. Subsequent to the above-mentioned early reports, it was found by many researchers that distortion of layers could be universally confirmed based on crystal structure analysis of clay minerals. Recently, Tateyama et al. (1977) proposed a method for calculating the tetrahedral rotation angle based on the following parameters: the thickness of the unit-structure, the bparameter, and the potassium-oxygen bond length which can be calculated experimentally from the observed frequency of the K-0 stretching vibration measured from infrared absorption peaks. The resultant values showed better agreement with those obtained by structural analysis than did those of previously reported methods. The above representation is still concerned with ideal images, and actually distortions may occur on variable fine scales particularly resulting from cation ordering (see next section). The distortions of layers revealed by refinement of the crystal structures often lead to the conclusion that layer displacements denoted by definite amounts such as a0/3 or bo/3 are no more than approximate values. Refinement of the structure of kaolinite revealed a departure of the layer structure from the ideal geometry (Brindley and Nakahira, 1958). The observed value of the Bangle, 104.8", is larger than the value of 103.5' which would be expected from an exact displacement of -a0/3. One sample of kaolinite from Pugu, Tanganyika (Robertson er al., 1954) is regarded to be a typical b-axis disordered kaolinite. The B-angle (404.5") also does not correspond exactly with displacements of -a0/3. Further, many broadened lines corresponding to pairs of the

22


Fig. 1.14 (A) Rotations and displacements creating the main distortion features of the tetrahedral sheet. (B) Normal projection of two distorted tetrahedral sheets onto the (001)plane as illustrated in the structure of muscovite-2M,.The basal oxygen nets only are shown. K : Potassium ion

type 201 and 13(-1- 1) with the line diffusion from the k = 0 member to the k = 3 member of a pair, may indicate that the layer displacement only approximates to bo/3. It has been reported that, in some micas, the stagger between two tetrahedral sheets is slightly larger than a,/3, due to the location of a large vacant octahedral site on the symmetry plane of the 2: 1 layer (e.g. Giiven, 1971). Recent refined structure analyses have revealed more variable distortions within layers at smaller magnitudes. Such distortions have been demonstrated from different sizes, shapes, and orientations of individual tetrahedra or octahedra at small magnitudes resulting in oxygen network distortion. Such distortions on a fine scale have been indicated by the careful examining of non-Bragg satellite reflections, and diffuse scattering in regions between the normal Bragg reflections. The origin of the distortions may be related to factors involving (a) order-disorder relation in isomorphous substitutions (b) the existence of octahedral vacancies and their distribution. 4. Order-disorder relation in isomorphous substitution

Isomorphous substitution is common in layer silicates. Isomorphous ions usually involve cations or anions of different sizes. Moreover, it is natural to consider that a tetrahedron having a larger cation would be larger in size than one having a smaller cation, and that the linking of larger and smaller tetrahedra would result in complicated distortions of the tetrahedral sheet. A similar situation would hold true for the octahedral sheet.


23

The mode of distribution of isomorphous ions can be divided into disordered and ordered fashions. A disordered fashion is represented by atom A substituting for atom B randomly at different but structurally equivalent sites in adjacent unit-cells. An ordered fashion is represented by an ordered distribution of A and B atoms over all the available sites. For example, there may be two kinds of crystallographically distinct tetrahedral cation sites T( 1) and T(2), one of which is occupied by A1 and the other by Si, and such a systematic substitution may be maintained between adjacent unit-cells. Cation ordering may be expressed on a unit-cellby-unit-cell basis of a single crystal and tends to take place for A and B ions of different sizes or different bonding characters. The existence of cation ordering has commonly been indicated in X-ray analysis as a result of careful refinement of crystal structures, particularly by finding differences of cation-to-oxygen distances in different tetrahedra or octahedra. In layer silicates, the tetrahedral cation oxygen bond length (T-0)is 1.62 A on average when the sites are all occupied solely by Si, and is 1.77 A when the sites are all occupied by A1 (Smith and Bailey, 1963). A linear relationship has been confirmed between these values for intermediate compositions. Similarly, the octahedral cation oxygen bond length (M-0, OH) varies according to composition. In cases where there is no cation ordering, it is considered that an average unit-cell may contain hybrid atoms of A and B; as a result, differences of mean bond lengths are unspecified in magnitude. In general, it can be considered that the symmetry of an average structure without cation

-- 4 3

I

1

Fig. 1.15 The structure of Llano vermiculite (Shirozu and Bailey, 1966). The structure was described as being of the s-type by Mathieson and Walker (1954). The structure involves a shift within each layer of - U o / 3 and shifts of alternatelayers relative to one another by --bo/3 and +bo/3. Tetrahedral cation ordering is significant. Broken lines indicate vertical alignments of exchangeable cations (mostly magnesium ions) between aluminium-rich tetrahedra of the layers above and below.

24

CRYSTAL MORPHOLOGY A N D STRUCTURE OF CLAY MINERALS

ordering is higher than that of one having cation ordering. Some of the space groups may be termed “ideal space groups” (Bailey, 1975) or “space groups of average structure,” which can be regarded as having their basis in the assumption of no vacancy, no cation ordering and no significant disortions, as illustrated by the space groups of mica and chlorite polytypes. The tetrahedral cation ordering of muscovite-2M remains problematical. Radoslovich (1960) first reported that within the space-group C2/c, T(l) sites are occupied by Sio.sAlo.s on average, and T(2) sites are almost fully occupied by Si. This may represent partial ordering, and the maximum ordering within the symmetry requirements of C2/c. It was also reported that there is no evidence of lower symmetry to allow higher ordering. Subsequent re-examination (Burnham and Radoslovich, 1965) has indicated that both sites have similar bond lengths statistically suggesting occupancy by Si0.,5A10.2s. An interesting local charge balance in relation to cation ordering is seen in Cr-chlorite of onelayer structure (Ia-4) (Brown and Bailey, 1963), and in vermiculite of two-layer structure (Shirozu and Bailey, 1966). In the former, Cr-ions are concentrated in one of the octahedral sites in the interlayer hydroxide sheet, which is located vertically between orderd Al-rich tetrahedra in the layers above and below. In the latter, a vertical alignment of exchangeable Mg cations between Al-rich T(1) tetrahedra of the layers above and below is seen (Fig. 1.15). Full refinement of structure analyses has often sought the influence of cation ordering in decreasing the ideal space group symmetry to that of a sub-group. Illustrations are known in some micas and chlorite. Guggenheim and Bailey (1977) refined the structure of a zinnwaldite-1M in C2/m, C2, and C1 symmetries, and found the best C2-model in which octahedral cation ordering had come to be clearly demonstrated (Fig. 1.16). In some instances, cation ordering results in the formation of superlattices. The above examples concerned the cation ordering occurring within a homogeneous single

Fig. 1.16 nlustrations of tetrahedral and octahedral distortions. (A) Distorted features of the tetrahedral and octahedral sheets in the dickite structure (Newnhamand Brindley, 1956). The octahedra having vacant sites occur with larger dimensions compared to the octahedra having occupied sites. (B) Octahedral ordering pattern of zinnwaldite-lM in sub-group C2 (Guggenheim and Bailey, 1977). M(2) sites are completelyoccupied by aluminium.Iron, lithium and other cations, and vacancies are almost randomly distributed in M(l) and M(3).


25

crystal. Cation ordering has also been sought within local domains by studying diffuse streaks in regions between the normal Bragg reflections. Gatineau (1964) reported that the tetrahedral cation ordering of mica occurs essentially in atom-rows in which all tetrahedral sits are entirely substituted by A1 or Si. The rows are arranged along one of the 3 possible directions, [lo], [Ill, [li]. As a result, the crystal is divided into domains in each of which the rows run in one of the directions. Each domain can be differentiated into two types of bands. One type contains equal numbers of rows of entirely A1 and entirely Si but without regular alternation, and the other is composed entirely of Si. These two types of bands have equal size and alternate regularly, thereby giving an overall Si: A1 ratio of 3: 1. Between two adjacent layers, an unsubstituted band comes immediately above a substituted band and vice versa resulting in local charge balance. 5. Distribution of vacant sites

The effect of order-disorder in the distribution of vacant sites on distortion of the structures is similar to that of isomorphous ions. This effect is well demonstrated in dickite and kaolinite which have no isomorphous substitution. Octahedra having vacant sites are larger than those occupied by aluminium. The liking of large and smaller sized octahedra results in a distorted octahedral sheet. The distortion is represented mostly by a shortening of the shared edges between occupied tehtrahedra in order to prevent repulsion between two adjacent octahedral cations, and also lengthening of the shared edges between occupied and unoccupied octahedra. The tetrahedral sheets linking to such distorted octahedral sheets are also distorted in such a way as to cause tilting of the tetrahedra since the apical oxygen nets are distorted from the ideal hexagonal net. As a result, the basal oxygens are no longer co-planar. The symmetry of a particular ordering pattern of octahedral cations and vacancies may change from that of “the ideal space group”, which may be considered in structures without defects. 6. Diffuse scattering by X-rays and electrons Recently, Kodama et al. (1971) and Kodama (1975, 1977) studied the diffuse scattering from muscovite, pyrophyllite and talc. Muscovite has essentially tetrahedral cation substitution and octahedral vacancies, pyrophyllite has octahedral vacancies but no isomorphous substitution, while talc has essentially no vacancies and no substitution, although a few iron ions may substitute for octahedral magnesium ions. Diffuse streaks are displayed by all these minerals in X-ray and electron-diffraction patterns. The streaks (between Bragg spots) appear in the direction [OI], [ 1 11, [TI], and also along the c*-direction, and intense parts appear near the reciprocal nodes 40, 26 and 26. Such streaks were also enhanced after an exchange reaction of the interlayer potassium of muscovite. Kodama suggested that the streaks may be due to out-of-plane distortions of the basal oxygens rather than Si-A1 substitutions in the tetrahedral sheets. 11. LAYER-RIBBON-STRUCTURES

Based on detailed studies of the disordered crystal morphology and structure of clay minerals with layer-structures, it seems reasonable to conclude that a similar disorderliness may exist in clay minerals with layer-ribbon-structures. However, the details have not yet been established. Both sepiolite and palygorskite occur as slender fibres with well-defined straight edges, although curved fibres are occasionally observed particularly in palygorskite. There is no evidence of a tubular morphology, so that the fibres are best described as slender laths. In the olden days of mineralogy, Fersman (1913) defined the group by its chemical composition, and differentiated it into 2 morphological types : a fibrous type (alpha-sepiolite) and a laminar type (beta-sepiolite). It has subsequently been reported by several workers that the sample called beta-sepiolite is compact and massive consisting of aggregates of platy particles without definite

26


outlines, and shows broader X-ray powder reflections than those of alpha-sepiolite (Brindley, 1959). Although it seems natural to consider that the broadness is due to finely divided particles, it has been also suggested that the broad reflections are due to “the degree of crystallinity” which may vary from sample to sample in sepiolite. Although the nature of beta-seopiolite has not yet been established, it provides interesting material for future study.

D. Interstratification and Intergrowth 1. INTERSTRATIFIED (or MIXED-LAYER) STRUCTURES

As stated above, in the structure of clay minerals with layer-structures, the distortions within layers are usually of slight degree although they need to be specified in order to understand the nature of the structures fully. In the general sense, therefore, it may still be said that the structures are similar as regards the dimensions and structure of the ab-plane (basal plane), in contrast to the thickness of the unit-structure (Fig. 1.4). Interstratified (or mixed-layer) clay minerals can thus exist in which unit-structures of different mineral groups, or different varieties of the same group, are stacked in parallel to the basal plane. Such mixed-layer structures were reported many years ago by Gruner (1934). Since then, many types have been found in nature and some have been synthesized. They are now regarded as common constituents of clays, whereas at the time of their discovery they were considered rare. The essential points may be summarized as follows. (1) Most studies have concerned the two-component layers, A and B. Three-component systems have increasingly been reported. (2) Most data have so far been related to the finding of combinations of minerals belonging to two different mineral groups (e.g. chlorite-smectite, mica-smectite, vermiculite-smectite),as shown in Table 1.2. Occasionally, interstratifications exist between minerals belonging to two different layer-types (2: 1 and 1 : 1). These were first described as a complicated combination of minerals of the kaolinite sub-group (probably involving both halloysite (10 A) and halloysite (7 A)) and montmorillonite (Sudo and Hayashi, 1956) and later as kaolinite-montmorillonite (Schultz et al. 1971 and Wiewibra, 1973). Further, interstratifications have been confirmed to be combinations of minerals belonging to two varieties of the same group, e.g. hydrated with dehydrated vermiculite (Walker, 1956), and K-vermiculite with Ca-vermiculite (Sawhney, 1972). The latter may be regarded as a state resulting from the unmixing with regard to interlayer cations. (3) It has been demonstrated that the occurrence of combinations of non-expandable and expandable layers is common, although combinations of expandable mineral layers belonging to the same or different groups are known, as stated above. Combinations of two kinds of nonexpandable layers are rarely found, e.g. “sericite-sudoite” (Shirozu et a[., 1971) and paragonitephengite (Frey, 1969). In the former, the component layers belong to different mineral groups, whereas in the latter they belong to the same group, i.e. mica. (4) The interstratified mineral can be described on the basis of two criteria: (a) the ratios of the component layers of different types, and (b) the stacking sequence of these layers. The stacking sequence is described by the terms “random” and “regular.” In current models, the sequence is considered to lie in the range of partial randomness of various degrees between the two limiting cases, completely random and completely regular. Detailed crystal structure analysis of the threedimensional aspect has not yet been performed. It should be emphasized also that the terms “random” and “regular” refer only to the stacking sequence. A completely random model is defined as showing no detectable influence of one kind of layer on the kinds of its neighbours. In this

Interstratifcation and Intergrowth

27

case, PAB(i.e. the probability that B-kind layer succeeds A), is equal to the probability of existence of B-kind layer, W,.The range of influence or correlation may vary, and is expressed by the “Reichweite, g” of Jagodzinski (1949). The completely random type is given by g = 0. g = 1 means that PAB is defined by the kind of the immediately preceding one-layer, and g = 2 that it is defined by the kinds of the immediately preceding two-layers (pair-layer). Various randomregular sequences can be modelized by the parameters defining the probability. Completely regular models (e.g. ABABAB. . . in g = 1 and AABBAABB. . . , AABAAB. . . , ABBABB. . . . in g = 2) can be regarded as being structures of single crysyals. Several interstratified minerals described as being of “regular type” have received individual mineral names. Even so, based on detailed analysis of the status accorded to the interstratified structure, it is also advantageous to consider the completely regular type as being a limiting case. The regular type reported so far is commonly regarded to be 1 : 1 regular type having component layers in equal proportion. (5) The unit thickness of a completely regular interstratified structure is given by the sum of the thicknesses of component unit-structures. The sum is given by the basal spacing d(001) calculated from integral series of basal reflections. A random interstratified structure gives a nonintegral series of basal reflections owing to the interference effects between the basal reflections assigned to the structures having one component unit-structure and the others. The nature of the interference peaks is examined by the positions, relative intensities, line profiles and the mode of peak migration between these two positions as expressed by variations in the apparent basal spacings of the interference peaks. These are affected by the factors such as the proportions of the component layers, the degree of randomness, particle-size distribution, and the thickness of crystallites. (6) A theoretical background to the structure of interstratified structures was first given by Hendricks and Teller (1942). General formulations for deriving the X-ray intensity from onedimensionally disordered crystals were successively reported by several authors, of whom Kakinoki and Komura (1952,1954) provided the most general formulation. The theories have been practically applied in clay mineralogy providing many useful data concerning such as peak migration diagrams, calculated diffraction patterns, newly proposed methods, and suggestive concepts (e.g. MacEwan et al. 1961; Sato, 1969; Reynolds and Hower, 1970; Tettenhorst and Grim, 1975). (7) Recent strict mathematical analysis has revealed that earlier descriptions in terms of the regular type do not necessarily mean a completely regular type, but may involve less regularity. Similarly, the random type may be involved in a range from a completely random type to a partially random type with a variable tendency of alternation. Sato and Kizaki (1972) analysed the structure of a 38 8, interstratified mineral of mica-montmorillonite and concluded that it has a structure with g = 2. (8) Recent crystallochemical studies on interstratified minerals have revealed that the properties of the component layers are not necessarily the same as the properties of the same layers in the monomineralic crystals. Modifications in properties are particularly observable with respect to thermal activity and the results of behavioural tests. The modifications can be said to represent intermediate (intergradient) properties between the two different mineral groups. Subtle variabilities in the structure and composition of interstratified minerals may provide an important key for solving their origin (Sudo and Shimoda, 1977). It is an interesting point for future studies to determine whether layers having intermediate (intergradient) properties can exist only in interstratified structures, or whether they can also exist in the monomineralic state. As one origin for interstratified minerals, particularly of the regular type, Sudo et al. (1962) proposed a structure model with an asymmetrical arrangement of two tetrahedral sheets within individual layers as regards the Si : A1 ratio. The possible validity of this model, in some instances, has been suggested by the swelling behaviour (Tettenhorst and Johns, 1966), by analysis of the

TABLE 1.2 Classification scheme for interstratified minerals

(A) Rg: minerals described as “1 : 1 regular type (unless specified)” which may involve a completely regular type and less regularity. Rm: minerals described as “random type” which may involve layer sequences in the range from completely random to partially random types with a variable tendency of alternation. (B) It is not uncommon to find that the properties of the component layers are slightly modified from the properties of the same layers in the monomineralic crystals. Modification has been reported particularly in the expandable layers of some samples of combinations between non-expandable and expandable layers. (C) /, Interstratified minerals having component layers belonging to two different groups. :’ , Interstratified minerals consisting of layers belonging to two different varieties of the same group. 1 : Aliettite (Veniale and van der Marel, 1969). 2: Hydrobiotite (Gruner, 1934). 3: Allevardite or rectorite. Brown and Weir (1963) indicated that the name “rectorite” has priority. Rectorite is a combination of paragonite-like and dioctahedral smectite-like layers (Kodama, 1966). A potassium mica analogue was also reported (Kodama, 1966). It has been particularly noted that the expandable layers tend to be vermiculitic. Tarasovite (Lazarenko and Korolev, 1970) is a 3 : 1 interstratification of dioctahedral mica-smectite. 4: Data obtained to date have indicated that the nature of the expandable layers is variable in the range “swelling chlorite”-vermiculite- smectite. The corrensite originally described by Lippmann (1954) is a combination of trioctahedral chlorite and “swelling chlorite”. The nature of the “swelling chlorite” needs further clarification, but it may at present be said to be closer to chlorite than vermiculite but still expandable in various degrees on the basis of X-ray, thermal analyses and other behavioural tests (Shimoda, 1974). 5 : Blatter, Roberson and Thompson (1973). 6: Tosudite. Although, in the original description (Sudo ef al., 1954), the exact chemical analysis of the mineral itself was not available because of finding an impurity of a small amount of kaolinite, it was reported that MgO ranges 0.02-0.71 %. Later, a combination of di.-, ti-chlorite and montmorillonite was established (Sudo and Kodama, 1957), and a combination of di.-chlorite and montmorillonite was reported (Shimoda, 1969). Recently, a sample containing lithium was identified as Li-tosudite (Nishiyama et al., 1975). This suggests that one of the component layers is a cookeite (di.-, tri.-chlorite)-like layer. The existence of a random type has not yet been fully

Non-crystalline and Poorly Crystalline Clay Minerals

29

established. Data obtained to date indicate that the nature of the expandable layers is not so variable as those combined with mica layers or trioctahedral chlorite layers. Usually, the expandable layers of tosudite may be called montmorillonite layers, except for the description of a sample as a “high-aluminous chlorite-swelling chlorite” combination (Heckroodt and Roering, 1965). 7: The sample first described by Sudo et af. (1956)is a very Complicated interstratification of minerals of the kaolinite sub-group (probably involving both halloysite (10A) and halloysite (7A))-montmorillonite. Electron microscopy revealed that it was largely composed of aggregates of platy particles. 8: e.g. Wiewi6ra (1973). 9: A combination of hydrated and dehydrated vermiculite developed at a stage of dehydration of vermiculite by heating, (Walker, 1956), and a combination of K-vermiculite-Ca-vermiculiteobtained when a Ca-vermiculite was exchanged progressively with potassium ions (Sawhney, 1972). 10: A combination of biotite and chlorite (Eroshchev-Shak, 1970). 11 : A combination of “sericite-sudoite.” This may correspond to the combination of di.-mica and di.(or di.-tri.)-chlorite in the table (Shirozu et af., 1971). 12: A combination of paragonite-phengite (Frey, 1969).

stacking sequence of a long spacing mica-like mineral (Cole, 1966) and by a consideration of the structural aspect of rectorite (Lippmann and Johns, 1969). In this connection, studies of cation ordering on the basis of refined structure analysis are of interest in relation to the origins of interstratified structures. (9) Earlier studies of interstratified structures have mostly been concerned with one-dimensional aspect. More complicated interstratified structures have been suggested by models as termed “frayed-edge’’ (Bray, 1937; Jackson, 1963), core-rind” (Gaudette et al. 1966), and reported by Sudo and Hayashi (1956). From these models, it is envisioned that interstratified structures with variable layer proportions may exist within local domains of a crystal particle. Tettenhorst and Grim (1975) reported calculated diffraction patterns from particular models having particular distribution of layers within a crystal particle as expressed by the position of one of the component layers gradually changing from the outside toward the middle of a crystallite, or having particular distribution of spacings as shown by a gradual increase from a core outward. These authors indicated that positions and shapes (line-profiles) of X-ray diffraction peaks are determined by these distributions in addition to the kind of interlayering. 11. SEPIOLITE-PALYGORSKITE INTERGROWTHS

Martin Vivaldi and Linares Gonzdlez (1962) postulated the existence of a random intergrowth of sepiolite-palygorskite on the basis of anomalous X-ray spacings (Fig. 1.17). These intergrowths may be analogous to interstratified structures of clay minerals with layer-structures. Shimoda (1964) proposed a similar structure for aquacreptite, but this structure can be regarded as a hybrid structure between layer- and layer-ribbon-structures.

Fig. 1.17 Schematic view of random intergrowths of sepilolite-palygorskite (cf. Fig. 1.11) (Martin Vivaldi and Linares Gonzklez (1962)).

E. Non-Crystalline and Poorly Crystalline Clay Minerals Clay minerals such as allophane, hisingerite, penwithite are regarded as non-crystalline clay

30


minerals. Although their degree of crystallinity is quite low compared to the other minerals, studies on various properties still provide a sufficientbasis for distinguishing crystal structure versions. The structure model of allophane proposed by Udagawa et al. (1966) consists of a kaolinmineral like layer-structure containing occasional A1 with a 4-fold co-ordination. Brindley and Fancher (1966) proposed a defect kaolin-mineral structure with vacant Si-sites mostly on the basis of the chemical composition. A structural image of hisingerite has been assigned to a defect nontronite structure (Sudo and Nakamura, 1952; Kohyama and Sudo, 1975). Imogolite is a poorly crystalline mineral and has been established as a new clay mineral. It was first reported by Yoshinaga and Aomine (1962) from volcanic ash soil in Kumamoto Prefecture, Kyushu. In contrast to allophane, imogolite is dispersible in acid medium (pH 3.5-4.0), and shows broad but clearly visible X-ray powder diffraction peaks. It occurs as slender fibres, as revealed by electron microscopy. The fibres consist of bundles of fine tubes, each about 20 A in diameter. The unique structure has been confirmed by Cradwick et al. (1972). It belongs to the orthosilicates. The structure may be derived from an association of the oxyanion of orthosilicate (SiOz-) within the gibbsite structure (Fig. 1.18). One oxyanion is located at each vacant site of the gibbsite structure, when the oxyanion displaces hydrogen from 3 hydroxyl groups around each vacant octahedral site, and the apical oxygen is substituted by (OH) and points away from the octahedral sheet of gibbsite. A considerable shortening of the 0-0distances round the vacant octahedral site from the distance in gibbsite (about 3.2 A) to that of the edges of an SiO, tetrahedron (less than 3 A) would account for the rolling of the gibbsite sheet to form a tube.

F. Electron-Optical Investigations The X-ray powder diffraction technique has been universally applied to the study of clay minerals from the crystal structural viewpoint. It is well known that single-crystal X-ray-diffraction studies represent a powerful tool in crystal structure analysis and its refinement. Some clay minerals occur as single crystals of macroscopic sizes. Crystal structure analysis of “macrocrystalline analogues of clay minerals” is of great value in the study of the structures of clay minerals in general, provided care is taken to determine whether or not a modification to be specified in magnitude exists between the crystal structural behaviour of the clay mineral and its macro-crystalline analogue. The electron-optical method is of considerable importance in studying clay minerals. It may be

Gibbsite b irnogolite 2n/n

” a” Fig. 1.18 The structure of imogolite as related by that of gibbsite (Cradwick er al., 1972)

Electron-optical Investigations

31

divided into the observation of electron micrographs and the analysis of electron-diffraction patterns. Recent advances have provided many useful innovations in the techniques concerned. Basically, it can be said that the purpose of the method is to provide accurate information concerning (a) the size, shape, and crystal-structural detail of individual crystals, and also to reveal the micro-texture of their aggregations and (b) impurities of minor amounts, and their modes of association with principal constituents. The kind of information gained on the size and shape of individual crystals will be linked to the method of sample preparation, and the techniques used. One common technique is transmission electron microscopy of a powder sample, which is usually prepared by successive processes such as pulverization, dispersion (mostly in water) and then collection on a substrate. In some instances, the grinding tends to break down individual crystals, resulting in complete or partial obliteration of their crystal shape. Expandable clay minerals also tend to be separated into thin flakes when dispersed in water or other media. In some samples of smectite (e.g. montmorillonite or hectorite), dispersed thin flakes may tend to aggregate on the substrate by edge-to-edge association (Mering and Oberlin, 1967; 1971). However, these problems should not detract from the wealth of useful and accurate information that can be obtained. Scanning electron microscopy aims to view the surface features of clays, providing information on their crystal sizes and shapes, states of orientation and aggregations, i.e. the micro-textural features of the clays as exposed at the surface. Borst and Keller (1969) made a systematic survey of clays using this technique, and indicated that the results were valid for interpreting the modes of occurrence and origin of clays. Replica techniques with metal shadowing are valid for revealing fine textures of the crystal surfaces. They can provide information on nascent crystal growth patterns, etched figures, crystallographic directions due to twinning or cleavage planes, and various defects resulting from lattice imperfections or the presence of occluded impurities. Bassett (1958) developed the decoration method, whereby gold evaporated on to crystal surfaces preferentially migrate to cleavage and growth step and causes fine surface textures to reveal. This technique has successively been applied to clay minerals particularly kaolin minerals (e.g. Gritsaenko and Samotoyin (1966); Sunagawa, et al., 1975, a, b) revealing interesting features such as growth spirals and modes of layer packing which may be interpreted as related by crystal growth mechanisms and polytypes. Single-crystal electron diffraction has important applications in the single-crystal structural analysis of fine-grained materials. Particles of clay minerals with layer-structures are usually collected on a substrate as very thin platy particles oriented with the flaky planes parallel to the substrate surface and so normal to the electron beam. Crystal particles with a fibrous crystal habit are usually collected on a substrate so that the fibres are oriented parallel to the substrate surface. The electron-diffraction patterns from thin flakes are composed of spots occurring in a hexagonal arrangement (Fig. 1.19). These correspond to the nodes of the two-dimensional lattice, a*b*, reciprocal to the two-dimensional real lattice. The nodes are elongated along the c*-axis to various degrees, like spikes, or as diffuse streaks joining the spikes still with distinct maxima at the lattice points, or further as entirely continuous streaks, together with merging thin or disordered crystals having turbostratic structures. The Ewald sphere is regarded as being almost planar in the electron diffraction. The electron-diffraction pattern corresponds to the cross-section of the set of reciprocal lines given by the Ewald sphere. The intensity variation along each of these reciprocal lines can be expressed by the term, IF(hk)z12,where the Z-value is measured from a plane passing through the origin parallel to the principal plane of the crystal. Identical or nearly identical hexagonal spot patterns are obtained from all clay minerals having the layer-structure. From the patterns, it is difficult to find characteristics which are useful to differentiate one clay mineral from another. A specimen with preferred orientation yields powder rings having symmet-

32

CRYSTAL MORPHOLOGY AND STRUCTURE OF CLAY MINERALS A

-b*

I

I1001 b, b”

Fig. 1.19 Two-dimensionalelectron-diffraction.(A) Real and reciprocal lattices of a monoclinic pseudo-hexagoMI lattice which is commonly found in the clay minerals with the layer-structure. The electron beam, A, is normal to ub-plane, i.e. parallel to c*-axis. E: Ewald sphere. The reciprocal lattice points are extended as continuous streaks parallel to c*-axiscutting the Ewald sphere as spots to form the diffraction pattern as (B).

rical profiles in contrast to asymmetrical bands as observed in X-ray diagrams. Therefore, from both hexagonal spot patterns and powder rings, the parameters a, and bo can be directly measured in reference to the powder rings of gold used as an internal standard. Where a crystal is so thin as to be composed of a single layer, or is completely disordered in the direction normal to the layer, the intensity along each streak depends solely on the atomic arrangement within each twodimensional unit-cell. Comparing the intensities at the level 2 = 0 with observed ones, MCring and Oberlin (1967; 1971) analysed the symmetry of a single layer of smectite projected onto its plane. The general configuration of the projected structure is, in the dioctahedral sub-group, divided into 2 structures: one is called a non-centro-symmetrical structure denoted by the plane group clml when the vacant sites are located at one of the mirror images (Fig. 1.6(B)), and the other is a centro-symmetrical structure, c2mm, when the vacant sites are located on the mirror plane (Fig. 1.6(A)). These authors indicated that the studied samples of montmorillonite favoured clml and those of nontronite favoured c2mm. Specific differences have still been found between the calculated and observed intensities, and this may be reasonably considered as due to distortion of the structures. Referring to the distorted structure patterns of smectite reported earlier, these authors calculated the intensities from a distorted structure and found that a better agreement was obtained between the calculated and observed intensities in a sample of nontronite which was studied. According to these authors the intensities of all the spots having hexagonal symmetry as shown by a montmorillonite sample from Camp-Berteaux, were considered to be due to “twins” formed by edge-to-edge associations, and the turbostratic structure is mostly due to a rotational disorder in smectite. Crystal-structure analysis techniques for clay minerals by electron diffraction have been developed particularly by Russian workers (Pinsker, 1953; Zvyagin, 1967). Significant results have been obtained from an analysis of oblique-texture electron-diffraction patterns. Clay mineral particles with the layer-structure tend to lie with their flake faces in contact with the plane of the substrate, but they will have random orientation around an axis normal to those faces (c*-axis). An electron-diffraction pattern obtained from a specimen with the substrate normal to the electron beam may be called as “normal-texture electron-diffraction pattern” consisting of a set of powder rings each of which represents a perpendicular section of 2 = 0 of the system of concentric cylinders generated by the rotation of the reciprocal lines. When the texture is tilted through an

(b) (4 Fig. 1.20 Polycrystalline electron-diffraction patterns.(A) Platy crystals with a monoclinic structure. The principle is only shown schematically. c*-axis is parallel to the electron beam before tilting. pqrs: A cylinder generated by rotation of the 021 reciprocal lattice points around the texture-axis c*. Most clay minerals with the layer-structure have pseudo-hexagonal unit-cells, then 111 ring nodes appear also on the cylinder surface at different heights, but the 111 ring nodes are omitted in this figure C : Powder ring indexed as (20,l I). On tilting through an angle ((p), Ewald sphere of reflection is located at “t” as shown in the figure. The cross section of the cyIinder and the sphere is represented by an ellipse, E, on which the ring nodes are separated and arranged as arcs. (B) and (C) Mg-chlorite. Photographs of powder rings and an oblique texture pattern ((p = 60”)respectively.

34


angle rp, oblique-texture diffraction pattern is obtained, whereby each ring node is separated into 2 points centered on the direct beam spot on the diffraction pattern, and arranged on an ellipse corresponding to the cross-section of the Ewald sphere cut by the cylinder (Fig. 1.20). Actually, however, due to some misorientation of crystals, the spots occur as short arcs with intensity maxima extending along the ellipse, and in disordered crystals, the arcs are much more diffuse. In this way, overlapping reflections from polycrystalline specimens may be divided and measured, and it has been reported that this technique can reveal the distortion of the structures of some clay minerals, e.g. the direct determination of the deformed structure of kaolinite by Zvyagin (1960). The vacuum conditions employed form an obstacle to the determination of the "natural" forms of some clay minerals having interlayer water, such as smectite and halloysite (10 A), since these minerals dehydrate under vacuum conditions. Minor but specified modifications can occur on dehydration. Recently, environmental cells have been used for the electron-diffractionanalysis of fine-grained hydrated crystals and biological paracrystals. The crystal structure of halloysite (10 A) has been performed using a conventional electron microscope equipped with an environmental cell (Kohyama et al., 1978). One of the significant and interesting results obtained by these authors was that the halloysite (10 A) had a two-layer structure defined by the following lattice parameters: a, = 5.14 A, b, = 8.90 A, c, = 20.7 A, 1 = 99.7'. Nishiyama et al. (1974) studied the oblique-texture patterns of some mica-polytypes and randomly interstratified minerals of Al-mica-montmorillonite in conjunction with their crystalmorphological properties. The interstratified minerals, particularly with large proportions of expandable layers, show continuous diffuse streaks on ellipses with several intensity-maxima which can be assigned to each mica polytype. The unit-cell data for a new clay mineral, surite, (a, = 5.22 A, b, = 8.97 A, c, = 16.3 A, 1= 96.1') was determined from its oblique-texture electron-diffraction pattern (Hayase, et al., 1978). REFERENCES Bailey, S. W. and Brown, B. E. (1962) Amer. Miner., 47, 819. Bailey, S. W. (1963) Amer. Miner., 48, 1196. Bailey, S. W. (1969) Clays and Clay Miner., 17, 355. Bailey, S. W. (1975) Amer. Miner., 60, 175. Bailey, S.W. (1980) Clays Clay Miner., 28, 73. Bassett, G. A. (1958) Phil. Mag., Ser. 8,3, 1042. Bates, T. F., Hildebrand, F. A. and Swineford, A. (1950) Amer. Miner., 35, 463. Bates, T. F. (1959) Amer. Miner., 44,78. Biscoe, J. and Warren, B. E. (1942) J . Appl. Phys., 13, 364. Blatter, C. L., Roberson, H. E. and Thompson, G. R. (1973) Chys Clay Miner., 21, 207. Borst, R. L. and Keller, W. D. (1969) Proc.Int. Clay Conf., Tokyo (ed. L. Heller), 1, 871, Israel Univ. Press. Bradley, W. F. (1940) Amer. Miner., 25, 405. Brauner, K. and Preisinger, A. (1956) Tschermuks Miner. Petrogr. Mitt., 6, 120. Bray, R. H. (1937) Soil Sci., 43, 1. Brindley, G. W. and Robinson, K. (1946) Miner. Mug., 27, 242, Brindley, G. W. and Mering, J. (1951) Acta Crysr., 4, 441. Brindley, G. W. and Comer, J. J. (1956) Clays Clay Miner., 4, 61. Brindley, G. W. and Nakahira, M. (1958) Miner. Mug., 31, 781. Brindley, G. W. (1959) Amer. Miner., 44,495. Brindley, G . W. (1961) In The X-ruy Identification and Crystal Structures of Clay Minerals (ed. G . Brown), Ch. XII, Mineralogical Society, London. Brindley, G. W., Souza Santos, P. de and Souza Santos, H. L. de (1963) Amer. Miner., 48, 897; idem. (1964) Amer. Miner., 49, 1543. Brindley, G . W. and Souza Santos, P. de (1966) Proc.2nd Int. Clay Codf., Jerusalem (ed. L. Heller) 1, 3. Brindley, G. W. and Fancher, D. (1969) Proc.I t . Clay Conf., Tokyo (ed. L. Heller), 2,29, Israel Univ. Press. Brown, B. E. and Bailey, S. W. (1963) Amer. Miner., 48,42.

References

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Brown, G. and Weir, A. H. (1963) Proc. Int. Clay Conf., Stockholm (ed. I.Th. Rosenqvist and P. Graff-Petersen), 1, 27. Burnham, C. W. and Radoslovich, E. W. (1965) Yb. Carnegie Inst. Wash., 63, 232. Chukhrov, F. V. and Zvyagin, B. B. (1966) Proc. Int. Clay Conf., Jerusalem (ed. L. Heller), 1, i I . Cole, W. F. (1966) Clay Miner., 6, 261. Cradwick, P. D. G., Farmer, V. C., Russell, J. D., Masson, C. R., Wada, K. and Yoshinaga, N. (1972) Nature, Phys. Sci., 240, 187. Donnay, G., Donnay, J. D. H. and Takeda, H. (1964) Acta Cryst., 17, 1374. Eroshchev-Shak, V. A. (1970) Sedimentology, 15, 115. Fersrnan, A. F. (1913) Zap. Imp. Akad. Nauk., 32, 321. Frey, M. A. (1969) Contr. Miner. Petrol., 24, 63. Gatineau, L. (1964) Bull. SOC. Franc. Miner. Crystallog., 87, 321. Gaudette, H. E., Grim, R. E. and Metzger, C. F. (1966) Am. Minner., 51, 1649. Grim, R. E. (1953) Clay Mineralogy, McGraw-Hill, New York. Gritsaenko, G. S. and Samotoyin, N. D. (1966) Proc. Int. Clay Conf., Jerusalem (ed. L. Heller), 1, 391. Gruner, J. W. (1934) Amer. Miner., 19, 557. Guggenheim, S. and Bailey, S . W. (1977) Amer. Miner., 62, 1158. Giiven, N. (1971) 2. Krist., 134, 196. Hayase, K., Dristas, J. A., Tsutsumi, S.,Otsuka, R., Tanabe, S., Sudo, T. and Nishiyama, T. (1978) Amer. Miner., 63, 1175. Heckroodt, R. 0. and Roering, C. (1965) Clay Miner., 6, 83. Hendricks, S . B. (1939)Z. Krist., 100, 509. Hendricks, S . B. (1940) Phys. Rev., 57, 448. Hendricks, S. B. and Teller, E. (1942) J. Chem. Phys., 10, 147. Honjo, G. and Mihama, K. (1954) Act0 Cryst., 7, 511. Jackson, M. L. (1963) Clays and Clay Miner., 11, 29. Jagodzinski, H. (1949) Acta Cryst., 2, 201, 208, 298. Kakinoki, J. and Komura, Y. (1952) J. Phys. SOC. Japan, 7, 30. Kakinoki, J. and Komura, Y. (1954) J. Phys. SOC. Japan, 9, 169, 177. Kodama, H. (1966) Amer. Miner., 51, 1035. Kodama, H., Alcover, J. F., Gatineau, L. and Mkring, J. (1971) Structures et Proprittks de Surface des Mineraux Argileux, Symposium, Louvain, 15. Kodama, H. (1975) In Contributions to C/ay Mineralogy, Dedicated to Prof. Toshio Sudo on the Occasion of His Retirement, 7. Kodama, H. (1977) Miner. Mag., 41,461. Kohyama, N. and Sudo, T. (1975) Clays and Clay Miner., 23, 215. Kohyama, N., Fukushima, K. and Fukami, A. (1978) Clays and Clay Miner., 26, 25. Kunze, G. (1956) 2. Krist., 108, 82. Laue, M. von (1932) Z. Krist., 82, 127. Lazarenko, E. K. and Korolev, Yu. M. (1970) Zapiski Vses. Mineral. Obschch., 99, 214. Lippmann, F. (1954) Heiderberg. Beitr. Miner., Petrogr., 4, 130. Lippmann, F. and Johns, W. D. (1969) N. Jb. Miner. Mh. Jg., 1969, H.5, 212. Lister, J. S. and Bailey, S . W. (1967) Amer. Miner., 52, 1614. MacEwan, D. M. C., Ruiz Amil, A. and Brown, G. (1961) In The X-ray Identification and Crystal Structures of C/ay Minerals (ed. G . Brown), Ch. XI, 393, Mineralogical Society, London. Mackenzie, R. C. (1963) Clays and Cluy Miner., 11, 11. Martin Vivaldi, J. L. and Linares GonzAlez, (1962) Clays and Clay Miner., 9, 592. Mathieson, A. McL. and Walker, G . F. (1954) Amer. Miner., 39, 231. Mkring, J. and Oberlin, A. (1967) Clays and Clay Miner., 15, 3. Mkring, J. and Oberlin, A. (1971) In The Electron-Optical Investigation of Clays (ed. J. A. Gard), Ch. 6. 193, Mineralogical SOC.,London. Nagy, B. and Bradley, W. F. (1955) Amer. Miner., 40, 885. Nakahira, M. (1952)J. Sci. Res. Inst. Tokyo, 46, 268. Newnham, R. E. and Brindley, G. W. (1956) Acta Cryst., 9, 759. Nishiyama, T. and Shimoda, S. (1974) J. Toyo Univ., General Education (Nut. Sci.), NO. 17, 1. Nishiyama, T., Shimoda, S.,Shimosaka, K. and Kanaoka, S . (1975) Chys Clay Miner., 23, 337. Norrish, K. (1954) Disc. Faruday SOC., 18, 120. Pedro, G. (1967) Bull. Groupe Franc. Argiles, XZX,69. Pinsker, Z. G. (1953) Electron Diffraction (trans]. J. A. Spink and E. Feigl), Buttenvorths, London. Radoslovich, E . W. 1960) Acta Crysf., 13, 919. Radoslovich, E. W. 1961) Nature, 191, 67. Radoslovich, E.W. 1962) Amer. Miner., 47, 617. Radoslovich, E. W. and Norrish, K. (1962) Amer. Miner., 47, 599. Radoslovich, E. W. (1963) Roc. Int. Clay Conf. Stockholm (ed. I. Th. Rosenqvist and P. Graff-Petersen), 1, 3, Pergamon Press, Oxford. Ramsdell, L. S. (1947) Amer. Miner., 32, 64.

i

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Rayner, J. H. and Brown, G. (1964) Clays and Clay Miner., 13, 73. Reynolds, R. C. and Hower, J. (1970) Clays and Clay Miner., 18, 25. Robertson, R. H. S., Brindley, G. W. and Mackenzie, R. C. (1954) Amer. Miner., 39, 118. Roy, D. M. and Roy, R. (1954) Amer. Miner., 39, 957. Sato, M. (1969) Z. Krist., 129, 388. Sato, M. and Kizaki, Y. (1972) Z. Krist., 135, 219. Sawhney, B. L. (1972) Clays and Clay Miner., 20, 93. Schultz, L. G., Shepard, A. O., Blackmon, P. D. and Starkey, H. C. (1971) Clays and Clay Miner., 19, 137. Shimoda, S. (1964) Clay Sci., 2, 8. Shimoda, S. (1974) J. Clay Sci. SOC.Japan, 14, 79. Shimoda, S. (1969) Clays and Clay Miner., 17, 179. Shirozu, H. and Bailey, S. W. (1965) Amer. Miner., 50, 868. Shirozu, H. and Bailey, S. W. (1966) Amer. Miner., 51, 1124. Shirozu, H., Ozaki, M. and Hayashi, S. (1971) Clay Sci., 4, 45. Smith, J. V. (1954) Acra Cryst., 7, 479. Smith, J. V. and Yoder, H. S. (1956) Miner. Mag., 31, 209. Smith, J. V. and Bailey, S. W. (1963) Acta Cryst., 16, 801. Steinfink, H. (1958) Act. Cryst., 11, 191, 195. Sudo, T. and Nakamura, T. (1952) Amer. Miner., 37, 618. Suto, T., Takahashi, H. and Matsui, H. (1954) Jap. J . Geol. Geograph., 24, 71. Sudo, T. and Hayashi, H. (I 956) Nature, 178, 1 1 15. Sudo, T. and Kodama, H. (1957) Z. Krist., 109, 279. Sudo, T., Hayashi, H. and Shimoda, S. (1962) Clays and Clay Miner., 9, 378. Sudo, T. and Shimoda, S. (1977) Miner. Sci. Engng., 9, 3. Sudo, T. and Yotsumoto, H. (1977) Clays Clay Miner., 25, 155. Sunagawa, I. and Koshino, Y. (1975a) Am. Miner., 60,407. Sunagawa, I., Koshino, Y., Asakura, M. and Yamamoto, T. (1975b) Fortschr. Miner., 52, 217. Takeuchi, Y. (1965) Clays and Clay Miner., 13, 1. Takeuchi, Y. and Sadanaga, R. (1959) Act. Cryst., 12, 945. Tateyama, H., Shimoda, S. and Sudo, T. (1977) Amer. Miner., 62, 534. Tettenhorst, R. and Johns, W. D. (1966) Clays and Clay Miner., 13, 85. Tettenhorst, R. and Grim, R. E. (1975) Am. Miner., 60,49, 60. Udagawa, S., Nakada, T. and Nakahira, M. (1969) Proc. Int. Clay Conf., Tokyo (ed. L. Heller), 1, 151, Israel Univ. Press. Veniale, F. and Marel, H. W. van der (1969) ROC.Int. Clay Conf., Tokyo(ed. L. Heller), I, 233, Israel Univ. Press. Wada, K. (1961) Amer. Miner., 46, 78. Walker, G. F. (1956) Clays and Clay Miner.. 4, 101. Warren, B. E. (1941) Phys. Rev., 59, 693. Weiss, A. (1961) Angew. Chem., 73, 736. Weiss, A., Thielepapo, W., Goring, W., Ritter, H. and Schafer, H. (1963) Proc. Int. Clay Conf., Stockholm (ed. I. Th. Rosenqvist and P. Graff-Petersen), 1, 287, Pergamon Press, Oxford. Wiewibra, G. F. (1973) Proc. Int. Clay Conf., Madrid (ed. J. M. Serratosa), 75. Wilson, A. J. C. (1949a) X-ray Optics, Methuen, London. Wilson, A. J. C. (1949b) Acta Cryst., 2, 245. Yada, K. (1967) Act. Cryst 23, 704. Yoshinaga, N. and Aomine, S. (1962) Soil Sci. Plant Nutr. (Tokyo, 8, 6, 114. Zvyagin, B. B. (1954)Doki. Akad. Nauk, SSSR, 96, 806. Zvyagin, B. B. (1960) Soviet Phys. Crystallog., 5 , 32 (transl. from KristaIloarafixa, 5, 40). Zvyagin, B. B. (1960) Electron-Diffraction Analysis of Clay Mineral Structures (trans]. S. Lyse), Plenum Press, New York.

Chapter 2

Photographic and Specimen Preparation Techniques in Electron Microscopy

A. General considerations €3. Photographic techniques I. Selection of magnification 11. Dispersed powders 111. Replica specimens IV. Lattice images 1. Instrument conditions 2. Alignment 2.1. Axial illumination 2.2. Tilted illumination 3. Selection of aperture 4. Astigmatism correction 5. Conditions of the specimen and selection of the field 6. Selection of magnification 7. Focusing 7.1. Axial illumination 7.2. Tilted illumination V. Multi-beam lattice images (structure images) I . Specimen preparation 2. Crystal structure and orientation of the molecule 3. Observation through an electron microscope C. Electron diffraction I. Introduction 11. The scan-micro method 111. The Geiss method IV. Field limiting method V. Angular resolution (aperture angle) D. Specimen preparation I. Supporting the specimen 11. Hydrophilic treatment of carbon film 111. Microgrids 1. Principle of making microgrids 2. Preparatory procedures for making microgrid 3. Preparation of the microgrid 4. Mounting and after-treatment IV. Powder specimen dispersion method 1. Plate-shaped specimens 2. Rod-shaped specimens V. Replica techniques 1. One-stage method (preshadowed carbon replica method) 2. Decoration replica method VI. Ultrathin sectioning 1. Preparation of the embedding material 37

38

PHOTOGRAPHIC AND SPECIMEN PREPARATION TECHNIQUES

Embedding Polymerization Handling of the specimen to be put in a capsule Shaping of blocks The knife and boat Cutting Mounting on the specimen grid 9. After-treatment 10. Special considerations E. Conclusion Acknowledgment General References References 2. 3. 4. 5. 6. 7. 8.

A. General Considerations The performance of today’s electron microscopes is so advanced that images of atoms and molecules can be observed. In order to take advantage of this high performance and to obtain fine micrographs, the researcher needs to become fully acquainted with the construction and function of electron microscopes and operate them in a manner suitable for obtaining the desired image. For example, when taking micrographs of a powder specimen at 10,OOO times or of a lattice image at 100,OOO times or more, the electron microscope must be operated from completely different viewpoints with respect to the illumination method for the electron beam, specimen orientation, problems related to the number of diffraction patterns and size of the objective lens aperture, specimen drift, specimen damage, etc. On the other hand, good micrographs cannot of course be obtained if the specimen is poor. The more the performance of an electron microscope is improved, the greater should be the consideration given to specimen preparation. Satisfactory micrographs are those which adequately provide the extent of information desired. Preferably, also, they should be of pleasing appearance. Several books have been published on the general principles and structure of electron microscopes and on the various problems related to them, all of which are now probably well known to researchers. With respect to specimen preparation techniques also, the basic form is widely established and many refinements of application have been reported. It is simply necessary to determine which method is the most suitable for one’s specimen, i.e. to decide on a method which can be easily and reliably applied or to improve an existing method in an appropriate way. This Chapter outlines various electron micrographic and specimen preparation techniques which were used for taking the micrographs collected in this book, and includes detailed descriptions of the preparation methods for microgrids and sectioned specimens employed in micrography at high magnifications.

B. Photographic Techniques I. SELECTION OF MAGNIFICATION

Bearing in mind the aims of taking the particular micrograph, the photographic magnification

Photographic Techniques

39

should be selected according to the state of the specimen, resolution of the photographic emulsion, exposure time, etc. More precisely, the following questions should be asked. (1) How large are the known or expected structures to be identified in the specimen? (2) What level of resolution can be expected with the given specimen preparation technique? (3) Since intense illumination by the electron beam is required for micrography at high magnifications, is there any risk of resultant specimen damage, specimen drift or specimen contamination? (4) The resolution of the photographic emulsion is usually about 20-30 pm: what is the relation of this to the structures in the specimen? (5) The exposure time is determined by the sensitivity of the photographic emulsion and density of the incident electron beam : what is the relation between the amount of specimen drift and the exposure time? In general, there is probably a tendency to take micrographs at higher magnifications than are actually required, and in that case the image quality is usually poor. 11. DISPERSED POWDERS

The main purpose of observing a powder specimen is to determine its external shape and size. For clay particles, the photographic magnification should be a few thousand times to about 30,000 times. Any current electron microscope will have no particular problem in photographing within this range of magnification. In exceptional cases, e.g. when high magnifications in excess of 50,000 times (direct magnification) are required as in the case of imogolite and allophane, the instrument conditions should be carefully established and a microgrid (see section D) for supporting the specimen should be used to reduce the specimen drift due to the electron beam illumination, as required for obtaining lattice

Fig. 2.1 Contrast change obtained by tilting a replica film. Specimen: dickite.

40


images. The contrast should be determined by considering the accelerating voltage and the objective lens aperture for each specimen. In the case of a specimen having very little contrast such as montmorillonite, the accelerating voltage should be set at 50 kV and as small an aperture as possible should be used. While contrast can be adjusted to some extent by the photograph processing technique, it can also be enhanced by providing a very thin metal coating using the shadowing technique. 111. REPLICA SPECIMENS

In the photography of replica specimens, it is again preferable that the replica film be supported on a microgrid, as in the case of dispersed powder specimens, when a particularly high magnification is required. Recently, scanning electron microscopy (SEM) has been increasingly used for the observation of specimen surfaces. This is because of problems in specimen preparation, the enhanced performance of SEM, and the difference between images of three-dimensional structures. However, the replica technique excels SEM observation in the resolution of the 2-direction, i.e. the contrast to display fine topographies, and therefore surface observations by the replica technique may represent an effective procedure depending on the actual purpose of observation. By making stereoscopic images it is also possible to understand three-dimensional structures well, and by tilting a replica film, it is possible to enhance very weak contrasts (Fig. 2.1). Moreover, the decoration replica is the only method for observing the minute steps of crystal growth. IV. LATTICE IMAGES

1. Instrument conditions

Careful consideration should be given to the accelerating voltage, stability of the objective lens current, specimen drift, specimen contamination, vibrations, astigmatism, contamination of various parts of the column, etc., to ensure that the electron microscope can be operated in its optimum state. 2. Alignment When a lattice image is photographed, transmission waves and diffraction waves are made to interfere with each other, according to the following two methods. 2.1 Axial illumination As shown in Fig. 2.2A, alignment is effected so that transmission waves may be brought to the optical axis. The state in which only transmission waves T and diffraction waves D , interfere with each other, is called “two-wave interference.” On the other hand, when the crystal plane becomes parallel to the incident beam resulting in simultaneous reflection, T, D , and D , interfere with each other. This state is called “three-wave interference.” In the case of axial two-wave or three-wave interference, the voltage axis should be aligned while observing the image caused by the transmission waves. 2.2 Tilted illumination In Fig. 2.2B, the incident beam is tilted, and the transmission wave and the diffraction wave are placed in positions symmetrical to the optical axis. This method, advocated by Dowel1 (1962), is called “tilted illumination.” Another method (Komoda, 1966a) has been developed from this method, which is a technique for photographing crossed lattice images by means of tilted illumination. The feature of this tilted illumination method is to prevent the decrease in image contrast due to chromatic aberration, by placing the diffraction patterns at equal distances from the optical axis.


llllm[Tllllllllllnlllllll

D,

41

Specimen

D? 0bj.lens

\/

Fig. 2.2 Illumination methods for photographing lattice images. (A) Axial illumination; (B) tilted illumination.

F!g. 2.3 Positional relationship between the optical axis and diffraction spots in tilted illumination (observation of a (020) plane lattice image).

As shown in Fig. 2.3, the alignment in this case is made by tilting the incident beam so that the (OOO) spot and the diffraction pattern may be equidistant from the optical axis. 3. Selection of aperture The reflection of the lattice spacing d makes a diffraction pattern on the back focal plane of the object lens at a distance of r = fo.I/d, wherefo is the focal length of the objective lens and 1 is the wavelength. For photographing a lattice image of lattice spacing d, therefore, the aperture diameter D should be selected so as to satisfy fo.l/d < D < 3f0.1/d for two-wave interference, and 2f0*I/d< D < 4f0.1/dfor three-wave interference. It should be remembered, however, that if the aperture is too small, the image will be subject to the influence of astigmatism. Furthermore, the upper limit will be to prevent the influence of spherical aberration due to higher-order reflections. 4. Astigmatism correction In the case of axial illumination, there is no problem of astigmatism. However, in the case of

Two-wave interference

Seven- wave interference Fig. 2.4 Relationship between diffraction pattern and objective aperture in lattice image photography.


Two-wave

43

Seven-wave

1 interference 1 I interference Fig. 2.5 Reference photos of two-wave (A) and seven-wave (B) interference lattice images. Specimen: pV0phyllite.

l0Oi

l0Oi

tilted illumination, the image obtained gives the appearance of having astigmatism. Since this is actually not due to astigmatism, it is not necessary to adjust the stigmator. In other words, all that has to be done is to tilt the illumination system after correcting the astigmatism by axial illumination. In the observation of a one-direction lattice image, the astigmatism, if any, may be ignored. However, when a multi-direction lattice image is being observed, astigmatism corrections should fully be made. 5. Conditions of the specimen and selection of the field Since the specimen receives an intense beam of illumination when a lattice image is being photographed, it must be stable against the electron beam. Naturally, the specimen must be sufficientlythin to be regarded as a phase object. Also, the crystal plane to be observed must be nearly parallel to the incident beam. In a crystal, the intensity of both the transmission waves and diffraction waves varies periodically as a function of the crystal thickness. Thus, the contrast of the lattice image also varies according to the thickness of the crystal. In the case of two-wave interference, if the Bragg condition is completely satisfied, maximum contrast is obtained where the specimen thickness is 1/4 of the extinction distance; and as the Bragg condition becomes less satisfied (Komoda, 1966b), so the maximum contrast is obtained in a somewhat thicker part of the specimen. A microgrid should of course be used as a specimen support, to prevent the specimen drift. In selecting the field, a search should be made for a shining part in the dark field image, or for a part where an image caused by diffraction waves can be seen in the bright field. 6. Selection of magnification

It is necessary to select a magnification which satisfies Md > 6,, where d denotes the spacing of the lattice image to be observed, M the photographic magnification, and 6, the resolution of the

44


emulsion. If Md is very close to 6,, the lattice image loses contrast. In general, the magnification should be determined by considering 6, as nearly 40 pm. 7. Focusing When an image formed by diffraction waves can be observed on the fluorescent screen, focusing should be carried out so that an image formed by transmission waves is brought onto the image formed by diffraction waves. This condition is discussed below. Even if an image caused by diffraction waves is not seen on the fluorescentscreen, focusing should be made. Placing transmission waves and diffraction waves together means that the displacement of the image due to the spherical aberration is corrected by defocusing. Thus, when the spherical aberration coefficientis denoted as C,, the angle of the diffraction beam against the optical axis as a, and the amount of defocus as A f , C,u3 - Afu = 0, i.e. A f = C,u2 represents the above-mentioned focusing conditions. 7.1 Axial illumination Denoting the Bragg angle by 8, u = 28 for axial illumination. Therefore, the focus is given by A f = 4C,02 = C,(A’/d2), where A is the wavelength of the electron beam and d is the lattice spacing. Since A f > 0, the focus is under-focus. 7.2 Tilted illumination In the case of tilted illumination, u = 8, and therefore Af = C,02 = 1/4C,(Az/d2).Although the focus is under-focus because A f > 0, the image actually obtained by the transmission waves becomes an over-focused image. This is because the incident beam has an aperture angle. It is apparently to be an over-focus of A f = -2CS8’ on the image obtained (Yada and Hibi, 1968).

V. MULTI-BEAM LATTICE IMAGES (STRUCTURE IMAGES) The photographing of this kind of image is basically the same as that for the lattice image mentioned above. However, the points of difference may be summarized as follows. (1) The voltage axis should be better aligned since the interference is all due to axial illumination. (2) The aperture size, which is limited by the spherical aberration of the objective lens, should be selected in accordance with the number and size of diffraction patterns. (3) The orientation of the specimen should be adjusted so that the diffraction pattern may be symmetric. (4) In order to obtain a micrograph having an optimum focus, which is important in image interpretation, several through-focus micrographs before and after optimum focus should be taken. Research on structure images began with the photography of the phthalocyanine molecule by Uyeda et al. in 1970 and the observations on the block structure of Nb20,, etc. by Cowley and Iijima in a similar manner at about the same time (1971). The photographicp rocedures for molecular images employed by Uyeda et al. will now be introduced. 1. Specimen preparation In order to increase the scattering power of the electron beam and enhance the contrast, copper hexachloro-phthalocyanine,a derivative of copper phthalocyanine, in which the 16 hydrogens around the macromolecular ring are substituted with chlorines (Fig. 2.6), was used. Such C1 substitution imparts a resistance to the electron beam which is 40 times greater. Since the molecule is thermally stable, it was evaporated on a KCl single crystal in a vacuum of lo-’ torr, yielding a crystal of 100 A or less in thickness.


45

C

C CI

CI

Fig. 2.6 Structural formula of chlorinated copper phthalocyanine molecules. (Source: Uyeda et a/., 1972. Reproduced by kind permission of the American Institute of Physics.)

\

y : o\\. 1

a

-+4--*-

y : y \

y:y.1

b

\

w

\

\

\

\ \

\

--+$

\

\

---Pc ring Fig. 2.7 Stacking of evaporated phthalocyanine molecules (Uyeda et a/., 1970).

2. Crystal structure and orientation of the molecule It is necessary to determine whether planar molecules assume a column structure, and whether their arrangement is properly oriented with respect to the habit plane. However, since the crystal was too small to undertake a structural determination using X-rays, two other methodsselected aread iffraction and optical diffraction-were employed. In this way, the dimensions of the unit cell were determined as a = 19.62 A, b = 26.04 A, c = 3.76 A, and /? = 116.5'. It was also inferred that the molecules were piled up in the direction of the c-axis as a column axis at an angle of 25" (Fig. 2.7). Thus, when observed in the direction of the column axis, all the molecules in the column lie one on another and become equal to the micrography of one molecule, as illustrated in Fig. 2.8. 3. Observation through an electron microscope

To elucidate the fine structure of molecules, use of as much effective reflection as possible is desirable, but it is impossible to admit waves of an unlimitedly large scattering angle into the lens because of spherical aberration. In other words, to form an image through recombination of dif-

46


-b

Fig. 2.8 Projection of phthalocyanine molecules. (Source: Uyeda et a/.,1972. Reproduced by kind permission of the American Institute of Physics.)

Fig. 2.9 Phase factor vs. optimum aperture angle a, (Uyeda et al., 1970). CS,1.4 mm; A,, 870 A; 1, 0.037A.

fraction waves, the intensity and phase of all the respective diffraction waves must be retained at the image forming plane. Fig. 2.9 plots the phase factor cos x corresponding to the amount of defocus Af = 870& which is the optimum image forming condition for the wavelength 1 = 0.037 A and spherical aberration coefficient C, = 1.4 mm. The objective lens aperture was selected so that the reflection waves were within the scattering angle a,, beyond which the phase factor oscillates rapidly. As the crystal plane is inclined at 25"to the column axis, the specimen was tilted with a specimen tilting device so that the electron beam became parallel to the column axis. This allowed the diffraction pattern in Fig. 2.10.A to be obtained. Fig. 2.1 1 shows a molecular image of phthalocyanine obtained by this method. Four-leaf clovershaped molecules form a two-dimensional face centered cubic lattice, but the details are not resolved. This is presumably due to spherical aberration, the large thickness of the crystal, and the thermal vibration of the molecules. Results obtained with the same specimen cooled with liquid helium and observed at a 500 kV accelerating voltage are shown for reference in Fig. 2.12. Fig. 2.13 illustrates how much the image quality depends on differences in the spherical aberration coefficient.

Fig. 2.10 Diffraction patterns of a copper phthalocyanine crystal. (A) Incident electron beam is inalignment with the crystal surface. (Source: Uyeda et a/.,1972. Reproduced by kind permission of the American Institute of Physics.)

Fig. 2.11 Molecular image of copper phthalocyanine obtained at 100 kV (contrast-reversed image). (Source: Uyeda ef al., 1972. Reproduced by kind permission of the American Institute of Physics.)

48


Fig. 2.12 Molecular image of copper phthalocyanine obtained with an HVEM (500 kV). (After Uyeda ef af.)

C. Electron diffraction I. INTRODUCTION

The history of using transmission electron microscopy to obtain an electron diffraction pattern corresponding to an image (TEM image), began when Boersch (1936) developed the three-stage lens system. Subsequently, Le Poole (1947) established the technique of selected area diffraction, which is widely used today and has made enormous contributions to research on crystalline materials. This technique, however, has the disadvantage that the correspondence of the fields between the image and its diffraction pattern is limited by the “diffraction error” (Agar, 1960) due to the objective lens spherical aberration, and that the minimum effective selected area is usually about 1 pm* in the case of a TEM with an accelerating voltage of 100 kV. This means that corresponding to advancement of the performance of the TEM, images of a micro-area

* This “diffraction error” phenomenon is reduced by using a shorter wavelength electron beam at a higher accelerating voltage. At lo00 kV, diffraction patterns from an effective area of 500 A diameter can thus be obtained.

Fig. 2.13 Image quality difference with spherical aberration difference. A, Nb205;C. = 10 mm, C, = 4.3 mm, Fo = 6.8 nun, lo00 kV (photo by Higata et al., Tohoku Univ.); B, C. = 0.7 mm, C, = 1.1 nun, FO = 1.5~nm; C, C , = 2 . 8 ~ ,C C = 1 . 6 m , Fo=2.0mm; D, C 8 = 4 . 8 ~ ,C c = 3 . 9 m , F o = 5.1 mm at 100 kV.

50


can be clearly observed, but the diffraction area does not keep pace. Recently, this contradiction has been solved, and the method of taking diffraction patterns from a much smaller area has been practically applied. The techniques for photographing a diffraction pattern of an ultra-micro area, will now be discussed. Riecke (1961) advocated selecting the field directly by using a finely focused electron beam, instead of selecting the aperture, and he succeeded in obtaining a diffraction pattern from a ca. 100 A diameter area in a specimen placed at the lens center, by using a condenser-objective lens c/o lens) Riecke (1969).* This method is called micro beam diffraction. Since the specimen is directly irradiated with a finely focused electron beam, a diffraction pattern from the irradiated area only is obtained. It is possible therefor to obtain diffraction pattern of an ultra-micro area which are free from “diffraction error.” The field area in this case is determined by the diameter of the extreme tip of the electron beam. In practice, however, it is rather difficult to make a finely focused electron beam hit the precise ultra-micro area of interest, and applications in research are generally technically difficult. In both the selected area diffraction method and the micro beam method, the lens system of the TEM can be utilized as it is. In recent years, however, a TEM provided with the capability of a scanning electron microscope (SEM), viz. an instrument capable of scanning transmission (STE-) work, has been developed (Koike et al., 1970). (Fig. 2.14). By making effective use of this

Fig. 2.14 General view of a 200 kV TEM with a scanning and analytical attachment (JEM-ZOOCX, JEOL).

* The functions of both the condenser lens and objective lens are performed simultaneously in the magnetic field of a single strongly-excited lens (pole piece).

I

Objective lens Specimen

w1

Scanning coil

inerator Deflection I dark field

Intermediate aperture Intermediate

Fluorescent

I

I

CRT

Detect0

Fig. 2.15 Ray diagram of the scan-micro method.

Fig. 2.16 Ray diagram of the Geiss method.

52


STEM capability, the correspondence in field of view between an image and its diffraction pattern has been greatly improved, and diffraction patterns from areas as small as 200 A in diameter can now be easily obtained Koike et al., (1972). Van Oostrum et al. (1973) obtained a diffraction pattern from a 100 A area, using the beamrocking method, in which the incident angle of the primary electron beam is changed linearly to display synchronously the detected intensity of the diffracted electrons on CRT, through brightness modulation. Further, Geiss (1975) succeeded in obtaining a diffraction pattern from a 30 A area, using a method which combined the c/o lens with the rocking technique. The diffraction method using a STEM is called the microbeam diffraction-STEM method or the scan-micro method. The combination of beam-rocking and the c/o lens developed by Geiss, is called the beam-rocking-microdiffractionmethod or simply the Geiss method.

Fig. 2.17 Example of changing the visual field area with beam diameter in the Geiss method: micro-micro-area diffraction patterns from areas of different size, (A), (B), (C) and @). on an Ag/MoS, specimen.

Electron Diffraction

53

11. THE SCAN-MICRO METHOD

The use of a scanning attachment in the TEM specimen chamber produces an electron beam which is a few tens of an angstrom or less in diameter on the specimen, but it increases the aperture angle of the incident beam to the order of rad, leading to poor resolution in the diffraction pattern obtained. In practice, therefore, the SEM should be used under conditions with a beam diameter of about 200 A and an aperture angle of 1 x rad. As shown in Fig. 2.15, the diffraction pattern formed on the back focal plane of the objective lens is projected as an enlarged pattern onto the fluorescent screen by the intermediate lens. A stationary diffraction pattern can be observed on the screen even when the electron beam is scanned across the specimen. When the electron beam is kept at an optional position on the specimen with a beam spot positioning device while observing the after-image of an STEM image on the CRT, a diffraction pattern from a 200 A area is projected onto the fluorescent screen. 111. THE GEISS METHOD

Fig. 2.16 gives a ray diagram of the Geiss method. The specimen is placed at the center of the magnetic field of the c/o lens. The electron beam passing through the condenser lens is deflected by the rocking coil placed beneath it and is redeflected in a contrary direction by the front magnetic field of the c/o lens. As a result, the beam incident on the specimen is rocked at a point of the specimen. That point of the specimen is projected as an enlarged image on the fluorescent screen through the rear magnetic field of the c/o lens, the intermediate lens and the projector lens. Moreover, on the back focal plane of the c/o lens a diffraction pattern is formed from the specimen and an objective lens aperture is in that plane. As is clearly apparent from the ray diagram, all the electrons contributing to the image-formation pass near the lens axis, and so the enlarged image on the screen remains still even though the incident beam is rocked on the specimen. Only the contrast of the image varies according to the rocking angle, i.e. the angle of illumination to the specimen. Thus, a diffraction pattern is obtained by (1) inserting a detector into part of this enlarged image

10-3

20

40

60

80

100

120

140pm

OL aperture diameter

Fig. 2.18 Relationship between objective aperture diameter and angular resolution of diffraction pattern.

Fig. 2.19 Application of scan-micro diffraction to clays. (A) Specimen is sepiolite (section) ( x 90,000).

Fig. 2.19-continued. (B) Specimen is a halloysite particle. ( x i75,OOO).

56


for detection of the image signals, and then (2) displaying the detected signals on the CRT after brightness modulation in synchronism with the rocking. The changeover between the diffraction pattern and the ordinary TEM image can be performed instantly by rocking on and off. IV. FIELD LIMITING METHOD

When the size of the detector is denoted as d and the magnification of the enlarged image at that plane as M , the selected area is given by d/M (Riecke, 1961 and 1969). The detector is the photomultiplier tube used in the STEM. A hole of 1 mm in diameter is made in the center of the fluorescent screen, to allow only those electrons which have passed through the hole to be admitted into the detector. This is the manner in which the field limiting is performed. Since the magnification of the enlarged image on the screen can be continuously changed as a TEM image, the use of an optional magnification permits the selection of a wide range of areas from the cu. 1 pm areas obtainable in ordinary selected area diffraction down to ca. 30 A micro-areas. Field selection is performed by moving the field from which to obtain a diffraction pattern to the position of the hole on the screen with the specimen movement mechanism. Fig. 2.17 shows an experimental example obtained with silver particles epitaxially grown on molybdenite (MoS,). The diffraction patterns from the areas indicated by the concentric circles A, B, C, and D in the TEM image, are labelled with the same symbols. The selected areas measured 50 A, 100 A, 300 A and lo00 A. In image A there exist only 3 approx. 17 A moir6 patterns of Ag/MoS,, indicating that the diffraction pattern A was obtained from an ultra-micro area of ca. 50 A In this case, an objective aperture of 7 pm in diameter was used. V. ANGULAR RESOLUTION (APERTURE ANGLE )

When the focal length of the objective lens is denoted as f and the diameter of the objective aperture as a, the angular resolution of the diffraction pattern 28 is given by 28 = a f (Riecke, 1961 and 1969). Fig. 2.18 shows both theoretical and experimental values. The larger the objective aperture, the poorer is the angular resolution. The minimum aperture used in Fig. 2.17 was 7 pm, and the rad. In the diffraction pattern D in Fig. 2.17, the aperture angle in such a case is about 2 x Ag (220) pattern and the MoS, (1 120) pattern are clearly separated, indicating that the value of this angular resolution is obtained. For reference, an application to a clay specimen is shown in Fig. 2.19.

D. Specimen preparation I. SUPPORTING THE SPECIMEN

A carbon film can be used for dispersed powder specimens, and a microgrid with or without a carbon film for specimens requiring high magnification photography. The role of the supporting film is not only to support the specimen. For example, in high-magnification observations of a fibre-structure specimen such as imogolite or chrysotile mounted across a small hole in a microgrid having no supporting film, the specimen is in many cases cut off at the position near the centre of the hole by the electron beam. If such a specimen is mounted on a supporting film, however, no cutting-off occurs. The supporting film thus helps to limit specimen damage due to the electron beam. On the other hand, a supporting film, if used in high-magnification observations, can often be an obstacle to image interpretation, since the image due to the phase contrast of the film itself, i.e. the background noise, is superimposed on the fine structure of the specimen.

Specimen Preparation

51

According to the particular research purpose, therefore, it is necessary to decide whether or not the use of a microgrid with a supporting film is desirable. 11. HYDROPHILIC TREATMENT OF CARBON FILM

The carbon film is physically and chemically stronger than plastic films such as Collodion and Formvar. However, it has the disadvantage of being hydrophobic. When a droplet of diluted suspension containing a powder specimen dispersed by ultrasonic vibration is placed on a carbon film, it does not dry up in a homogeneously spread state due to the high surface tension of the droplet on the carbon film. When a carbon film is used, therefore, hydrophilic treatment of its surface is required. The treatment methods include using an anion surface active agent (Van Oostrum et al., 1973) or vacuum-evaporating silicon monoxide, although method of ion-

Fig. 2.20 Low magnification view of microgrids ( x 500).

58


bombarding the surface of the carbon film (Fleisher et al., 1967)is usually employed. This can be done easily with a compact sputter coating device available commercially for the coating of SEM specimens. We utilize an FC-1100 Sputter Coating Device (JEOL), which performs ion bombardment for 40-60 sec at voltages of 500-700 V and currents of 4-5 mA. 111. MICROGRIDS

Ideally, the smaller the specimen, the thinner should be its supporting film. However, the thinner the supporting film, the more fragile it is. This not only renders it difficult to support a specimen with a conventional supporting grid, but also gives rise to the greatest cause of specimen drift. To overcome this drawback, a microgrid with holes of 0.1-10 pm diameter, which is far smaller than the mesh of a supporting grid, should be prepared on the supporting grid (Fig. 2.20). Such a microgrid should be used as it is or with a specimen supporting film placed on it. Numerous methods for making microgrids have been utilized since the microgrid was first advocated by Sjostrand(l956). We employ the method of Fukami and Adachi (1965 and 1966). 1. Principle of making microgrids

As illustrated in Fig. 2.21, a glass slide is cooled down below the dew-point in an electric refrigerator. The glass slide is then removed and exposed to the room atmosphere to form dew-drops on the slide surface. Next, a diluted solution of plastic is poured on until it is spread over the dewdrops on the glass slide. Along with volatilization of the solvent, a plastic film is formed. However, due to the presence of the dew-drops the plastic film becomes perforated by micro-holes, which are in positions corresponding to and of the same sizes as the original dew-drops. In order to maintain the hemispherical shape of the dew-drops, a cation surface active agent is used which functions to render the glass slide surface hydrophobic to the extent desired. The size and density of the dew-drops so formed on the glass slide can be controlled by a combination of the following two factors: (1) the hydrophobicity of the glass slide, which differs according to the kind of surface active agent employed, and (2) the temperature of the glass slide surface below the dew-point. It is, thus, possible to manufacture a microgrid with holes of any desired size and distribution density. 2. Preparatory procedures for making microgrid A glass slide of excellent quality (about 0.8 mm in thickness) is cleaned by means of ultrasonic cleaning in a solvent such as carbon tetrachloride or l,l, I-trichloroethane. The following materials

/ //

///

///

/I/

Thin adsorption layer of water-repellent Microscope slide glass Minute water-droplet Liquid layer of dilute solution of plastic Thin deposited layer of plastic

-

Self-perforated micro grid

Fig. 2.21

Microgrid preparation procedure (Fukami et al., 1965).


59

TABLE 2.1 Plastic material, solvent and agents for making microgridst (1) Plastic material and solvent Plastic material: cellulose acetobutyrate (Triafol) Solvent: ethyl acetate (extra purity) Concentration: 0.1-0.5 % W/V) (2) Hydrophobic agents (i) To make a microgrid with holes of 1.5 pm or less in diameter. @ Distearyl dimethyl ammonium chloride (Softex KWO) @ Benzalkonium chloride (ii) To make a microgrid with large holes of 1.5-lOpm in diameter. @ Polyoxyethylene stearyl propylene diamine (Diamiet 315) @ Polyoxyethlene lauryl amine (Amiet 105) (iii) Concentration of agent to be dissolved in distilled water: 0.03 % (W/V)

(3) Hydrophilic agent Anion surface active agent for rehydrophilic treatment of the hydrophobic-treated glass slide. (i) Sodium diakyl sulphosuccinate (Pelex OTP). (ii) Concentration of agent to be dissolved in distilled water: 0.5 %

w/v)

t The materials, dubbed “microgrid-making materials,” are available commercially in Japan. TABLE 2.2 Pre-cooling temperature of the metal plate Dew point (“C)

Pre-cooling temp. (“C)

10 or higher

5-10 0-5

5 0 -5

should be prepared (see Table 2.1) : (1) Plastic material and solvent, (2) Aqeuous solution for hydrophobic treatment of the glass slide, and (3) Hydrophilic solution for separating the microgrid film. To cool the slide glass homogeneously, a well-polished metal plate such as of brass, measuring 1 cm in thickness and 15 cm in length and width, is pre-cooled according to the conditions indicated in Table 2.2. 3. Preparation of the microgrid (1) For the hydrophobic treatment of the surface of the glass slide, a hydrophobic agent suitable for preparing the microgrid with holes of the desired diameter is selected from among those indicated in Fig. 2.22. This aqueous solution is placed in a dyeing bottle which can accommodate 5 to 10 glass slides. Clean glass slides are immersed in the aqueous solution for 10-30 min. Then, they are removed, placed in distilled water in a 500 ml beaker, and shaken slowly to eliminate the excess hydrophobic agent. After repeating this operation 2 or 3 times in new distilled > water, the slides are dried. The slides should be picked up slowly from both the hydrophobic agent and the distilled water, so as not to leave any droplets of the agent or the water on them. If droplets of the agent do remain on the slides, it will mean an excessive or insufficient thickness of adsorption layer of the

60

PHOTOGRAPHIC AND SPECIMEN PREPARATION TECHNIQUES Hole size of self-perforated micro grid

0.03 % aqueous solution of water-repellents

Concentration of the solution of Triafol

I

I

0.1%

I I I I

I

0.1%

I

Difference between dew-point and cooling temprature

very little

I I I

I

iI

I I

little

I I

I

Amietl05

I I

I

0.2%-0.3%

i

i

I

0.3%LO.5%

I I little-medium I I I I medium-large

Fig. 2.22 Relation between hole diameter of microgrid and materials used (Fukami et at., 1965).

hydrophobic agent, for which the dipping time should be adjusted. If droplets of water remain, it will mean excessive or insuffcient washing of the hydrophobic agent with the distilled water. (2) The glass slides after completion of such hydrophobic treatment are placed on a metal block pre-cooled in a refrigerator, and cooled to below the dew-point. They are then removed from the refrigerator into the room atmosphere, and dew-drops are allowed to form rapidly on them. Since the size of the dew-drops will be directly related to that of the holes of the microgrid, the time for cooling will be shorter when a microgrid of smaller hole size is required, and longer when a larger hole size is required. The cooling time is generally in the range of 3-50 sec. (3) Since dew-drops are formed rapidly on the glass slides after their removal from the refrigerator, the Triafol solution is poured on quickly until it becomes spread out. The superfluous solution is drained off with a filter paper and the glass slides are kept horizontal to volatilize the solvent. The microgrid layer is formed within a few minutes after the solvent has volatilized before the disappearance of the dew-drops. The concentration of the Triafol solution must be changed according to the hole size desired (see Fig. 2.22). For checking the microgrid so prepared, an optical microscope can be used. This enables the holes to be observed directly, except for those of especially small size. If the relative room humidity is 50%, the Triafol solution should be pre-cooled to below the dew-point. When it is above 70%, spreading of the Triafol solution and volatilization of the solvent under lighting with a 250 W infrared lamp successfully prevents the formation of holes of an extraordinary size due to the high humidity. (4) For separation of the microgrid layer from the glass slide, “on-water-surface separation” is carried out. However, this separation technique is often difficult to perform in practice since the surface of the glass slide has been subjected to hydrophobic treatment, which effectively prevents water from infiltrating into the space between the microgrid layer and the slide surface. The glass slide with the microgrid is therefore immersed in a hydrophilic agent for 3-10 min, the dipping time varying according to the strength of the hydrophilic agent. Even without the hydrophilic treatment, however, separation can sometimes occur. Next, to wash away the hydrophilic agent from the surface of the microgrid, the glass slide is placed in distilled water. This washing should be carried out carefully so that the microgrid layer does not peel off from the glass slide. Then, after removal from the water, the slide is dried ready for the on-water-surface separation.


4.

61

Mounting and after-treatment

The following procedures should be carried out in advance. Specimen supporting grids are placed on the glass slide. A toluene solution containing 0.2 % or 0.3 % (W/V) chloroprene rubber (Neoprene W; Dupont Co.) is dropped onto the glass slide in such a manner that a single drop of the solution covers 3 or 4 supporting grids (see Fig. 2.23). As soon as the grids are covered, the superfluous solution is drained off with a filter paper. The solution volatilizes in about 1 min, and the supporting grids are left covered with a thin tacky layer, so adhering to the surface of the glass slide. The solution of chloroprene rubber is used for adhesion of the suporting grid and the microgrid layer. This adhesive treatment can be adopted for both a supporting grid and a carbon film. The bonding agent has a thermosetting characteristic, so that it increases in adhesive effect on irradiation with the electron beam. Our experience indicates that no trouble is caused with the specimen due to the bonding agent. (2 % solution dubbed “Mesh Cement” is commercially available in Japan.) As shown in Fig. 2.24, the glass slide with adhesive-treated supporting grids is placed into a Petri dish filled with distilled water, in order to permit the microgrid layer to be pealed off and floated onto the water surface. While the distilled water is drained off slowly, the microgrid layer is deposited on and fastened to the surface of the supporting grid. The superfluous water is absorbed with a filter paper so as to dry the microgrid layer. The dried microgrid layer is then vacuumevaporated with carbon rather thickly to reinforce its physical and chemical strength.

Y -\Chloroprene

rubber solution

Supporting grid

How to stick supporting grids onto a glass slide (Fukami e t a / . , 1965).

Fig. 2.23

I

--------> Adhesive treated supporting grids

==-Fig. 2.24

How to put a microgrid film on a supporting grid.

62

PHOTOGRAPHIC AND SPECIMEN PPREARATION TECHNIQUES

To provide the completed microgrid with a carbon supporting film, carbon film vacuumevaporated on a cleavage surface of mica is floated onto the surface of the water and deposited on and fastened to the microgrid surface. More than 100 supporting grids with microgrids can be prepared on a single glass slide. IV. POWDER SPECIMEN DISPERSION METHOD

The conditions for dispersing a powdered specimen differ according to the bond between the single particles or their size and morphological habit. Powdered specimens which are well dispersed in a test tube or mortar frequently reaggregate when dried on a supporting film. It is difficult, therefore, to decide which method is best of those now available for preparing powdered specimens. An appropriate method should of course be adopted to place the single particles homogeneously on the supporting film without aggregation. Practically, however, success may be governed more or less by chance. It is necessary to locate, in the image projected onto the fluorescent screen, a field of view with the best or at least well dispersed particles. The dispersion methods can be classified broadly into wet- and dry-methods. For clay specimens, the wet-method is generally used, as outlined below. 1. Plate-shaped specimens A few mg of clay powder is placed in a mortar and ground well with only a little distilled water. One drop of the resultant emulsion is spread over a carbon film subjected to hydrophilic treatment, and the supporting film is tilted to allow the emulsion specimen to flow away with washing water. The residual water on the film surface is adsorbed away with a filter paper. It should be noted that the water existing between the tweezers also, has to be taken away. This dispersion method utilizes the adhesion of a plate-shaped specimen to the thin film. The single platy clay which adheres to the film surface survives the washing water and remains on the film, while most aggregated clay is washed away.

2. Rod-shaped specimens Rod-shaped specimens are dispersed in distilled water, using a mortar or ultrasonic vibration. One drop of the suspension is spread over a supporting film, and the dropped suspension is drained off with a strip of filter paper. If the specimen on the film is found to be too large after drying, one drop of distilled water is placed on the dried suspension, and the superfluous specimen is drained off with a filter paper. V. REPLICA TECHNIQUES

Several replica techniques are available such as the one-stage and two-stage methods. The former is probably the most frequently used, and is used mainly by us. In the one-stage method (often called the preshadowed carbon replica method), the specimen is directly shadowed with platinum, covered with a carbon replica film, and then dissolved. For bulk structures of solids, however, it is necessary to utilize the two-stage method. 1.

One-stage method (preshadowed carbon replica method)

The powdered specimen is dispersed on a glass slide and then shadowed with platinum. The shadowing angle is about 45" for general specimens, and 50-60" for fine structures, depending on their size. The carbon replica film should be given the minimum necessary thickness for maintaining specimen strength during specimen preparation. Usually, hydrofluoric acid is employed as a specimen solvent. In cases where a high-magnification replica image is required, the platinum particles used for

Specimen Preperation

63

shadowing sometimes create problems. In such cases, a replica film obtained only by direct carbon shadowing is used. With carbon shadowing, small amounts of the carbon reach even the shaded portions of the specimen, allowing a carbon replica film to be formed simultaneously. In such cases, the dissolving time is within 30 min with hydrofluoric acid. 2. Decoration replica method

This is a modification of the one-stage replica method. With the specimen heating device installed in the evaporation equipment which we use, it is impossible to measure the specimen temperature accurately. Therefore, we evaporate gold after keeping the specimen for about 30 min at an indicated, empirically selected temperature of 350°C. The evaporation distance is 10 cm and the amount of evaporation is controlled by adjusting the opening time of a shutter provided between the source of evaporation and the specimen. Generally, the opening times are 1 or 2 sec. After the carbon replica film has been prepared, the specimen is dissolved. VI. ULTRATHIN SECTIONING

With biological materials, ultrathin sectioning using an ultramicrotome is indispensable in specimen preparation for electron microscopy. Also, biological specimens require such pretreatments as fixation and dehydration to prevent morphological changes. For clays, however, such pretreatments are not carried out at present. It is desirable that the clay, which contains water, can be observed as it is. However, such observations are still in the experimental stage, and a primary ultrathin sectioning suitable for clays is described here. The sectioning procedure is the same as for biological specimens: a powdery specimen is solidified in a plastic and formed into small blocks, which are sectioned with a diamond knife.

1

I

1.

Preparationmaterial of embedding After-treatment

I-I-FI

I-I

Embedding

Observation

I

Polymerizing

F1-I

Trimming

-TISectioning

Preparation of the embedding material

For biological specimens, a variety of resins .are used. For clays, it is convenient for after-treatment to use methacrylate resins. In general, 2 types of resins, n-butylmethacrylate and methylmethacrylate, are employed in different mixing ratios to adjust the hardness after polymerization. The monomers of commercially available methacrylate resins are delivered with hydroquinone added as an inhibitor. The hydroquinone and water must therefore be removed from the monomers immediately before use. Hydroquinone removal and dehydration (1) Prepare an aqueous solution of 3-5 % NaOH. (2) Place the same amounts of monomers and NaOH solution in a separating funnel. (3) Shake the funnel well to mix the monomers and NaOH solution. Then leave the funnel to stand for several minutes until the mixture solution has separated into two layers (see Fig. 2.25). (4) Drain the lower layer which consists of NaOH solution containing hydroquinone. (5) If some hydroquinone still remains in the monomers, the NaOH solution will be coloured brown. In that case, repeat steps 2 4 until the color has disappeared. (6) After the NaOH solution has become colourless, place distilled water in the solution, shake the solution well, leave it to stand until the solution has separated into two layers, and then drain off the lower layer. Repeat this procedure 3-5 times. (7) Finally, remove the water from the monomers in the following manner. Place the mono-

64


Monomer

aOH solution

Fig. 2.25 Separation of monomer and water content in a separating funnel.

mers in a dark bottle, add calcium chloride and leave the mixture to stand overnight to dehydrate. Since the monomers are polymerized by temperature and ultraviolet rays, they must be put in a dark bottle and stored in a cool, dark place. (8) Immediately before use, mix n-butylmethacrylate and methyl-methacrylate, and add 1 to 1.5 % (W/V) benzoyl-peroxide or 2,4-dichlorobenzoyl-peroxideas a catalyst. (9) Next, filter the monomers through a filter paper to remove undissolved catalyst, etc. The filtered solution so obtained can be used for embedding. In the case of biological specimens, the resin mixing ratio between the n-butylmethacrylateand methyl-methacrylate is about 7: 3. However, the hardness of the mixture should be adjusted by varying the mixing ratio, depending on the season or the specimen. It is desirable that the hardness of the polymerized resins be equal to that of the specimen. However, if the hardness of the resins is increased by the use of more methyl-methacrylate, the specimen blocks will become brittle and difficult to cut. Embedding To embed the specimen in resin, use No.0 or No.00 gelatin capsules as containers. Embedding procedure (I) As shown in Fig. 2.26,arrange the gelatin capsules on their stands, and place the specimens in the capsules with the portion of interest directed towards the capsule bottom. Biological specimens are usually cut into 1 x 1 x 2-3 mm blocks. In the case of clays (powder), the cutting direction and crystal orientation are important, as will be described later. (2) Place the previously prepared monomers containing polymerization catalyst into the capsule up to the brim and put a cover on. Most types of specimens are placed directly in the capsules and the monomers are then poured into the capsules for polymerization purposes. In the 2.

Specimen /

Inject the monomer.

Insert the specimen.

I

Put on the lid .,

(a) (b) (C) Fig. 2.26 Embedding procedure.

Conclusion

65

case of porous specimens, the following methods are effective for allowing the embedding agent to permeate fully into the pores within the specimen.

(i) First place the monomers in a capsule, and then drop the specimen into them. The specimen will begin to polymerize after it has fallen to the bottom of the capsule. (ii) Alternatively, after placing the specimen and monomers in a capsule, carry out vacuum treatment to replace the air in the pores with the monomers.

3. Polymerization For polymerization, the specimen is kept overnight in a thermostat adjusted to 45°C to 60°C. With biological specimens, a method for stepwise increase in the temperature is sometimes used, but clay specimens do not require such a procedure. The resin which is polymerized and hardened into a capsular shape is called a “block.” If the block is too soft to cut easily, it may be hardened by being kept overnight in the thermostat maintained at ca. 60°C.If this fails to ensure sufficient hardness, a new block must be prepared as is often necessary with specimens containing water. 4.

Handling of the specimen to be put in a capsule

The preparation of clay sections using a microtome is mainly for the observation of c-axis images. Therefore, when a clay specimen is put in a capsule, it is necessary to orient each particle so that the c-axis plane is cut out. In the case of plate-shaped specimens, as mentioned in connection with the powder dispersion method, the dried suspension piles up in the c-axis direction with the a - b plane at the bottom. Repeating the operation of applying the suspension on a thin paper and drying it results in clay layers being formed on the paper surface. The paper is cut into pieces 2 mm wide and 3-5 mm long, and 4 or 5 pieces are placed in a capsule with the 2 mm sides directed to the bottom, as shown in Fig. 2.27. In the case of fibrous specimens also, the fibers are arranged in the same direction on the paper surface with the assistance of a bonding agent, and then the paper is cut and placed in a capsule in the manner mentioned above.

Fig. 2.27 Example of clay specimen embedding.

5.

Shaping of blocks Each block is shaped so that the desired portion appears in the cross section and so that it has a

lmm (front)

(side) Before trimming

After trimming

Fig. 2.28 Example of block trimming.

66


cross section which can be easily cut by a microtome. That is to say, each block is shaped into an external form such as that shown in Fig. 2.28 using of a file, razor blade or a small-sized saw, as occasion demands. If the desired crystal orientation cannot be readily cut out because the embedded specimen is not properly oriented, the following operations permit the specimen orientation to be changed. (1) Cut off the end of a block containing the specimen, as shown in Fig. 2.29.

P

j

Cut along the dotted lines. (a)

@El Bonding

Trimming

(b)

(C)

Fig. 2.29 How to correct the crystal orientation of an embedded specimen.

(2) Using a strong bonding agent, cement this end to the remaining portion of the block or to one end of a block prepared previously without a specimen in it, in such a manner that the specimen is placed in the desired orientation. (3) Scratching the ends to be bonded together with a coarse file, etc., increases the bonding strength. (4) After bonding, shape the block as shown in Fig. 2.28.

6. The knife and boat A glass knife is suitable for biological specimens, but with clay specimens, a diamond knife with a boat attached to its edge must be used. Sections must normally be cut to a thickness of less than lo00 A, and the boat is designed to allow the cut sections to be floated onto the surface of a solution which fills the boat. If the boat is filled with water only, the water rises above the knife edge due to surface tension. To reduce this effect, some alcohol is added to the water. The amount of solution in the boat should be such that the knife edge is slightly wetted. Excess solution will mean that the specimen surface is wetted during the cutting operation, rendering it hard to cut or the cut section hard to collect as it goes round behind the knife. If the amount of solution is too small, the cut section will not be well stretched and may become wrinkled, since it remains on the knife edge. The purpose of adding the alcohol is to prevent the solution from rising at the knife edge. However, the surface tension of the solution is utilized to obtain well-stretched sections without wrinkles. The amount of admixed alcohol must therefore be decided carefully: 10 to 30% alcohol is usually mixed with the water. 7. Cutting The cutting procedure is not described in detail here since it differs with microtomes of different makes. 8. Mounting on the specimen grid

When a section is floated on the surface of the solution in the boat, its thickness is judged from the interference colour of reflected light coming from the section. Besides its thickness, the state of stretching of the section and the presence or absence of scratches should be assessed using a binocular attached to the microtome. In the observation of clay sections, a microgrid with a carbon film is used since lattice images are

Conclusion

67

often required. The section floating on the solution cannot be placed on a supporting grid by merely dipping the grid into the solution to scoop the section up, because the section will flow out of the grid. It is advisable therefore to hold the grid with tweezers and to bring the carbon film on the grid into light touch with the section, from above. This causes the section to stick to the carbon film. The water remaining on the section is removed with filter paper. If the section is not fully stretched on the solution in the boat, a piece of filter paper moistened with chloroform can be brought close to the section to expose it to chloroform vapor, so allowing the section to stretch into a wrinkle-free state. The relationship between the section’s thickness and its interference color is generally as follows: 600 A. Silver: 600 900 A. Gold: 900 1500 A. Purple: 1500 1900 A. Gray: Blue: 1900 2400 A. Green: 2400 2800 A. Yellow: 2800 3200 A. Orange: 3200 3600 A. Red: 3600 4000 A.

--

--

- -

- -

9. After-treatment

If the resin used as an embedding agent causes difficulty in specimen observation, it can generally be removed by gently dipping the section (placed on a supporting grid with a carbon film) in acetone or chloroform. The solvent should be changed 2 or 3 times to dissolve the resin fully. The resin embedding a section irradiated with an electron beam tends to be difficult to dissolve. The embedding agent can be removed by sublimating it under intense electron beam irradiation in the EM column during specimen observation. This method, however, is not recommended since it may damage the specimen itself, break the supporting film, and contaminate the inside of the EM column with resin. 10. Special considerations (1) In order to avoid deformation of the embedding resin by electron beam irradiation during observation, a method is employed first to observe the entire section under weak beam irradiation and, after field selection, to increase the intensity of the beam irradiation gradually. (2) Since clay sections are apt to be thicker than biological ones, they easily become expanded or contracted under electron beam irradiation, resulting in specimen drift. (3) It should be noted, moreover, that there is very little probability of the a . b plane being cut vertically or of the fibre axis being cut perpendicularly. During observation, therefore, a specimen tilting device should be used to compensate for the crystal orientation of the section.

E. Conclusion This completes the description of the procedures used to take the photographs introduced in this book. The fact that high resolution photographs can now be easily obtained is due, of course, to the improvements in TEM performance. However, it should also be remembered that the microgrid has played an important role in such improvement. Sectioning techniques are becoming an indispensable research tool for clay specimens, as well as for biological specimens. The micro-area diffraction technique, which is a new method not yet in full use, was briefly discussed: however, it is likely to become the only means of crystal analysis in research on cross-sectional images of narrow clay sections. The so-called “analytical electron microscope” for use in micro-area analysis was not described due to the limitations of space: for its applications to clay specimens, the reader is referred to the available literature (Koike et al., 1973; Suzuki et al., 1974a, b; Hayashi et al., 1978).

68


ACKNOWLEDGMENT

The kind assistance of Messrs. A. Ono, Y. Harada and E. Watanabe is gratefully acknowledged. GENERALREFERENCES a) Hirsch, P. B., Howie, A, Nicholson, R. B., Pashley, D. W. and Whelan, M. J. (1965) Electron Microscopy of Thin Crystals, Buttenvorths, London. b) Hall, C. E. (1967) Introduction to Electron Microscopy, 2nd ed., McGraw-Hill, New York, London. c) Sjostrand, F. S. (1967) Electron Microscopy of Cells and Tissues, vol. 1, Academic Press, New York, London. d) Gard, J. A. (1971) The Electron-Optical Investigntion of Clays, Mineralogical SOC.,London. e) Adachi, K., Ishihara, S.,Okada, M., Ono, A., Tanabe, Y.and Yotsumoto, H. (1975) Basis of Electron Microscopy, Kyoritsu Shuppan, Tokyo. f) Wenk, H. R., (coordinating editor) (1976) Electron Microscopy in Mineralogy, Springer-Verlag, Berlin, Heidelberg, New York.

REFERENCES Agar, A. W. (1960) Brit. J . Appl. Phys., 11, 185. Boersch, H. (1936) Ann. Phys., 27, 75. Cowley, M. J. and Iijima, S. (1971) 29th Ann. Proc. Electron Microscope SOC.Amer., 168. Dowell, W.C.T. (1962) J. Phys. SOC.Japun, 17, Suppl. B-11, 175. Fleisher, S., Fleischer. B. and Stoeckenius, W. (1967) J. Cell Biol., 32, 193. Fukami, A. and Adachi, K. (1963) Proc. 19th Electron Microscopy, D-4. Fukami, A., and Adachi, K. (1965) J. Electron Microscopy, 14, 122. Fukami, A., Adachi, K. and Katoh, M. (1966) Proc. 6th Int. Congr. Electron Microscopy, Kyoto, 1, 263. Fukami, A., Adachi, K. and Katoh, M. (1972) J. Electron Microscopy, 21, 99. Geiss, R. H. (1975) Appl. Phys. Lett., 27, 174. Hayashi, H., Aita, S. and Suzuki, M. (1978) Clays and Clay Miner., 26, 181. Iijima, S. (1972) 30th Ann. Proc. Electron Microscope SOC.Amer. Koike, H., Ueno, K. and Watanabe, M. (1970) Proc.7th Int. Congr. Electron Microscopy, Grenoble, 1,24. Koike, H., Matsuo, T., Ueno, K. and Suzuki, M. H., (1972) JEOL News, 10e(3), 6. Koike, H., Narnae, T. Watabe, T. and Mikajiri, A. (1973) JEOL News, 1Oe(4), 2. Komoda, T. (1966a) J. Electron Microscopy, 15, 197. Komoda, T. (1966b) Japan J. Appl. Phys., 5,419. Le Poole, J. B. (1947) Philips Techn. Rev., 9, 33. Riecke, W. D. (1961) Optik, 18,278. Riecke, W. D. (1969) Z. Angew. Phys., 27, 155. Sjostrand, F. S. (1956) Exp. Cell Res., 10, 657. Suzuki, Y., Aita, S., Hoshino. T. and Iwata, H. (1974) JOEL News, 12e(2), 2. Suzuki, R., Yotsumoto, H. and Shibatomi, K. (1974) JEOL News, 12e(2), 5 . Uyeda, N., Kobayashi and T. Suito, E., Harada, Y. and Watanabe, M. (1970) Proc. 7th Int. Congr. Electron Microscopy, Grenoble, 1, 23. Uyeda, N., Kobayashi, T., Suito, E., Harada, Y.and Watanabe, M. (1972) J . Appl. Phys., 43, 5181. Van Oostrum K. J., et al. (1973) Appl. Phys. Lett, 23,283. Yada, K. and Hibi, T. (1968) J . Electron Microscopy, 17, 97.

Chapter 3

Electron Micrographs of the Principal Clays and Clay Minerals and Other Related Mineral Species

A. Brief guide to the clays and clay minerals appearing in the photographs I. Toseki 11. Roseki

Kuroko Greentuff Loam Note of the mineral names used in this chapter B. Electron micrographs of clays and clay minerals I. Kaolinite-serpentinegroup-Kaolinite Sub-group 11. Kaolinite-serpentine group-Serpentine Sub-group 111. Pyrophyllite and talc IV. Mica clay group V. Chlorite group VI. Vermiculite group VII. Smectite group VIII. Interstratified minerals IX. Sepiolite and palygorskite X. Zeolites XI. Other clays and clay minerals References 111. IV. V. VI.

A.

Brief Guide to the Clays and Clay Minerals Appearing in the Photographs

The clays and clay minerals illustrated here were mostly collected in Japan, and their modes of occurrence were highly complex. When discussing modes of occurrence and origins, certain specific names such as “Toseki,” “Roseki,” “Kuroko,” “green tuff,” and “loam” are generally applied in Japan. Many kinds of clay minerals have been found in these materials, and the clay minerals themselves have been studied in detail. The above names are widely used in the ceramic industry, mining, mineralogy, petrology, geology, agriculture, etc. Toseki and Roseki (generally employed as commercial names) are raw materials of pottery. They are refractory and composed mainly of kaolinite, mica clay minerals, and pyrophyllite. Kuroko is a type of ore deposit. Abundant clay and Al-clay minerals are found in the alteration areas of such deposits. Green tuff represents Miocene formations composed mainly of volcanic rocks and related pyroclastics, and is widely distributed on the Japan Sea side of northwest Japan. It contains many kinds of green-coloured clay minerals and zeolites. The Kuroko deposits occur 69

70

ELECTRON MICROGRAPHS

only in the green tuff region. Volcanic ash soil usually termed “loam” consists essentially of clay minerals such as allophane, halloysite, kaolinite and imogolite. I. TOSEKI

Toseki is one of the raw materials used in Japan for pottery and porcelain. It is composed mainly of mica, kaolinite, and quartz. The Al-mica clay minerals in Toseki are known as “sericite” (cf.A. VI). Toseki is generally distinguished into “kaolin-toseki” and “sericite-toseki” based on its main constituent clay mineral. Some samples of Toseki also include interstratified Al-mica/montmorillonite and tosudite, although the amounts of such interstratified minerals are usually small. The origin of Toseki is considered to be a hydrothermal alteration product of acidic rocks such as acidic tuffs, liparite and porphyry. Clay mineralogy has revealed the essential properties of the clay minerals in Toseki. Kanaoka (1972) found that the sericite in Toseki shows the IM, 2M, and 2M, polytypes, and that sericite-toseki can be grouped into 3 types based on the polytypes of the sericite. Ichikawa and Shimoda (1976) and Shimoda et al. (1978) demonstrated the existence of lithium (Li)-tosudite in certain samples of Toseki. Some Toseki deposits show a zonal distribution of clay minerals. In the Izushi Toseki deposit, for example, the mineral zones from the outer to inner part of the deposit can be summarized as follows: (1) pitchstone, (2) montmorillonite with small amounts of cristobalite and mordenite, (3) interstratified Al-mica/montmorillonite, (4) lithium-bearing tosudite, and (5) sericite. The clay used as the Toseki ore from this mine is a mixture of interstratified Al-mica/montmorillonite and tosudite. The names “Gaerome” and “Kibushi” clays are also widely used for plastic kaolin clays distributed in and around Aichi, Gifu, and Mie Prefectures. These areas are composed of granitic rocks as the basement and lacustrine sediments of Pliocene age deposited in numerous small basins on this basement. The lower part of the lacustrine sediments consists mainly of quartz sand including kaolin clay. The upper part is a silty clay composed mainly of kaolinite and small amounts of halloysite and montmorillonite, and usually contains carbonized woody fragments. The former part is known as Gaerome and the latter as Kibushi. 11. ROSEKI

Roseki (lit. waxy stone) is one of the raw materials of refractory products and also is used as a paper clay. It is composed mainly of pyrophyllite, kaolinite and sericite, and can be broadly divided into “pyrophyllite-roseki,” “kaolin-roseki” and “sericite-roseki” based on its main constituent mineral. Roseki deposits are distributed in the western part of Japan (Chugoku and north Kyushu districts), and the north-central part (Hokushin district), as shown in Fig. 3.1. The Roseki deposits from these two districts have slightly different mineral assemblages : the former consists mainly of pyrophyllite and diaspore with small amounts of corundum, and the latter of pyrophyllite, sericite and kaolinite. Although they are considered to have formed by hydrothermal alteration of acidic rocks of Cretaceous to Miocene age, the difference in mineral assemblage appears to indicate some difference in origin. In addition to the above-mentioned minerals, some Roseki ores contain dickite, nacrite and halloysite, which usually occur in clayey veins cutting the Roseki ore. Shimoda and Sudo (1960) and Sudo et al. (1962) found an interstratified Al-mica/montmorillonite in the Yonago Roseki deposit of the Hokushin district, and in the Goto Roseki deposit of north Kyushu. Recently, Kakitani and Morita (unpublished data) found an interstratified mineral in a Roseki deposit from the Chugoku district. Sudo et al. (1954) reported the occurrence of tosudite in a lenticular veinshaped mass in the clayey part of the Kurata kaolin-roseki mine. Nishiyama et al. (1975) found

Brief Guide to Clays and Clay Minerals in Photographs

71

' A-

HOKKAIDO

0s i IMA- FUKUSHIMAdisf:rict

t

KlTAKAMl district

HOKUROKU district

B

0

-h

B

NORTH KYUSH. "

0

HlTOYOSHl district

-0

200 km

Fig. 3.1 Distribution of Roseki and Kuroko deposits, green tuff and volcanic ash soils (loam) in Japan (modified from data of Fujii (1976). Shirozu (1978) and Nagasawa (1978)). Roseki dep.

Kuroko dep.

Green tuff.

Volcanic ash soil(loam)

Volcanoes

a lithium(Li)-bearing tosudite in a clayey vein cutting the pyrophyllite mass of the Tohoo Roseki mine. Although tosudite is widely recognized in Toseki, its occurrence is rare in Roseki. Sudo et al. (1962) described the zonal distribution of clay minerals found in the Yonago Roseki deposit of the Hokushin district. However, subsequently there have been no similar reports on the mineral distribution of other Roseki deposits. 111. KUROKO

As mentioned, Kuroko deposits occur only in the green tuff region (Fig. 3.1). They are considered to have formed originally in sedimentary basins in relation to submarine volcanic activity.

72

ELECTRON MICROGRAPHS

The ore minerals found in the deposits are mainly sphalerite, galena, chalcopyrite, pyrite, and gypsum. Ore composed principally of sphalerite, galena and chalcopyrite is called black ore, and that composed of pyrite and chalcopyrite is called yellow ore. Kajiwara (1970) has given a geologic profile for one typical Kuroko deposit, in the Shakanai mine of the Hokuroku district, as shown in Fig. 3.2. The stockwork mineralization resulted from the passage of hydrothermal solutions

A//;A \F:?\Y

A

I

I\

I

I\

I\

,~

,\

,\

Rhyolite volcanic breccia

~

s,

.I

\I

\I

Tuff breccia

A

A

A

A

~

0

20, d((

A

A

A

I

Electron micrographs of clay minerals

The chemistry of clay minerals

The Origin of Clay Minerals in Soils and Weathered Rocks

Clay minerals: a physico-chemical explanation of their occurrence

Clays and Clay Minerals of Japan, #26 (Developments in Sedimentology)

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