Advanced techniques for clay mineral analysis: invited contributions from the symposium held at the 7th International Clay Conference, September 6-12, 1981, Bologna and Pavia, Italy

DEVELOPMENTS IN SEDIMENTOLOGY 34

ADVANCED TECHNIQUES FOR CLAY MINERAL ANALYSIS Invited contributions from the Symposium held at the 7th International Clay Conference, September 6-12,1981, Bologna and Pavia, Italy

edited by

JmJm FRlPlAT Centre National de la Recherche Scientifique, Centre de Recherche sur les Solides 6 Organisation Cristalline Imparfaite, Orl6ans Cedex (France)

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam - Oxford - New York 1982

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Molenwerf 1 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada:

E LSEV IER/NORTH-HOLLAND INC 52, Vanderbilt Avenue New York, N.Y. 10017

L i b r a r y 01 Congress Cataloging i n Publication Data

Main entry under title: Advanced techniques for clay mineral analysis. (Developments in sedimentology ; v. 34) Bibliography: p. Includes index. 1. Clay minerals--Analysis--Congresses. I. Fripiat, J. J. 11. International Clay Conference (7th : 1981 : Bologna and Pavia, Italy) 111. Series. ~~389.62 .A38 553.6'1'028 81-9881 ISBN O-d4-420@-9 (U.S.) M C W

0 Elsevier Scientific Publishing Company, 1982 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying. recording or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, P.O. Box 330, 1000 AH Amsterdam, The Netherlands Printed in The Netherlands

1

PREFACE One o f t h e i m p o r t a n t aims o f an i n t e r n a t i o n a l congress devoted t o n a t u r a l o r s y n t h e t i c m a t e r i a l s i s t o i n f o r m t h e researcher

about t h e p o t e n t i a l use o f new

p h y s i c a l t e c h n i q u e s employed f o r c h a r a c t e r i z i n g t h e s e m a t e r i a l s . T h i s s p e c i f i c t a s k i s becoming more and more u r g e n t because t h e number o f p h y s i c a l t e c h n i q u e s and t h e i r new a p p l i c a t i o n s a r e i n c r e a s i n g v e r y r a p i d l y . The o r g a n i z i n g committee o f t h e 7 t h I n t e r n a t i o n a l C l a y Conference has t h o u g h t t h a t a s p e c i a l symposium on Advanced Methods i n C l a y M i n e r a l s Research s h o u l d be o f g r e a t i n t e r e s t f o r many p a r t i c i p a n t s and I have been asked t o t a k e t h e r e s p o n s i b i l i t y f o r i t s organization. The f i r s t d i f f i c u l t y o f t h a t t a s k was t o s e l e c t t h e methods t o be reviewed. There a r e t e c h n i q u e s which have been known f o r many y e a r s but,because o f t h e r e c e n t i n s t r u m e n t a t i o n p r o g r e s s , t h e r e a r e new r e s u l t s which a r e w o r t h w h i l e t o summarize. There a r e a l s o t e c h n i q u e s w h i c h a r e n o t o f g e n e r a l use i n t h e f i e l d o f c l a y r e s e a r c h b u t which can p r o v i d e i m p o r t a n t supplementary i n f o r m a t i o n . F i n a l l y , t h e r e a r e r e c e n t t e c h n i q u e s which have n o t y e t developped o r demonstrated t h e i r f u l l c a p a c i t y . A r e v i e w o f p r e l i m i n a r y r e s u l t s o b t a i n e d by t h e s e new t e c h n i q u e s can s t i m u l a t e o t h e r s c i e n t i s t s t o use them more b r o a d l y . Thus t h e c h o i c e i s n o t easy and, i n a d d i t i o n , t h e space and t i m e a l l o t t e d f o r t h a t t y p e o f r e v i e w a r e n e c e s s a r i l y l i m i t e d . N i n e t e c h n i q u e s have been s e l e c t e d f o r one o r s e v e r a l o f t h e reasons e x p l a i n e d above; t h e y c o n s t i t u t e t h e n i n e chapt e r s o f t h i s monograph. The a u t h o r s have been asked t o a v o i d as much as p o s s i b l e t h e o r e t i c a l d i s c u s s i o n s and t o c o n c e n t r a t e - t h e i r p r e s e n t a t i o n on e x p e r i m e n t a l r e s u l t s and t h e i r p h y s i c a l meaning. There was a good reason t o o p e r a t e i n t h a t way. Indeed i n 1979, a t Urbana ( I l l i n o i s ) , J.W. S t u c k i and W.L. Banwart o r g a n i z e d a Nato School devoted t o Advanced Chemical Methods f o r S o i l and C l a y M i n e r a l Research (Nato advanced s t u d y i n s t i t u t e s : S e r i e s C: Mathematical and P h y s i c a l Sciences, D. R e i d e l P u b l i s h . Co,

1980). The o b j e c t i v e o f t h a t school was t o t e a c h a l i m i t e d number o f p a r t i c i p a n t s t h e b a s i c p r i n c i p l e s o f Mossbauer spectroscopy, N e u t r o n S c a t t e r i n g , X-Ray Photoe l e c t r o n Spectroscopy, N u c l e a r Magnetic Resonance, E l e c t r o n S p i n Resonance and Photo A c o u s t i c Spectroscopy, and t o show t h e way t o a p p l y t h e i r b a s i c p r i n c i p l e s t o t h e s t u d y o f c l a y m i n e r a l s . Thus a r e c e n t book c o n t a i n i n g t h e t h e o r i e s d e a l i n g w i t h s e v e r a l c h a p t e r s o f t h e p r e s e n t monograph i s a v a i l a b l e . The p r e s e n t monograph has

as

goal t o r e a c h a l a r g e r community o f s c i e n -

t i s t s and t o d i s s e m i n a t e i n f o r m a t i o n a b o u t a p p l i c a t i o n s o f p h y s i c a l t e c h n i q u e s on

2 a more general

basis.

I t i s why modern developments o f Thermal Methods Ana-

l y s i s and o f E l e c t r o n Microscopy have been i n c l u d e d t o g e t h e r w i t h c h a p t e r s deal i n g w i t h MGssbauer Spectroscopy, N u c l e a r Magnetic Resonance, E l e c t r o n S p i n Resonance, Neutron S c a t t e r i n g and X-Ray P h o t o e l e c t r o n Spectroscopy. I t appeared a l s o d e s i r a b l e t o have a c h a p t e r on UV and V i s i b l e Spectroscopy f o r w h i c h , t o my b e s t knowledge, no r e v i e w on t h e i r a p p l i c a t i o n t o t h e s t u d y o f c l a y m i n e r a l s

e x i s t s . The same i s t r u e f o r f a r i n f r a r e d s p e c t r o s c o p y .

A l l c r i t i c i s m s c o n c e r n i n g t h e c h o i c e o f t h e t o p i c s have t o be adressed t o t h e e d i t o r . The a u t h o r s o f i n d i v i d u a l chapte,rs s h o u l d be g i v e n a l l c r e d i t f o r making t h e r e a d e r aware o f t h e new e x c i t i n g developments i n t h e p h y s i c a l t e c h n i q u e s used i n c l a y m i n e r a l s r e s e a r c h .

J . FRIPIAT.

5

Chapter 1

THERMOANALYTICAL METHODS IN CLAY STUDIES Robert C. MACKENZIE The Macaulay Institute for Soi Research, Craigiebuckl er , Aberdeen , Scot1and , UK. 1.1 INTRODUCTION There is nothing new in the thermal study of clays. Indeed, as early as about 315 BC, Theophrastus refers to the effect of "fire" (i.e. heat) on talc (as steatite) and on palygorskite (as "mountain wood") (Eichholz, 1965) and development in the use of heat as a discriminator can be traced from that time on (Mackenzie, 1981a). Even evolved gas analysis, which would be considered by some as a relatively recent technique for clays, has its roots in the eighteenth century, when the Rev. Stephen Hales (1727) found that "a cubick inch of fresh u n t r i e d earth" (his italics) yielded "43 cubick inches of air" on heating and Josiah Wedgwood (1782) detected only carbon dioxide on firing china clay in a closed system, the evolved water having condensed and gone unnoticed. The first thermoanalytical study of clays was performed in 1887, when Henri Le Chatelier recorded what were essentially heating-rate curves for halloysite, allophane, kaolinite, pyrophyllite and montmoril lonite, over the approximate temperature range 20-llOO°C, in an attempt to use their behaviour on heating as a classificational criterion. His results suggest that the samples used were remarkably authentic - surely a tribute to the mineralogists o f the time who had none of the modern methods of diagnosis available to them. Despite the differences observed, 1 ittle advance, apart from the pub1 ication of some so-called "dehydration curves" (Samoilov, 1909) and some heating curves (e.g. Mellor and Holdcroft, 1911; Ashley, 1911; Brown and Montgomery, 1912), occurred until Wallach in 1913 first applied differential thermal analysis (DTA) to clays. Even this, however, seems to have elicited little response and, although the OTA studies o f Satoh (1918, 1921) aroused more attention, it was not until the early 19405, subsequent to the detailed studies of Norton (1939) and Hendricks and Alexander (1939), that DTA blossomed forth as an investigational technique. The reason is simple: at that period clays excited much interest as the general structure of the clay minerals had been establisned and the species collected into groups, with the reLult that methods of identification and estimation additional or complementary to X-ray diffraction were being sought. Unfortunately, the indiscriminate application of DTA to problems that it could not possible solve,and even the use of unsuitable equipment and technique, led to the method being discarded by some as useless in clay mineralogy. However, by no means all clay mineralogists were

6 so d i s i l l u s i o n e d and much p a i n s t a k i n g work over t h e years (by e.g.

Ralph E. G r i m ,

Paul F. Kerr, Toshio Sudo and o t h e r s ) g r a d u a l l y demonstrated t h a t DTA d i d have a place i n c l a y m i n e r a l o g i c a l studies.

A t t h i s p o i n t t h e reader m i g h t w e l l ask why t h e r m o a n a l y t i c a l s t u d i e s (discussed i n t h e paragraph above) should be separated from p u r e l y thermal s t u d i e s ( r e f e r r e d t o i n the f i r s t paragraph).

The reason i s t h a t thermal methods have t o s a t i s f y

c e r t a i n c r i t e r i a before they can be termed thermoanalytical.

These c r i t e r i a ,

some o f t h e thermoanalytical techniques c u r r e n t l y a v a i l a b l e and t h e i r a p p l i c a t i o n and/or a p p l i c a b i l i t y i n c l a y i n v e s t i g a t i o n s a r e t h e s u b j e c t o f the remainder o f t h i s paper.

1.2

THERMAL ANALYSIS Over t h e past f i f t e e n years much a t t e n t i o n has been devoted t o nomenclature,

d e f i n i t i o n and c l a s s i f i c a t i o n o f thermoanalytical techniques w i t h t h e r e s u l t t h a t the methods included can now be c l e a r l y recognized and named. According t o t h e I n t e r n a t i o n a l Confederation f o r Thermal A n a l y s i s (ICTA), thermal a n a l y s i s covers (Lombardi,

1980):

"A group o f techniques i n which a p h y s i c a l p r o p e r t y o f a substance and/or i t s r e a c t i o n products i s measured as a f u n c t i o n o f temperature, w h i l s t t h e substance i s subjected t o a c o n t r o l l e d temperature programme". The t h r e e c r i t e r i a t h a t d i s t i n g u i s h a t h e r m o a n a l y t i c a l method are, t h e r e f o r e , t h a t a p h y s i c a l p r o p e r t y i s measured as a f u n c t i o n of t e m p e r a t u r e under a c o n t r o l l e d t e m p e r a t u r e programme.

Thus, a s i n g l e isothermal d e t e r m i n a t i o n i s n o t

thermoanalytical b u t assessment o f the r e s u l t s o f a s e r i e s o f isothermal determinations a t d i f f e r e n t temperatures as a f u n c t i o n o f temperature i s . S i m i l a r l y , non-thermal methods, such as X-ray d i f f r a c t i o n , performed under a c o n t r o l l e d temperature programme become t h e r m o a n a l y t i c a l determinations.

I n the

account t h a t f o l l o w s , however, o n l y those methods normally i n c l u d e d i n thermal a n a l y s i s w i l l be considered:

i t should be observed t h a t c l a s s i c a l c a l o r i m e t r y i s

excluded, d e s p i t e i t s c l o s e r e l a t i o n s h i p t o some thermoanalytical methods.

1.2.1

A v a i l a b l e t h e r m o a n a l y t i c a l techniques.

A general c l a s s i f i c a t i o n o f methods c u r r e n t l y recognized as t h e r m o a n a l y t i c a l i s given i n Table 1.1 along w i t h t h e p h y s i c a l p r o p e r t y on which they depend and,

'for common methods where i t i s g e n e r a l l y i n use, t h e acceptable a b b r e v i a t i o n (Lombardi, 1980).

Most o f t h e techniques can be d e f i n e d i n e x a c t l y t h e same way

as thermal a n a l y s i s , t h e p h y s i c a l p r o p e r t y i t s e l f

-

"mass" f o r thermogravimetry,

"an e l e c t r i c a l c h a r a c t e r i s t i c " f o r "thermoelectrometry': "a physical p r o p e r t y " i n t h e d e f i n i t i o n . wording i s necessary.

etc.

-

r e p l a c i n g t h e words

I n some instances, however, more p r e c i s e

For example, s i x methods a r e l i s t e d as being dependent on

change i n mass, b u t o n l y two a r e so dependent d i r e c t l y : i s o b a r i c mass-change

7

TABLE 1.1 Classification of thermoanalytical techniques Physical property

Derived techniques

Mass

Isobaric mass-change determination Thermogravimetry Evolved gas detection Evolved gas analysis Emanation thermal analysis Thermoparticulate analysis Heating-curve determination* Differential thermal analysis Differential scanning calorimetry? Thermodilatometry Thermomechanical measurement+ Thermosonimetry5 Thermoacoustimetryg Thermoptometry Thermoel ectrometry Thermomagnetometry

Temperature

En tha 1 py Dimensions Mechanical characteristics Acoustic characteristics Optical characteristics Electrical characteristics Magnetic characteristics

Abbreviation TG EGD EGA

DTA

osc

*

I n t h e c o o l i n g mode t h i s becomes Cooling-curve determination.

t

Two t y p e s , Power-compensation DSC and Heat-flux DSC, can be d i s t i n g u i s h e d .

?

T e s t s under o s c i l l a t o r y l o a d come under t h e heading Dynamic thermomechanical

measurement.

5 Thermosonimetry r e f e r s t o sound e m i t t e d by t h e sample whereas Thermoacoustimetry i n v o l v e s measurement o f changes i n t h e c h a r a c t e r i s t i c s o f imposed a c o u s t i c waves p a s s i n g through t h e sample.

determination, which covers equilibrium techniques, such as the once common "dehydration curves" under a constant partial pressure of water vapour, and thermogravimetry- (TG), which uses a dynamic temperature programme. Evolved gas detection (EGO) and evolved gas analysis (EGA) are secondary techniques whereby materials evolved during heating are detected or analysed, respectively, and the remaining two, emanation thermal analysis and thermoparticulate analysis, are tertiary techniques, being special instances of EGA related to radioactive emanation and particulate matter, respectively. A common method that is not listed i n Table 1.1 is derivative thermogravimetry (DTG), the reason being that derivative curves can be calculated for most measurements and it would be invidious to include only one. Attention should also be drawn to the distinction between derivative and differential, the former applying to the mathematical process and the latter being used only as the adjectival form of "difference" (Lombardi, 1980). Thus, in "differential thermal analysis" (and "differential scanning calorimetry") the "difference in temperature between" (and "the difference in energy inputs into"), "a substance and a reference material is measured". Moreover, heating

a curves

-

i.e.

derivatives (T) T

curves f o r sample temperature against tlme

- "heating-rate

curves", where

dT/dt

- give r i s e t o

two

i s p l o t t e d against temperature

o r time ( t ) , and "inverse heating-rate curves" where

dt/dT

i s p l o t t e d against

o r t: both these have been e x t e n s i v e l y used i n the past. The i n f o r m a t i o n given above, together w i t h t h a t i n Table 1.1, i s probably

adequate t o a l l o w appreciation o f t h e enormous s t r i d e s t h a t have been made over the past decade o r so i n o b t a i n i n g i n t e r n a t i o n a l agreement on a general nomenc l a t u r e and c l a s s i f i c a t i o n system f o r thermoanalytical techniques.

This e f f o r t ,

however, has covered n o t o n l y nomenclature o f methods b u t a l s o t h a t o f components o f equipment, o f aspects o f experimental" technique, o f c r i t i c a l p o i n t s on curves

and o f symbols (Lombardi, 1980) and has been f o r t u n a t e enough t o r e c e i v e the backing o f n a t i o n a l and i n t e r n a t i o n a l standards i n s t i t u t i o n s , such as AFNOR, ASTM and ISO, as w e l l as o f major i n t e r n a t i o n a l bodies such as IUPAC (1974, 1980). Moreover, t h e recommendations i n English have been converted i n t o forms acceptable i n many other language-speaking areas (Lombardi, 1980; Mackenzie, 1981b), since d i r e c t t r a n s l a t i o n i s n o t always p o s s i b l e because o f already accepted conventions i n other languages. 1.2.2 Simultaneous techniques I t i s o f t e n convenient t o make two o r more measurements on one sample a t the same time, leading t o "simultaneous determinations" such as DTA-EGA, TG-EGA, DTA-TG-DTG,

etc.

This has advantages and disadvantages, and one has t o study n o t

only t h e bases o f the techniques themselves b u t a l s o t h e nature o f t h e samples involved before deciding on t h e i r use. For example, EGA i s most p r o f i t a b l y employed i n conjunction w i t h DTA o r TG, as one can then r e l a t e the evolved v o l a t i l e s t o s p e c i f i c changes i n t h e sample; s i m i l a r l y , by comparing simultaneous DTA and DTG curves one can r e a d i l y r e l a t e r e a c t i o n s i n v o l v i n g mass change w i t h

s p e c i f i c enthalpy changes.

And, o f course, t h e r e i s a considerable saving i n

both time and m a t e r i a l .

The major disadvantage i s t h a t optimum c o n d i t i o n s f o r one technique may n o t necessarily be those f o r another. However, t h i s can be f o r example, i n minimized by c a r e f u l s e l e c t i o n of experimental c o n d i t i o n s

-

simultaneous DTA-TG, by using a small sample and/or employing a slow heating r a t e . 1.2.3

Standardization o f techniques

Since thermoanalytical r e s u l t s can vary w i t h experimental technique, t h e Standardization Committee o f ICTA have published a code o f p r a c t i c e l i s t i n g t h e i n f o r m a t i o n t h a t should be supplied w i t h every curve published: they have a l s o been instrumental i n p r o v i d i n g m a t e r i a l s f o r temperature c a l i b r a t i o n o f apparatus (Lombardi, 1980). These aspects should be thoroughly studied by anyone considering a p p l i c a t i o n o f thermal analysis.

9 1.3

APPLICATIONS TO CLAYS Emphasis in this article I ' s , quite deliberately, on the applications or potential applications of the various techniques now available to clays. It is, therefore, impossible to deal adequately with Instrumentation, experimental technique, or even with some basic principles, although all these are critical in determining the quality of thermoanalytical results. To overcome this deficiency the reader is referred to the books of Daniels (1973) and Wendlandt (1974) and to the excellent reviews that have appeared biennially in A n a l y t i c a l C h e m i s t r y Fundamental R e v i e w s (e.g. Murphy, 1978) for a considerable period. In the account that follows, clay mineralogical applications take priority, but due consideration must also be given to the wider field of applications to clays and clay rocks of technological or industrial importance and to accessory minerals, since the presence or absence of these may well determine the suitability of a clay for a particular application. With this wide field in mind, it will be appreciated that the references given are illustrative only: an exhaustive study of all published work would be inordinately long. 1.3.1

IIsobaric) mass-change determination In isobaric mass-change determination the sample is heated at each selected temperature until there is no further mass change and the e q u i l i b r i u m mass is plotted against the temperature: the partial pressure o f the evolved volatile (e.g. water or carbon dioxide) is maintained constant throughout the determination An excellent description of the technique has been given by Weiser and Milligan (1939).

In the past this technique, although not perhaps in an isobaric mode, was widely applied to clays in the derivation of so-called "dehydration curves". An excellent collection of these was given by Nutting (1943) and the technique was still employed for characterization purposes in the 1950s (see e.g., Mackenzie, 1957a). It is rather time-consuming and with the advent of thermogravimetry seems to have fallen into disuse. However, families of isothermal mass-change curves, particularly in their isobaric mode, can probably yield more reliable information on the kinetics of reactions than the TG curves so commonly in use (see below). 1.3.2

Thermogravirnetry (TG) Although DTA has been the most widely used technique in clay mineralogy, the use o f TG and DTG has grown markedly, particularly since the introduction of the Derivatograph, which provided simultaneous DTA-TG-DTG curves, and its commercial production in Hungary in the mid-1950s. This instrument, which is by far the most widely used, and apparently the only one comnercially produced in eastern Europe, has been upgraded several times and now has various optional additional

i 0 attachments f o r thermodilatometry, EGA, etc.

( P a u l i k and Paulik, 1978).

I t s value

i n c l a y mineralogy i s r e a d i l y assessed from t h e simultaneous curves f o r a l a r g e number o f c l a y s and c l a y m i n e r a l s published by Langier-Kuzniarowa (1967).

Out-

s i d e e a s t e r n Europe, thermogravimetry was s t i m u l a t e d by t h e commercial p r o d u c t i o n o f t h e r o b u s t Stanton thermobalance i n 1954 and simultaneous techniques by t h e i n t r o d u c t i o n o f the M e t t l e r Thermoanalyzer (Wiedemann, 1964).

A wide range o f

thermobalances and simultaneous DTA-TG instruments s u i t a b l e f o r c l a y s t u d i e s can now be purchased (e.g.

Dunn, 1980). t h e number o f the l a t t e r tending t o increase

as a v a i l a b l e s e n s i t i v i t y has increased.

O f the various types of balance system

used (Keattch and Dollimore, 1975), t h e n u l l - p o i n t e l e c t r o b a l a n c e now seems t h e most common.

When simultaneous equipment i s n o t used and a comparison i s made

between DTA and DTG curves, g r e a t care must be taken t o ensure t h a t a l l experimental v a r i a b l e s are i d e n t i c a l f o r b o t h determinations

-

f o r example, use o f a d i f f e r e n t

h e a t i n g r a t e can d i s p l a c e peaks and even a l t e r peak shape a p p r e c i a b l y ( A l i e t t i , B r i g a t t i and Poppi, 1979). The main uses o f TG add DTG (which must be considered t o g e t h e r ) i n c l a y mineralogy have been i n determining t h e reasons f o r DTA peaks, assessing t h e range over which r e a c t i o n s occur and o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n .

The methods

a r e n o t p a r t i c u l a r l y s u i t a b l e f o r i d e n t i f i c a t i o n studies, although t h e occurrence o f one o r two peaks on a DTG curve, and the r e l a t i v e s i z e s o f t h e two peaks when they appear, can apparently be employed i n c h a r a c t e r i z i n g s e r p e n t i n e m i n e r a l s (Morandi and F e l i c e , 1979) and t h e disappearance o f the hygroscopic m o i s t u r e peak a f t e r K - s a t u r a t i o n can be used t o d i s t i n g u i s h h y d r o b i o t i t e from m o n t m o r i l l o n i t e

i n some s o i l s (Ryzhova, 1980).

These must be regarded as r a t h e r i s o l a t e d instances

and t h e main use o f the techniques has undoubtedly been t o o b t a i n q u a n t i t a t i v e i n f o r m a t i o n on evolved v o l a t i l e s , e t c .

I n such a p p l i c a t i o n s , however, g r e a t care

must be exercised, as t h e mass change, during, f o r example, a d e h y d r o x y l a t i o n r e a c t i o n , could be s e r i o u s l y affected iff e r r o u s i r o n i n t h e l a t t i c e were simultaneously o x i d i z e d t o f e r r i c .

For t h i s reason too, q u a n t i t a t i v e determina-

t i o n o f m i n e r a l s by DTG (Smalley and Xidakis, 1979) should be undertaken o n l y when s u f f i c i e n t c o n f i r m a t o r y evidence t h a t n o t h i n g l i k e l y t o i n t e r f e r e w i t h t h e DTG peak area i s present and when comparison can be made w i t h a mineral t h a t i s

i d e n t i c a l w i t h t h a t i n the c l a y .

I t i s noteworthy i n t h i s respect t h a t even t h e

s a t u r a t i n g c a t i o n o f m o n t m o r i l l o n i t e a f f e c t s t h e c h a r a c t e r and temperature o f the DTG dehydroxylation peak (Schomburg and S t b r r ,

1978a).

TG has proved

valuable i n e l u c i d a t i n g t h e nature of DTA peaks f o r p a l y g o r s k i t e and s e p i o l i t e (Fernandez Alvarez, 1978) and f o r m o n t m o r i l l o n i t e ( I l i u t a , Drimus and Preda, 1978) and OTG i n r e v e a l i n g m u l t i p l e r e a c t i o n s n o t obvious on t h e TG curve (e.g. Rautureau and Fornes, 1978).

Mifsud,

Changes i n t h e temperature range and magnitude o f

t h e step on t h e TG, o r peak on t h e DTG, curve can p r o v i d e v a l u a b l e c o n f i r m a t o r y evidence f o r a p a r t i c u l a r phenomenon, such as t h e occurrence o f NH,+

i n some

11

Japanese dioctahedral micas (Higashi, 1978): the presence o f t h i s i o n not o n l y moves t h e dehydroxylation r e a c t i o n t o a lower temperature b u t a l s o increases t h e mass l o s s because o f the e v o l u t i o n o f NH,. TG and DTG also y i e l d useful i n f o r m a t i o n on accessory minerals.

Thus, t h e

dehydroxylation o r decarbonation o f accessory hydroxide o r carbonate minerals i s u s u a l l y obvious on TG and DTG curves (e.g. Iwasa, 1978) and, provided c a r e f u l c a l i b r a t i o n i s performed i n advance, even s a l t s , such as sodium carbonate and sodium c h l o r i d e , can be q u a n t i t a t i v e l y determined i n s a l i n e c l a y s (Asomoza et ai., 1978). TG has been e x t e n s i v e l y i n v e s t i g a t e d as a means f o r studying t h e k i n e t i c s o f r e a c t i o n s because ( a ) determinations are l e s s time-consuming than isothermal i n v e s t i g a t i o n s , (b) w i t h isothermal methods, some r e a c t i o n occurs before the temperature o f i n t e r e s t i s reached and ( c ) the whole temperature range i s covered w i t h o u t any missing regions (see Sharp, 1972).

While these comments are c o r r e c t ,

actual l i m i t a t i o n s on the d e r i v a t i o n o f k i n e t i c parameters are formidable, n o t o n l y because o f the occurrence of temperature gradients w i t h i n t h e sample but a l s o because o f other more fundamental aspects d e a l t w i t h below.

Most methods

f o r i n t e r p r e t i n g TG curves are based on t h e simple r a t e equation da/dt

k(1

- a)"

(where a i s the f r a c t i o n decomposed i n time t , n the order o f r e a c t i o n and

k

the

r a t e constant) combined w i t h t h e Arrhenius equation (2)

k = Aexp (-E/RT)

(where A i s t h e pre-exponential f a c t o r , E the a c t i v a t i o n energy and R the gas constant), t h e temperature

(where

T~

T

being defined by

i s the i n i t i a l temperature and 8 , = d T / d t , the heating r a t e ) : note t h a t

the heating r a t e i s assumed t o be c o n s t a i t .

The two usual procedures are then

e i t h e r t o f o l l o w t h e d i f f e r e n t i a t i o n method o f Freeman and C a r r o l l (1958) o r the i n t e g r a t i o n method o f Coats and Redfern (1963).

The l a t t e r i s generally regarded

as y i e l d i n g most r e l i a b l e r e s u l t s - see, f o r example, t h e recent study o f Boy and BMhme (1979) and compare t h e i r r e s u l t s f o r t h e dehydroxylation o f k a o l i n i t e w i t h those determined by a v a r i e t y o f experimental methods (Sharp, 1972). I n a l l these studies approximations a r e involved and, although the d i f f i c u l t i e s inherent i n dealing w i t h a dynamic system (see Sharp, 1972) are imnense, other more fundamental aspects must a l s o be remembered.

One need o n l y consider t h e

1 2

equations above.

The f i r s t two a r e gas-phase equations and t h e r e i s no guarantee

they would apply t o a s o l i d : moreover, one could j u s t i f i a b l y query t h e physical s i g n i f i c a n c e o f the derived "order o f r e a c t i o n " and " a c t i v a t i o n energy" when so applied. Indeed, recent EGA studies have shown t h a t decomposition r e a c t i o n s can, n o t unexpectedly, change mechanism during the r e a c t i o n (Garn, Kawalec and Chang, 1978; P r i c e e t al., 1980).

Moreover, many s o l i d - s t a t e decompositions i n powder

systems a r e d i f f u s i o n c o n t r o l l e d ( s e n s u l a t o , covering both i n t e r - and i n t r a p a r t i c l e d i f f u s i o n ) , o r f o l l o w some o t h e r law, so t h a t t h e "order o f r e a c t i o n " i s r a t h e r meaningless (Sharp, 1972).

Consequently, Garn (1979) has q u i t e r i g h t l y

suggested t h a t t h e term " a c t i v a t i o n energy" should be replaced by "temperature c o e f f i c i e n t o f reaction".

While t h e use o f small samples and/or o f a constant

temperature regime over the decomposition i n t e r v a l (Rouquerol, 1970; P a u l i k and Paulik, 1972) would reduce temperature gradients t h a t i n t e r f e r e w i t h i n t e r p r e t a t i o n , these v a r i a n t s do n o t o b v i a t e t h e more fundamental o b j e c t i o n s and much study i s s t i l l r e q u i r e d d e s p i t e t h e l a r g e numbers o f papers t o be found i n t h e J o u r n a l of Thermal A n a l y s i s , Thermochimica A c t a and Thermal A n a l y s i s A b s t r a c t s

over the past decade.

It should be noted, however, t h a t the basic o b j e c t i o n s

r a i s e d above do n o t necessarily mean t h a t t h e numerical vaiues obtained f o r c e r t a i n k i n e t i c parameters have no p r a c t i c a l value. 1.3.2.1

Evolv-d gas d e t e c t i o n (EGD)

I n EGD one determines whether o r n o t gas e v o l u t i o n i s associated w i t h a thermal effect.

I t i s customary, therefore, t o use i t i n conjunction w i t h DTA (mass loss

being adequate i n d i c a t i o n o f gas e v o l u t i o n i n TG) and t h e s i m p l e s t method i s t o i n s e r t a thermal c o n d u c t i v i t y c e l l i n the c a r r i e r gas stream coming from t h e equipment (Ingraham, 1967), although several o t h e r methods are a l s o a v a i l a b l e (Daniels, 1973). An EGD technique t h a t has proved very useful i n studying polymer degradation on heating i s thermal v o l a t i l i z a t i o n a n a l y s i s (McNeill, 1977).

I n t h i s t h e sample

i s heated i n a high vacuum chamber connected t o a vacuum pump though a t r a p cooled i n l i q u i d nitrogen, the pressure between t h e sample and the t r a p being measured by a P i r a n i gauge.

Whenever t h e sample decomposes, the pressure increases and

the decomposition o f the polymer can thus be followed.

The method i n t h i s form

does n o t g i v e any i n d i c a t i o n o f t h e v o l a t i l e products formed b u t t h e equipment has been modified so t h a t the condensed v o l a t i l e s i n t h e c o l d t r a p b o i l o f f as t h e t r a p i s allowed t o heat up.

Since t h e b o i l i n g p o i n t permits each component

t o be i d e n t i f i e d (McNeill, 1980), t h e method i s upgraded t o EGA

- always

assuming

t h e r e i s no i n t e r a c t i o n between condensed v o l a t i l e s .

A method f o r simultaneous DTA and EGD by measuring the i n t e n s i t y o f a l a s e r beam t r a v e r s i n g t h e sample c e l l above the sample has r e c e n t l y been described i n N e t s u s o k u t e i (1980).

Although a p p l i e d t o d e r i v e the phase diagram o f a known

13 b i n a r y l i q u i d system, t h e method i s c l e a r l y non-specific and could probably be adapted t o DTA-EGD o f clays o r clay-organic complexes: i t s advantage l i e s i n t h e f a c t t h a t i t gives a very sharp i n f l e c t i o n immediately vapour e v o l u t i o n commences even w i t h very slow heating rates. 1.3.2.2

Evolved g a s a n a l y s i s (EGA)

EGA i s much more useful, and indeed

usual, than EGD, since i t enables

determination o f t h e i d e n t i t y and/or amount o f the evolved v o l a t i l e material.

It

i s normally employed along w i t h DTA, TG o r DTA-TG-DTG, thus enabling q u a n t i t a t i v e e v a l u a t i o n q f the e f f e c t s on t h e curves i n terms o f s p e c i f i c v o l a t i l e s . Basic i n f o r m a t i o n on various methods o f EGA, such as mass spectrometry, gas chromatography, i n f r a - r e d absorption and s e l e c t i v e sorption, w i l l be found i n the books o f Lodding (1967), Mackenzie (1970-72), Daniels (1973) and Wendlandt (1974), i n t h e Proceedings o f the various I n t e r n a t i o n a l Conferences on Thermal Analysis (Redfern, 1965; Schwenker and Garn, 1968; Wiedemann, 1972, 1980; B U Z ~ S , 1975; Chihara, 1977), i n t h e Proceedings o f the F i r s t European Symposium on Thermal Analysis (Dollimore, 1976) and i n various s c i e n t i f i c j o u r n a l s . B r i e f l y , t h e methods of EGA f a l l i n t o two classes: ( a ) those f o r which p r i o r knowledge of t h e nature o f the v o l a t i l i z e d m a t e r i a l i s unnecessary and ( b ) those f o r which such knowledge i s essential. While the former, which i n c l u d e mass spectrometry and gas chromatography, are by f a r the most generally useful, t h e l a t t e r have a d e f i n i t e place i n studies, such as those on clays, where a l i m i t e d number of v o l a t i l e m a t e r i a l s are t o be expected. Both have indeed been a p p l i e d i n c l a y studies: f o r example, Mhller-Vonmoos and M h l l e r (1975) have demonstrated how mass spectrometry combined w i t h DTA can reveal the presence o f organic carbon, p y r i t e and various carbonate minerals i n a clay, whereas Morgan (1977) has used DTA and separate detectors t o determine when and how much water, carbon d i o x i d e and- sulphur d i o x i d e (from o x i d a t i o n o f p y r i t e ) a r e evolved from clays, shales and s c h i s t s and P a u l i k and P a u l i k (1978) have used t h e i r technique o f thermal gas t i t r i m e t r y (i.e.

sorption o f the v o l a t i l e i n a suitable solution

o r solvent f o l l o w e d by t i t r a t i o n ) along w i t h DTA-TG-DTG t o determine the amounts o f contaminating a l u n i t e and c a l c i t e i n bauxites. The optimum technique depends on circumstances

-

and, n o t infrequently,on

finance.

While a s e r i e s o f detectors s p e c i f i c f o r one v o l a t i l e o n l y can be r e a d i l y and r e l a t i v e l y cheaply purchased and attached i n s e r i e s t o a s u i t a b l e thermal analysis instrument, t h e more f l e x i b l e and universal system using mass spectrometry i s expensive.

I n s e t t i n g up equipment and assessing r e s u l t s several important aspects must be kept i n mind: for example, t h e free, volume around the sample must

be r e l a t i v e l y small t o avoid undue d i l u t i o n o f evolved m a t e r i a l w i t h c a r r i e r gas, t h e i n t e r f a c e between a mass spectrometer and t h e thermal analysis instrument must be chosen w i t h care so t h a t one v o l a t i l e i s n o t p r e f e r e n t i a l l y enriched a t

1 4 t h e expense o f another, and t h e l i k e l i h o o d of two v o l a t i l e s r e a c t i n g b e f o r e measurement must be assessed.

A v a r i a n t o f EGA, pyrolysis-gas chromatography-mass spectrometry, which was developed m a i n l y f o r t h e study o f s y n t h e t i c polymers, has proved extremely u s e f u l i n t h e i n v e s t i g a t i o n and c h a r a c t e r i z a t i o n o f s o i l organic m a t t e r (Bracewell and Robertson, 1977) and should a l s o be a p p l i c a b l e t o o r g a n i c m a t t e r i n c l a y deposits. I n t h i s method the sample i s very r a p i d l y pyrolysed i n an i n e r t atmosphere, t h e products being separated by gas chromatography and i d e n t i f i e d by mass spectrometry. Such a procedure y i e l d s f a i r l y l a r g e fragments o f t h e o r i g i n a l molecules, thus g i v i n g an i n s i g h t i n t o t h e n a t u r e o f t h e organic polymers present.

I n some

circumstances, and p a r t i c u l a r l y when i n t e r a c t i o n between products i s l i k e l y t o occur, a p r e f e r a b l e system i s pyrolysis-mass spectrometry (Bracewell and Robertson, 1980), d e s p i t e i t s h i g h e r c o s t because o f t h e more e l a b o r a t e data h a n d l i n g system required. 1.3.2.2.1 Ema n a t ion t h e r m a l a n a l y s i s . T h i s i s e s s e n t i a l l y a v a r i a n t o f EGA where t h e r a d i o a c t i v e emanation evolved d u r i n g h e a t i n g o f t h e sample i s measured. While t h i s d e f i n i t i o n would n o r m a l l y i n c l u d e o n l y radon isotopes, i n p r a c t i c e t h e method has been extended t o non-radioactive i n e r t gases, such as argon, k r y p t o n o r xenon, and t h e i r r a d i o a c t i v e isotopes: t h e r e i s thus a g r a d a t i o n i n t o normal EGA.

Materials

n o t c o n t a i n i n g i n e r t gas can be l a b e l l e d by d i f f u s i n g gas i n t o t h e s o l i d a t h i g h pressures and temperatures, by i n c l u d i n g t h e gas d u r i n g synthesis o r by bombardi n g t h e s u r f a c e o f the sample w i t h a c c e l e r a t e d ions o f i n e r t gas.

Changes i n gas

r e l e a s e r a t e on h e a t i n g can then be c o r r e l a t e d w i t h dehydration, decomposition, r e c r y s t a l l i z a t i o n , phase t r a n s i t i o n , s o l i d - s t a t e r e a c t i o n s and changes i n s u r f a c e properties.

Development o f emanation thermal a n a l y s i s i n c o n j u n c t i o n w i t h DTA i s

due mainly t o Balek and h i s c o l l a b o r a t o r s i n Czechoslovakia and i n f o r m a t i o n on theory, i n s t r u m e n t a t i o n and a p p l i c a t i o n s i s b e s t obtained from a r e c e n t exhaustive review (Balek, 1977).

Although no s e r i o u s s t u d i e s seem y e t t o have been made on

c l a y s , accessory minerals such as i r o n oxides, q u a r t z and z e o l i t e s have r e c e i v e d attention.

Several p o s s i b l e a p p l i c a t i o n s t o c l a y minerals, p a r t i c u l a r l y h o l l o w

f i b r o u s and h i g h l y disordered species, s p r i n g t o mind, b u t i t has y e t t o be e s t a b l i s h e d whether t h e i n f o r m a t i o n obtained would be s u p e r i o r t o t h a t f r o m more conventional methods. So-called "temperature-programmed d e s o r p t i o n curves" might we1 1 be regarded as r e l a t e d t o t h e above, even although t h e gases i n v o l v e d a r e n o t i n e r t .

Recent . _

work w i t h these (Criado et ai., 1980) has c l e a r l y shown t h e i r value i n s t u d y i n g t h e k i n e t i c s o f d e s o r p t i o n o f sorbed gas. 1.3.2.2.2

T h e m o p a r t i c u l a t e a n a l y s i s . Since t h e degradation o f s y n t h e t i c polymers

y i e l d s condensation n u c l e i as w e l l as molecular species, t h e r m o p a r t i c u l a t e a n a l y s i s ,

1 5

whereby t h e amount (and sometimes i d e n t i t y ) o f such n u c l e i a r e measured as a f u n c t i o n o f temperature, i s c l e a r l y c l o s e l y r e l a t e d t o EGA. P a r t i c l e s o f t h e order o f 1-100 nm i n s i z e a r e i n v o l v e d and an e a r l y review o f equipment, technique and a p p l i c a t i o n s was g i v e n by Murphy (1967).

A more r e c e n t v a r i a n t ( g i v e n various

names by i t s o r i g i n a t o r s ) i s a c l o s e r e l a t i v e o f EGD, s i n c e n e i t h e r the amount nor i d e n t i t y o f t h e p a r t i c l e s i s measured, b u t when combined w i t h mass spectrometry becomes again e s s e n t i a l l y EGA (see Smith, P h i l l i p s and Kaczmarek, 1976; Smith, Meier and P h i l l i p s , 1977). Although t h e technique has been a p p l i e d o n l y t o l a r g e organic molecules t h e r e a r e p o s s i b i l j t i e s o f a p p l i c a t i o n i n c o n d i t i o n s where chemical t r a n s p o r t occurs. Since chemical t r a n s p o r t has r e c e n t l y been observed on h e a t i n g c e r t a i n i r o n oxides

(E. Paterson, personal communication), i t i s , therefore, applications e x i s t i n the c l a y f i e l d . 1.3.3

marginally possible t h a t

Heating curve determination

Heating curves and the two d e r i v a t i v e s , h e a t i n g - r a t e curves and i n v e r s e heati n g - r a t e curves, were a t one time w i d e l y used i n c l a y s t u d i e s : indeed, t h e f i r s t thermoanalytical records f o r c l a y s were, as mentioned above, h e a t i n g - r a t e curves. However, h e a t i n g curves f e l l i n t o disuse, as t h e low s e n s i t i v i t y o f r e c o r d i n g necessary t o cover the whole temperature range o f i n t e r e s t precluded d e t e c t i o n o f small thermal e f f e c t s , and i n t e r e s t i n t h e two d e r i v a t i v e curves waned as DTA became established, s i n c e e s s e n t i a l l y t h e same i n f o r m a t i o n c o u l d be obtained more readily.

So f a r as t h e author i s aware, no heating-curve determinations on clays

a r e now performed.

1.3.4 D i f f e r e n t i a l thermal a n a l y s i s (DTA) DTA i s by f a r t h e best known and most w i d e l y used thermoanalytical technique i n c l a y studies, whether on i t s own o r simultaneously w i t h o t h e r methods such as

TG, EGA, e t c .

I t s main uses a r e i n " f i n g e r - p r i n t i n g " specimens, i n d e t e c t i n g

c e r t a i n accessory m i n e r a l s and "abnormal" species o f c l a y minerals, i n d e t e c t i n g changes i n mineralogy w i t h depth o r distance, i n q u a n t i t a t i v e (or, more f r e q u e n t l y , s e m i - q u a n t i t a t i v e ) s t u d i e s and i n determining heat s t a b i l i t y o r t h e occurrence o f solid-state reactions.

L i m i t a t i o n s imposed by i n s t r u m e n t a t i o n , technique and

n a t u r a l v a r i a t i o n s i n m i n e r a l s make accurate q u a n t i t a t i v e work d i f f i c u l t and preclude i t s use d i a g n o s t i c a l l y , except i n s p e c i a l circumstances.

A s these d e t a i l s

and p o s s i b l e methods o f m i n i m i z i n g l i m i t a t i o n s a r e so wel1,known (see Mackenzie, 1957b, 1970-72),

t h e method w i l l n o t be d e a l t w i t h here a t any g r e a t l e n g t h and

reference w i l l be made o n l y t o some r e c e n t developments. Commercial equipment design has improved enormously over t h e p a s t decade and, although t h e r e may n o t be so many instruments on t h e market now as a few years ago, those companies t h a t c u r r e n t l y manufacture equipment have introduced many

16 improvements,

While no instrument i s u n i v e r s a l and t h e instrument must be chosen

having regard t o t h e proposed a p p l i c a t i o n s , t h e general t r e n d towards t h e use o f small samples o f o n l y a few m i l l i g r a m s i s t o be welcomed, as i t minimizes e r r o r s due t o thermal g r a d i e n t s and, provided r e c o r d i n g s e n s i t i v i t y i s adequate, enables simultaneous DTA-TG t o be v a l i d l y used.

The f a c t t h a t two markedly d i f f e r e n t DTA

instruments y i e l d e s s e n t i a l l y t h e same curves f o r a d e h y d r o x y l a t i o n r e a c t i o n when sample s i z e i s l e s s than 30 mg (Broersma et ai., very small samples.

1978) confirms t h e value o f using

Many manufacturers a l s o produce several types o f specimen

holders f o r e a c h instrument ,so t h a t t h e optimum (Wilburn, 1972) can be selected, and c o n t r o l o f atmosphere around the Sample i s now u n i v e r s a l .

These advances i n

i n s t r u m e n t a t i o n enable DTA t o be much more w i d e l y and v a l i d l y used i n c l a y s t u d i e s than ever before. An advance i n methodology has been the i o t r o d u c t i o n o f stepwise h e a t i n g (Staub and Perron, 1974; Simonsen and Zaharescu, 1979), by which temperature i s increased i n small steps (0.5-10°C,

depending on t h e r e a c t i o n ) i n s t e a d o f continuously,

e q u i l i b r i u m being a t t a i n e d a t each step.

This i s e s s e n t i a l l y e q u i v a l e n t t o

o b t a i n i n g a s e r i e s o f isothermal measurements d u r i n g one d e t e r m i n a t i o n w i t h t h e r e s u l t t h a t heats o f m e l t i n g , t r a n s i t i o n , etc.,

can be determined more a c c u r a t e l y .

To t h e a u t h o r ' s knowledge, t h i s technique has n o t y e t been a p p l i e d t o clays: t h e r e i s no reason t o suppose i t i s i n a p p l i c a b l e , although any apparent advantages would have t o be c r i t i c a l l y checked.

The sample c o n t a i n e r can sometimes have an undesir-

a b l e e f f e c t ( M a r t i n V i v a l d i , G i r e l a V i l c h e z and F e n o l l Hach-Ali, 1964; D o l l i m o r e and Mason, 1981): t o prevent c o n t a i n e r e f f e c t s i n m e t a l l u r g y , electromagnetic l e v i t a t i o n of the sample has been introduced (Jorda, F l U k i g e r and MUller, 1978) b u t t h i s would u n f o r t u n a t e l y be impossible w i t h c l a y s and t h e m a t e r i a l o f the specimen holder must be c a r e f u l l y selected.

The n e c e s s i t y f o r care i n sample

p r e p a r a t i o n has been emphasized by work on q u a r t z (Moore and Rose, 1979), where small exothermic peaks observed a f t e r g r i n d i n g i n s t e e l and agate v i b r a t i o n m i l l s have been a t t r i b u t e d t o o x i d a t i o n o f contaminating i r o n and r e l e a s e o f s t o r e d energy, r e s p e c t i v e l y .

On t h e o t h e r hand, t h e doubling o f t h e d e h y d r o x y l a t i o n

endotherm o f g o e t h i t e on g r i n d i n g has been a t t r i b u t e d t o p a r t i c l e s i z e e f f e c t s (Murad, 1979).

The value o f c o n t r o l l e d atmosphere has been confirmed by s t u d i e s

on carbonate minerals, which show t h a t i n a carbon d i o x i d e atmosphere t h e m i n e r a l s present can be i d e n t i f i e d and t h e amounts estimated f o r o n l y 0.125 mg i n a 50 mg sample (Warne and M i t c h e l 1 , 1979). The main use o f DTA i n c l a y studies, r e c e n t l y as i n t h e past, has been as an a d j u n c t t o o t h e r techniques i n c h a r a c t e r i z a t i o n s t u d i e s .

Yet some i n v e s t i g a t i o n s

r e v e a l t h e value o f the technique i n i t s own r i g h t , and i t may be a p p o s i t e t o c i t e some examples.

Thus, peak temperature o f the low-temperature endotherm f o r

H-saturated allophane and t h a t o f the high-temperature exotherm f o r Na-allophane increases and decreases, r e s p e c t i v e l y , w i t h i n c r e a s i n g SiO,:Al,O,

r a t i o (Henmi,

17 1980).

Moreover, t h e exotherm o f allophane i s lowered i n temperature and

broadened on admixture w i t h i r o n o x i d e g e l s o r hydrous i r o n o x i d e minerals, whereas i t i s u n a f f e c t e d by oxide m i n e r a l s such as hematite o r maghemite (Suzuki and Satoh, 1980).

For d i c k i t e s , i n c r e a s i n g breadth o f t h e dehydroxylation

endotherm seems t o be approximately associated w i t h i n c r e a s i n g degree of disorder, although some exceptions suggest t h i s r u l e i s n o t r i g i d ( B r i n d l e y and P o r t e r , 1978).

I t i s w e l l knownthat DTA curves o f i n t e r s t r a t i f i e d m i n e r a l s a r e d i f f i c u l t

to interpret

-

presumably because d e h y d r o x y l a t i o n o f one l a y e r n e c e s s a r i l y a f f e c t s

t h a t o f neighbouring l a y e r s whether o r n o t they a r e o f the same t y p e

-

b u t i t has

r e c e n t l y been shown t h a t the shape and s i z e of t h e hygroscopic m o i s t u r e peak f o r Sr-saturated mica-montmorillonites can be used t o assess t h e p r o p o r t i o n o f expansi b l e l a y e r s present (Inoue, Minato and Utada, 1978).

Since s o l i d - s t a t e r e a c t i o n s

t h a t occur between carbonate minerals, s o l u b l e s a l t s and mica on h e a t i n g a l s o take place w i t h m i c a - m o n t m o r i l l o n i t e b u t n o t w i t h p a l y g o r s k i t e (Mashhady et al., 1980) and manganese oxides r e a c t w i t h k a o l i n i t e i n t h e s o l i d s t a t e a t e l e v a t e d temperatures (Holland and Segnit, 1976), c a r e must be taken i n i n t e r p r e t i n g curves f o r systems I n p o l l u t i o n studies, t h e area o f t h e dehydroxylation

c o n t a i n i n g such mixtures.

endotherm o f c h y s o t i l e can a p p a r e n t l y be used t o assess asbestos content (Menis, Mackey and Garn, 1978).

The e f f e c t s o f sorbed organic m a t e r i a l s on DTA curves

f o r c l a y m i n e r a l s have been examined by, i n t e r alia, Hllbner (1927) and Eltantawy (1979), w h i l e c l a y - o r g a n i c r e a c t i o n products have been i n v e s t i g a t e d by Kuroda and Kato (1979). Although DTA has been w i d e l y used i n s t u d y i n g t h e k i n e t i c s o f r e a c t i o n s , t h e r e i s no doubt t h a t t h e techniques used a r e f r a u g h t w i t h even more d i f f i c u l t i e s than mentioned above f o r TG (Sharp, 1972).

However, a considerable p r o p o r t i o n o f

thermoanalytical papers do deal w i t h t h i s s u b j e c t and a t r e a t i s e on non-equilibrium k i n e t i c s has appeared (Koch, 1977).

As mentioned above under TG, i t i s u n l i k e l y

t h a t decomposition o f s o l i d s f o l l o w s t h e same k i n e t i c s throughout t h e whole range from a = 0 t o a = 100 (where a i s t h e f r a c t i o n reacted) and thus t h e best t h a t can be expected would be t o deduce values f o r a l i m i t e d a range. 1.3.5

D i f f e r e n t i a l scanning c a l o r i m e t r y (DSC)

Two types o f DSC a r e recognized, power-compensation DSC and h e a t - f l u x DSC (Table 1.1). The r e l a t i o n s h i p s between these and DTA have r e c e n t l y been discussed i n d e t a i l (Mackenzie, 1980) and need n o t be repeated here.

It need only be

mentioned t h a t DSC was o r i g i n a l l y l i m i t e d t o a maximum temperature o f about 450°C because o f t h e i n c r e a s i n g importance above t h a t temperature o f r a d i a t i v e heat t r a n s f e r : however, r e c e n t t e c h n o l o g i c a l advances have enabled t h e temperature range t o be extended t o about 750-800°C so t h a t a l i m i t e d a p p l i c a t i o n i n c l a y mineralogy i s now possible. The main advantage o f DSC over DTA i s t h a t i t i s i n h e r e n t l y q u a n t i t a t i v e f o r

18 change i n enthalpy, heat capacity, e t c .

Consequently, i t can, f o r example, be

used t o determine n o t o n l y t h e enthalpy change o c c u r r i n g d u r i n g a r e a c t a l s o t h e t o t a l energy r e q u i r e d t o f i r e a c l a y over any s p e c i f i c temperatur, up t o about 750°C.

The method has, however, n o t been w i d e l y used i n t h e study

0,

clays, although i n v e s t i g a t i o n s on m o n t m o r i l l o n i t e (Homshaw and Chaussidon, 1979), on smectites, z e o l i t e s , h a l l o y s i t e and opal (Eger, Cruz-Cumplido and F r i p i a t , 1979) and on s y n t h e t i c g o e t h i t e (Paterson, 1980) suggest i t can y i e l d v a l u a b l e r e s u l t s , both q u a l i t a t i v e and q u a n t i t a t i v e , i n connection w i t h s o l v a t i o n problems.

I t has

a l s o been u s e f u l i n d e t e c t i n g d i f f e r e n t types o f surface hydroxyl groups on s y n t h e t i c g o e t h i t e (Paterson and S w a f f i e l d , 1980) and i n r e v e a l i n g major d i f f e r e n c e s i n s o i l and rock q u a r t z samples (Barwood and Hajek, 1979). I n view o f these i n d i c a t i o n s , i t would appear t h a t DSC i s l i k e l y t o be increasi n g l y used i n c l a y s t u d i e s over t h e n e x t few years, p a r t i c u l a r l y i n q u a n t i t a t i v e and surface i n v e s t i g a t i o n s .

An extension o f t h e c u r r e n t upper temperature l i m i t

would open even g r e a t e r prospects, b u t t h i s does n o t seem l i k e l y i n t h e near f u t u r e . 1.3.6

Thermodilatometry

I n thermodilatometry t h e volume or, more u s u a l l y , t h e l i n e a r dimensionsal change on h e a t i n g i s s t u d i e d as a f u n c t i o n o f temperature under n e g l i g i b l e load. The method i s an o l d one and indeed t h e o b s e r v a t i o n o f shrinkage o f c h i n a c l a y on f i r i n g gave Wedgwood (1782) t h e idea f o r h i s famous pyrometer, which, although g r o s s l y i n a c c u r a t e i n absolute terms, was t h e o n l y means a v a i l a b l e f o r a c c u r a t e l y comparing h i g h temperatures f o r 50 years o r more from t h e 1780s.

Thermodilatometry i s a very common method i n ceramic technology, where a knowledge o f t h e f i r i n g shrinkage o f c l a y i s e s s e n t i a l and where b l e n d i n g can be used t o minimize such shrinkage.

I t has a l s o been a p p l i e d t o c l a y minerals, b u t

t h e determination i s so s e n s i t i v e t o degree o f o r i e n t a t i o n o f p l a t y minerals, t h e nature and amounts o f accessories, etc., o f g r e a t value f o r i d e n t i f i c a t i o n ,

t h a t t h e r e s u l t s have n o t been considered

There have been r e c e n t i n d i c a t i o n s , however,

t h a t t h e method can have some l i m i t e d d i a g n o s t i c uses, s i n c e i t would appear t h a t small amounts o f d i c k i t e o c c u r r i n g i n a k a o l i n can be i d e n t i f i e d by an expansion e f f e c t a t about 650°C t h a t i s p a r t i c u l a r l y s t r o n g f o r d i c k i t e (Schomberg and Schtlrr, 1978b).

Thermodilatometry has a l s o r e c e n t l y been used t o o b t a i n informa-

t i o n on t h e mechanism o f i n i t i a t i o n o f t h e d e h y d r o x y l a t i o n and " m u l l i t i z a t i o n " r e a c t i o n s o f k a o l i n i t e (Flank, 1979) and t h e r e i s no doubt t h a t , because o f i t s s e n s i t i v i t y t o o r i e n t a t i o n e f f e c t s , i t has p o t e n t i a l a p p l i c a t i o n i n assessing the degree o f o r i e n t a t i o n o f p l a t y p a r t i c l e s i n c l a y s t r a t a . Thermodilatometers and d i f f e r e n t i a l thermodilatometers a r e r e a d i l y a v a i l a b l e commercially and d e r i v a t i v e thermodilatometric curves can be e a s i l y recorded from t h e r m o d i l a t o m e t r i c measurements.

When performed simultaneously w i t h DTA-TG,

thermodilatometry and d e r i v a t i v e thermodilatometry can y i e l d r e s u l t s t h a t h e l p t o

19

explain features on DTA and TG curves ( P a u l i k and Paulik, 1978). Such simultaneous measurements may, therefore, be worthy o f f u r t h e r study by c l a y mineralogists. 1.3.7

Thermomechanical measurements

AS noted i n Table 1.1, thermomechanical measurements can be c a r r i e d out w i t h a

n o n - o s c i l l a t o r y (i.e.

s t a t i c ) o r w i t h an o s c i l l a t o r y ( i . e . dynamic) load. The former, which are c u r r e n t l y commonly r e f e r r e d t o as "thermomechanical analysis" o r "TMA" (despite t h e e r r o r o f using "analysis" i n such a connotation) can have various modes depending on whether the s t r e s s applied t o t h e sample i s compression, tension, f l e x u r e o r t o r s i o n and on whether deformation o r p e n e t r a t i o n i s measured (Daniels, 1973). Clearly, t h i s technique i s c l o s e l y r e l a t e d t o thermodilatometry, t h e d i f f e r e n c e being i n the use o f a f i n i t e load, whether p o s i t i v e o r negative: indeed, t h e same equipment can be used f o r both measurements (see Flank, 1979) and t h e r e i s some argument as t o whether thermodilatometry should be separately recognized.

Dynamic thermomechanical measurements enable a d i f f e r e n t s e t o f

parameters, such as the. shear modulus, the mechanical damping index, the loss tangent ( t a n 6 ) , etc.,

t o be measured as a f u n c t i o n o f temperature.

I t should

be noted t h a t i n d i v i d u a l isothermal measurements a r e n o t included. Thermomechanical measurements, both s t a t i c and dynamic, f i n d extensive use i n polymer science, b u t the p o t e n t i a l f o r t h e i r use i n c l a y mineralogy appears s l i g h t although i t i s f a s c i n a t i n g t o speculate on the p o s s i b l e r e s u l t o f applying the t o r s i o n a l pendulum (so-called " t o r s i o n a l b r a i d analysis"

- see Daniels,

1973) t o

c l a y samples.

I n c i v i l engineering and ceramic technology many o f the techniques are, i n f a c t , applied, o f t e n i s o t h e r m a l l y b u t sometimes, as when t e s t i n g hightemperature r e f r a c t o r i e s , using a c o n t r o l l e d temperature programme.

I n ceramic

and r e f r a c t o r y studies, measurements may be made o f deformation under load, shear modulus, modulus of e l a s t i c i t y , crushing strength, r e f r a c t o r i n e s s under load, creep, s t r e s s r e l a x a t i o n and toughness and of t h e i r v a r i a t i o n w i t h temperature. I n c i v i l engineering, load bearing properties, p e n e t r a t i o n measurements, etc., are u s u a l l y c a r r i e d o u t a t room temperature b u t may have t o be made over a l i m i t e d temperature range. Despite t h e i r p r a c t i c a l importance, these aspects are n o t s t r i c t l y w i t h i n the province o f t h i s review o r indeed o f the f i e l d o f experience o f the author

-

and w i l l n o t be f u r t h e r considered.

-

A much more r e l e v a n t a p p l i c a t i o n i s i n the

i n v e s t i g a t i o n o f o i l shales, where thermomechanical properties, such as deformation on heating, anisotropy o f compressive strength, etc.,

a r e important i n determining

the release o f t h e o i l product i n the r e t o r t (Rajeshwar, Nottenburg and DuBow, 1979).

A t present, therefore, thermomcchanical measurements are o f use mainly

i n technology and i n d u s t r y r a t h e r than i n mineralogy.

-

20 1.3.8

Thermosonimetry and thermoacoustimetr,y

These techniques are q u i t e d i s t i n c t .

I n thermosonimetry one measures t h e

sound emitted by a sample on heating, whereas i n thermoacoustimetry one measures t h e changes i n t h e c h a r a c t e r i s t i c s o f imposed sound waves caused by passage through t h e sample. As w i t h t h e two types o f thermomechanical measurement, therefore, t h e type o f i n f o r m a t i o n gained i s d i f f e r e n t . Sound emission from, o r d e c r e p i t a t i o n of, rocks was studied by Smith and Peach i n 1949 and associated by Smith (1957) w i t h t h e i r p h y l l o s i l i c a t e components.

This

l e d t o a survey o f micas (Hutchison, 1966) t h a t detected two temperature ranges o f sound emission

-

a t 305-340°C associated w i t h trapped water and a t 600-1000°C

associated w i t h p a r t i a l dehydroxylation.

The apparatus used i n these studies was

very simple and l i t t l e f u r t h e r ensued u n t i l L$nvik i n 1972 (see L l n v i k , 1974) devised a much more r e f i n e d sound-measurement arrangement using a s p e c i a l l y designed wave-guide system: sound emission was recorded e i t h e r as r a t e o r as amplitude against temperature.

Although frequency analysis o f t h e emitted sound i s possible,

i t i s r a t h e r more d i f f i c u l t t o perform.

The method has more r e c e n t l y been taken

up by Clark (1978) and co-workers i n the UK, who have developed a concurrent ( n o t simultaneous, as two samples a r e used) thermosonimetry-DTA apparatus (Clark and Garlick, 1979).

A l e s s elaborate, b u t a l s o l e s s v e r s a t i l e , system has been

described by Poulou and Baudracco-Gritti

(1978) and a S o v i e t instrument, whereby

pressure changes i n h i g h vacuum are measured as t h e sample decrepitates, i s comnercially a v a i l a b l e (Pawlikowski, 1979).

The l a s t i s s p e c i f i c a l l y designed

f o r measurement o f release o f m a t e r i a l from i n c l u s i o n s and i t i s doubtful whether i t would p i c k up o t h e r causes o f sound emission such as s t r a i n release, microcrack

propagation, etc. LBnvik (1974, 1978) has a p p l i e d the technique to, i n t e r alia, q u a r t z i t e s and boehmite and has noted very s t r o n g sound emission associated w i t h phase t r a n s i t i o n s and w i t h decomposition reactions.

C l a r k and G a r l i c k (1979), w i t h t h e i r combined

equipment, have come t o the conclusion t h a t maximum sound emission occurs a t t h e

-

i.e. sound extrapolated onset o f t h e DTA peak associated w i t h such processes emission i s v i r t u a l l y complete a t t h e p o i n t a t which measurable enthalpy changes occur

- and have r e l e a t e d t h e emissionof

sound e s s e n t i a l l y t o t h e release o f s t r a i n :

once the s t r a i n i s released, through t h e commencement o f a phase change o r a decomposition reaction, sound emission ceases.

A s i m i l a r mechanism would presumably

apply t o microcrack propagation b u t other r e l a t i o n s h i p s c o u l d w e l l h o l d f o r other phenomena such as t h e release o f inclusions. The r a p i d development i n t h i s f i e l d over t h e l a s t few years suggests t h a t t h e time i s now r i p e t o apply i t t o c l a y minerals: i n i t s most advanced form i t might w e l l h e l p t o e l u c i d a t e t h e mechanisms o f some r e a c t i o n s s t i l l i l l - u n d e r s t o o d . Thermoacoustimetry has so f a r been a p p l i e d mainly t o polymers and, l i k e a l l measurements i n v o l v i n g o s c i l l a t i o n o r waves, whether sound o r electromagnetic,

21

gives i n f o r m a t i o n on t h e shear modulus, t h e modulus o f e l a s t i c i t y , the l o s s tangent and such l i k e .

An e x c e l l e n t d e s c r i p t i o n has been given by Perepechko (1975).

I t s a p p l i c a t i o n t o clays a t t h e moment i s uncertain: i t s most l i k e l y f i e l d would be i n r e l a t i o n t o f i r e d c l a y products but, l i k e dynamic thermomechanical measurements and a l t e r n a t i n g e l e c t r i c a l measurements, i t has already found a p p l i c a t i o n i n o i l - s h a l e technology (Mraz, Rajeshwar and DuBow, 1980). 1.3.9 Therrnoptornetry Thermoptometry i s a wide term covering a whole range o f techniques, such as

thermophotometry (measurement o f t o t a l 1 i g h t ) , thermospectrometry (measurement of l i g h t o f a s p e c i f i c wavelength), thermorefractometry (measurement o f r e f r a c t i v e index) and thermomicroscopy f o r e i t h e r emitted o r r e f l e c t e d l i g h t .

Thermo-

luminescence i s a special case o f thermophotometry where emitted l i g h t only i s measured a t temperatures below r e d heat. Thermoluminescence i s probably t h e most widely used l o f thesetechniques, having been widely a p p l i e d t o l u n a r and m e t e o r i t e samples (Nambi, Bhasin and Bapat, 1978) as w e l l as t o c h a r a c t e r i z a t i o n o f marbles and limestones (Afordakos, Alexopoulous and M i l i o t i s , 1974; Nambi and Mitra, 1978; Chistyakova, 1979). i s a l s o used f o r age determination i n geology

-

It

f o r example, t o date the baking

o f a sediment (Huxtable, A i t k e n and Bonhommet, 1978)

-

and i n a,rchaeology

-

to

date ceramics and other a r t i f a c t s (Cairns, 1976). I t s r e l a t i o n s h i p t o DTA, DTG and other techniques has been discussed by Chen (1976). Basic equipment f o r simultaneous thermoluminescence-DTA has been described by David (1972) and the more s e n s i t i v e and elaborate equipment used i n achaeological and chemical studies by Cairns (1976) and Wendlandt (1980), r e s p e c t i v e l y . David (1972) reproduces a glow curve f o r "a c l a y " (which from i t s DTA curve i s a g i b b s i t i c bauxite) and Cairns (1976) discusses i n d e t a i l t h e o r i g i n o f thermoluminesce i n c l a y s and i n ceramic and other materials. Thermomicroscopy has been widely applied i n studies on glasses, r e f r a c t o r i e s and ceramics, and neat instruments f o r simultaneous DTA, o r DSC, and thermomicroscopy have been described and successfully used ( M i l l e r and Sommer, 1966; Sommer and Jochens, 1971; Kunihisa, 1979). Several standard hot-stage microscopes w i t h temperature c o n t r o l are a l s o commercially a v a i l a b l e b u t clays as such, presumably because o f t h e i r small p a r t i c l e size, have been r a t h e r neglected. Equipment f o r use w i t h some o f the o t h e r techniques r e f e r r e d t o above w i l l be found ( b u t n o t necessarily under t h e same names) i n standard t e x t s (e.g. Wendlandt and Hecht, 1966; Wendlandt, 1974).

It seems t o t h e author t h a t some o f these

methods have as y e t untapped p o t e n t i a l i-n studying, f o r example, changes i n t h e c o l o u r o f smectites on s a t u r a t i o n w i t h non-chromophoric ions and moderate heating.

2 2

Therrnoelectrometry Like thermoptometry, thermoelectrometry i s essentially a portmanteau term covering measurement of any e l e c t r i c a l property as a function of temperature. Thus i t covers variations in resistance, conductance, inductance and capacitance, f o r both d.c. and a.c., as well as variations in d i e l e c t r i c constants, thermoe l e c t r i c i t y , thermally stimulated current, etc. The value of d.c. conductance (or resistance) in showing up decomposition reactions, solid-state transitions, etc., was f i r s t appreciated by Berg and Burmistrova (1960), who constructed an instrument f o r simul taneous d.c. conductance measurement and DTA. Developments in :equipment and applications up t o 1974 have been discussed by Wendlandt (1974): more recently, a.c. conductance measurements have been favoured (Wendlandt, 1979). The terms "thermally stimulated conductivity" and "thermally stimulated current" have come into very widespread use, particularly in relation t o semi-conductor research: unfortunately, the f a c t t h a t both a r e given the abbreviation "TSC" has led t o widespread confusion in indexing - even in C h e m i c a l A b s t r a c t s . Freund and co-workers, in t h e i r fundamental study of proton tunneling i n hydroxides, have recently used d.c. conductance and thermally stimulated depolarization measurements t o provide evidence of the occurrence o f proton conductivity and transitory HOH species in gibbsite, brucite and portlandite before the comnencement of dehydroxylation (Freund and Wengeler, 1980; Wengeler, Martens and Freund, 1980). In these studies, they observed t h a t the d.c. conductance depends on the nature of the electrodes used and that thermopotential measurements are sensitive t o electron acceptors or donors sorbed on p a r t i c l e surfaces. D.c. conductance has also been used t o investigate water sorbed on goethite (Kaneko and Inouye, 1979). Similar applications are likely with clays. In o i l shales, the d.c. resistance decreases exponentially w i t h increasing temperature - an observation attributed t o carbonate ions being the transporting species o r t o breakdown of hydrocarbon units i n the trapped kerogen (Rajeshwar, Nottenburg and DuBow, 1979). Such systems are extremely complicated and i t i s d i f f i c u l t t o separate the effects due t o the clay matrix from those due to the kerogen. Attempts have been made t o solve the problem using a.c. measurements involving conductance i.e. t h a t involve d i e l e c t r i c constant, d i e l e c t r i c loss factor and loss tangent and equipment for simultaneous and concurrent a.c. capacitance measurement and DTA has been described (Rajeshwar, Nottenburg and DUBOW, 1978; Nottenburg e t ai., 1979). The l a t t e r i s considered the more r e l i a b l e as the optimum sample s i z e f o r DTA and for capacitance measurement i s quite different. I t seems t o the author that examination of non-oil-bearing shales of similar mineralogical composition could a s s i s t in resolving some of the d i f f i c u l t i e s of interpretation encountered, b u t there may be practical d i f f i c u l t i e s related t o p a r t i c l e orientation, pore space, etc. 1.3.10

23

Thermally stimulated c u r r e n t s have been r e l a t e d t o various types o f l a t t i c e defects i n i o n i c c r y s t a l s and a review has been given by Radhakrishna and Haridoss (1978).

These c u r r e n t s can a l s o be r e l a t e d t o thermoluminescence e f f e c t s ( F i e l d s

and Moran, 1974)

-

as, indeed, can thermally stimulated conductance (Chen, 1976;

Braeunlich, K e l l y and F i l l i a r d , 1979)

-

b u t no a p p l i c a t i o n t o clays can be traced.

I t may be o f i n t e r e s t t o some t o note t h a t values o f t h e thermoelectric power

( i n uV/"C against lead) have been measured by Lee (1973) f o r a considerable number o f minerals, i n c l u d i n g some that, l i k e hematite, magnetite and ilmenite, can occur as accessories i n clays: i n d i c a t i o n s are given o f t h e spread o f values f o r the samples t e z t e d as w e l l as mean values. I n conclusion, i t would appear t h a t some thermoelectrometric techniques may have considerable p o t e n t i a l i n c l a y studies

-

e s p e c i a l l y i n r e l a t i o n t o water-

loss reactions and l a t t i c e d e f e c t i n v e s t i g a t i o n s

- q u i t e a p a r t from t h e i r p o t e n t i a l

value i n technological assessments. Thermomagnetometry

1.3.11

The most common magnetic c h a r a c t e r i s t i c t o be measured as a f u n c t i o n o f temperature i s magnetic s u s c e p t i b i l i t y , w i t h consequent d e r i v a t i o n o f t h e Curie point soils.

-

a parameter t h a t has proved extremely useful i n some studies on clays and Thus, on t h e basis t h a t t h e very l a r g e v a r i a t i o n i n the Curie p o i n t o f

i l m e n i t e (below 77 K t o 841 K) can be c o r r e l a t e d w i t h i t s o r i g i n , i t has been claimed t h a t samples w i t h a Curie p o i n t i n t h e r e g i o n 103-223 K are l i k e l y t o be associated w i t h k i m b e r l i t e deposits (Garanin, Kudryavtseva and Soshkina, 1979). Moreover, f o r red-yellow d e s e r t i c s o i l s , i t has been possible t o i n t e r p r e t Curie p o i n t s i n terms o f the presence o f hematite and maghemite (Timofeev and Smirnov, 1980): i n t h i s study, non-coincidence of the thermomagnetic curve on a second heating has been i n t e r p r e t e d as i n d i c a t i n g t h e presence o f f e r r i h y d r i t e

-

although

formation o f i r o n oxides from some other minerals present could p o s s i b l y a l s o contribute.

Be t h a t as i t may, i t would seem t h a t thermomagnetometry may w e l l

have s i g n i f i c a n t a p p l i c a t i o n s i n c l a y mineralogy, e s p e c i a l l y i n the i n v e s t i g a t i o n o f c e r t a i n accessory minerals. 1.4

CONCLUSIONS

From the above account, i t i s evident t h a t , although TG and DTA a r e by f a r the most widely used thermoanalytical techniques i n c l a y mineralogy a t present, many o t h e r methods t h a t can e l u c i d a t e s p e c i f i c features o r behaviour a r e available. Thus, EGA and DSC a r e p r e s e n t l y r e c e i v i n g greater a t t e n t i o n and thermosonimetry along w i t h some thermoptometric, thermoelectrometric and thermomagnetic methods deserve consideration i n t h e f u t u r e .

Methods using o s c i l l a t i o n s o r waves t h a t

j i v e i n f o r m a t i o n on t h e shear modulus, the modulus o f e l a s t i c i t y , etc.,

are

r e l e v a n t e s s e n t i a l l y t o technology and industry, although t h e i r possible uses i n

24 more fundamental studies should not be neglected. All-in-all, however, thermoanalytical methods are not in themselves necessarily good in mineral identification: theirforte is in yielding information on characteristics or detail that it would be difficult, if not impossible, to obtain by any other method. REFERENCES Afordakos, G. , Alexopoulos, K. and Miliotis, D., 1974. Using artificial thermoluminescence to reassemble statues from fragments. Nature, Lond., 250: 47-48. Alietti, A., Brigatti, M.F. and Poppi, L . , 1979. An illite/montmorillonite interlayer mineral in porphyritic rock 'Alpe di Siusi' (BZ-Italy): The Chemistry of I/M interlayer minerals. Clay Miner., 14: 39-46. Ashley, H.E., 1911. The decomposition of clays and the utilization of smelter and other smoke in preparing sulphate from clays. J. ind. Engng Chem., 3: 91-95. Asomoza, P.M., Razo, M.L., Chaidez, T.L. and Casillas, S.R., 1978. Quantitative evaluation of Na,CO, and NaCl content of the clays of the ex-lake of Texcoco (Valley of Mexico) by means of thermogravimetry. J. therm. Analysis, 13: 327-339, Balek, V., 1977. Emanation thermal analysis. Thermochim. Acta, 22: 1-156. Barwood, H.L. and Hajek, B.F., 1979. Differential thermal characteristics of soil and reference quartz. J. Soil Sci. SOC. Am., 43: 626-627. Berg, L.G. and Burmistrova, N.P., 1960. [Thermographic analysis of salts with simultaneous determination of temperature effects and electrical conductivity.1 Zh. neorg. Khim. 5: 676-683. Boy, S. and BMhme, K. , 1979. GegenUberstellung verschiedener LBsungsverfahren zur Ermittlunq kinetischer Parameter anhand von Experimentalwerten. Thermochim. Acta, 28: 2491258. Bracewell, J.M. and Robertson, G.W., 1977. Pyrolysis studies on humus in freely drained Scottish soils. In C.E.R. Jones and C.A. Cramers [Editors). Analytical Pyrolysis: Proc. 3rd Int. Symp. Analyt. Pyrol. , Amsterdam. ' Elsevier, Amsterdam, pp. 167-178. Bracewell, J.M. and Robertson, G.W. , 1980. Pyrolysis-mass spectrometry studies of humification in a peat and a peaty podzol. J. analyt. appl. Pyrol., 2: 53-62. Braeunlich, P., Kelly, P. and Filliard, J.P., 1979. Thermally stimulated luminescence and conductivity. Top. appl. Phys. ,.37: 35-92. Brindley, G.W. and Porter, A.R.D., 1978. Occurrence of dickite in Jamaica ordered and disordered varieties. Am. Miner., 63: 554-562. Broersma, A., de Bruyn, P.L., Jens, J.W. and Stol, R.J., 1978. Simultaneous DTA and DTG measurements on aluminium oxide monohydroxides. J. therm. Analysis, 13: 341-355. Brown, G.H. and Montgomery, E.T., 1912. The dehydration of clays. Trans. Am. Ceram. SOC., 14: 709-721. Buzds, I. (Editor), 1975. Thermal Analysis: Proc. 4th ICTA, Budapest. Heyden, London, 3 vols. Cairns, T. , 1976. Archaeological dating by thermoluminescence. Analyt. Chem. , 48: 266A-280A. Chen, R., 1976. Methods for kinetic analysis o f thermally stimulated processes. 3. Mater Sci., 11: 1521-1541. Chihara, H. (editor), 1977. Thermal Analysis: Proc. 5th ICTA, Kyoto. Heyden, London, 573 pp. Chistyakova, A.A., 1979. [Investigation of the thermoluminescence of limestones and limes.] Zap. vses. Miner. Obshch., 108: 91-99. Clark, G.M., 1978. Instrumentation for thermosonimetry. Thermochim. Acta, 27: 19-25. Clark, G.M. and Garlick, R., 1979. Thermosonimetry of the NBS-ICTA temperature standards. Thermochim. Acta, 34: 365-375. Coats, A.W. and Redfern, J.P. , 1963. Thermogravimetric analysis. Analyst, Lond. , 88: 906-924.

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....

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26 Ingraham,,T.R. , 1967. Thermal c o n d u c t i v i t y d e t e c t o r s and t h e i r a p p l i c a t i o n t o some thermodynamic and k i n e t i c measurements. I n W. Lodding ( E d i t o r ) . Gas E f f l u e n t A n a l y s i s . Marcel Dekker, New York, pp. 25-69. Inoue, A., Minato, H. and Utada, M., 1978. M i n e r a l o g i c a l p r o p e r t i e s and occurrence o f i l l i t e / m o n t m o r i l l o n i t e mixed l a y e r m i n e r a l s formed f r o m Miocene v o l c a n i c g l a s s i n Waga-Omoho d i s t r i c t . Clay Sci., 5: 123-136. IUPAC, 1974. Recomnendations f o r nomenclature o f thermal a n a l y s i s . Pure appl. Chem., 37: 439-444. IUPAC, 1980. Nomenclature f o r thermal a n a l y s i s I 1 and 111. Pure appl. Chem., 52: 2385-2391. Iwasa, Y., 1978. Occurrence o f l e p i d o c r o c i t e i n a y e l l o w o r g a n i c c o l o r e d s o i l i n humid s u b - t r o p i c a l region. Nendo Kagaku, 18: 135-144. Jorda, J.L., FlUkiger, R. and MUller, J., 1978. Thermal a n a l y s i s o f l e v i t a t i o n heated samples: A p p l i c a t i o n t o t h e niobium-germanium system. J. Mater. Sci., 13: 2471-2476. Kaneko, K. and Inouye, K., 1979. A d s o r p t i o n o f water on i r o n h y d r o x i d e as s t u d i e d by e l e c t r i c a l c o n d u c t i v i t y measurements. B u l l . chem. SOC. Japan, 52: 315-320. Keattch, C.J. and Dollimore, D., 1975. I n t r o d u c t i o n t o Thermogravimetry, Second E d i t i o n . Heyden, London, 164 pp. Koch, E., 1977. Non-Isothermal Reaction A n a l y s i s . Academic Press, London, 607 pp 1979. An apparatus f o r t h e r m o a n a l y t i c a l microscopy and i t s Kunihisa, K.S., a p p l i c a t i o n s . Thermochim. Acta, 31: 1-11. Kuroda, K. and Kato, C., 1979. Synthesis o f t h e t r i m e t h y l s i l y l a t i o n d e r i v a t i v e o f h a l l o y s i t e . Clays Clay Miner., 27: 53-56. 1967. Mass Spectrometric i d e n t i f i c a t i o n of gaseous Langer, H.G. and Gohlke, R.S., products from thermal a n a l y s i s . I n W. Lodding ( E d i t o r ) . Gas E f f l u e n t A n a l y s i s . Marcel Dekker, New York, pp. 71-100. Langier-Kuzniarowa, A., 1967. Termogramy Mineralov I l a s t y c h . Wydawn. Geologiczne, Warsaw, 316 pp. Le C h a t e l i e r , H., 1887. De l ' a c t i o n de l a c h a l e u r s u r des a r g i l e s . B u l l . SOC. fr. Miner. C r i s t a l l o g r . , 10: 204-211. Lee, Chia-Chou, 1973. Measurements o f t h e r m o e l e c t r i c i t y o f m i n e r a l s . S c i e n t i a Geol. S i n i c a , 4: 317-326. Lodding, W. ( E d i t o r ) , 1967. Gas E f f l u e n t Analysis. Marcel Dekker, New York, 220 pp. Lombardi, G., 1980. For B e t t e r Thermal Analysis, Second E d i t i o n , I s t i t u t o d i M i n e r a l o g i a d e l l ' U n i v e r s i t a d i Roma and ICTA, Rome, 46 pp. Ldnvik, K., 1974. Thermosonimetry: A method o f m a t e r i a l c h a r a c t e r i z a t i o n . I n I. Buzds ( E d i t o r ) . Thermal A n a l y s i s : Proc. 4 t h ICTA, Budapest. Heyden, London, 3: 1089-1105. Ldnvik, K., 1978. An experimental i n v e s t i g a t i o n o f t h e thermal decomposition o f b r u c i t e by thermasonimetry. Thermochim. Acta, 27: 27-44. and Berezowski, R.M., 1980. Thermal and MLIssbauer s t u d i e s o f MacKenzie, K.J.D. i r o n - c o n t a i n i n g hydrous s i l i c a t e s . 11. H i s i n g e r i t e . Thermochim. Acta, 41: 335-355. ... - - - Mackenzie, R.C. , 1957a. Saponite from A l l t Ribhein, F i s k a v a i g Bay, Skye. Mineralog. Mas. 31: 672-680. Mackenzie, R.C. ( E d i t o r ) , 1957b. The D i f f e r e n t i a l Thermal I n v e s t i g a t i o n o f Clays. M i n e r a l o g i c a l Society, London, 456 pp. Mackenzie, R.C. ( E d i t o r ) , 1970-72. D i f f e r e n t i a l Thermal A n a l y s i s . Academic Press, London, 2 v o l s . 1980. D i f f e r e n t i a l thermal a n a l y s i s and d i f f e r e n t i a l scanning Mackenzie, R.C., c a l o r i m e t r y : S i m i l a r i t i e s and d i f f e r e n c e s . A n a l y t . Proc. , Lond., 17: 217-220. Mackenzie, R.C. , 1981a. De c a l o r e : Prelude t o thermal a n a l y s i s . Thermochim. Acta, i n press. Mackenzie, R.C., 1981b. Nomenclature i n thermal analysis, P a r t V . Symbols. J. therm. Analysis, i n press. 1977. I n s t r u m e n t a l methods f o r t h e study o f thermal degradation o f McNeill, I.C., polymers. I n N. Grassie ( E d i t o r ) . Developments i n Polymer Degradation. A p p l i e d Science, London, pp. 43-66.

-

2 7

McNeill, I.C., 1980. The use o f subambient thermal v o l a t i l i z a t i o n a n a l y s i s t o study v o l a t i l e products o f polymer degradation. I n H.G. Wiedemann ( E d i t o r ) . Thermal Analysis: Proc. 6 t h ICTA, Bayreuth. B i r k h l u s e r Verlag, Basel, 1: 319-324. M a r t i n V i v a l d i , J.L., G i r e l a Vilchez, F. and F e n o l l Hach-Ali, P., 1964. The thermal decomposition o f NH,-montmorillonites. P a r t 11. Clay Miner. B u l l . , 5: 401-406. Reda, M., Wilson, M.J. and Mackenzie, R.C., 1980. Clay and s i l t Mashhady, A.S., mineralogy o f some s o i l s f r o m Qasim, Saudi Arabia. J. S o i l Sci., 31: 101-115. M e l l o r , J.W. and H o l d c r o f t , A.D., 1911. The chemical c o n s t i t u t i o n o f t h e k a o l i n i t e molecule. Trans. Eng. Ceram. SOC., 10: 94-120. Menis, O., Mackey, J.A. and Garn, P.D., 1978. Measurement o f peak area o f c h y s o t i l e i n 640°C r e g i o n t o assess asbestos i n p o l l u t i o n s t u d i e s . Proc. 4 t h J o i n t Conf. Sensing Environm. P o l l u t . , New Orleans, Am. Chem. Sot., Washington, pp. 899-908. Mifsud, A., Rautureau, M. and Fornes, V., 1978. Etude de l ' e a u dans l a p a l y g o r s k i t e a l ' a i d e des analyses thermiques. Clay Miner., 13: 367-374. M i l l e r , R.P. and Sommer, G., 1966. A h o t stage microscope i n c o r p o r a t i n g a d i f f e r e n t i a l thermal a n a l y s i s u n i t . J. s c i e n t . Instrum. , 43: 293-297. Moore, G.S.M. and Rose, H.E. , 1978. Thermal e f f e c t s of contamination, adsorbed J. therm. Analysis, 15: 37-45. water and annealing on t h e DTA o f powdered q u a r t z . Morandi, N. and F e l i c e , G., 1979. Serpentine m i n e r a l s from v e i n s i n s e r p e n t i n i t e rocks. Mineralog. Mag., 43: 135-140. Morgan, D.J., 1977. Simultaneous DTA-EGA o f m i n e r a l s and n a t u r a l m i n e r a l mixtures. J. therm. Analysis, 12: 245-263. Mraz, T., Rajeshwar, K. and DUBOW, J., 1980. An automated technique f o r thermoacoustimetry o f s o l i d s . Thermochim. Acta, 38: 211-223. MUller-Vonmoos, M. and MUller, R., 1975. A p p l i c a t i o n o f DTA-TG-MS i n t h e i n v e s t i g a t i o n o f c l a y s . I n I. Buzds ( E d i t o r ) , Thermal Analysis: Proc. 4 t h ICTA, Budapest. Heyden, London, 2: 521-530. Murad, E. , 1979. Mbssbauer s p e c t r a o f g o e t h i t e : evidence f o r s t r u c t u r a l i m p e r f e c t i o n s . Mineralog. Mag., 43: 355-361. Murphy, C.B., 1967. T h e r m o p a r t i c u l a t e a n a l y s i s . I n W. Lodding ( E d i t o r ) . Gas E f f l u e n t A n a l y s i s . Marcel Dekker, New York, pp. 195-209. Murphy, C.B., 1978. Thermal a n a l y s i s . A n a l y t . Chem., 50: 143R-153R. Nambi , K.S.V. and M i t r a , S., 1978. Thermoluminescence i n v e s t i g a t i o n s o f o l d carbonate sedimentary rocks. Neues Jb. Miner. Abh., 133: 210-226. Nambi, K.S.V., Bhasin, B.D. and Bapat, V.N., 1978. Thermoluminescence c h a r a c t e r i s t i c s o f USGS standard b a s a l t i c r o c k BCR-1. Thermochim. Acta, 25: 126-129. [ Netsusokutei , 19801. Q u e s t i o n and Answer. Netsusokutei 7: 100-101. Norton, F.H., 1939. C r i t i c a l study o f t h e d i f f e r e n t i a l thermal method f o r t h e i d e n t i f i c a t i o n o f t h e c l a y minerals. J. Am. Ceram. SOC., 22: 54-63. Nottenburg, R., Freeman, M., Rajeshwar, K. and DUBOW, J . , 1979. Concurrent d i e l e c t r i c a n a l y s i s - d i f f e r e n t i a l thermal a n a l y s i s . A n a l y t . Chem., 51: 1149-1155. N u t t i n g , P.G., 1943. Some standard thermal d e h y d r a t i o n curves. P r o f . Pap. US Geol. Surv., No. 197-E, 197-217. Paterson, E., 1980. Use of thermal methods i n t h e study of surface phenomena. Analyt. Proc., Lond., 17: 234-236. Paterson, E . and S w a f f i e l d , R., 1980. I n f l u e n c e o f adsorbed i o n s i n t h e d e h y d r o x y l a t i o n o f s y n t h e t i c g o e t h i t e . 3. therm. A n a l y s i s , 18: 161-167. Paulik, F. and P a u l i k , J., 1972. Quasi-isothermal thermogravimetry. I n H.G. Wiedemann ( E d i t o r ) . Thermal Analysis: Proc. 3 r d ICTA, Davos. B i r k h l u s e r Verlag, Basel, 1: 161-167. Paulik, F. and Paulik, J., 1978. Simultaneous techniques i n thermal a n a l y s i s . Analyst, Lond., 103: 417-437. Pawlikowski, M., 1979. Wyniki wstepnych badan d e k r e p i t o m e t r y c z w c h galeny i d o l o m i t u z k o p a l n i "Boleslaw". Inform. Zjazdu Polsk. Towarzyst. Miner. , Krakow-Wiel iczka-Trzebionka. Wydawn. Geologiczne, Warsaw, p . 26. Perepechko, I., 1975. A c o u s t i c Methods. of I n v e s t i g a t i n g Polymers (Translated by G. L e i b ) . M i r Publishers, MOSCOW, 312 pp. Poulou, R. and Baudracco-Gritti, C., 1978. Un e n r e g i s t r e u r d e c r e p i t o m e t r i q u e a f i l t r a g e e l e c t r o n i q u e . B u l l . Miner., 101: 402-405.

Price, D., Dollimore, D., Fatemi, N.S. and Whitehead, R., 1980. Mass s p e c t r o m e t r i c d e t e r m i n a t i o n of k i n e t i c parameters f o r s o l i d s t a t e decomposition r e a c t i o n s . Thermochim. Acta, 42: 323-332. Radhakrishna, S. and Haridoss, S., 1978. Thermo c u r r e n t s i n i o n i c c r y s t a l s . C r y s t . L a t t i c e Defects, 7: 191-207. Rajeshwar, K., Nottenburg, R.N. and DUBOW, J.B., 1978. Simultaneous d i e l e c t r i c a n a l y s i s - d i f f e r e n t i a l thermal a n a l y s i s o f s o l i d m a t e r i a l s . Thermochim. Acta, 26: 1-17. Rajeshwar, K., Nottenburg, R. and DUBOW, J., 1979. Thermophysical p r o p e r t i e s of o i l shales. J. Mater. Sci., 14: 2025-2052. Redfern, J.P. ( E d i t o r ) , 1965. Thermal A n a l y s i s 1965: Proc. 1 s t ICTA, Aberdeen. Macmillan, London, 293 pp. Rouquerol, J., 1970. L ' a n a l y s e thermique a v i t e s s e de decomposition constante. J. therm. Analysis, 2: 123-140. Ryzhova, L.V., 1980. [Use o f X-ray d j f f r a c t o m e t r i c and thermogravimetric methods i n t h e i n v e s t i g a t i o n o f ion-exchange r e a c t i o n s i n s o i l s . 1 Pochvovedenie, No. 1, 143-151. Samoilov, Ya.V., 1909. [On t h e water of k a o l i n i t e . 1 I z v . imp. Akad. Nauk. S. Petersb., 3: 1137-1152. Satoh, S., 1918. On t h e endo- and exothermal change o f t h e k a o l i n i t e i n Japan. Kogyo-Kwagaku-Zasshi, 21 : 631-648. Satoh, S . , 1921. A study o f t h e h e a t i n g and c o o l i n g curves o f Japanese k a o l i n i t e . J. Am. Ceram. SOC., 4: 182-194. Schomburg, J. and S t M r r , M., 1978a. Kombinierte thermische Analyse a n D r e i s c h i c h t tonmineralen. Thermochim. Acta, 25: 313-324. Schomburg, J. and S t M r r , M., 1978b. Nachweis D i c k i t g e h a l t e i n Kaolinen m i t t e l s D i l a t o m e t r i e . Chem. Erde, 37: 107-108. Schwenker, R.F. and Garn, P.D. ( E d i t o r s ) , 1969. Thermal A n a l y s i s . Academic Press, New York, 2 v o l s . Sharp, J.H., 1972, Reaction k i n e t i c s . I n R.C. Mackenzie ( E d i t o r ) . D i f f e r e n t i a l Thermal Analysis. Academic Press, London, 2: 47-77. Simonsen, K.A. and Zaharescu, M., 1979. A new d i f f e r e n t i a l thermal a n a l y s i s method. J. therm. A n a l y s i s , 15: 25-35. Smalley, I.J. and Xidakis, G.S., 1979. Thermogravimetry of a n expansive c l a y s o i l from Adelaide: Approximate m i n e r a l o g i c a l a n a l y s i s u s i n g standard m o n t m o r i l l o n i t e s . Clay Sci., 5: 189-193. Smith, F.G., 1957. D e c r e p i t a t i o n c h a r a c t e r i s t i c s o f igneous rocks. Can. Miner., 6: 78-86. 1949. Apparatus f o r t h e r e c o r d i n g o f d e c r e p i t a t i o n Smith, F.G. and Peach, P.A., i n m i n e r a l s . Econ. Geol., 44: 449-454. Smith, J.D.B., Meier, J.F. and P h i l l i p s , D.C., 1977. T h e r m o p a r t i c u l a t i n g o r g a n i c compounds. J. therm. Analysis, 11: 423-430. Smith, J.D.B., P h i l l i p s , D.C. and Kaczmarek, T.D., 1976. T h e r m o p a r t i c u l a t i o n analyses o f malonic a c i d compounds. A n a l y t . Chem., 48: 89-95. 1971. Techniques of microthermal a n a l y s i s based on Sommer, G. and Jochens, P.R., a thermocouple as h e a t i n g source and specimen h o l d e r , Miner. S c i . Engng, 3: 3-16. Staub, H. and Perron, W., 1974. New Method o f p u r i t y d e t e r m i n a t i o n by means of c o l o r i m e t r i c d i f f e r e n t i a l thermal a n a l y s i s . A n a l y t . Chem., 46: 128-130. Suzuki, S. and Satoh, R., 1980. Effect o f i r o n on t h e exothermic peak temperature of allophane from a M i s o t s u c h i weathered pumice bed. S o i l S c i . P1. Nutr., 26: 441-445. Timofeev, B.V. and Smirnov, Yu.A., 1980. [ C h a r a c t e r i s t i c s o f i r o n accumulation and t h e n a t u r e o f i t s compounds i n s o i l s o f t h e h i g h p l a t e a u x o f A l g i e r s . ] Pochvovedenie, No. 11, 131-136. Wallach, R., 1913. L'analyse thermique des a r g i l e s . C.r. hebd. Seanc. Acad. Sci., P a r i s , 157: 48-50. Warne, S.St.J. and M i t c h e l l , B.D., 1979. V a r i a b l e atmosphere DTA i n i d e n t i f i c a t i o n and d e t e r m i n a t i o n o f anhydrous carbonate m i n e r a l s i n s o i l s . J. S o i l Sci., 30: 111-116. Wedgwood, J., 1782. An a t t e m p t t o make a thermometer f o r measuring t h e h i g h e r degrees o f h e a t from a r e d h e a t t o t h e s t r o n g e s t t h a t vessels made o f c l a y can support. P h i l . Trans. R. SOC., 72: 305-326.

29 Weiser, H.B. and M i l l i g a n , W.O., 1939. The c o n s t i t u t i o n o f t h e c o l l o i d a l systems o f t h e hydrous oxides. Chem. Rev., 25: 1-30. Wendlandt, W.W., 1974. Thermal Methods o f Analysis, Second E d i t i o n . Wiley, New York, 505 pp. 1979. Concurrent TG-DTA measurements using a Cahn RG balance. Wendlandt, W.W., Thermochim. Acta, 30: 361-363. Wendlandt, W.W., 1980. An apparatus f o r thermophotometry. Thermochim. Acta, 35: 255-257. Wendlandt, W.W. and Hecht, H.G., 1966. Reflectance Spectroscopy. Interscience, New York, 298 pp. Wengeler, H., Martens, R. and Freund, F., 1980. Proton c o n d u c t i v i t y o f simple i o n i c hydroxides. P a r t 11. I n s i t u formation o f water molecules p r i o r t o dehydration. Ber. Bunsenges. phys. Chem., 84: 873-880. Wiedemann, H.G., 1964. U n i v e r s e l l e s Messgerlt fur g r a v i m e t r i s c h e Untersuchungen u n t e r v e r l n d e r l i c h e n Bedingungen. Chemie-Ingr-Technik, 11: 1105-1114. Wiedemann, H.G. ( E d i t o r ) , 1972. Thermal Analysis: Proc. 3 r d ICTA, Davos, B i r k h h s e r Verlag, Basel, 3 vols. Wiedemann, H.G. ( E d i t o r ) , 1980. Thermal Analysis: Proc. 6 t h ICTA, Bayreuth. B i r k h l u s e r Verlag, Basel, 2 vols. Wilburn, F.W., 1972,. Glass-making r e a c t i o n s i n three-component systems t h a t i n c l u d e s i l i c a . Ph.D. Thesis. U n i v e r s i t y o f S a l f o r d , UK.

31 CHAPTER 2 H I G H RESOLUTION ELECTRON-MICROSCOPY APPLIED TO CLAY MINERALS Jean P i e r r e EBERHART L a b o r a t o i r e de C r i s t a l l o g r a p h i e , M i n e r a l o g i e e t P e t r o g r a p h i e U n i v e r s i t e L o u i s P a s t e u r , S t r a s b o u r g , France. 2.1

INTRODUCTION Clay m i n e r a l s a r e f i n e g r a i n e d m a t e r i a l s ; t h e average s i z e o f t h e i r l a m e l l a r

p a r t i c l e s l i e s i n t h e m i c r o m e t e r range, u s u a l l y a t t h e r e s o l u t i o n l i m i t of l i g h t microscopes ., U n t i l t h e h e v e l o p m e n t o f e l e c t r o n microscopy, a t o m i c - s c a l e i n f o r m a t i o n s have been p r o v i d e d by X-ray d i f f r a c t i o n , w h i c h has l e d t o t h e knowledge o f t h e b a s i c c l a y miner a l s s t r u c t u r e s . However X-rays can g i v e o n l y an averaged i n f o r m a t i o n i s s u e d from a g r e a t number o f u n i t - c e l l s , a s s u m i n g

t h a t they are s t a t i s t i c a l l y i d e n t i c a l .

T h e o r e t i c a l l y , owing t o t h e s m a l l e l e c t r o n wavelength, t h e e l e c t r o n - m i c r o s c o p e should be a b l e t o p r o v i d e d i r e c t s t r u c t u r a l i n f o r m a t i o n s on i n d i v i d u a l c l a y c r y s t a l s w i t h an a t o m i c r e s o l u t i o n . I n f a c t , f o r many y e a r s , E.M.

r e s e a r c h on c l a y

m i n e r a l s has been l i m i t e d t o m i c r o m o r p h o l o g i c a n a l y s i s w i t h a r e s o l u t i o n o f about 10-20

4,

w e l l below t h e t h e o r e t i c a l .one. I d e n t i f i c a t i o n o f m i n e r a l s has been ena-

b l e d by t h e r e c o g n i t i o n o f c a r a c t e r i s t i c a l shape f e a t u r e s ; c a t a l o g u e s o f m i c r o graphs have been p u b l i s h e d t o t h a t end. The main reasons o f t h a t l i m i t a t i o n have been t h e l o w i n s t r u m e n t a l r e s o l u t i o n and t h e l a c k o f knowledge o f t h e imaging process. Even nowadays a g r e a t d e a l o f clay-E.M.

r e s e a r c h i s s t i l l devoted t o mor-

phol ogy . I n t h e s e v e n t i e s appeared a new g e n e r a t i o n o f E.M. w i t h an e f f e c t i v e r e s o l u t i o n o f 2-5

w,

c l o s e t o t h e t h e o r e t i c a l l i m i t imposed by s p h e r i c a l a b e r r a t i o n . I t opened

t h e way t o t h e s o - c a l l e d h i g h - r e s o l u t i o n e l e c t r o n - m i c r o s c o p y (H.R.E.M.)

and t o

atomic r e s o l u t i o n . Numerous micrographs o f m i n e r a l s , showing a c o r r e l a t i o n w i t h s t r u c t u r e p r o j e c t i o n , were soon p u b l i s h e d (e.g. Buseck and I i j i m a , 1974 on s i l i cates). Furthermore a b e t t e r knowledge o f e l e c t r o n - i m a g i n g and t h e use o f computer p r o c e s s i n g o f t h e image c o n t r a s t have l e d i n r e c e n t y e a r s t o new developments. The p r e s e n t c o n t r i b u t i o n w i l l be m a i n l y d e v o t e d t o h i g h - r e s o l u t i o n aspects i n t r a n s m i s s i o n microscopy. A f t e r an e l e m e n t a r y a b s t r a c t on image f o r m a t i o n and p r o c e s s i n g , t h e s p e c i f i c problems o f c l a y m i n e r a l s w i l l be o u t l i n e d . 2.2

PRINCIPLES OF ELECTRON MICROSCOPE I M A G I N G . A m o s t l y d e s c r i p t i f o u t l i n e o f t h e t h e o r y w i l l be g i v e n i n t h i s c h a p t e r . I t

aims t o be comprehensive t o e v e r y e a r t h s c i e n t i s t who has some knowledge o f t h e X-ray d i f f r a c t i o n t h e o r y . F o r f u r t h e r d e t a i l s and a complete t r e a t m e n t , t h e r e a d e r can c o n s u l t r e c e n t s p e c i a l i z e d books. Some r e f e r e n c e s a r e g i v e n a t t h e end.

32 Approximations and c o n v e n t i o n s .

2.2.1

-

-

(a) I m p l i c i t approximations : S c a t t e r i n g i s supposed t o t a k e p l a c e i n k i n e m a t i c a l c o n d i t i o n s . The waves and t h e Ewald-sphere a r e supposed t o be p l a n e w i t h i n t h e a n g u l a r

aperture. ( b ) Conventions : Wave-functions i n 2-D d i r e c t space ( o b j e c t and image p l a n e ) a r e n o t e d by

-

-t

s m a l l l e t t e r s , e.g. f ( x y ) = f ( r ) . - The c o r r e s p o n d e n t d a t a i n 2-D r e c i p r o c a l space (back f o c a l p l a n e ) a r e n o t e d -+ by c a p i t a l l e t t e r s , e.g. F ( h k ) = F ( R ) . The F o u r i e r t r a n s f o r m s a r e n o t e d by t h e symbol plr. T h e i r f o r m u l a t i o n has been

-

s i m p l i f i e d by o m i s s i o n o f c e r t a i n f a c t o r s w h i c h a r e n o t e s s e n t i a l f o r imaging comprehension. -+ -+ Wave v e c t o r s a r e n o t e d ko ( i n c i d e n t ) and k ( s c a t t e r e d ) , w i t h k

-

ko = 1 / X .

4.2.2

Wave t r a n s m i s s i o n t h r o u g h t h e microscope. I n an i d e a l i z e d imaging p r o c e s s , t h e c o h e r e n t e l e c t r o n - w a v e s t r a n s m i t t e d and

s c a t t e r e d f r o m a p o i n t o f t h e o b j e c t a r e focused by t h e o b j e c t i v e and b r o u g h t t o g e t h e r i n t h e c o r r e s p o n d i n g image-plane p o i n t w i t h t h e r e l a t i v e phases. The d i f f e r e n c e s i n a m p l i t u d e and phase f r o m one p o i n t t o a n o t h e r g i v e r i s e t o t h e image c o n t r a s t . T h i s process may be decomposed f o r t h e a n a l y s i s i n s e v e r a l s t e p s ( s e e F i g . 1): (a)

O b j e c t s t r u c t u r e a s "seen" by t h e e l e c t r o n s . O b j e c t w a v e - f u n c t i o n i . e . a m p l i t u d e and phase d i s t r i b u t i o n a t t h e e x i t

(b) face o f t h e o b j e c t .

( c ) Wave t r a n s m i s s i o n t h r o u g h t h e microscope. F i l t e r i n g by l e n s c a r a c t e r i s t i c s ( a p e r t u r e , a b e r r a t i o n s and d e f o c u s ) ; r e c o m b i n a t i o n t o t h e image-plane wave-function. (d)

Observed image. I n t e n s i t y d i s t r i b u t i o n i n t h e r e c o r d e d m i c r o g r a p h .

The aim i s t o c o r r e l a t e ( d ) t o ( a ) . I n p r a c t i c e t h e imaging p r o c e s s i n v o l v e s o n l y t h e o b j e c t i f l e n s ; t h e c o n t r i b u t i o n o f t h e o t h e r l e n s e s i s n e g l i g i b l e i n terms o f r e s o l u t i o n and c o n t r a s t . (a) Object. Viewed f r o m t h e e l e c t r o n - i n t e r a c t i o n , t r i b u t i o n v(x,y,z).

t h e o b j e c t a c t s as a p o t e n t i a l d i s -

I n f a c t , as t h e image i s two-dimensional,

a c t s l i k e a 2-0 p o t e n t i d ptojection

the object i t s e l f

v ( x y ) = v(P) on t h e e x i t p l a n e .

A c t y b X u U i ~ ~object e c a n be expanded i n t o a F o u r i e r s e r i e s as f o l l o w s v(:)

= E E v(;) h k

exp.(-Zai

li) = S(6)

33

OBJECT

F i g . 2.1.

1

FOCAL PLANE

PLANE IMAGE

P r i n c i p l e o f . imaging.

+ The p o t e n t i a l p r o j e c t i o n v ( r ) can be c o n s i d e r e d as a sum o f sine-waves w i t h spa+ + t i a l f r e q u e n c y R and a m p l i t u d e V ( R ) . R corresponds t o r e c i p r o c a l l a t t i c e p o i n t s

w i t h i n t e g e r c o o r d i n a t e s h,k. (b)

O b j e c t wave.

D u r i n g i t s t r a v e l t h r o u g h t h e o b j e c t , t h e n c i d e n t wave undergoes an a b d o h p t h n d k i d t e66ec.t; t h e second i s t h e most i m p o r t a n t i n t h i n c r y s -

eddect and a p h a c

+

t a l l i n e o b j e c t s w i t h o u t heavy atoms. A t a PO n t r ( x y ) in t h e e x i t p l a n e t h e -t e f f e c t can be r e p r e s e n t e d by a Rhanbpahency ~ u n c t i a na ( r ) {l-s(G)}exp i$(G) + + where s ( r ) i s t h e a b s o r p t i o n o r a m p l i t u d e component, @ ( r ) t h e phase component. o(;)

=

I f t h e emergent w a v e - f u n c t i o n f(;)

i s r e f e r e d t o an u n i t y i n c i d e n t a m p l i t u d e +

-

f

w h i c h i s modulated by t h e above t r a n s p a r e n c y f u n c t i o n , i t w r i t e s s i m p l y f ( r ) = o ( r ) . I n a s o - c a l l e d weakey-dcattehing a b j e c t t h e t r a n s p a r e n c y components a r e assumed +. t o be s m a l l ( s < < l , $ < < l )so , t h a t f ( r ) can be approximated t o + + + + f(r) a(r)= 1 s(r) + i$(r) (1) + The phase 4 k i d . t $ ( r ) f o r a g i v e n i n c i d e n t e l e c t r o n - e n e r g y Eo and wavelength h + i s d i r e c t l y r e l a t e d t o t h e o b j e c t p o t e n t i a l p r o j e c t i o n v ( r ) by

-

3 4 c ) Wave t r a n s m i s s i o n t h r o u g h t h e o b j e c t i v e . A c c o r d i n g t o Abbe's t h e o r y o f a pendect L e u , a l l t h e beams i s s u e d f r o m a -+

e x i t - p l a n e come t o f o c u s i n a p o i n t '; o f t h e image-plane + + (Gaussian p l a n e ) so t h a t r ' ( x ' , y ' ) = -m r ( x , y ) , where m i s t h e m a g n i f i c a t i o n o f t h e l e n s . To s i m p l i f y t h e f o r m u l a t i o n , t h e image w i l l be r e f e r e d t o t h e o b j e c t point r i n the object

s c a l e ; t h e image wave f u n c t i o n t h e n becomes i d e n t i c a l t o t h e o b j e c t w a v e - f u n c t i o n and can be w r i t t e n fa(;)

= f(7.). F o r a b e t t e r u n d e r s t a n d i n g o f t h e imaging p r o c e s s i n a r e a l l e n s , i t i s u s e f u l

t o c o n s i d e r t h e t r a n s m i s s i o n i n two s t e p s : di66huc,7%tion i n t h e back f o c a l p l a n e and .imaging i n t h e Gaussian p l a n e . Back f o c a l p l a n e o f a p e r f e c t o b j e c t i v e - l e n s . -+ The beams s c a t t e r e d w i t h an a n g l e 28 ( d i r e c t i o n k) a r e focused a t a p o i n t + R(hk) i n t h e f o c a l p l a n e . The observed d i f f r a c t i o n p a t t e r n , l o c a t e d i n t h a t p l a n e , i s o f t h e F r a u n h o f f e r t y p e . The Ewald-sphere w i l l be a s s i m i l a t e d t o t h e back f o c a l - p l a n e a t a d i s t a n c e f o f t h e l e n s ; i t f o l l o w s (see F i g . 4) + - + + R (k ko) X f , R 2f8

-

+ The s c a l e f a c t o r h f w i l l be o m i t t e d h e n c e f o r t h . The s c a t t e r e d a m p l i t u d e F(R) a t + a p o i n t R(hk) i n t h e b a c k - f o c a l p l a n e becomes now + + ++ 'L+ F(R) = 11 f ( r ) exp 2a i r R d r 2 = f ( r ) (3) + The f o c a l - p l a n e w a v e - f u n c t i o n a t R i s t h u s t h e F o u r i e r - t r a n s f o r m o f t h e o b j e c t

w a v e - f u n c t i o n . A c c o r d i n g t o E q . ( l ) , i t can be w r i t t e n f o r a w e a k - s c a t t e r i n g o b j e c t as + + 'L 'L = 6 ( ~ ) S(R) t i Q(R) where S = s F(;) and O = $ (4)

-

A

The d i f f r a c t i o n p a t t e r n F(R) i n t h e back f o c a l p l a n e r e p r e s e n t s t h e s p a t i a l frequency-spectrum o f t h e o b j e c t - f u n c t i o n : The 6 - f u n c t i o n r e p r e s e n t s a sharp c e n t r a l peak c o r r e s p o n d i n g t o u n s c a t t e r e d

-

e l e c t r o n s (6 = o f o r R # 0). The second and t h i r d terms r e p r e s e n t t h e s c a t t e r e d a m p l i t u d e . F o r a c r y a l i t

-

I s m a i n l y c o n c e n t r a t e d i n sharp peaks a t t h e r e c i p r o c a l l a t t i c e p o i n t s ( w i t h i n t e g e r hk c o o r d i n a t e s ) , each r e l a t e d t o a l a t t i c e p l a n e p e r i o d i c i t y = f X/R(hko). Around t h e d i s c r e t e peaks o c c u r s a d i f f u s e s c a t t e r i n g due d ( hko) + t o t h e c r y s t a l morphology ( f o r a s i n g l e u n i t - c e l l , F(R) would r e p r e s e n t t h e structure factor). Back f o c a l - p l a n e o f a r e a l o b j e c t i v e - l e n s .

I n f a c t an e l e c t r o n i c l e n s , l i k e a l i g h t r e f r a c t i o n l e n s , i n t r o d u c e s s e v e r a l l i m i t a t i o n s t o t h e wave p r o p a g a t i o n , and a c t s l i k e a f i l t e r .

-

Aperture l i m i t a t i o n .

The presence of a c i r c u l a r diaphragm o f r a d i u s Ro i n

35

t h e b a c k - f o c a l p l a n e can be r e p r e s e n t e d by t h e a p e r t u r e - f u n c t i o n A ( R ) ,

A = 1 for

R


Ro

- S p h e r i c a l a b e r r a t i o n ( s e e F i g . 2 . 2 ) . , Defocus due t o s p h e r i c a l a b e r r a t i o n grows as t h e 3rd power o f t h e s c a t t e r i n a n g l e 28; f o r an a n g u l a r a p e r t u r e a i t l e a d s t o a smearing-disc o f r a d i u s Cs a ( r e f e r e d t o t h e o b j e c t p l a n e ) ; C s i s t h e spher i c a l a b e r r a t i o n c o n s t a n t . A t a d i s t a n c e R i n t h e f o c a l p l a n e i t produces a phase s h i f t y,

3

-

A d e v i a t i o n A f r o m t h e i d e a l f o c u s b r i n g s a phase s h i f t yA a t a distance R from the o p t i c a l a x i s Defocus. =

yA (R)

- -ax ( 2 8 ) 2 =nX R 2

m

OBJECTIVE

F i g . 2.2.

-

c,a3

GAUSSIAN PLANE

S p h e r i c a l a b e r r a t i o n and o p t i m a l defocus.

A s t i g m a t i s m and c h r o m a t i c a b e r r a t i o n w i l l be n e g l e c t e d ; t h e f i r s t c a n be c o r -

r e c t e d ; t h e o m i s s i o n o f t h e second i s c o n d i t i o n n e d by t h e w e a k - s c a t t e r i n g o b j e c t hypot h e s i s

.

The t o t a l phase s h i f t comes t h e r e f o r e Y

=y,

+ yA =

aX

(1 C 2 s

A2 R4

-

AR2)

I t can be assumed t h a t t h e whole spread a c t i o n due t o a p e r t u r e , a b e r r a t i o n

and f o c u s i s l o c a t e d i n t h e b a c k - f o c a l p l a n e where t h e aperture-diaphragm a c t u a l l y l i e s . T h i s a c t i o n a t a d i s t a n c e R ca? be r e p r e s e n t e d by t h e s o - c a l l e d + T(R) m u l t i p l y i n g t h e i d e a l d i f f r a c t i o n w a v e - f u n c t i o n F ( R ) ,

&anddc~ d u n d o n

t h e l e n s a c t i n g l i k e a space-frequency f i l t e r . The a c t u a l d i f f r a c t i o n f u n c t i o n becomes

36 + + -f F1(R) = F(R)T(R) = F(R) A(R) exp { - i y ( R ) } (6) + For a weakly-scattering o b j e c t , F ' ( R ) can be developed as f o l l o w s , according t o Eq.(4) and ( 6 )

- SAcosY

Is'

F'(R)

t

QAsinY)

+

i { S A s i n y t @A COSY)

(7)

d i t h i n t h e small angular range corresponding t o an a x i a l aperture i n c l u d i n g +

6'(R) ( b r i g h t f i e l d image), the imaginary p a r t o f F ' ( R ) may be neglected, l e a ding t o F'(Z)

5

6'

-

SA cosy

+

OAAsiny

(8)

Image plane.

+ The amplitude a t a p o i n t r i n t h e image plane i s the summation o f t h e ami n t h e f o c a l plane : p l i t u d e s scattered a t a l l p o i n t s

;

The image wave-function i s the convolution o f t h e o b j e c t - f u n c t i o n w i t h a smearing+ f u n c t i o n t(r) ?(R). For t h e weakly-scattering approximation, f ' ( r ) can be w r i t t e n as f o l l o w s , according t o Eq. (8) +

' L +

f ' ( r ) = F'(R) = 1

-

'L

(SAcosy)

+

%

(OAsiny)

(d) Image i n t e n s i t y d i s t r i b u t i o n

-+ What i s a c t u a l l y observed o r recorded i s an i n t e n s i t y d i s t r i b u t i o n j ( r )

j(F)

=

=

If(;)

t(r)I2

According t o Eq. ( l o ) , the b r i g h t - f i e l d image i n t e n s i t y o f a weakly-scattering o b j e c t can be approximated t o %

j(;)

= l-Z(SAcosy)

=

+

%

2(4Asiny)

The image c o n t r a s t c i s the d i f f e r e n c e between non scattered and scattered i n t e n s ity + + c(r) = j(r)

-

%

%

1 = -2(SAcos~) + 2(@AsinY)

(11)

Spatial-frequency spectrum o f t h e image c o n t r a s t +

ry-,

C(R) = c ( r ) =

-2SAcosy

+

2OAsiny

The Fourier-transform o r spatial-frequency spectrum o f t h e image-contrast i n weakly-scattering c o n d i t i o n s i s a 1 i n e a r combination o f t h e spatial-frequency spectra o f t h e amplitude and phase transparency f u n c t i o n s s and @, modulated by the amp1 i t u d e and phase c o n t r a s t f u n c t i o n s (-2cosy) and ( 2 s i n y ) r e s p e c t i v e l y . This r e s u l t c a l l e d the &fieah h u g e a p p o x i m a t i o n i s important f o r image processing,as w i l l be mentioned l a t e r . For a given microscope, t h e values of -b s i n y and cosy a t a p o i n t R o f the f o c a l plane depend on defocus A,as can be

3 7

seen from Eq. ( 5 ) . As those values determine t h e respective weight of t h e two contrast components, t h e image c o n t r a s t w i l l be very sensible t o defocus. Two l i m i t instances can be o u t l i n e d , according t o Eq. (12). I f s i n y = o f o r a g r e a t p a r t o f t h e d i f f r a c t i o n p a t t e r n i n s i d e the aperture,

-

the absorption o r amp1 i t u d e c o n t r a s t i s enhanced + c(r) = 2s (13) The image c o n t r a s t i s then i n t e r p r e t a b l e i n terms o f t h e o b j e c t absorption

-

-

I f siny =

2

1 f o r a g r e a t p a r t o f t h e d i f f r a c t i o n p a t t e r n i n s i d e the aperture,

the phase c o n t r a s t p r e v a i l s .(see F i g . 3) -+

c ( r ) = 24 =

27T -

v();

EO

The image c o n t r a s t i s then d i r e c t l y i n t e r p r e t a b l e i n terms o f t h e o b j e c t potent i a l p r o j e c t i o n , e.g. the s t r u c t u r e . It must be pointed o u t t h a t f o r a d a r k - f i e l d image the above approximation

does n o t hold any longer. The imaging then i n v o l v e s o n l y a d i f f r a c t e d beam + + R(hk) and t h e r e f o r e the imaginary p a r t i n F’(R) cannot be neglected any more. The foregoing treatment was based on coherent plane waves. L i m i t e d coherence due t o energy- and angular d i s p e r s i o n can be taken i n account by an envelope f u n c t i o n mu1 t i p l y i n g t h e t r a n s f e r f u n c t i o n , t h e r e f o r e reducing the c o n t r a s t a t high resol u t i o n and important d e f o c a l i s a t i o n . CRYSTAL I M A G I N G .

2.3

2.3.1 D i r e c t l y i n t e r p r e t a b l e images. The r e s o l u t i o n d o f a t h i n o b j e c t i s l i m i t e d by t h e o b j e c t i v e angular aperture u and spherical a b e r r a t i o n c o e f f i c i e n t C,. Aperture d i f f r a c t i o n leads t o a smearing d i s k w i t h diameter 0.6 A/a Spherical a b e r r a t i o n leads t o a smearing d i s k w i t h diameter 2 Csu 3.

-

-

.

When a increases, t h e d i f f r a c t i o n term decreases, t h e a b e r r a t i o n term increases; the best compromise according t o Rayleigh i s achieved when t h e t w o terms are equal, thus leading t o what can be c a l l e d t h e RayLeigh h e 6 o W o n do and aperture a, f o r the Gaussian focus 0.8 (A Cs) 1’4 w i t h a. 0 6X 1/4 do

=[c)

5

Defocusing reduces t h e a b e r r a t i o n smearing (see F i g . 2 ) . thus a l l o w i n g a l a r g e r dedocud) aperture. Scherzer (1949) has shown t h a t f o r an optimum defocus ( S c h M z ~ Az 1.2( C,X) 1/2

-

t h e r e s o l u t i o n improves t o dIo

z

0.6(X Cs) 14’

with

a l o e

4

?ao

T h i s r e s o l u t i o n can be c a l l e d t h e Schehzeh de60cu-6 hedoLuLion. I t can be seen from F i g . 2.3 t h a t i t corresponds a p p r o x i m a t e l y t o t h e f i r s t z e r o o f t h e phasec o n t r a s t t r a n s f e r - f u n c t i o n and t h a t t h e siny-component i s c l o s e t o u n i t y w i t h i n a good p a r t o f t h e a p e r t u r e , p r o v i d i n g an a l m o s t p h a s e - o b j e c t imaging a c c o r d i n g

.

t o Eq (14). I t 6 0 U o w ~t h a t in 6uch opehating c o n d X i o n ~ , t h e image can be dihectLy intmpmted i n tenms 06 t h e ~&uctwre.The Scherzer d e f o c u s r e s o l u t i o n can t h e r e f o r e be c a l l e d t h e b ~ & , ' L e h u o U o n .

I sin 7

.

100 k e V

F i q . 2 . 3 . O v e r a l l a s p e c t o f t h e phase c o n t r a s t t r a n s f e r f u n c t i o n i n t h e f o c a l p l a n e f o r Scherzer d e f o c u s , p l o t t e d a g a i n s t s p a t i a l p e r i o d i c i t i e s , f o r 100 and 1000 keV. I t i s easy t o understand t h a t i n t h o s e c o n d i t i o n s t h e l i g h t c o n t r a s t f r o m

small phase s h i f t corresponds t o a l o w p o t e n t i a l p r o j e c t i o n (e.g.

structural

" t u n n e l s " ) , whereas t h e d a r k c o n t r a s t f r o m l a r g e phase s h i f t corresponds t o a h i g h p o t e n t i a l p r o j e c t i o n (e.g. a t o m i c rows). F o r a 100 kV microscope w i t h Cs = 0.16 cm t h e f o l l o w i n g v a l u e s a r e achieved: Gaussian f o c u s

: A

Scherzer defocus : A =

0

- 900

: do

= 4.4.

: d I o = 3.3.

;;

w i t h a0 = 5.3.10m3 r d w i t h a'0 = 6.7.10m3rd

F o r a L g h uoMage m i a o ~ c o p et h e s t r u c t u r e r e s o l u t i o n i s a g r e a t deal b e t t e r . F o r a 600 kV E.M.

0

w i t h Cs = 0.33 cm, i t g e t s down t o a b o u t 2 A ( s e e F i g . 3 ) .

F u r t h e r m o r e s c a t t e r i n g and energy l o s s e s a r e reduced, so t h a t t h e t h i c k n e s s can be i n c r e a s e d w i t h i n t h e weak-object a p p r o x i m a t i o n . These q u a l i t i e s e x p l a i n t h e growing i n t e r e s t f o r h i g h - v o l t a g e h i g h - r e s o l u t i o n E.M. J o u f f r e y e t a l . 1979).

( C o s s l e t and Smith, 1979;

3 9

S i l i c a t e s t r u c t u r e - i m a g e s have been performed u s i n g f o r e g o i n g o p e r a t i n g c o n d i t i o n s , e.g.

by Buseck e t a l . (1974), Thomas e t a1.(1979).

The c r y s t a l o r i e n t a t i o n s

which a r e most l i k e l y t o p r o v i d e u s e f u l and c a r a c t e r i s t i c s t r u c t u r e - p r o j e c t i o n s a r e t h o s e f o r w h i c h t h e beam i s p a r a l l e l t o an i m p o r t a n t z o n e - a x i s ( a s f o r example A 2a

Both t h e d i r e c t 000 beam and t h e d i f f r a c t e d hkO beam c a n t h e n pass t h r o u g h t h e a p e r t u r e diaphragm when o p e r a t i n g w i t h s y m n e t r i c d i r e c t - b e a m d e f l e c t i o n (see F i g . 2.4) and i n t e r f e r e t o f o r m image p l a n e - f r i n g e s w i t h s p a c i n g m d ( h k l ) . T h a t corresponds t o l i m i t t h e F o u r i e r s e r i e s i n Eq. ( 9 ) t o t h e c o n s t a n t t e r m and one 0 s i n e term. W i t h 100 keV, a 3.5 A s p a c i n g c a n t h u s be r e s o l v e d . F u r t h e r m o r e , f o r a g i v e n a p e r t u r e g r e a t e r t h a n a O s t h e s p h e r i c a l a b e r r a t i o n c a n be e x a c t l y compensated b y a c o r r e s p o n d i n g d e f o c u s , t h a n k s t o t h e 2-beam symmetry. L a t t i c e

F i g . 2.4. Symmetri c a l 2-beam o p e r a t i o n .

40

r e s o l u t i o n down t o 1

and even l e s s can be achieved; t h e o n l y l i m i t a t i o n s are

Instrumental s t a b i l i t y ( e l e c t r o n i c a l and mechanical) and chromatic a b e r r a t i o n . That r e s o l u t i o n can t h e r e f o r e be c a l l e d the i m m e n t d te.bo.&u%n. The p o s i t i o n o f t h e f r i n g e s depends on the operating parameters (plane o r i e n t a t i o n , phase s h i f t s , e t c ...) and cannot be d i r e c t l y c o r r e l a t e d t o t h e o b j e c t s t r u c t u r e planes. However such observations can g i v e valuable informations on e x i s t i n g c r y s t a l l a t t i c e - d e f e c t s and disorder; f o r example w e l l c r y s t a l l i z e d mica gives good hkO l a t t i c e - i m a g e s whereras smectites almost never g i v e any. Indirect interpretation o f imaqs. Apertures which are s i g n i f i c a n t l y l a r g e r than

2.3.3

t r a n s m i t a g r e a t number o f

d i f f r a c t e d beams hk0 together w i t h t h e d i r e c t beam, leading t o a n image F o u r i e r series w i t h numerous sine-terms i n c l u d i n g s p a t i a l p e r i o d i c i t i e s d(hk0) smaller than d l 0 . D e t a i l s as f i n e as t h e instrumental r e s o l u t i o n can then be observed i n t h e image. However d i r e c t i n t e r p r e t a t i o n o f those d e t a i l s i n terms of s t r u c t u r e - p r o j e c t i o n i s no longer possible. Thanks t o t h e knowledge o f the imaging theory and t o the a v a i l a b l e means o f computing, an i n d i r e c t image-object c o r r e l a t i o n can now be achieved u s i n g image processing. Several methods have been proposed and applied i n recent years. Only a rough o u t l i n e w i l l be given; r e f e rences w i l l be found a t t h e end. (a)

Image improvement.

A noisy background o f t e n a f f e c t s h i g h - r e s o l u t i o n images. I t can be removed

by e l i m l n a t i n g high s p a t i a l frequencies i n processed image-contrast F o u r i e r transforms (low-pass f i l t e r ) , provided t h a t t h e noise g r a i n s are smaller than the i n t e r e s t i n g s t r u c t u r a l d e t a i l s . The p e r i o d i c i t y o f b a d l y c r y s t a l l i s e d objects can be enhanced i n the image by Fourier-transform o f d i s c r e t e d i f f r a c t i o n spots (equivalent t o o p t i c transforms). U n t i l

now these techniques have mainly been used f o r b i o l o g i c a l o b j e c t s . However they could be u s e f u l f o r c e r t a i n c l a y studies. Thus C o l l i e x e t a1 (1980) have enhanced t h e imaging o f f a u l t e d areas i n l a y e r - s i l i c a t e s by o p t i c a l transforms. (b) Image processing based on image f i l t e r i n g . I n the micrograph o f a weakly-scattering specimen i t i s t h e o r e t i c a l l y possible t o recover S and @, hence t h e o b j e c t - f u n c t i o n , from t h e l i n e a r c o n t r a s t transform + C(R) -(Eq.12). The simplest case concerns a phase-object where @ = C/siny. Several images are recorded w i t h d i f f e r e n t defocus (through-focal series); t h e F o u r i e r transforms are performed i n c l u d i n g an a d d i t i o n a l noise-term. An optimal estimate o f 0 can be obtained by a least-mean-square method. Image processing needs t h e knowledge o f t h e microscope t r a n s f e r - f u n c t i o n values. They are u s u a l l y obtained from the o p t i c a l Fourier-transform o f a t h i n amorphous carbon o r s i l i c o n f i l m image. Least square f i t t i n g o f t h e experimental and t h e t h e o r e t i c a l r a d i a l i n t e n s i t y d i s t r i b u t i o n s leads t o t h e r e q u i r e d values o f y.

41

Several l i n e a r f i l t e r i n g attempts have been published. Processing methods f o r more general non l i n e a r cases by i t e r a t i o n have been e s t a b l i s h e d . A 3-dimensional r e c o n s t r u c t i o n has been proposed on t h e b a s i s o f t h e microscope's d i f f r a c t o m e t e r f e a t u r e s ( r e v i e w i n Hawkes , ( 3 ) ) . ( c ) Matching o f computed and observed images. U n t i l now t h e most successful way o f image i n t e r p r e t a t i o n has been a t r i a l and-error method i n which an i n i t i a l s t r u c t u r e - h y p o t h e s i s (e.g. based on X-Ray data) i s r e f i n e d u n t i l t h e t h e o r i c a l t r a n s m i t t e d wave-image matches w i t h t h e observed image. The most used object-wave c a l c u l a t i o n i s t h e s o - c a l l e d muetis&e

method

(Cowley and Moodie, 1957; Goodman and Moodie, 1974). The c r y s t a l i s assumed t o be a s t a c k i n g o f i n d i v i d u a l t h i n s l i c e s s c a t t z r i n g one a f t e r another t h e i n c i d e n t wave. The t h e o r e t i c a l d i f f r a c t e d amplitude F(R) i n t h e back-focal plane i s obt a i n e d by an i t e r a t i o n formula f o r t h e N s c a t t e r e d waves which c o n t r i b u t e t o t h e image and i s then m u l t i p l i e d by t h e t r a n s f e r - f u n c t i o n . The squared F o u r i e r - t r a n s form r e s t o r e s t h e t h e o r e t i c a l image i n t e n s i t y - d i s t r i b u t i o n which can now be compared w i t h t h e observed image. Computation programmes have been c a r r i e d o u t on s i l i c a t e m i n e r a l s , p a r t i c u l a r l y by O'Keefe and Buseck (1978). An example i s shown i n F i g . 2.5.

Fig.2.5. 2ML-mica viewed along ,w i t h p r o j e c t e d s t a c k i n g sequences: a) S t r u c t u r e model p r o j e c t i o n b) Computed charge-density p r o j e c t i o n 0 c) M u l t i s l i c e - p r o essed image f o r Eo = 100 keV, C s = 0.18 cm,A = 1200 A, t h i c k n e s s 100 d ) Observed " w h i t e - d o t t e d " s t r u c t u r e image. The d o t s correspond t o s t r u c t u r e channels i n t h e K-layer. ( x 8,880,000) (Amouric e t a1 1981a).

F\

-

42 The same t h e o r y can t a k e i n t o a c c o u n t t h e e f f e c t o f c r y s t a l d e f e c t s . I t a p p l i e s t o t h e g e n e r a l case, w i t h o u t r e s t r i c t i v e h y p o t h e s i s on t h e o b j e c t . I t s o n l y l i m i t i s determined by t h e c o m p u t a t i o n means. However t h e q u e s t i o n o f t h e uniqueness o f t h e s o l u t i o n must n o t be o v e r l o o k e d . I n t h e m u l t i s l i c e method each s l i c e s t r u c t u r e i s approximated t o i t s p r o j e c t i o n . That a p p r o x i m a t i o n f a i l s w i t h l a r g e u n i t - c e l l s o r d i s o r d e r a l o n g t h e i n c i d e n t b e a m - d i r e c t i o n . A somewhat d i f f e r e n t t r e a t m e n t has been proposed by Van Dyck (1980), based on a d i r e c t - s p a c e quantum-mechanical c a l c u l a t i o n i n w h i c h t h e f i r s t - o r d e r m u l t i s l i c e expansion i s t r a n s f o r m e d i n t o a second-order expansion. It should t h e r e f o r e be more a c c u r a t e , ,'should a p p l y t o t h i c k e r s l i c e s and s h o u l d

be a b l e t o account f o r d i s o r d e r e d s t r u c t u r e s . F u r t h e r m o r e t h e c a l c u l a t i o n t i m e 2 becomes p r o p o r t i o n a l t o N ( i n s t e a d o f N w i t h t h e f o r e g o i n g t e c h n i q u e ) , w h i c h s h o u l d enable i t t o be performed on s m a l l cwnputers. 2.4

CLAY SPECIMENS I N H.R.E.M.

2.4.1

Structure

U n t i l now H.R.E.M.

has been m a i n l y a p p l i e d t o w e l l c r y s t a l l i z e d m a t e r i a l s ,

s e l e c t e d and s y n t h e s i z e d i n v i e w o f an easy s t r u c t u r e imaging, i . e . w i t h s i m p l e 0

s t r u c t u r e s , l a r g e atom spacings (4-5 A ) , and good electron-beam r e s i s t a n c e . The c l a y m i n e r a l o g i s t must a c c e p t t h e m a t e r i a l as he f i n d s i t , w i t h a r a t h e r 0

complex s t r u c t u r e and main a t o m i c d i s t a n c e s down t o 2.6 A, w i t h d i s o r d e r and d e f e c t s acquired d u r i n g growth o r a l t e r a t i o n , with a h i g h s e n s i t i v i t y t o e l e c t r o n bombardment and a l o w e l e c t r o n - c o n d u c t i v i t y .

The b e s t c r y s t a l 1 i z e d c l a y m i n e r a l ,

mica, i s t h e r e f o r e t h e one w h i c h has been t h e most f r e q u e n t l y s t u d i e d by means o f H.R.E.M. 2.4.2. (a)

Specimen o r i e n t a t i o n Normal o r i e n t a t i o n

Due t o t h e i r l a y e r e d s t r u c t u r e , c l a y m i n e r a l s a r e prone t o a p r e f e r r e d l a m e l l a r o r i e n t a t i o n when d e p o s i t e d on a m i c r o g r i d . Thus t h e c l a s s i c a l p r e p a r a t i o n mode which makes use o f g r i n d i n g , suspension, g r a n u l o m e t r i c a l s e p a r a t i o n and sediment a t i o n b r i n g s most o f t h e (001) l a m e l l a s p e r p e n d i c u l a r t o t h e beam and l e a d s t o t h e imaging o f t h e < O O b zone p l a n e s . A x i a l i l l u m i n a t i o n w i t h Scherzer defocus i n c l u d i n g t h e i n n e r hexagon (020 and 110 t y p e r e f l e c t i o n s c o r r e s p o n d i n g t o a 0

4.5 A

s e p a r a t i o n ) p r o v i d e s a hexagonal l a t t i c e image w h i c h i n c e r t a i n c o n d i t i o n s

i s r e l a t e d t o a s i n g l e - l a y e r s t r u c t u r e - p r o j e c t i o n as shown by J e f f e r s o n and Thomas (1974). However such l a t t i c e - f r i n g e images a r e a l m o s t t h e same f o r a l l c l a y miner a l s and can h a r d l y g i v e any i n t e r e s t i n g s t r u c t u r a l i n f o r m a t i o n . T h e i r occurence i n d i c a t e s t h a t t h e s t a c k i n g i s n o t a t u r b o s t r a t i c one ( s e e F i g . 6 ) . H i g h e r r e s o l u t i o n imaging combined w i t h computer m a t c h i n g seems n o t t o have been a t t e m p t e d so f a r i n t h a t o r i e n t a t i o n .

43

Fig. 2.6.Crossed l a t t i c e images due t o r e f l e c t i o n s on (020) and (110). a M o n t m o r i l l o n i t e (Yoshida, 1976) ( x 6,900,000) b{ Mica muscovite (Ehret, Lab. de Mineralogie, Strasbourg). ( x 7,670,000)

44

( b ) Layer-imaging o r i e n t a t i o n The most i n t e r e s t i n g s t r u c t u r a l f e a t u r e o f c l a y m i n e r a l s i s t h e l a y e r - s t a c k i n g . I t d e t e r m i n e s a p a r t o f t h e i r p r o p e r t i e s and i s r e l a t e d t o t h e i r o r i g i n . I n o r d e r t o v i s u a l i z e t h e l a y e r s , t h e y must be b r o u g h t t o become p a r a l l e l t o t h e beam. F o r s e p a r a t e c l a y p a r t i c l e s t h o s e c o n d i t i o n s can b e s t be achieved b y t h i n sect i o n n i n g normal t o t h e i r b a s a l p l a n e s u s i n g an u l t r a m i c r o t o m e w i t h diamant k n i f e . Whereas v e r y common f o r b i o l o g i c a l m a t e r i a l , t h a t method i s d i f f i c u l t t o a p p l y t o r a t h e r b r i t t l e m a t e r i a l s l i k e c l a y c r y s t a l s . A g r e a t a t t e n t i o n must be payed t o a p e r f e c t and o r i e n t e d embedding i n an e p o x i c o r m e t h a c r y l i c r e s i n w h i c h s h o u l d have a hardness c l o s e t o t h a t o f t h e m i n e r a l . V a r i o u s t e c h n i q u e s have been d e s c r i b e d , f i t t e d t o t h e r e s p e c t i v e m a t e r i a l (e.g.

E b e r h a r t and T r i k i , 1972;

Lee e t a1 , 1975a; Tchoubar e t a1 , 1973). A one-dimensional s t r u c t u r e - p r o j e c t i o n i s observed i n a g e n e r a l a z i m u t h ( w i t h o n l y 001 beams). 2-D s t r u c t u r e - p r o j e c t i o n s

demand a h i g h l y a c c u r a t e a z i m u t h a l o r i e n t a t i o n (e.g. o r p a r a l l e l t o t h e beam f o r mica, see F i g . 2.5. The average t h i c k n e s s o f t h e m i c r o t o m e - s e c t i o n s i s about 500

A , too

l a r g e f o r s t r u c t u r a l imaging. T h e r e f o r e t h e o b s e r v a t i o n s

have t o be achieved on t h e edges o f wedge-shaped ' c r y s t a l l i t e s . A c c u r a t e o r i e n t a t i o n a l o n g dense rows can t h e o r e t i c a l l y be performed by u s i n g a t i l t i n g s t a g e and c h e c k i n g by means o f t h e d i f f r a c t i o n p a t t e r n . However, owing t o t h e beams e n s i t i v i t y , t h i s method i s t o o slow t o be p r o p e r l y used f o r c l a y m i n e r a l s . The s i m p l e s t way seems t o r e c o r d images o f a g r e a t number o f p a r t i c l e s h a v i n g random azimuth; t h e p r o b a b i l i t y o f f i n d i n g d o t t e d s t r u c t u r e - i m a g e s becomes t h u s f a i r l y h i g h . S t r u c t u r e - i m a g e s o b t a i n e d i n such c o n d i t i o n s and combined w i t h computer s i m u l a t i o n have b r o u g h t t h e most v a l u a b l e r e s u l t s u n t i l now i n H.R.E.M.

clay

study.

E . M . specimens o f i n r o c k s embedded c l a y s a r e r e a d i l y p r e p a r e d f r o m a c c u r a t e l y o r i e n t e d p e t r o g r a p h i c t h i n s e c t i o n s w h i c h have been l o c a l l y t h i n n e d by a r g o n i o n - m i l l i n g under g r a z i n g i n c i d e n c e . (e.g. Amouric e t a l , 1981a). 2.4.3

Beam s e n s i t i v i t y

Electron-beam induced d e h y d r a t i o n and d e h y d r o x y l a t i o n l e a d s soon t o s t r u c t u r a l damage and t h e r e f o r e t o an image b l u r r i n g , i n c o m p a t i b l e w i t h h i g h r e s o l u t i o n . T h i s comnands q u i c k o p e r a t i o n , g e n e r a l l y e x c l u d i n g c a r e f u l o r i e n t a t i o n and t h r o u g h - f o c u s s e r i e s ( s e e f o r e g o i n g p a r a ) . P r o c e s s i n g methods a r e t h e r e f o r e spec i a l l y u s e f u l t o e x t r a c t t h e maximum amount o f i n f o r m a t i o n o u t o f a s i n g l e m i c r o g r a p h. Well adapted i m a g e - a m p l i f i e r s (e.g. equiped w i t h h i g h - r e s o l u t i o n micro-channel p l a t e s ) c o u l d be v e r y u s e f u l i n a l l o w i n g l o w e r e l e c t r o n i n t e n s i t i e s .

45

2.5

APPLICATIONS OF H.R.E.M.

TO CLAY MINERALOGY

The main us e o f H.R.E.M. i n c l a y m i n e r a l o g y c o n s i s t s i n i n v e s t i g a t i n g l o c a l s t r u c t u r e f e a t u r e s d e p a r t i n g f r o m t h e p e r f e c t c r y s t a l s t r u c t u r e as shown by X-ray d i f f r a c t i o n . Viewing a l o n g t h e (001) l a y e r s can g i v e an i n s i g h t i n t o t h e most f r e q u e n t d e f e c t s t r u c t u r e s i n v o l v i n g t h e s t a c k i n g sequence o f t h e 1 ayers, t h e knowledge o f w h i c h i s o f paramount i m p o r t ance f o r t h e comprehension o f growt h and w eat h erin g mechanisms and f o r t h e i n f e r e n c e o f surrounding c o n d i t i o n s . The answer t o t h e d i f f i c u l t q u e s t i o n o f a l t e r a t i o n e i t h e r t h r o u g h repeat ed d i s s o l u t i o n and neoro rma t i o n , o r t h r o u g h s o l i d - s t a t e t r a n s f o r m a t i o n should t o be found by means o f t h a t t e c h n i q u e . Numerous works on E.M. r e l a t e d t o c l a y m i n e r a l o g y have been p u b l i s h e d i n t h e l a s t t e n y e a r s . Most o f them a r e based on l a t t i c e - f r i n g e images o r on one-dimens i o n a l s t r u c t u r e p r o j e c t i o n s . Very few have achieved a r e a l 2-D s t r u c t u r e imaging. Some examples o f t y p i c a l a p p l i c a t i o n s w i l l be o u t l i n e d h e r e a f t e r . On a g i v e n t o p i c , o n l y t h e most r e c e n t s t u d i e s w i l l be quoted. M e r e l y morphological lowr e s o l u t i o n works on c l a y s w i l l n o t be mentioned. Examples o f mic r o m o r p h o l o g i c a l s t u d i e s by means o f E.M.

a r e g i v e n by t h e now

w e l l known s e r p e n t i n e polymorphism, i.e. t h e t u b u l a r s t r u c t u r e o f c h r y s o t i l e (e.g. Yada, 1972), t h e c o r r u g a t i o n o f a n t i g o r i t e and t h e p l a t y morphology o f l i z a r d i t e (e.g. Thomas e t a l , 1979 and i n ( 7 ) ) . I n t h e same way t h e c h a n n e l - s t r u c t u r e o f s e p i o l i t e has been shown by Rautureau e t a1 (1974, 1977). S o l i d - s t a t e t r a n s f o r m a t i o n s o f c l a y m i n e r a l s i s one o f t h e i m p o r t a n t t o p i c s 0 approached by E.M. A g r a d a t i o n o f a l l o p h a n e t o f i b r o u s 10 A - h a l l o y s i t e has been p o i n t e d o u t b y Sudo and Yotsumoto (1977). Ordered and unordered i n t e r s t r a t i f i c a t i o n can be s t u d i e d on an i n d i v i d u a l l a y e r s c a l e by l a t t i c e - f r i n g e imaging (e.g. Lee e t a l , 1975b). - I n t e r l a y e r s o r p t i o n o f o r g a n i c compounds has been v i s u a l i z e d as shown i n F i g . 2.7. Lay er-by -la y er imaging o f t h e c r y s t a l g r o w t h o f c l a y m i n e r a l s i s p r o b a b l y one of t h e most i n t e r e s t i n g f i e l d o f a p p l i c a t i o n o f E.M., e s p e c i a l l y when combined w i t h t h i n sectioning. The f i r s t s t a ges of s y n t h e t i c a l c h r y s o t i l e g r o w t h have been shown by Yada and I i s c h i (1977) i n d i f f e r e n t pH and hydrothermal c o n d i t i o n s . Layer f o r m a t i o n and s t a c k i n g f o l l o w e d b y c u r l i n g t o t u b u l a r f i b e r s c o u l d be d i r e c t l y observed on lattice-fringe

images w i t h i n l e s s t h a n an hour, whereas X-ray d i f f r a c t i o n c o u l d

n o t d i s c l o s e any t r a c e o f c h r y s o t i l e c r y s t a l l i z a t i o n b e f o r e one o r several days. The p o s s i b i l i t y o f d i r e c t imaging o f s t a c k i n g f a u l t s i n s e l e c t e d l a y e r e d mat e r i a l s has been shown f i r s t by J e f f e r s o n and Thomas (1974). Real sub-atomic

46

f

Fig. 2.7. I n t e r l a y e r s o r p t i o n o f lauryl-ammonium ions on i n t e r s t r t i f i e d micasmectite as shown by a l a t t i c e image. Unexpanded l a y e r s have 10 spacing; expanded l a y e r s have 24 1 spacing. (Yoshida, 1973). ( x 1,300,000) s t r u c t u r e r e s o l u t i o n has been achieved r e c e n t l y , mostly on mica, associated w i t h computer simulation. I n d i v i d u a l l a y e r stacking can be d i r e c t l y viewed i n mica c r y s t a l s by means o f " b r i g h t - d o t t e d " s t r u c t u r e images (see F i g . 5); the observed stacking sequences can then be c o r r e l a t e d t o the growth c o n d i t i o n s by comparing s y n t h e t i c a l c r y s t a l growth I n we1 1 defined c o n d i t i o n s and n a t u r a l c r y s t a l growth. Various ordered and disordered stacking sequences have been observed i n t h a t way i n s y n t h e t i c a l and n a t u r a l m a t e r i a l s by Amouric e t a1 (1978, 1981) and by I i j i m a and Buseck (1978). Mica i n t e r g r o w t h i n pyroxene has even been pointed o u t by Buseck and Veblen (1978). Nucleation o f s y n t h e t i c a l mica i n hydrothermal c o n d i t i o n s i s being studied by Amouric (1981) who has n o t i c e d on s t r u c t u r e 0 images t h a t few-layered nucleous are o f t e n made up o f an i n i t i a l 30 A l a y e r sur0 rounded symmetrically by 10 A l a y e r s . Disordered stacking i n c h l o r i t e has a l s o been observed (see F i g . 8 ) . I t must be pointed o u t t h a t i n kinematical d i f f r a c t i o n c o n d i t i o n s ( i . e . on very t h i n c r y s t a l edges) o n l y t h e basic p e r i o d i c i t y o f the layers can be observed, owing t o the absence o f the s u p e r l a t t i c e r e f l e c t i o n s which are then forbidden by the space group (e.g. the 001 r e f l e x i o n s w i t h 1 odd i n 2M micas). Thus the polytype sequence can o n l y appear i n t h i c k e r areas where the mu1 t i p l e s c a t t e r i n g p r e v a i l s and t h e space-group e x t i n c t i o n breaks down. Recent microscopes a1 low the combined study o f h i g h - r e s o l u t i o n images and energy-dispersive X-ray microanalysis. Using t h a t method,a c o r r e l a t i o n o f major and t r a c e element content w i t h c r y s t a l d e f e c t s and stacking sequences has been pointed o u t by Buseck and Veblen (1978).

0

F i g . 2 . 8 . Chlorite structure image with 14 A layer spacing and random stacking. (Amouric et a1 , 1981b). ( x 6,140,000)

48

2.6

CONCLUSION

F u t u r e p r o g r e s s i n H.R.E.M. c a n be expected i n s e v e r a l d i r e c t i o n s . F u r t h e r r e f i n e m e n t o f image p r o c e s s i n g c o u l d l e a d t o a b e t t e r mat ching w i t h o b s e r v a t i o n i n g en e r a l cases, e s p e c i a l l y w i t h d i s o r d e r e d s t r u c t u r e s f r e q u e n t l y encountered i n c l a y s . I n - l i n e p r o c e s s i n g c o u l d make e a s i e r t h e d i r e c t i n t e r p r e t a t i o n o f s t r u c t u r e images i n p e c u l i a r cases (e.g. L a b e r r i g u e e t a l , 1980). 0

X-ray m i c r o a n a l y s i s t o g e t h e r w i t h m i c r o d i f f r a c t i o n on areas smal l e r t han 100 A i n diame t e r, ac hie v e d w i t h f i e l d e m i s s i o n cathodes and combined w i t h H.R.E.M. , c o u l d make i t p o s s i b l e t o i n v e s t i g a t e t h e r e l a t i o n s between l o c a l s t r u c t u r e , c o m p o s i t i o n and r e a c t i o n mechanisms, t h u s l e a d i n g t o u s e f u l c o r r e l a t i o n s w i t h geochemical c o n d i t i o n s o f c l a y m i n e r a l s growth. However a f a i r s i g n a l - t o - n o i s e r a t i o f r o m such a s m a l l number o f atoms needs a h i g h e l e c t r o n i n t e n s i t y incompat i b l e w i t h t h e electron-beam s e n s i t i v i t y o f c l a y s . E l e c t r o n - e n e r g y - l o s s s p e c t r o m e t r y c o u l d b r i n g an answer t o h i g h - r e s o l u t i o n a n a l y s i s ( J o u f f r e y , 1977). A w e l l designed h i g h - r e s o l u t i o n e l e c t r o n - i m a g e amp1 i f i c a t i o n c o u l d a1 l o w t o r e c o r d s t r u c t u r e images i n o p t i m a l o p e r a t i n g c o n d i t i o n s w i t h o u t beam-damaging t h e clays mineral s. H igh-v o lt a ge E.M. c o u l d a f f o r d a much b e t t e r s t r u c t u r e - r e s o l u t i o n t o g e t h e r w i t h reduced energy-spread and o b j e c t - h e a t i n g and w i t h a t h i c k e r specimen. However s e v e r a l equipment d i f f i c u l t i e s must be overcome t o r e a c h t h e t h e o r e t i c a l r e s o l u t i o n a t 1 MeV o r h i g h e r . The s c a nning-t ra n s m i s s i o n - e l e c t r o n - m i c r o s c o p e i s n o t hampered by e i t h e r spher i c a l o r c h r o m a t i c a l a b e r r a t i o n , i s l e s s specimen damaging and b e t t e r s u i t e d f o r i n - l i n e pro c es s i n g . I t c o u l d t h e r e f o r e c h a l l e n g e t h e t r a d i t i o n a l t r a n s m i s s i o n microscope. However, above a l l i t must be k e p t i n mind t h a t t h e use o f r e a l h i g h - r e s o l u t i o n elec t ron-mic ro s c opy t o i n v e s t i g a t e c l a y - m i n e r a l s t r u c t u r e s on a n e a r l y at omic s c a l e needs a t h oro u g h knowledge o f t h e imaging process. Observed s t r u c t u r e images must be compared w i t h computed images r a t h e r t h a n l o o k e d a t as e x c i t i n g p i c t u r e s . F or c l a y m i n e r a l o g i s t s , w h o agree t o t a k i n g i n account t hose i m p e r a t i v e s , H.R.E.M. may w e l l r e v e a l a most p o w e r f u l t o o l f o r e x p l o r i n g l o c a l s t r u c t u r e - f e a t u r e s on an atomic s c a l e , as i t can be expected f r o m t h e f i r s t s e r i o u s i n v e s t i g a t i o n s summarized i n paraqraph 2.5. On t h e o t h e r hand,a merely d e s c r i p t i f i n t e r p r e t a t i o n can l e a d t o g r o s s m i s i n t e r p r e t a t i o n s . ACKNOWLEDGEMENTS The a u t h o r expresses h i s g r a t i t u d e t o a l l t h e e l e c t r o n - m i c r o s c o p i s t s who have k i n d l y s e n t him documents and h e l p e d him w i t h t h e i r advices.

49

REFERENCES Books and genera l r e v i e w s (1) E b erh art , J.P. , 1976. Methodes p h y s i q u e s d ' e t u d e des mineraux e t m a t e r i a u x s o l i d e s . Doin, P a r i s , 507 pp. (2) Gard, J.A. ( E d i t o r ) , 1971. The e l e c t r o n - o p t i c a l i n v e s t i g a t i o n o f c l a y s . Min era l .Soc. London , 381 pp. (3) Hawkes, P.W. ( E d i t o r ) , 1980. Computer p r o c e s s i n g o f e l e c t r o n microscope images. S p r i n g e r , B e r l i n , 296 pp. (4) He idenre ic h, R.D. , 1964. Fundamentals o f t r a n s m i s s i o n e l e c t r o n microscopy. Wiley, New-York, 405 pp. (5) K i h l b o r g , L. ( E d i t o r ) , 1979. D i r e c t imaging o f atoms i n c r y s t a l s and molecu les . Nobel Symposium 47. The Royal Swedish Academy o f Sciences, 295 pp. (6) Saxton, W.O. , 1978. Computer t e c h n i q u e s f o r image p r o c e s s i n g i n e l e c t r o n microscopy. Academic Press, London, 304 pp. (7) Sudo T., Shimoda S., Yotsumoto H., A i t a S., 1981. E l e c t r o n micrographs o f c l a y m i n e r a l s . Kodansha-Elsevier, 203 pp. (8) Wenk, H.R. ( E d i t o r ) , 1976. E l e c t r o n m i c r o s copy i n mineralogy. S p r i n g e r , B e r l i n , 564 pp.

Papers Amouric, M. , 1981. P r i v a t e communication. Amouric, M., Baronnet, A,, F i n c k , C., 1978. P o lyt ypisme e t desordre dans l e s micas d i o c t a e d r i q u e s s y n t h e t i q u e s . Etude p a r i m a g e r i e de reseau. Mat.Res. B u l l . , U.S.A. , 13: 627-634. Amouric, M., M e r c u r i o t , G., Baronnet, A., 1981a. On computed and observed H.R.T.E.M. images o f p e r f e c t mica p o l y t y p e s . Bull. Minera1. . t o be p u b l i s h e d . Amouric, M. , Baronnet, A. , C a t t i , M. , 1981b. P r i v a t e communication. Buseck, P.R., I i j i m a , S. , 1974. H i g h r e s o l u t i o n e l e c t r o n microscopy o f s i l i c a t e s . Amer.Mineral., 59: 1-21. Buseck, P.R., Veblen, D.R., 1978. Trace elements, c r y s t a l d e f e c t s and h i g h reso1u t i o n e l e c t r o n microscopy. Geochim .Cosmochim .Acta, 42 : 669-678. C o l l i e x , C., G a it e , J.M., Mory, C., Rautureau, M., Tchoubar, C., 1980. 0pt ical f i l t e r i n g o f f a u l t e d a r e a s i n e l e c t r o n m i c r o graphs o f l a y e r s i l i c a t e s . J.Microsc.Spectrosc. E l e c t r o n . , 5: 33-40 Coss le t , V.E. , Smith, D.J. , 1979. H i g h r e s o l u t i o n imaging o f amorphous m a t e r i a l s and c r y s t a l l i n e d e f e c t s . I n (5), pp. 39-45. Cowley, J.M. , Moodie, A.F. , 1957. The s c a t t e r i n g o f e l e c t r o n s by atoms and c r y s t a l s . I . A new t h e o r e t i c a l approach, A c t a Cryst . , 10: 609-619. E berh art , J.P. , T r i k i , R. , 1972. D e s c r i p t i o n d ' u n e t e c h n i q u e p e r m e t t a n t d ' o b t e n i r des coupes minces d e mineraux a r g i l e u x p a r u l t r a m i c r o t o m i e . J. Microsc. (F ) ,

15: 111-120.

Goodman, P., Moodie, A.F., 1974. Numerical e v a l u a t i o n o f N-beam wove f u n c t i o n i n e l e c t r o n s c a t t e r i n g by t h e m u l t i s l i c e method. Act a Cryst.A 30: 280-290. I i j i m a , S., Buseck, P.R., 1978. Experimental s t u d y o f d i s o r d e r e d mica s t r u c t u r e s by h i g h - r e s o l u t i o n e l e c t r o n microscopy. A c t a C r y s t . , A 34: 709-719. Jeffe rs o n, D.A., Thomas, J.M. , 1974. H i g h - r e s o l u t i o n e l e c t r o n m i c r o s c o p i c s t u d i e s o f s t r u c t u r a l f a u l t s i n l a y e r e d s i l i c a t e s . J.Chem.Soc.Faraday Trans.G.B.

70: 1691-1695.

J o u f f r e y , B., 1977. I n e l a s t i c s c a t t e r i n g and e l e c t r o n spect roscopy. P r o c . E l e c t r . Microsc.Soc.Amer. , 35: 692-695. J o u f f r e y , B., Do rig n a c , 0. , Tanaka, M., 1979. Atomic l e v e l e l e c t r o n microscopy as a f u n c t i o n o f a c c e l e r a t i n g v o l t a g e . I n (5), pp. 63-73. Labe rrig ue, A., B a l o s s i e r , G . , B e o r c h i a , A., Bonhomme, P., Bonnet, N., Trayon, M., 1980. T r a i t e m e n t d i r e c t en MET. D e c o n v o l u t i o n holographique a 1 ' a i d e de 1 ' e l e c t r o t i t u s . U t i l i s a t i o n d ' u n diaphragme de phase de t y p e e l e c t r o s t a t i q u e . J.Microsc.Spectrosc.Electron., 5: 651-660.

50

Lee, S.Y., Jackson, M.L., Brown, J.L., 1975a. Micaceous v e r m i c u l i t e , g l a u c o n i t e and mixed -1 ayered k a o l i n i t e - m o n t m o r i l l o n i t e e x a m i n a t i o n by u l t r a m i c r o t o m y and h i g h - r e s o l u t i o n e l e c t r o n microscopy. Proc.Soi1 Science Soc.Amer., 39: 793-800. Lee S.Y., Jackson, M.L., Brown, J.L., 1975b. Micaceous o c c l u s i o n s i n k a o l i n i t e observed by u l t r a - m i c r o t o m y and h i g h - r e s o l u t i o n e l e c t r o n m i c r o s c o p y . Clays, c l a y m i n e r . , 23: 125-129. Lynch, D.F., Moodie, A.F., O'Keefe, M.A., 1975. The u s e o f t h e c h a r g e - d e n s i t y a p p r o x i m a t i o n i n t h e i n t e r p r e t a t i o n o f l a t t i c e images. Acta C r y s t . , A31:300-307. O'Keefe, M.A., Buseck, P.R. , I i j i m a , S, 1978. Computed c r y s t a l s t r u c t u r e images f o r h i g h - r e s o l u t i o n e l e c t r o n microscopy. N a t u r e 274: 322-324. Rautureau, M., C l i n a r d , C., Tchoubar, C., 1974. M i s e en e v i d e n c e des canaux d e l a s e p i o l i t e p a r examen d e coupes u l t r a - m i n c e s en m i c r o s c o p i e e l e c t r o n i q u e a haute r e s o l u t i o n . J.App1 .Cryst.; 7: 293-294. Rautureau, M., M i f s u d , A., 1977. Etude p a r m i c r o s c o p i e e l e c t r o n i q u e des d i f f e r e n t s e t a t s d ' h y d r a t a t i o n de l a s e p i o l i t e . C l a y Min.Bul1 12: 309-318. Scherzer, 0. , 1949. The t h e o r e t i c a l r e s o l u t i o n o f t h e e l e c t r o n microscope. J.Appl.Phys., 20 : 20-29. Sudo, T., Yotsumoto, H., 1977. The f o r m a t i o n o f h a l l o y s i t e t u b e s f r o m s p h e r u l i t i c h a l l o y s i t e . Clays and c l a y miner., 25: 155-159. Tchoubar, C., Rautureau, M., C l i n a r d , C., Ragot, J.P., 1973. Technique d ' i n c l u s i o n a p p l i q u e e a 1 ' e t u d e des s i l i c a t e s l a m e l l a i r e s e t f i b r e u x . J.Microsc.(F), 18: 147-154. Thomas, J.M., J e f f e r s o n , D.A. , M a l l i n s o n , L.G., Smith, D.J., Crawford, E . S . , 1979. The e l u c i d a t i o n o f t h e u l t r a s t r u c t u r e o f s i l i c a t e m i n e r a l s by h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p y and X-ray e m i s s i o n m i c r o a n a l y s i s . I n ( 5 ) , pp.167-179. Van Dyck, D., 1980. F a s t c o m p u t a t i o n a l procedures f o r t h e s i m u l a t i o n o f s t r u c t u r e images i n complex o r d i s o r d e r e d c r y s t a l s : a new approach. J.Microsc. G.B., 119: 141-152. Yada K. , 1972. Study o f m i c r o s t r u c t u r e o f c h r y s o t i l e asbestos by h i g h r e s o l u t i o n microscopy. A c t a C r y s t . , A 27: 659-664. Yada, K., I i s c h i , K., 1977. Growth and m i c r o s t r u c t u r e o f s y n t h e t i c c h r y s o t i l e . Amer.Minera1. , 62: 958-65. Yoshida, T., 1973. Elementary l a y e r s i n t h e i n t e r s t r a t i f i e d c l a y m i n e r a l s as r e v e a l e d by e l e c t r o n microscopy. Clays and C l a y Min., 21: 413-420. Yoshida, T. , 1976. Study o f m i c r o s t r u c t u r e o f mica and m o n t m o r i l l o n i t e by h i g h r e s o l u t i o n e l e c t r o n microscopy. C l a y S c i . , 5 : l - 7 .

.,

51 Chapter 3 NEUTRON SCATTERING TECHNIQUES FOR THE STUDY OF CLAY MINERALS.

P e t e r L. HALL University of Birmingham, Edgbaston, Birmingham B 1 5 2TT, U.K. INTRODUCTION

3.1

3 . 1 . 1 General background Among t h e advanced methods forming t h e s u b j e c t of t h i s symposium, neutron s c a t t e r i n g is perhaps one of t h e l e a s t f a m i l i a r t o e a r t h s c i e n t i s t s , Although t h e f i r s t reported a p p l i c a t i o n of t h e technique t o c l a y mineralogy d a t e s back 15 years (Naumann e t a l . , 1966), t h e t o t a l number of p u b l i c a t i o n s i n t h e f i e l d t o d a t e remains f a i r l y small. This may be a t t r i b u t e d p r i n c i p a l I y t o two f a c t o r s . F i r s t l y , t h e experimental f a c i l i t i e s a r e a v a i l a b l e a t only a few l o c a t i o n s , normally a t research r e a c t o r s o r p a r t i c l e a c c e l e r a t o r s . Secondly, t h e development of purpose-huilt neutron s c a t t e r i n g c e n t r e s having s u p e r i o r neutron f l u x e s , improved instrument design and d a t a handling f a c i l i t i e s is a r e l a t i v e l y recent occurrence. These c e n t r e s , p a r t i c u l a r l y t h e high-flux r e a c t o r s a t t h e Brookhaven and Oak Ridge Laboratories i n t h e USA and a t t h e I n s t i t u t Laue Langevin i n Grenoble, France, and a l s o t h e pulsed neutron sources a t Argonne and Los Alamos in t h e USA (and under c o n s t r u c t f o n a t t h e Rutherford Laboratory i n England), i l l u s t r a t e t h e continuing e v o l u t i o n of t h e technique. The widening range of i t s f i e l d s of a p p l i c a t i o n during t h e p a s t few years make t h i s an a p p r o p r i a t e time t o review t h e progress i n c l a y r e s e a r c h using neutron s c a t t e r i n g methods. Section 1.2 o u t l i n e s t h e b a s i c p r o p e r t i e s which make thermal neutrons a u s e f u l probe of t h e microscopic p r o p e r t i e s of condensed matter. Section 2 d i s c u s s e s b r i e f l y t h e b a s i c p r i n c i p l e s of t h e technique and t h e types of experimental measurement which may be made. Section 3 d i s c u s s e s i n g r e a t e r d e t a i l t h e g e n e r a l n a t u r e of q u a s i - e l a s t i c s c a t t e r i n g s p e c t r a and t h e i r dependence on t h e d e t a i l s of t h e molecular d i f f u s i o n processes, and reviews t h e a p p l i c a t i o n of t h e incoherent q u a s i - e l a s t i c s c a t t e r i n g method t o s t u d i e s of the motions of water a s s o c i a t e d with expanding l a t t i c e c l a y s . Section 4 then b r i e f l y d e s c r i b e s t h e progress made by coherent s c a t t e r i n g methods ( d i f f r a c t i o n and small a n g l e s c a t t e r i n g ) i n studying s t r u c t u r a l a s p e c t s of c l a y minerals and t h e i r i n t e r c a l a t i o n complexes.

No d e t a i l e d treatment of t h e theory of t h e s u b j e c t is attempted h e r e f o r reasons of space. The r e a d e r is r e f e r r e d i n s t e a d t o a number of books and review a r t i c l e s which cover t h i s ground (Turchin, 1965; Marshall and Lovesey, 1971; W i l l i s , 1973; Lovesey and Springer, 1977 and Squires 1978). P a r t i c u l a r a s p e c t s of t h e s u b j e c t a r e a l s o well-documented, e.g. d i f f r a c t i o n (Bacon, 1975); q u a s i - e l a s t i c s c a t t e r i n g (Springer, 1972, 1977) and small angle s c a t t e r i n g (Schmatz e t a l . , 1974; Kostorz, 1979). A previous review of t h e a p p l i c a t i o n of neutron s c a t t e r i n g t o c l a y research a l s o c o n t a i n s t h e general theory of t h e s u b j e c t (Ross and Hall, 1980). 3 . i .2 The use of neutrons a s a spectroscopic probe I n common with electromagnetic r a d i a t i o n and e l e c t r o n s , neutrons e x h i b i t wave p r o p e r t i e s and t h e a s s o c i a t e d d i f f r a c t i o n and i n t e r f e r e n c e phenomena. A neutron of momentum E has a d e Broglie wavelength h

h

I !

mv

A=-=-

and an energy

E =

-

P2

hk

2m

2m

- =(-I

52

where m and v are the neutron's mass and velocity, k is the magnitude of the wavevector (2n/X) and I( is Dirac's constant. 'Ihe neutron has a halfintegrerspin and an associated magnetic moment. Neutrons produced in a reactor or particles accelerator initially have relatively large energies which subsequently decrease rapidly through collisions with light nuclei in a suitable moderating material (A20, D20 or graphite). When the neutrons reach thermal equilibrium with the moderator, their e n e r w spectrum is a is Maxwellfan function whose peak flux occurs at an energy of %T, where Boltzmann's constant and T is the moderator temperature. F i g . 3 . 1 illustrates flux distributions for moderator temperatures of 20, 300 and 2000OK. For a moderator at ambient temperature, the maximum flux corresponds to an energy For hot and cold moderators of approximately 0.038 eV (wavelengthWO.15 nm). the peak flux shifts to correspon,dinglyhipher or lower energies respectively, e.g. a 200K cold source (typically liquid hydrogen or deuterium) yields a peak flux at approximately 0.003 eV (wavelength 0 . 5 nm).

5

T 300 K

0

5

10

Velocity, k m s

15

-1

+

Fig. 3.1 Neutron f Z u distributions for moderator temperatures of 20, 300 and ZUU@'K.

Both "thermal" and "cold" neutrons are commonly used in neutron scattering experiments on condensed matter. They are unique probes of structural and dynamic properties, excitations and interatomic forces for two principal reasons. Firstly, their wavelengths are comparable to interatomic separations, and secondly their energies correspond to those characteristic of vibrational modes of crystals and molecular solids and liquids. This dual comparability may be contrasted with electromagnetic radiation such as Xrays (satisfying only the first condition) and infra-red radiation (satisfying only the second). Another essential difference between neutrons and electromagnetic radiation lies in the fact that, except in the case of magnetic materials, neutrons are scattered by nuclei and not electrons. The nature of the neutron-nucleus interaction depends on the relative orientations of the neutronic and nuclear spins. As will be shown in the next section, this implies the existence of a randomly varying component of the scattered wave amplitude in addition to a uniformly varying component.

53 The l a t t e r alone is r e s p o n s i b l e f o r i n t e r f e r e n c e phenomena l e a d i n g t o coherent d i f f r a c t i o n of neutrons ( p r e c i s e l y analogous t o X-ray d i f f r a c t i o n ) . The e x i s t e n c e of two s c a t t e r e d components, among o t h e r reasons, l e a d s t o a number of d i s t i n c t experiments o r types of measurement which can he made with neutrons. These w i l l be discussed i n t h e next s e c t i o n .

3.2 3.2.1

BASIC

PRINCIPLES OF NEUTRON SCATTERING

Types of scattering experiment

A s s t a t e d above, as a consequence of t h e e x i s t e n c e of t h e s p i n magnetic moment of t h e neutron, t h e s c a t t e r i n g process is s e n s i t i v e t o t h e r e l a t i v e d i r e c t i o n s of t h e s p i n s of t h e n u c l e i and t h e neutron. I n general t h i s r e s u l t s i n a v a r i a t i o n from atom t o atom of t h e amplitude of t h e s c a t t e r e d wave, not only f o r d i f f e r e n t i s o t o p e s of t h e same element, b u t a l s o f o r Roth i s o t o p i c and p a r a l l e l and a n t i p a r a l l e l s p i n s t a t e s of a given isotope. s p i n d i f f e r e n c e s t h u s c o n t r i b u t e t o an i n c o h e r e n t l y s c a t t e r e d component, i n addition t o t h e coherently s c a t t e r e d component a r i s i n g from i s o t o p i c and s p i n uniformity ( i n g e n e r a l , zero nuclear s p i n ) . The majority of m a t e r i a l s , including those having i d e a l , s t o i c h i o m e t r i c c r y s t a l s t r u c t u r e s , t h e r e f o r e In c o n t r a s t , X-rays e x h i b i t only give r i s e t o both types of s c a t t e r i n g . coherent s c a t t e r i n g from p e r f e c t c r y s t a l s , incoherent o r d i f f u s e s c a t t e r i n g being a consequence of c r y s t a l l i n e imperfections o r s t r u c t u r a l d i s o r d e r ( a l l o y s , amorphous s o l i d s ) . It follows t h a t t h e r e l a t i v e i n t e n s i t i e s of neutron coherent and incoherent s c a t t e r i n g can be varied by a l t e r i n g t h e i s o t o p i c composition of a A technique of some importance i n neutron s c a t t e r i n g experiments, material. p a r t i c u l a r l y d i f f r a c t i o n and small-angle s c a t t e r i n g measurements ( s e e Section 4 ) , is s e l e c t i v e o r complete d e u t e r a t i o n of hydrogenous m a t e r i a l s , f o r reasons which w i l l be explained below. The two components of t h e s c a t t e r i n g provide d i s t i n c t information: only the coherent component can produce i n t e r f e r e n c e phenomena such a s Bragg The incoherent component cannot c o n t a i n such information, diffraction. although under a p p r o p r i a t e c o n d i t i o n s i t can provide information concerning v i b r a t i o n a l and o t h e r dynamic p r o p e r t i e s of m a t e r i a l s , a s discussed l a t e r . A t t h i s p o i n t , a f u r t h e r d i s t i n c t i o n between t h e various types of This d i s t i n c t i o n , independent of t h e coherent o r s c a t t e r i n g must be made. We incoherent n a t u r e of t h e s c a t t e r i n g , is on t h e b a s i s of energy t r a n s f e r . therefore distinguish (no energy t r a n s f e r ) from i n e l a s t i c s c a t t e r i n g (one o r more energy quanta exchanged between neutron and s c a t t e r i n g m a t e r i a l ) . The term "neutron s c a t t e r i n g " t h e r e f o r e covers four c a t e g o r i e s of measurement, which may be summarized as follows:( i ) Coherent, e l a s t i c s c a t t e r i n g , i.e.

neutron d i f f r a c t i o n (ND).

(ii)

Coherent, i n e l a s t i c s c a t t e r i n g , abbreviated INS.

(iii)

Incoherent, e l a s t i c s c a t t e r i n g , abbreviated ENS.

(iv)

Incoherent, i n e l a s t i c s c a t t e r i n g (IINS).

Before summarizing t h e information provided by each experiment, t h e c l a s s i f i c a t i o n must be widened s l i g h t l y . In t h e f i r s t category, i n a d d i t i o n t o Bragg d l f f r a c t i o n , m u s t be included coherent low-angle s c a t t e r i n g due t o microscopic inhomogenities such a s pores o r c o l l o i d a l p a r t i c l e s . This is t h e small-angle neutron s c a t t e r i n g (SAYS) technique. F i n a l l y , in t h e t h i r d

54 category (ENS), there must be included a closely related phenomena where the scattering is not quite elastic due to relative motions between scattered neutrons and nuclei which are mobile on the appropriate time-scale (lo-'' seconds). The small energy changes (essentially a neutron Mppler effect) contain information regarding the motions of such nuclei. The technique is called quasi-elastic neutron scattering (QENS) because the energy changes are extremely small, of the order of micro electron-volts, i.e. a small percentage of the incident energies of the neutrons. QENS provides information complementary to pulsed NMR, although on a somewhat shorter time-scale. The information yielded by neutron scattering methods may therefore be summarised as follows:ND: Crystal structure,:including the positions of hydrogen atoms, structures of liquids and amorphous materials and magnetic structures. SANS: Inhomogenities such as pore sizes, particle sizes in colloidal materials, lattice defects etc.

INS: Collective excitations,.such as phonon dispersion relations, i.e. dependence of phonon energies on direction of propagation within the crystal (Squires, 1978; Ross and Hall, 1980). ENS + QENS: Study of vibrational, reorientational and diffusive motions of molecules and groups. IINS:

the energy spectrum of excitations (e.g. phonon density of states). Similar to the information provided by infrared and Raman spectroscopy but (1) heavily weighted towards hydrogen-containing groups due to the large incoherent cross-section of the proton (see below), and (ii) independent of the selection rules governing the coupling of vibrational excitations with electromagnetic radiation. The essential principles of the more important of the above types of neutron experiment will be discussed in section 2 . 3 . The next section gives a brief definition of coherent and incoherent scattering lengths and cross-sections. 3.2.2

Coherent and incoherent scattering lengths and cross-sections

The theory of the scattering of neutrons from individual nuclei and sets of fixed nuclei has been treated in detail elsewhere (e.8. Ross and Hall, 1980). Here a simple treatment leading to the definition of the scattering lengths and cross-sections is given, following "urchin (1965). Let the scattering nucleus have spin I. The "compound nucleus" existing momentarily during the scattering event therefore has a spin of J+ I + 1 / 2 or JI 1/2 in the parallel and antiparallel cases, having respective probabilities p+ = (I + 1)/(2I + 1) and p- = I/(2I + 1). By defining appropriate quantum mechanical operators, and utilizing the Schradinger equation for the neutron-nucleus system, it can be shown that the wave function consists of two terms: (a) a plane wave representing the incident neutrons, and (b) a spherical wave representing the scattered neutrons (see Fig. 3 . 2 ) . The scattered wave intensity is defined by a scattering amplitude or scattering length b, which is related to the above quantities by the expression

-

where

-

-

r and 2 are the spin operators for the nucleus and neutron

5 5

respectively and b+ and b- are the scattering lengths corresponding to the parallel and antiparallel spin orientations. The first two terms of the above equation are spin independent, and define f o r the nucleus a coherent scattering length:

I

I + 1

bcoh =

b+ + - b21 + 1 21 + 1

The corresponding coherent scattering cross-section i s 0coh =

(3)

4nbcoh2

The third term, when averaged over all possible spin directors, gives the incoherent component. From quantum mechanics the average value of (I.2)’ is equal to 1(1+1)/4. The incoherent scattering length is then given by the expression

and the corresponding incoherent scattering cross-section by 0

inc

=

4n

(b+

-

b-1’

(21

+

1>*

I(1

+

1)

(5)

e i k‘r-r’l

Scatted

beam

k

Fig.3.2 Wave representation of neutron scattering. The incident plane uave i s scattered by t h e isotropic potential due t o the p a r t i c l e a t P’, t h e scattered wave having spherical symmetry. The relative values of ocoh and Oinc vary randomly between nuclei with increasing mass number, depending on the relative magnitudes and signs of b+ and b-. This contrasts strongly with X-ray scattering amplitudes, which increase smoothly with the number of electrons per atom and are identical for different isotopes of the same element. For most isotopes, the coherent neutron scattering cross-sections are comparable with or larger than, the incoherent cross-sections. Two notable exceptions, however, are hydrogen ( l H ) and vanadium (51V) in which spin interactions

5 6

cause b and b- to be of opposite sign and of relative mapnitudes such that ucoh,.<sinc (Ross and Hall, 1980). Thus in structural work, it is useful to replace H by D, since for the latter 0coh‘

‘inc’

Vanadium is

conventionally used as a standard incoherent scatterer for calibration purposes in incoherent scattering experiments. Values of scattering crosssections for a few common isotopes and elements are listed in Table3.1. TABLE 3.1

Neutron scattering cross sections for common isotopes and elements

Isotope or element

Spin

(x 10-28m2)

“coh

1H

lI2

2H

1 0

1 2C

14N 160 Mg 27611

1 0 5i2

5 1v

?I2

0

si

0

Fe 3.2.3

Cross Sections

0

“inc

1.76 5.6

79.7 2 .o 0 0.3 0 0.36 0.01 0.03 4.8 0.44

5.517

11.1 4.24 3.34 1.49 2.17 0.033 11.36

Basis of experimental methods

The basis of the neutron scattering experiment is to prepare an initial state, 1.e. a monochromatic and collimated beam characterised hy an energy After scattering from the sample one detects a final E and wavevector k seats characterisepby an energy E/ and wavevector k/ at a scattering angle (This definition preserves 0 as the Bragg angle). The detector may of 28. or may not perform an energy analysis of the scattered beam.

.

Neutron scattering processes can most easily be represented as vector diagrams in reciprocal (momentum) space, since momentum = 1( x wavevector. Thus coherent Bragg scattering is illustrated in F i g . 3a, where lf, and k/ k / is the scattering are the incident and scattered wavevectors and 9 = k vector or wavevector transfer, corresponding to a msent& transfer KQ. Fig.3.3b illustrates a schematic view of a simple neutron diffractometer. Bragg diffraction i s only possible if 9 coincides with some reciprocal lattice vector of length 2 ~ r l d , ~ ~This condition is equivalent to Bragg’s law in momentum space. Since the process is elastic, E’ = Eo and therefore

-

.

62

- (k” - ko2)

= 0

2m

so that k/ = ko = k say, From Fig. 3.3a it therefore follows that, for elastic scattering,

4n

-sin8

(71 A Fig.3.3~ shows the reciprocal space diagram for coherent inelastic Q = 2k s i n 8 =

57

k/

scattering. Here the d i f f e r e n c e between k and does not simply correspond t o a reciprocal l a t t i c e vectorTxcept when the wave vector of a c o l l e c t i v e e x c i t a t i o n ( e . g . a l a t t i c e phonon) i s incorporated. Here

-0

D

/’

=YM C

L

Ch.

F i g . 3 . 3 Reciprocal space s c a t t e r i n g diagrams and schematic experimental laoyouts. ( a ) and ( b ) : Coherent e l a s t i c s c a t t e r i n g and t h e neutron diffractornefier. (el and ( d ) : Coherent i n e l a s t i c s c a t t e r i n g and t h e t r i p l e

a x i s spectrometer. ( e l and (fl: Incoherent i n e l a s t i c s c a t t e r i n g and t h e t i m e - o f - f l i g h t spectrometer. C[ = collimator, M = monochromator, A = analyser c r y s t a l , D = d e t e c t o r , S = sample, Ch. = neutron chopper.]

5 8 where l'fy is t h e phonon momentum and Q =

hka

The corresponding schematic diagram (Fig.3.3d)corresponds t o t h e t r i p l e a x i s spectrometer. F i n a l l y , E'ig.3.3e r e p r e s e n t s t h e v e c t o r diagram f o r incoherent i n e l a s t i c s c a t t e r i n g , where k and k' d i f f e r i n magnitude, corresponding t o Fig. 3 . 3 f , which g i v e s t h e scT-tic o u t l i n e of t h e time-of-flight spectrometer, i n which t h e f i n a l energy E is measured by timing t h e f l i g h t of t h e pulsed In t h i s experiment i n c i d e n t beam between t h e chopper Ch. and t h e d e t e c t o r . both energy and momentum t r a n s f e r s occur, r e s p e c t i v e l y given by t h e expresions and

E s s e n t i a l l y , e l a s t i c s c a t t e r i n g experiments measure an i n t e n s i t y p r o p o r t i o n a l t o do/dQ where 0 is t h e a p p r o p r i a t e (coherent o r incoherent) c r o s s s e c t i o n . I n e l a s t i c experiments measure t h e douhle d i f f e r e n t i a l c r o s s s e c t i o n d20/dME' where d Q i s t h e s o l i d angle subtended by t h e d e t e c t o r a t t h e s a m p l e and I n the d i f f r a c t i o n dE' is t h e energy range analysed by t h e d e t e c t o r . experiment t h e s c a t t e r e d i n t e n s i t y i s p r o p o r t i o n a l t o t h e sauare of t h e ) o r S ( Q ) , which is t h e Fourier transform of s t a t i c structure factor, F(T +kQ t h e neutron s c a t t e r i n g d e n s i t y d i s t r i b u t i o n i n t h e c r y s t a l (Racon, 1975). In the case of i n e l a s t i c s c a t t e r i n g t h e double d i f f e r e n t i a l c r o s s s e c t i o n is p r o p o r t i o n a l t o a q u a n t i t y c a l l e d t h e incoherent ( o r coherent) s c a t t e r i n g These f u n c t i o n s d e s c r i b e w) o r Sc ,(q, o) r e s p e c t i v e l y . function S t h e d i s t r i & i o n of momentum and energy t r a n s f e r s among t h e s c a t t e r e d neutrons, and c o n t a i n information regarding t h e v i b r a t i o n a l o r d i f f u s i v e motions of atoms i n t h e s c a t t e r i n g m a t e r i a l (Van Hove, 1954). A b r i e f d e s c r i p t i o n of simple neutron d i f f r a c t o m e t e r s and i n e l a s t i c s c a t t e r i n g spectrometers has been given elsewhere (Ross and H a l l , 1980).

(A,

3.3 3.3.1

INCOHERENT NEUTRON SCATTERING

-

STUDY OF DYNAMICS OF ADSORBED MOLECULES

P r i n c i p l e s of t h e q u a s i - e l a s t i c s c a t t e r i n g (QENS) method.

(i) Van Hove s e l f - c o r r e l a t i o n function. In t h e QENS experiment, neutrons s c a t t e r e d incoherentlv i n t o a d e t e c t o r Dositionen a t a eiven a n e l e t o t h e i n c i d e n t beam a r e energy analysed (by r e f l e c t i o n from an a n a l y s e r c r y s t a l o r by time-of f l i g h t methods), t h e spectrum from each d e t e c t o r c o n s i s t i n g of t h e i n t e n s i t y of s c a t t e r e d neutrons versus energy ( o r time-of-flight). Apart from i n e l a s t i c processes, t h e s p e c t r a a r e u s u a l l y dominated by a l a r g e peak from e l a s t i c a l l y s c a t t e r e d neutrons. Neutrons s c a t t e r e d from a r i g i d l a t t i c e e x h i b i t an e l a s t i c peak whose width i n energy d e f i n e s t h e instrumental r e s o l u t i o n which depends on both design f a c t o r s and t h e i n c i d e n t neutron wavelength. This peak is r e f e r r e d t o a s t h e r e s o l u t i o n f u n c t i o n (Fig.3.4a). It is measured using a vanadium s h e e t a s a s t a n d a r d incoherent s c a t t e r e r . I n c o n t r a s t , when s c a t t e r i n g t a k e s p l a c e from n u c l e j which a r e d i f f u s i n g o r r e o r i e n t a t i n g on a time-scale comparable with t h e seconds) t h e e l a s t i c r e s o l u t i o n f u n c t i o n i n t e r a c t i o n time (lo-'' becomes broadened in energy because of t h e r e l a t i v e motions between t h e neutrons and t h e mobile n u c l e i (Fig.3.4b). This phenomenon is termed

5 9

q u a s i - e l a s t i c b r o a d e n i n p , and t h e dependence of t h e q u a s i - e l a s t i c l i n e widths and i n t e n s i t i e s on a n g l e ( i . e . momentum t r a n s f e r ) c o n t a i n s i n f o r m a t i o n concerning t h e s p a c e and time-dependence of t h e m o l e c u l a r motions. A f t e r c o r r e c t i o n s f o r a b s o r p t i o n , r e s o l u t i o n and v a r i o u s o t h e r i n s t r u m e n t a l e f f e c t s , d i s c u s s e d i n d e t a i l by H a l l e t a l . ( 1 9 7 9 a ) , t h e s p e c t r a p r o v i d e measurements of t h e double d i f f e r e n t i a l c r o s s - s e c t i o n d i s c u s s e d above i n section 2.3. T h i s q u a n t i t y can be r e l a t e d t o t h e i n c o h e r e n t s c a t t e r i n g function, S ( Q ,A) which d e s c r i b e s t h e d i s t r i b u t i o n of e n e r g y and momentum i n c -’ t r a n s f e r s i n t h e s c a t t e r i n g process. Van Hove (1954) showed t h a t f o r systems which can be d e s c r i b e d c l a s s i c a l l y ( t 3 s e c o n d s ) , Sinc(Q’ W ) i s r e l a t e d hy a d o u b l e (5.e. s p a c e and t i m e ) F o u r i e r t r a n s f o r m t o t h e s e l f - c o r r e l a t i o n f u n c t i o n , G - (r, t ) , d e f i n e d a s t h e p r o h a h i l i t y t h a t a n atom i n i t i a l l y a t t h e o r i g i n ” i s l a t e r found a t p o s i t i o n f a t time t. This connection i s e x p r e s s e d by t h e e q u a t i o n

I n p r i n c i p l e one could a p p l y t h e r e v e r s e t r a n s f o r m t o c a l c u l a t e G ( r t ) d i r e c t l y from t h e observed d a t a . Tn p r a c t i c e , however, i n s u f f i c i e n t EoTits w) s p a c e a r e e x p e r i m e n t a l l y a c c e s s i b l e , so t h a t i t i s i n g e n e r a l in n e c e s s a r y t o c a l c u l a t e t h e form of G ( r t ) f o r some assumed model of t h e molecular motions i n o r d e r t o o b t a i n s a - ; h e o r e t i c a l scattering function for comparison with e x p e r i m e n t .

(a,

( i i ) Behaviour of t h e c o r r e l a t i o n f u n c t i o n a t l o n g times. The n a t u r e of t h e s c a t t e r i n g a t n e a r - e l a s t i c e n e r g i e s ( W W 0) i s a s s o c i a t e d w i t h t h e hehaviour of t h e s e l f - c o r r e l a t i o n f u n c t i o n a t long times ( i n t h i s c o n t e x t , much g r e a t e r t h a n t h e i n t e r a c t i o n time ( ~ p i1c o s e c o n d ) ) . For unbounded o r macroscopic t r a n s l a t i o n a l d i f f u s i o n t h e p r o h a b i l i t y of a n atom r e t u r n i n g t o i t s o r i g i n a l p o s i t i o n decays with t i m e and become i n f i n i t e s i m a l , so t h a t G s ( ~ ,t ) tends t o zero a t t + I n c o n t r a s t , f o r r o t a t i o n a l motions ( o r s p a t i a l l y bounded t r a n s l a t i o n s ) t h e p r o b a b i l i t y of a r e t u r n t o t h e o r i g i n a l t ) does n o t v a n i s h a t l o n g times. In p o s i t i o n remains f i n i t e , and G t h e s e c a s e s t h e c o r r e l a t i o n f u z c t i o n may be d i v i d e d i n t o time-dependent and time-independent components:

=.

(z,

On s u b s t i t u t i n g t h i s e x p r e s s i o n i n t o e q u a t i o n (11) i t may be shown t h a t t h e corresponding s c a t t e r i n g f u n c t i o n a l s o c o n t a i n s two t e r m s , one c o r r e s p o n d i n g t o f i n i t e energy t r a n s f e r ( i . e . q u a s i - e l a s t i c s c a t t e r i n g ) and one, a r i s i n g In general, from t h e c o n s t a n t term i n G ( r t ) , which i s p u r e l y e l a s t i c .

s

-’

t h e r e l a t i v e a m p l i t u d e s of t h e e l a s t i c and a u a s i - e l a s t i c terms a r e f u n c t i o n , of a d i m e n s i o n l e s s v a r i a b l e Q a , where a i s a c h a r a c t e r i s t i c dimensions of t h e molecular system, e.g. t h e r a d i u s of a r o t a t i o n a l motion. (For example, f o r a water molecule undergoing C 2 r o t a t i o n s , t h e r o t a t i o n a l r a d i u s of t h e p r o t o n s i s a p p r o x i m a t e l y a = 0.758). In t h i s case t h e s c a t t e r i n g function may be r e p r e s e n t e d by t h e e x p r e s s i o n

Qua s j - e l a s t i c Elastic where A o ( Q a ) , t h e a m p l i t u d e of t h e e l a s t i c t e r m , is known a s t h e g l a s t i c . i n c o h e r e n t s t r u c t u r e f a c t o r ( E I S F L , which may h e r e g a r d e d as t h e F o u r i e r t r a n s f o r m of t h e f i n a l d i s t r i b u t i o n of d i f f u s i n g atoms. The spectrum t h u s

6 0 consists of a sharp line, represented by the delta function, plus a broadened (commonly a Lorentzian or sum of component corresponding to a function L(w), Lorentzians), of relative amplitudes A (Qa) and A 1 (Qa) respectively. The idealized situation i s illustrated in pig. 3.4~.Thg requirement that the scattering function is normalized implies that, with the correct definition of L(w), A 1 (Qa) = 1 - A. (Qa).

In practice, because of the additional broadening due to the finite instrument resolution, a separation as clear as that illustrated in Fig.3.4~ is rarely observed. In practice the situation i s more commonly like that illustrated in Fig.3.4d. ( I n fact the gradient change may be even less pronounced). Separation of elastic and quasi-elastic amplitudes then requires computational line-profile fitting methods, as discussed in detail elsewhere (e.g. Hall et al., 1979a). 'Ihe parameters extracted by such fitting methods include (i) the width of the quasi-elastic line, or the related energy AE = hAw and (if) the relative amplitudes A and A 1 . By collecting data at many scattering angles the dependence oP Aw and A on Q i s determined. A plot of Aw (or AE) versus Q2 is known as a quasicelastic broadening curve and provides information concerning the time-dependence of the diffusive process (i.e. a diffusion coefficient or rotational correlation time). A typical broadening curve is illustrated j n Fig.3.5a; t h e gradient of the curve at law Q may be related to the diffusion coefficient (see section 3 . 2 ) . A typical form for .A (Qa) is illustrated in Fig.3.5b; invariably A (0) = 1, i.e. the spectrum is purely elastic at 0 = 0. The rate of deca? of the EISF with Qa gives a measure of the parameter a, the distance scale of the bounded motion. The next section discnsses the Q-dependence of the quasi-elastic broadening and the EISF for various specific rotational and translatisnal diffusion models. F i g . 3.4 Typical incoherent

a

C

b

d

e l a s t i c arld q u a s i - e l a s t i c p r a k shapes. ( a ) E l a s t i c scntfrrq'nn onlu. ( b ) Puasielastically broadened Zinc. (el E l a s t i c and auasie l a s t i c components-idealised case. ( d l E l a s t i c and q u a s i - e l a s t i c componmts w i t h m s o l u t i o n broadening.

6 1

r

QZ

+

Qa

F i g . 3 . 5 T y p i c a l forms of:

(a) t h e quasi-elastic broadening curve, and (bl t h e eZastic incoherent s t r u c t u r e factor for spatiaZZy bounded motion.

3.3.2

Scattering function for translational and rotational diffusion models

In this section a number of models for the incoherent scattering function, leading to certain characteristic forms of the quasi-elastic spectra, are discussed. These correspond to a variety of simple diffusion models, the examples selected being those which have relevance to the possible motions of small adsoibed molecules, e.g. water in layer silicates. Only the general features of the results are given here together with references to the original papers where the theoretical details m y be found. The models which are discussed fall into three categories: (i) unbounded tranplational diffusion; (ii) spatially bounded translational diffusion; and (iii) reorientational motions. In addition, each type of motion may also be continuous (i.e. Fickian) or stochastic (i.e. rapid jumps between fixed sites). (i) Unbounded translational diffusion. In the case of continuous diffusion, Fick’s law predicts a correlation function of Gaussian shape: Gs

(fl,

t)

=

(4~Dt)-~” exp (-r2/4Dt)

(14)

whose space-time Fourier transformation (equation 11) yields a Lorentzianshaped scattering function

where D is the diffusion coefficient. Thus the Lorentzian HWHM is DQ’, and the quasi-elastic broadening curve is linear on a Q2 scale and of gradient D The model was derived by Vineyard (1958). (see curve c of Fig.3.6).

6 2 It has, however, been found by experiment t h a t t h e d i f f u s i v e processes which take place i n most m a t e r i a l s d e v i a t e from t h e p r e d i c t i o n s of t h e continuous d i f f u s i o n model above a c e r t a i n value of Q (1.e. when t h e measurement samples small d i s t a n c e s and times), and a r e more a c c u r a t e l y described by jump models, e s p e c i a l l y of rapid jumps between s i t e s a t which When t h e the d i f f u s i n g moecules remain f o r some mean residence time T. quasi-equilibrium s i t e s a r e d i s t r i b u t e d on a l a t t i c e of known geometry, r a t e equations i n c o r p o r a t i n g t h e known jump v e c t o r s may be solved t o c a l c u l a t e t h e s e l f - c o r r e l a t i o n f u n c t i o n (Springer, 1972). Alternatively random walk methods may be u t i l i z e d (Chandrasekhar, 1943). Assun.inp t h a t successive jumps a r e uncorrelated but descrihed by t h e same s p a t i a l d i s t r i b u t i o n p ( x ) , t h i s method p r e d i c t s a s c a t t e r i n g f u n c t i o n of t h e form

where F(Q) i s t h e Fourier transform of p ( r ) and r = l / ~ i.e. t h e mean A t high Q, where jump r a t e . This i s a Lorentzian of half-width r[l-F(Q)]. u s u a l l y F(Q) + 0, t h e q u a s i - e l a s t i c half-width tends t o an asymptotic The shape of t h e broadening curve depends on t h e form value .‘I of P(L). Fig.3.6 (curves a and b ) i l l u s t r a t e s c a l c u l a t e d broadening curves f o r two d i f f e r e n t forms of t h e d i s t r i b u t i o n of jump l e n g t h s ( S i n p i and Sjolander, 1960; H a l l and Ross, 1981). Both models d e v i a t e considerably from t h e p r e d i c t i o n s of t h e Vineyard model (curve c ) and tend t o asymptotic widths Au = r a t l a r g e Q.

( i i ) S p a t i a l l y r e s t r i c t e d t r a n s l a t i o n a l d i f f u s i o n . In many m a t e r i a l s t h e space a v a i l a b l e f o r d i f f u s i o n may be geometrically bounded, e.g. d i f f u s i o n For d i f f u s i o n may be confined t o a s u r f a c e o r r e s t r i c t e d t o a given volume. confined t o a plane s u r f a c e (freedom of motion in two dimensions), i t can be shown t h a t Sinc(Q, u) =

i.e.

1

n

B sin2 a

(B s i n 2 a l 2

+ w2

a Lorentzian of Hk7MI-I B s i n a, where a i s t h e angle between

0

and t h e

R = DQ2 s u r f a c e normal. For continuous two-dimensional d i f f u s i o n (Dianoux e t a l . , 1975) while f o r jumps of length a between s i t e s of mean residence time T , B = [ l - J o ( Q a ) ] / ~ where Jo i s t h e zeroth o r d e r Bessel function. (Stockmeyer, 1976). Both models p r e d i c t a n i s o t r o p i c q u a s i - e l a s t i c s c a t t e r i n g ; i f Q i s p a r a l l e l t o t h e plane Aw = 0, i.e. t h e s c a t t e r i n g i s purely e l a s t i c . As before, t h e jump motions a r e c h a r a c t e r i s e d by a maximum broadening a t high Q equal t o l / r ( o r H I T i n energy u n i t s ) . In a p o l y c r y s t a l l i n e m a t e r i a l t h e degree of anisotropy w i l l depend on t h e e x t e n t of p r e f e r r e d o r i e n t a t i o n . In a randomly o r i e n t a t e d specimen, t h e s c a t t e r i n g However, f u n c t i o n w i l l be i s o t r o p i c though not in g e n e r a l lorentzian-shaped. t h e best f i t l o r e n t z i a n i n t h i s case has w i d t h < B > = 2B/3, i.e. two-thirds of t h e in-plane component.

An a l t e r n a t i v e s i t u a t i o n occurs when t h e d i f f u s i n g molecules a r e bounded by impermeable s u r f a c e s . Here t h e a p p r o p r i a t e s o l u t i o n s derived from d i f f u s i o n equations o r random walk theory can be modified by assuming r e f l e c t i v e boundaries i n o r d e r t o o b t a i n t h e a p p r o p r i a t e form of t h e s e l f - c o r r e l a t i o n f u n c t i o n f o r restricted d i f f u s i o n . ‘Ihe following problems have been solved e x p l i c i t l y : (a) continuous d i f f u s i o n between p a r a l l e l boundaries ( H a l l and Ross, 1978); (b) random jump d i f f u s i o n between p a r a l l e l

63 boundaries ( H a l l and Ross, 1981); and ( c ) c o n t i n u o u s d i f f u s i o n w i t h i n a n impermeable s p h e r e (Volino and Dianoux, 1980). Each of t h e s e models c o n t a i n s a p u r e l y e l a s t i c component of t h e t y p e d i s c u s s e d i n s e c t i o n 3.1 which arises from t h e s p a t i a l l y r e s t r i c t e d c h a r a c t e r of t h e motion. The EISF is u n i t y a t Q = 0, and r e t a i n s a s i g n i f i c a n t magnitude up t o Q = 2n/R where R i s a c h a r a c t e r i s t i c l e n g t h , i.e. t h e i n t e r p l a n e s e p a r a t i o n or s p h e r e r a d i u s . A t h i g h e r Q ( i . e . small d i s t a n c e ) t h e measurements are n o t a f f e c t e d s i g n i f i c a n t l y by t h e e x i s t e n c e of t h e b o u n d a r i e s , and t h e models r e v e r t t o t h e behaviour of t h e analogous, unbounded d i f f u s i o n models ( H a l l and Ross, 1981). For both v e r s i o n s of t h e p a r a l l e l - b o u n d a r y d i f f u s i o n problem (i.e. t h e problem of d i f f u s i o n i n a n i n f i n i t e l y deep one-dimensional p o t e n t i a l w e l l ) , t h e s c a t t e r i n g f u n c t i o n i s of t h e form m

Sinc(Q9

w) =

.A

( 9 2 ) 6(w)

E l a s t i c term

+

L

n=1

An (QR) L (mn)

(18)

Q u a s i - e l a s t i c terms

The EISF i s given by Ao(QR) = S ~ ~ ~ ( Q R / ~ ) / ( Q = Rj o/ 2~ (QR/2) )~ where R i s t h e i n t e r p l a n a r s p a c i n g and j i s t h e s p h e r i c a l Bessel f u n c t i o n of o r d e r zero. The q u a s i - e l a s t i c componegt c o n s i s t s of a series of L o r e n t z i a n s of width Awn and amplitude An(QR). I n p r a c t i c e , t h e sum converges r a p i d l y because of t h e form of t h e g e n e r a l amplitudes. However, f o r most purposes t h e model may be s i m p l i f i e d by a d d i n g a l l t h e q u a s i - e l a s t i c terms t o p e t h e r t o o b t a i n a n average l i n e w i d t h < Au > whose amplitude w i l l be q u i t e c l o s e t o 1 - A (QE). The v a l u e ofCdw> w i l l , i n g e n e r a l , be Q-dependent, s i n c e t h e mixing of t h e component l i n e s i s weighted by t h e i r Q-dependent amplitudes. F i g u r e 7 i l l u s t r a t e s t h e dependence of d A w b on ( Q R ) 2 on a douhle l o g a r i t h m i c scale f o r t h e p a r a l l e l - b o u n d a r y models. A t low Q (01? Q11>2* t h e c o n t i n o u s d i f f u s i o n model ( a ) and t h e jump model (b) tend r e s p e c t i v e l y t o t h e p r e d i c t i o n s of t h e unbounded F i c k i a n d i f f u s i o n model (c) and t o t h a t of t h e r e l a t e d unbounded jump d i f f u s i o n model, i.e. t o widths Am = DQ2 and Am = r where, a s b e f o r e , r = l/r. The one-dimensional s o l u t i o n s c a n r e a d i l y be g e n e r a l i s e d t o t h r e e dimensions t o d e s c r i b e r e s t r i c t e d random motions i n any p a r a l l e l - s i d e d box. For example, f o r jumps of mean l e n g t h r w i t h i n a cube of s i d e R i t c a n show t h a t

where Qx, Qy,

and Q

are t h e components of Q a l o n g t h e s i d e s of t h e cube and

L (Aw ) d e n o t e s a L o r e n t z i a n of HWHM ij k

C l e a r l y Aw

000

-

0, i.e.

t h e r e i s s t i l l a n e l a s t i c component.

( i i i ) R e o r i e n t a t i o n a l motions. For a molecule o r f u n c t i o n a l proup undergoing f i x e d r o t a t i o n without any- centre-of-mass d i f f u s i o n , t h e o r y a g a i n p r e d i c t s a n e l a s t i c s c a t t e r i n g component s i n c e a n atom may r e t u r n t o its s t a r t i n g posi ti o n a t long t i m e s . Two important cases a r e t h e models f o r

6 4 jump r e o r i e n t a t i o n between N e q u i d i s t a n t s i t e s on a c i r c l e (Barnes, 1973), and t h e s p h e r i c a l r o t a t i o n a l d i f f u s i o n model o f S e a r s (1967) which h a v e been used t o model u n i a x i a l and i s o t r o p i c r o t a t i o n a l p r o c e s s e s r e s p e c t i v e l y . As i n t h e case of r e s t r i c t e d d i f f u s i o n above, t h e Q-dependence of t h e e l a s t i c i n t e n s i t y ( i . e . t h e a m p l i t u d e of t h e EI S F ) e n a b l e s t h e s p a t i a l dimensions of t h e motion, i n t h i s c a s e t h e r a d i u s of t h e r o t a t i o n a l motion, t o be determined. F u r t h e r d i s c u s s i o n s of r o t a t i o n a l d i f f u s i o n models, i n c l u d i n g t h o s e s u b j e c t t o p e r i o d f r p o t e n t i a l , are g i v e n elsewhere Dianoux e t a l . , 1975; Dlanoux and V n l i n n , 1977; Ross and Hall, 1980).

(a) Jump d i f f u s i o n w i t h ( b l Jwnp d i f f u s i o n w i t h guassian distribu?-ion of lengths (Hall and Ross, 1981). icl Continuous Fickian d i f f u s i o n . Curves ( a ) and ibl are calculated f o r mean square j m p Fig.3.6 Quasi-elastic p(rJ

=

broadening curves.

r e x p ( - r / r ) ( S i n - m i and Sjtilander, 19601.

l e n t h s 3.f

?.

1

of q u a s i - e l a s t i c broadenings f o r one-dimensionai! r e s t r i c t e d d i f f u s i o n models. ( a ) Continuous ( b l Jump d i f f u s i o n on segmmt of length d i f f u s i o n on segment of length 9,. I. (c) Fickian d i f f u s i o n . (d). and ( e l are t h e o r e t i c a l asymptotic values AW = 0,2112 and ~w = r r e s p e c t z v e l y .

F i g . 9.7 DoubZe logarithmic plot of v a r i a t i o n w i t h (QI)

65

1.0

X

0.5

3

Fig.3.8 QENS d a t a f o r Ca2' m o n t m o r i l l o n i t e t w o - l a y e r h y d r a t e ( p / p = 0.76) a t a n e n e r g y r e s o l u t i o n of 38 peV. S o l i d c u r v e s a r e s i m u l a t i o n s g a s e d on the t w o - p h a s e j u m p d i f f u s i o n model d e s c r i b e d i n t h e t e x t . -------AE.

_____ X .

1.0

A

4

u

.d .y

v1

id

4

w

0.4

0.2

Q2

(113

0.6

0. 8

Fig. 3.9 &ENS data for Ca2+ montmorilZonite two-layer hydrate at high energy ------- B E . . A ------- Elastic fraction. resolut-: ___.--............. _ic. .........

---

/. *+

0

Fig.7.3 Energy level diagrams for Ni(en)22+ on clays Octahedral complexes do not undergo significant band shifts on the surface. Mono-complexes such as [Cu(en) (H20)412+ and [Ni(en)(H20) 12+ disproportionate on the surface into bis-corplexes and aquo-complexes (Velghe, Schoonheydt, Lunsford, 1977; Velghe, Schoonheydt, Uytterhoeven, Peigneur and Lunsford, 1977; Velghe, Schoonheydt and Uytterhoeven, 1979). The stepwise adsorption of gaseous en on hydrated clays proceeds over the bis-complex : [M(H20)612+ + 2en

+ [M(en)2]2+ +

6H20

V(en),2+ (M=Ni, Cu) (9)

With ligands such as dien(diethy1enetriamine) and tetren(tetraethylenepentamine) an important amount of protonated 1igand.s is exchanged together with the complexes (Schoonheydt, Velghe, Baerts and Uytterhoeven, 1979). Dependkng on the water content the following equilibria are observed :

176

tetrenH

+

2dienH +

-HZO'

+ [Ni(tetren)(H20) ] 2 + s 2 0 t e t r e n

+

[ N i ( d i e n I 2 l2 +

-+H20

2dien

-H20

+

+

[I

> t r e n H )13+(10)

Pla

diamagnetic

[ N i ( d i e n H ) 2 ]4+

(11)

p l a n a r , diamagne

S i m i l a r r e a c t i o n s can b e w r i t t e n f o r Cu. P 1 1 t h e s e d a t a d e m o n s t r a t e t h e l a r g e a f f i n i t y of t h e b i d i

ion-

a 1 s u r f a c e o f c l a y s f o r " b i d i m e n s i o n a l " ( = p l a n a r ) .complexes. 7.3.3

Aromatic m o l e c u l e s

I n t h e e a r l y 7 0 ' s t h e most p o p u l a r s y s t e m w a s Cu2+

+

benzene,

forming on t h e s u r f a c e g r e e n , t y p e I , complexes and r e d , t y p e 11, complexes : g r e e n r t y p e 1971, Mortland, 1973).

+H 0

r e d , t y p e I1 ( P i n n a v a i a and -3 A u ( I I 1 ) i s a l s o able t o form t y p e s

+ 2+ N a , Ca , F13+,

3+

Mortland, I and 2+ 3+

I1 complexes (Vandepoel, 1 9 7 5 ) . Cr , Fe , Fe , 2+ + 2+ Mn2+, Zn , Ag , N i 2 + and Co o n l y form t y p e I complexes. A r o m a t i c l i g a n d s c a n b e d i v i d e d i n 2 g r o u p s a c c o r d i n g t o t h e i r a b i l i t y t o form

2+

.

The l a t t e r group c o n t a i n s t y p e I o r b o t h t y p e s o f complexes w i t h Cu benzene, d i p h e n y l , n a p h t a l e n e , a n t h r a c e n e and a n i s o l e , t h e f o r m e r t o l u e n e , x y l e n e s , p h e n o l , hexamethylbenzene and 1, 2 , 4 , 5 tetrachlorobenzene. complex. complexes.

-

Vandepoel e t a l . (1973) a l s o found a brown

T a b l e 7 . 3 summarizes t h e s p e c t r a l c h a r a c t e r i s t i c s of t h e

The a s s i g n m e n t s g i v e n i n T a b l e 7.3 a r e t h o s e of Vande

P o e l e t a l . (1973) and must b e r e g a r d e d as working h y p o t h e s e s . p r e s e n t t h e r e i s n o f i r m e x p l a n a t i o n of t h e s e s p e c t r a . Complexes w i t h 2 ,2 ' - b i p y r i d i n e

( b i p ) and 1 , l o - p h e n a n t h r o l i n e

P t

(phen)

a r e exchanged w i t h e x t r e m e l y h i g h s e l e c t i v i t i e s and w i t h p r e c i p i t a t i o n o f s a l t s i n t h e i n t e r l a m e l l a r s p a c e (Schoonheydt, P e l g r i m s , Heroes and U y t t e r h o e v e n , 1 9 7 7 ; B e r k h e i s e r and M o r t l a n d , 1 9 7 7 ) .

Epacinns are

a b o u t 1.8nm and even a f t e r c a l c i n a t i o n i n t e r e s t i n g a d s o r p t i v e p r o p e r -

t i e s a r e e v i d e n t ( T r a y n o r , M o r t l a n d and P i n n a v a i a , 1978; L o e p p e r t , The s p e c t r u m of [ R u ( b i p ) 12+ i s M o r t l a n d and P i n n a v a i a , 1 9 7 9 )

.

dominated by a ~ C band T a r o u n d 460nm and a n i n t r a l i w n d band around 280nm ( F e r g i s o n and H a r r i s , 1966; L Y t l e and

~ + n "

177 1 9 6 9 ; Crosby and K l a s s e n , 1 9 6 5 ) . TABLE 7 . 3

A b s o r p t i o n b a n d s and t e n t a t i v e a s s i g n m e n t s f o r Cu2+-benzene complexes Species

assignment

benzene 255 (39200)

TI

-+

71

+TI::

comp1e x

iT::

physisorbed benzene 284 (35200) type I (green) 390 (25640) 686 (14577)

charge t r a n s f e r d-d

H2°\ H20’

t y p e I1 ( r e d ) 510 (19608)

charge t r a n s f e r

900-5900

d-p

(11000-1700)

brown 355 (28169) 825 ( 1 2 1 2 1 )

charge t r a n s f e r

cu

;+/OH2 \

OH2

P H

d-d

On t h e c l a y t h e l a t t e r i s r e d u c e d i n i n t e n s i t y and a s h o u l d e r a p p e a r s around 320nm ( K r e n s k e , Abdo, Van Damme, Cruz and F r i p i a t , 1 9 8 0 ) . The f o l l o w i n g h y p o t h e t i c a l r e a c t i o n was p r o p o s e d t o a c c o u n t f o r t h i s new band :

R e a c t i o n ( 1 2 ) r e s u l t s i n a l o s s of aromatic c h a r a c t e r o f t h e ligand. A b l u e s h i f t o f t h e n+n’: band i s e x p e c t e d ( P l b e r t , 1 9 6 7 ) . T h i s is o p p o s i t e t o t h e experimental o b s e r v a t i o n . however, n o a l t e r n a t i v e i s a v a i l a b l e .

A t present,

178

The LMCT band i s a t 463nm on t h e h y d r a t e d h e c t o r i t e , a t 455nm on t h e d e h y d r a t e d c l a y and a t 452nm i n aqueous s o l u t i o n ( K r e n s k e , ?&do Van Damme, Cruz and F r i p i a t , 1 9 8 0 ) . Two e f f e c t s g o v e r n t h e s e s h i f t s : ( i ) t h e change o f t h e s o l v e n t "H20" t o t h e s o l v e n t " c l a y " and ( i i ) d i s t o r t i o n of t h e complex i n t h e i n t e r l a m e l l a r s p a c e .

The

l a t t e r i s e a s y t o imagine f o r 2 r e a s o n s : t h e " k e y i n u i n " phenomenon and t h e i n t e r a c t i o n of a f l a t s u r f a c e w i t h a corrplex o f d i f f e r e n t geometry.

The e x a c t n a t u r e o f t h e d i s t o r t i o n i s unknown a t p r e s e n t .

However d i s t o r t i o n s have b e e n invoked t o e x p l a i n s i r i l a r band s h i f t s ( F e l i x , F e r g u s o n , Giidel and L u d i , 1 9 8 0 ) .

'32

The l u m i n e s c e n c e p r o p e r t i e s o f R u ( b i p ) H 0-dependent. 2

t o c a . 15 %

on h e c t o r i t e a r e a l s o

The quantum y i e l d i n c r e a s e s by a b o u t 2 from ca. 3% H20 H20 ( 0 . 0 4 -+ 0 . 0 9 ) . Cr(bip):+ on h e c t o r i t e , i r r a d i a t e d

i n i t s d-d band, h a s a quantum y i e l d d e c r e a s i n g from ca. 0.08 t o c a . 0.002 w i t h i n c r e a s i n g w a t e r c o n t e n t ( K r e n s k e , Pbdo, Van Dame, Cruz, and F r i p i a t , 1 9 8 0 ) . F o r b o t h complexes t h e l u m i n e s c e n c e i s 3+ 3+ quenched i n a d i f f u s i o n - c o n t r o l l e d way by 02. Q u e n c h i n g by Eu , C r o r Fe3+ i s more e f f e c t i v e w i t h t h e s e c a t i o n s s u b s t i t u t e d i n t h e o c t a h e d r a l l a y e r s o f t h e l a t t i c e t h a n when t h e y a r e i n i o n exchange positions. These r e s u l t s open new r e s e a r c h a r e a s b u t many problems must b e s o l v e d b e f o r e t h e s p e c t r a o f t h e s e complexes a r e u n d e r s t o o d and r e l a t e d t o t h e i r p e c u l i a r i o n exchange b e h a v i o r .

The i m p o r t a n c e o f

a complete u n d e r s t a n d i n g of t h e s p e c t r o s c o p i c p r o p e r t i e s o f t h e s e monolayer complexes r e a c h e s f u r t h e r t h a n t h a t . I n d e e d , t h e photoc h e m i c a l s u b s t i t u t i o n o f 2H20 m o l e c u l e s on Cr(bip):+ on t h e s u r f a c e i s i n c i s - p o s i t i o n s , whereas t h e r e a c t i o n i n s o l u t i o n g i v e s trans-products

( C r u z , Pbdo, Van Damme and F r i p i a t , 1 9 8 0 ) .

This is

an example o f a s t e r e o s p e c i f i c , p h o t o c h e r r i c a l s u b s t i t u t i o n r e a c t i o n , t h e s t e r e o s p e c i f i c i t y b e i n g i n d u c e d by t h e s u r f a c e . Monolayer

'32

a r r a n g e m e n t s of Ru ( b i p ) d i s s o c i a t i o n of H 2 0 .

have been p r o p o s e d f o r p h o t o c a t a l y t i c

I t i s expected t h a t s t e r e o s p e c i f i c r e a c t i o n s

can b e performed on t h e c l a y s u r f a c e r o c c u p i e d by c a t i o n i c corrplexes with c a t a l y t i c a c t i v i t y . 7.4

LATTICE TRFNSITION METAL IONS

U.v-vis.-n.i.r. s p e c t r o s c o p y o f t r a n s i t i o n metal i o n s i n l a t t i c e s i t e s i s mainly c o n c e r n e d w i t h Fe. The d-d bands o f Fe a r e f o r b i d d e n and u s u a l l y n o t s e e n a t t h e low Fe c o n c e n t r a t i o n s of k a o l i n i t e and smectite materials.

a t 240-5nm and a t 190-5nmr due t o an 0

They have 2 weak bands

+Fe(II1)

charae t r a n s f e r

179 transitions

( K a r i c h k o f f and B a i l e y , 1 9 7 3 ) , w i t h F e ( I I 1 ) i n o c t a h e d r a l

sites s u b s t i t u t i n g f o r A l ( I I 1 ) . For suspensions t h e i n t e n s i t y of t h e b a n d a t 240nm i s d e p e n d e n t on t h e Fe c o n t e n t and on t h e t a c t o i d s i z e of t h e m i n e r a l s (Chen, Shaked and B a n i n , 1 9 7 9 ) . I n n o n t r o n i t e n e a r l y a l l F l ( I I 1 ) i s s u b s t i t u t e d by F e ( I I 1 ) and t h e 2 b a n d s a r e f o u n d a t 260nm and a t 200nm. F e ( I I 1 ) s u b s t i t u t i n g f o r M g ( I 1 ) g i v e s the c h a r g e t r a n s f e r b a n d i n t h e r e g i o n 245-250nm w h i l e i t i s a t 215nm f o r t e t r a h e d r a l F e ( I I 1 ) ( K a r i c h k o f f and B a i l e y , 1 9 7 3 ) . of t h e bands i s - p a r t i a l l y bond l e n g t h s : FeIV-O



C i s assumed e q u a l t o 4B. A l l A2 and A3 a r e

a r b i t r a r i l y t a k e n 0. d e f i n e d i n f i g u r e 7.1.

1 --[I5 2

A+4B+3C-E

4B+C 2Al+A+4B +3C-E

+ + 5 \+In n I1

C 4B+C

fi (3B+C)

2 A2+E+4B +3C-E

E = A +F+4B+2C 2

A2+A+4B-E

-6B 2A3+A-5B-E

185

- 2 6 B 2A 3+A+B+2C-E

A1+A+2C-E

A 3+A+B+ZC-E

A3+A-5B-E

-G/2 B

-3/2 B A 2+ A 3 +P+B+ZC-E

-5?/2

3B

-3fi

A 2+A 3+A-5B-E

- 3 0 B

A

2A3+A+B+2C-E

+A

B +P+ B-E

2fl B Al+A2+A+2C-E

B

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z',

188 Melamed, N . G . , 1 9 6 3 . O p t i c a l p r o p e r t i e s o f powders. I. O p t i c a l a b s o r p t i o n c o e f f i c i e n t s and t h e a b s o l u t e v a l u e o f t h e d i f f u s e reflectance. 11. P r o p e r t i e s of l u m i n e s c e n t powders. J. Appl. P h y s . , 34: 560-570. Monahan J r . , E.M. a n d N o l l e , P . W . , 1977. Q u a n t i t a t i v e s t u d y o f a p h o t o - a c o u s t i c s y s t e m f o r powdered samples. J . P p p l . P h y s . , 48: 3519-3523. M o r t l a n d , M.M., 1973. C l a y - a r e n e c o m p l e x e s and p r o c e s s of p r o d u c i n g same. U S P a t e n t N o 3 , 7 7 8 , 4 5 7 . P i n n a v a i a , T . J . and Mortland, M.M., 1971. I n t e r l a m e l l a r metal complexes on l a y e r s i l i c a t e s . I . C o p p e r ( I 1 ) - a r e n e complexes on m o n t m o r i l l o n i t e . J . P h y s . Chem., 7 5 : 3957-3962. Rao, C . N . R . , 1975. U l t r a - v i o l e t and v i s i b l e s p e c t r o s c o p y . Chemical A p p l i c a t i o n s . B u t t e r w o r t h s , London, 242 p p : S a e h r , D . , Le D r e d , R . a n d H o f f n e r D . , 1978. C o n t r i b u t i o n a l'gtude des interactions vermiculite-colorants cationiques. C l a y Min., 13: 411-426. S c h o o n h e y d t , R . A . , 1978. A n a l y s i s o f t h e e l e c t r o n p a r a m a g n e t i c r e s o n a n c e s p e c t r a o f bis(ethylehediamine)copper(II) on t h e s u r f a c e o f z e o l i t e s X a n d Y a n d o f a Camp B e r t e a u m o n t m o r i l l o n i t e . J. P h y s . Chem., 82: 497-498. S c h o o n h e y d t , R.A. , P e l g r i m s , J . , Heroes, Y . a n d U y t t e r h o e v e h J . B . , 1977. C h a r a c t e r i z a t i o n o f t r i s ( 2 , 2 I - b i p y r i d y l ) r u t h e n i u m ( I 1 ) on hectorite. C l a y Min. , 1 3 : 435-438. S c h o o n h e y d t , R.A. , V e l g h e , F. , B a e r t s R . and U y t t e r h o e v e n , J . B . , 1 9 7 9 . Complexes of diethylenetriamine(dien) a n d t e t r a e t h y l e n e p e n t a m i n e ( t e t r e n ) w i t h C u ( I 1 ) and N i ( I 1 ) on h e c t o r i t e . Clays and C l a y Min., 27: 269-278. S c h o o n h e y d t , R . A . , V e l g h e , F. a n U y t t e r h o e v e n J . B . , 1979. (x = 1, 2 , 3 ; e n = e t h y l e n e C h a r a c t e r i z a t i o n of [Ni(en),]'+ d i a m i n e ) on t h e s u r f a c e o f m o n t m o r i l l o n i t e s . I n o r g . Chem., 18: 1842-1847. S i f f e r t , B., 1978. P r B p a r a t i o n e t d t u d e s p e c t r o m g t r i q u e de c o m p l e x e s s i l i c a t e s p h y l l i t e u x c o l o r a n t s a z o T q u e s . C l a y M i n . , 1 3 : 147-166. S i l l e n , G.L. and M a r t e l l , A . E . , 1 9 6 4 . S t a b i l i t y C o n s t a n t s o f Metal I o n Complexes. The C h e m i c a l S o c i e t y , London, 754 p p . S m i t h , G . , 1977. L o w - t e m p e r a t u r e o p t i c a l s t u d i e s o f metal-metal charge-transfer transitions i n various minerals. C a n a d i a n Min., 15: 500-507. S t r o m e , D . H . and K l i e r , K . , 1980. The e f f e c t o f oxygen on photoluminescence and resonance e n e r g y t r a n s f e r i n c o p p e r ( 1 ) Y zeolite. A.C.S. Symp. S e r . N o 135: 155-176. S t u l , M.S. and Mortier, W.J., 1974. The h e t e r o g e n e i t y o f t h e charge density i n montmorillonites. C l a y s a n d C l a y Min., 2 2 : 391-396. T a r a s e v i c h , Y u . I . , 1975. The i n v e s t i g a t i o n o f c o o r d i n a t i o n compounds o n t h e s u r f a c e o f l a y e r s i l i c a t e s . P r o c . I n t . Conf. C o l l o i d and S u r f a c e S c i . , 1: 27-31. Tarasevich, Yu.1. and S i v a l o v , E . G . , 1 9 7 5 a . E l e c t r o n i c s p e c t r a of aquo-cations o f d i v a l e n t c o p p e r , s o r b e d by m o n t m o r i l l o n i t e . K o l l o i d . Z h . , 37: 814-817. T a r a s e v i c h , Yu.1. and S i v a l o v , E . G . , 1975b. The e l e c t r o n i c s p e c t r a o f aquo-cations o f d i v a l e n t cobalt, s o r b e d by m o n t m o r i l l o n i t e . T e o r . Eksp. Khim., 11: 4 1 0 - 1 5 . T a r a s e v i c h , Yu.1. and S i v a l o v , E . G . , 1 9 7 5 c . E l e c t r o n i c s p e c t r a o f a q u o - c a t i o n s o f b i v a l e n t n i c k e l a d s o r b e d on m o n t n o r i l l o n i t e . Ukr. Khim. Zh., 4 1 : 354-356 T a r a s e v i c h , Yu.1. S i v a l o v , E . G . , 1977. S p e c t r a l s t u d y of t h e e n v i r o n m e n t of i o n s i n k a o l i n i t e . Uks. Khim. Zh., 43: 367370.

3+

189 T a r a s e v i c h , Y u . 1 . and F e d o r o v a , V.P., 1 9 7 9 . E l e c t r o n i c s p e c t r a o f K o l l o i d Zh., a r y l c a r b i n o l s s o r b e d on a r g i l l a c e o u s m i n e r a l s . 4 1 : 812-814. T e l i c h k u n , V.P., T a r a s e v i c h , Yu.1. and Goncharuk, V . V . , 1 9 7 6 . E l e c t r o n s p e c t r a o f H a m m e t t d y e i n d i c a t o r s s o r b e d by m o n t m o r i l l o n i t e . T e o r . E k s . Khim., 1 3 : 131-134. T e l i c h k u n , V.P. and T a r a s e v i c h , Y u . I . , 1977. E l e c t r o n i c s p e c t s a o f a r y l c a r b i n o l s a d s o r b e d b y k a o l i n i t e s . Teor. E k s . Khim., 14: 401-404. Theng, B . K . G . , 1 9 7 4 . The C h e m i s t r y o f C l a y - O r g a n i c R e a c t i o n s . A d a m H i l g e r , London, 343 p p . T h i e l m a n n , V . J . and M c A t e e , J . L . , J r . , 1975. G a s c h r o m a t o g r a p h i c b e h a v i o r o f metal-tris(ethy1enediamine)complex c a t i o n - e x c h a n g e d montmorillonites. J . C h r o m a t o g r a p h y , 105: 115-123. Thompson, F.D., 1977. C o l o r d e v e l o p i n g c o a t i n g c o m p o s i t i o n s c o n t a i n i n g r e a c t i v e p i g m e n t s p a r t i c u l a r l y f o r m a n i f o l d copy UP P a t e n t 4 , 0 2 2 , 7 3 5 . paper. T r a y n o r , M.F., M o r t l a n d , M.M. a n d P i n n a v a i a , T . J . , 1978. Ion e x c h a n g e and i n t e r s a l a t i o n r e a c t i o n s of h e c t o r i t e w i t h trisb i p y r i d y l metal c o m p l e x e s . C l a y s a n d C l a y Y i n . , 26: 318-326. Van Dame, H . , C r e s p i n , M . , Cruz4 M . I . and F r i i a t , J . J . , 1977. A d s o r p t i o n o f s a f r a n i n e b y N a , N i 2 + a n d Fey+ m o n t m o r i l l o n i t e s . C l a y s a n d C l a y Min., 25: 19-25. Van Dame, H . , C r e s p i n , M . , O b r e c h t , F . , C r u z , M . I . and F r i p i a t , J . J . , 1978. F c i d - b a s e a n d c o m p l e x a t i o n b e h a v i o r of p o r p h y r i n s on t h e i n t r a c r y s t a l s u r f a c e o f s w e l l i n g c l a y s : meso-tetrap h e n y l p o r p h y s i n a n d m e s o - t e t r a ( 4 - p y r i d y l ) p o r p h y r i n o n rnontmorJ . C o l l o i d I n t e r f a c e S c i . , 66: 43-54. illonites. Van d e P o e l , D . , 1975. F d s o r p t i o n d e C O I P ~ O S ~ Sa r o m a t i q u e s s u r l a montmorillonite saturge en Cu(I1). Universitd Catholique d e L o u v a i n , Ph.D. Van d e P o e l , D . , C l o o s , P . , H e l s e n , J . a n d J a n n i n i , E . , 1973. Comportement p a r t i c u l i e r d u b e n z e n e a d s o r b & s u r l a m o n t m o r i l l o n i t e cuivrique. B u l l . Groupe F r a n q a i s F r g i l e s , 25: 115-126. V e l g h e , F. , S c h o o n h e y d t , R.A. a n d U y t t e r h o e v e n , J . B . , 1977. The c o o r d i n a t i o n of h y d r a t e d C u ( I 1 ) - and N i (11)- i o n s on montmorillonite surface. C l a y s and C l a y Min., 25: 375-380. V e l g h e , F. , S c h o o n h e y d t , R.A. , U y t t e r h o e v e n , J . B . , P e i g n e u r , P. a n d 1977. S p e c t r o s c o p i c c h a r a c t e r i z a t i o n and t h e r m a l Lunsford, J . H . , s t a b i l i t y of c o p p e r ( I 1 ) e t h y l e n e d i a m i n e complexes on s o l i d surfaces. 2. Montmorillonite. J . P h y s . Chem., 81: 1187-1194. Yariv, S . , Lahav, N . a n d L a c h e r , M . , 1 9 7 6 . On t h e mechanism of s t a i n i n g m o n t m o r i l l o n i t e by b e n z i d i n e . C l a y s and C l a y M i n . , 2 4 : 51-52.

191 Chapter 8

APPLICATION OF FAR INFRARED SPECTROSCOPY TO THE STUDY OF CLAY MINERALS AND ZEOLITES

Jose J . FRIPIAT C.N.R.S. - Centre de Recherche s u r l e s S o l i d e s a O r g a n i s a t i o n C r i s t a l l i n e I m p a r f a i t e , l b , Rue de l a F 6 r o l l e r i e , 45045 Orleans Cedex, France.

8.1

INTRODUCTION I n f r a r e d spectroscopy i n t h e range o f 250-4000 wave number r e p r e s e n t s an e s t a -

b l i s h e d t o o l w i d e l y used by c l a y s c i e n t i s t s f o r t h e s t u d y o f t h e m i n e r a l framework and o f molecules adsorbed on i t s s u r f a c e . I n 1974, V.C. sponsorship o f t h e B r i t i s h

Farmer e d i t e d , under t h e

M i n e r a l o g i c a l Soci t y , an e x h a u s t i v e r e v i e w on these

subjects. However, as i t w i l l be shown l a t e r below 250 cm-’

f o r v a r ous reasons, t h e s p e c t r a l domain

has n o t r e c e i v e d much a t t e n t i o n

The aim o f t h i s r e v i e w i s t o s t i -

m u l a t e t h e i n t e r e s t o f c l a y s c i e n t i s t s f o r f a r i n f r a r e d spectroscopy. As compared t o t h e o t h e r t o p i c s t r e a t e d i n t h i s book where numerous r e c e n t papers a r e c r i t i c a l l y reviewed t h e p r e s e n t c h a p t e r aims : ( i ) t o d e s c r i b e b r i e f l y t h e p r i n c i p l e o f t h e i n t e r f e r o m e t e r used t o r e c o r d s p e c t r a i n t h e range o f 10-300 cml; ( i i ) t o summarize t h e t h e o r e t i c a l background which p e r m i t s t o o b t a i n i n f o r m a t i o n f r o m these s p e c t r a and f i n a l l y ( i i i ) t o r e v i e w papers d e a l i n g w i t h c l a y m i n e r a l s and z e o l i t e s t h a t c o n t a i n e x p e r i m e n t a l r e s u l t s which i l l u s t r a t e t h e u s e f u l n e s s o f s p e c t r a l s t u d i e s i n the f a r i n f r a r e d region. The r a p i d p r o g r e s s o f f a r i n f r a r e d s p e c t r o s c o p y found i t s o r i g i n i n t h e a v a i l a b i l i t y o f F o u r i e r t r a n s f o r m s p e c t r o m e t e r s which became p o s s i b l e because o f t h e r e d u c t i o n o f t h e c o s t o f f a s t e l e c t r o n i c minicomputers w i t h l a r g e s t o r a g e c a p a c i t y . I t i s o n l y about t e n y e a r s t h a t t h i s new i n s t r u m e n t a t i o n has been c o m m e r c i a l l y

a v a i l a b l e and t h a t t h e s o l i d s t a t e p h y s i c i s t s and chemists have t a k e n advantage o f t h i s new r e s e a r c h t o o l

.

A r e f e r e n c e book on t h e F o u r i e r t r a n s f o r m s p e c t r o s c o p y has been p u b l i s h e d by B e l l (1972). As i t w i l l be shown l a t e r , t h e F o u r i e r t r a n s f o r m s p e c t r o s c o p y has s e v e r a l i n s t r u m e n t a l advantages o v e r t h e c l a s s i c a l I R s p e c t r o s c o p y which uses d i s p e r s i v e ( p r i s m s o r g r a t i n g s ) i n s t r u m e n t s . F o r c l a y s s c i e n t i s t s , t h e main s c i e n t i f i c i n t e r e s t i s i n t h e s t u d y o f v i b r a t i o n s o f exchangeable c a t i o n s i n t h e i n t e r l a y e r space o r i n t h e z e o l i t e cages and o f some l a t t i c e v i b r a t i o n s o c c u r i n g below 300 cm-l. I f t h e t i m e used f o r r e c o r d i n g s p e c t r a becomes c r u c i a l t h e n t h e use o f F o u r i e r

192 t r a n s f o r m i n t e r f e r o m e t e r a l s o o f f e r s t h e advantage o f b e i n g a v e r y f a s t t e c h n i q u e

if t h e r e g i o n between, say,400 t o 1500 cm-l, shows v i b r a t i o n a l bands which p e r m i t i d e n t i f i c a t i o n o f the mineral.

8.2

FOURIER TRANSFORM SCANNING INTERFEROMETRY The e q u i v a l e n c e o f s p e c t r a l i n f o r m a t i o n i n t h e t i m e domain and i n t h e f r e q u e n c y

domain r e p r e s e n t s t h e b a s i c p r i n c i p l e o f t h e F o u r i e r t r a n s f o r m I R s p e c t r o s c o p y . The i n t e r f e r o m e t e r encodes t h e s p e c t r a l i n f o r m a t i o n i n t h e t i m e domain and by F o u r i e r t r a n s f o r m a t i o n o f t h e i n t e r f e r o g r a m , one o b t a i n s t h e c o n v e n t i o n a l I R spect r u m i n t h e frequency domain.

Fig.8.1.

Schematic r e p r e s e n t a t i o n o f an i n t e r f e r o m e t e r w o r k i n g b y t r a n s m i s s i o n

The o p t i c a l p a r t o f a modern i n s t r u m e n t i s shown i n F i g . 8.1. The i n f r a r e d beam i s focused on t h e p l a n e o f a semi t r a n s p a r e n t m i r r o r c a l l e d a beam s p l i t t e r ( 6 s ) .

A f r a c t i o n o f t h e beam i s r e f l e c t e d on m i r r o r My, whereas a f r a c t i o n i s r e f l e c t e d by m i r r o r M2. From m i r r o r s M1 and Me t h e beams a r e r e f l e c t e d by a d o u b l e m i r r o r MM o s c i l l a t i n g on an a i r cushion. T h i s m o t i o n generates a d i f f e r e n c e i n t h e o p t i c a l p a t h o f t h e two beams w h i c h i n t e r f e r e s on t h e beam s p l i t t e r . About 50% o f t h e energy source reaches m i r r o r M3. L e t 6 be t h e d i f f e r e n c e i n t h e o p t i c a l pathway o f t h e two beams f o r a g i v e n p o s i t i o n of t h e m i r r o r and w be t h e f r e q u e n c y o f t h e photon. The s p e c t r a l s i g n a l a f t e r i n t e r f e r e n c e i s given by :

193

S ( W , ~ ) = A2(w)

[l

+

cos ( 2

IT w

6)l

(1)

A b e i n g t h e a m p l i t u d e o f two c o h e r e n t waves w i t h t h e phase d i f f e r e n c e 2

IT w 6.

Since MM o s c i l l a t e s a t c o n s t a n t speed v, 6 = 2 v t and t h e o p t i c a l i n t e n s i t y r e a c h i n g m i r r o r M3 i s

I

I(t) =

W=m

L)) =m

A2(.)

dw

+

cos(4

A2(.)

IT w

v t ) dw

0

W=O

The i n t e r f e r o g r a m c o n t a i n i n g a l l s p e c t r a l components i s w =m

F(t) =

J

A2(w)

cos(4

II w

v t ) dw

(3)

0

and t h e F o u r i e r t r a n s f o r m e d o f F ( t ) i s t h e s p e c t r a l d i s t r i b u t i o n o f t h e i n t e n s i t y , S(W)

L/2v F(v) exp(- i 4

IT w

v t) dt

(4)

-L/2v where L i s t h e maximum d i s p l a c e m e n t o f t h e m i r r o r . L/2v i s t h e scanning t i m e . Thus F ( v ) r e p r e s e n t s t h e I R spectrum i n t h e t i m e domain whereas S ( W ) corresponds t o t h e I R spectrum i n t h e f r e q u e n c y domain. I n summary, because o f t h e sequence o f c o n s t r u c t i v e and d e s t r u c t i v e i n t e r f e rences, any o p t i c a l energy a t a g i v e n wavelength ( p h o t o n ) which e n t e r s t h e i n t e r f e r o m e t e r i s t r a n s f o r m e d i n t o a s i n u s o i d o f g i v e n p e r i o d and a m p l i t u d e . The sample i s t h u s i r r a d i a t e d b y a f l u x o f s i n u s o i d a l waves w h i c h c o n t a i n components w i t h f r e q u e n c i e s c o r r e s p o n d i n g t o c h a r a c t e r i s t i c v i b r a t i o n f r e q u e n c i e s o f t h e sample. The i n t e r f e r o g r a m r e a c h i n g t h e d e t e c t o r ( a p y r o e l e c t r i c d e v i c e ) c o n t a i n s t h e s p e c t r a l i n f o r m a t i o n on t h e v i b r a t i o n a l modes o f t h e sample encoded i n t h e t i m e domain. T h i s i n f o r m a t i o n i s r e s t i t u t e d as t h e c o n v e n t i o n a l f r e q u e n c y domain spectrum a f t e r F o u r i e r transformation. The i n f r a r e d sources a r e e i t h e r a mercury lamp e m i t t i n g between 10 and 300 cm-’ o r a G l o b a r f o r t h e 200-4000 cm-’

domain.

The n a t u r e o f t h e beam s p l i t t e r depends upon t h e i n v e s t i g a t e d s p e c t r a l domain. I t c o n s i s t s o f a germanium f i l m d e p o s i t e d o n t o a KBr t h i n s l a b between 400 and

4000 cm-1, a m e t a l l i c g r a t i n g between 50-2000 cm-l and f i l m s o f m y l a r between

10-700 cm-’.

The m o t i o n o f m i r r o r MM i s m o n i t o r e d by an a u x i l i a r y i n t e r f e r o m e t e r

194 using

a

He-Ne l a s e r whereas t h e change o f t h e sources, o f t h e a p e r t u r e s , o f

t h e beam s p l i t t e r s and o f t h e o p t i c a l f i l t e r s d r e c o n t r o l l e d by t h e computer. Thus a complete r e c o r d o f t h e s p e c t r a can be d i g i t a l l y c o n t r o l l e d and m o n i t o r e d by t h e computer. A F o u r i e r t r a n s f o r m s p e c t r o m e t e r has two c1o;ely

r e 1 a t e d advantages w i t h r e s p e c t

t o a c l a s s i c a l d i s p e r s i v e i n s t r u m e n t . The f i r s t advantage, t h e s o - c a l l e d m u l t i p l e x o r F e l l g e t advantage, i s r e l a t e d t o t h e way one r e c o r d s

an

I R spectrum i n a FT

i n t e r f e r o m e t e r . I n a c o n v e n t i o n a l d i s p e r s i v e i n s t r u m e n t one scans t h r o u g h t h e s p e c t r a l range o f i n t e r e s t and observes each s p e c t r a l r e g i o n f o r a v e r y l i m i t e d t i m e depending on t h e t o t a l t i m e o f r e c o r d i n g t h e spectrum. By c o n t r a s t , i n a FT i n t e r f e r o m e t e r one observes t h e whole s p e c t r a l range a l l t h e t i m e . O b v i o u s l y , one can r e p e a t t h e number o f s p e c t r a l scans, o f say 1 sec, and t h u s accumulate a l a r g e number o f s p e c t r a . T h i s r e s u l t s i n a m a j o r improvement i n t h e s i g n a l t o t o t h e ope-

n o i s e r a t i o when one compares t h e o p e r a t i o n o f a FT i n t e r f e r o m e t e r r a t i o n o f a conventional d i s p e r s i v e I R spectrometer.

The second advantage, t h e s o - c a l l e d t h r o u g h p u t o r J a c q u i n o t advantage i s r e l a t e d t o t h e amount o f energy a v a i l a b l e a t t h e d e t e c t o r o f t h e i n t e r f e r o m e t e r . The amount o f energy i s much h i g h e r a t comparable r e s o l u t i o n . I n a d i s p e r s i v e i n s t r u m e n t , i f x i s t h e s p e c t r a l domain d i s p e r s e d by t h e g r a t i n g and dx i s t h e dx output s l i t width, i s t h e amount o f energy a c t u a l l y a v a i l a b l e . I n t h e h i g h wavelength r e g i o n , t h e l o w amount o f energy p e r p h o t o n would r e q u i r e s l i t w i d t h s i n c o m p a t i b l e w i t h a r e a s o n a b l e r e s o l u t i o n whereas i n an i n t e r f e r o m e t e r as sketched i n F i g . 8.1, a b o u t 50% o f t h e source energy i s used w i t h o u t t h e o r e t i c a l 1 i m i t a t i o n o f t h e r e s o l u t i on. The l i m i t a t i o n o f t h e F o u r i e r t r a n s f o r m i n s t r u m e n t l a y s i n t h e h i g h frequency r e g i o n ( s a y 3000-4000 cm-1) because o f t h e v a s t amount o f s p e c t r a l i n f o r m a t i o n cont a i n e d i n t h e i n t e r f e r o g r a m . I n t h i s case a d i s p e r s i v e i n s t r u m e n t becomes more c o m p e t i t i v e , s p e c i a l l y i f a c c u r a t e band shapes a r e needed. As usual t h e two t e c h n i q u e s a r e t h u s complementary,but f o r i n v e s t i g a t i o n of v i b r a t i o n a l s p e c t r a between 300 and 10 cm-1, t h e F o u r i e r t r a n s f o r m i n t e r f e r o m e t e r i s unique. 8.3

SAMPLE PREPARATION

P.s i t c o u l d be expected f r o m u s u a l p r a c t i c e i n spectroscopy, t h e s i m p l e s t way t o use a F o u r i e r t r a n s f o r m s p e c t r o m e t e r i s by t r a n s m i s s i o n t h r o u g h t h e sample.

A v e r y i n t e r e s t i n g r e v i e w o f t h e use o f r e f l e c t a n c e i n t e r f e r o m e t r y w i l l be p u b l i s h e d by F. G e r v a i s ( 1 9 8 1 ) . W i t h c l a y m i n e r a l s , a u t o - c o h e r e n t f i l m s o b t a i n e d by s e d i m e n t a t i o n f r o m suspens i o n s , a s t h o s e m o s t l y used i n c l a s s i c a l I R s p e c t r o s c o p y s t u d i e s , a r e

perfectly

s u i t a b l e . Mica f l a k e s produced by c l e a v a g e can a l s o be used. Wafers made f r o m

195 f i n e l y ground powders can be e q u a l l y adequate. I t i s d i f f i c u l t t o e s t a b l i s h a l i m i t t o t h e absorbance c h a r a c t e r i s t i c s o f samp l e s which p e r m i t t o use a t r a n s m i s s i o n i n t e r f e r o m e t e r . T o t a l absorbance n o t exceed i n g lo2 cm-1 can be r e g a r d e d as an a p p r o x i m a t e e s t i m a t e . I f t h i n a u t o - c o h e r e n t f i l m s o r w a f e r s cannot be o b t a i n e d , p e l l e t s made by d i s -

p e r s i n g t h e f i n e l y ground s o l i d i n t o a p o l y e t h y l e n e powder and by c o l d p r e s s i n g a r e w e l l adapted t o r e c o r d s p e c t r a below 300 cm-l. The s e d i m e n t a t i o n o f t h e s o l i d p a r t i c l e s on p o l y e t h y l e n e f i l m u s u a l l y produces i n t e r f e r e n c e f r i n g e s which may o v e r l a p w i t h t h e a b s o r p t i o n bands.

I n summary,the v a r i o u s ways t o p r e p a r e samples f o r f a r i n f r a r e d s t u d i e s a r e n o t d i f f e r e n t f r o m t h o s e used i n t h e h i g h e r wavelength r e g i o n s , e x c e p t t h a t p e l l e t s a r e made f r o m p o l y e t h y l e n e . W i t h a u t o - c o h e r e n t f i l m s w i t h a p r e f e r r e d o r i e n t a t i o n o f m i c r o c r y s t a l s around one a x i s ( t h e C a x i s i n l a y e r l a t t i c e m i n e r a l s f o r example), d i c h r o i c e f f e c t s can be observed j u s t as i t i s sometimes t h e case i n t h e OH s t r e t c h i n g r e g i o n . As t h e sample compartment i n a F o u r i e r t r a n s f o r m s p e c t r o m e t e r i s c o n t i n u o u s l y purged by a stream o f d r y n i t r o g e n , and because o f t h e l o w p h o t o n i c energy,there a r e severe l i m i t a t i o n s f o r t h e use o f c e l l s and sample h o l d e r s . T h e r e f o r e t h e c o n t r o l o f t h e w a t e r c o n t e n t o f t h e f i l m cannot be achieved v e r y a c c u r a t e l y . It i s o n l y by i n c r e a s i n q t h e t e m p e r a t u r e o f t h e f u r n a c e ( F i g . 8.1) and by r e -

p r o d u c i n g s i m i l a r c o n d i t i o n s o u t s i d e t h e s p e c t r o m e t e r t h a t t h e sample h y d r a t i o n can be e v a l u a t e d . The use o f c e l l s f i t t e d w i t h windows i n w h i c h t h e sample can be p r e t r e a t e d and conditioned,reduces

so much t h e energy a v a i l a b l e , t h a t no d e v i c e s which

c o u l d meet t h e d e s i r a b l e r e q u i r e m e n t s have so f a r been d e s c r i b e d .

8.4

THEORETICAL ASPECTS. I n a system c o n t a i n i n g N atoms,the number o f i n t e r n a l normal modes o f v i b r a -

tions i s (3 N

-

6):

The group t h e o r y f o r a known symmetry o f t h e m a t e r i a l p r e d i c t s

t h e number o f o p t i c a l ( e i t h e r Raman o r I n f r a r e d ) a c t i v e modes. F o r m i c r o c r y s t a l s w i t h l a r g e u n i t c e l l s such as found i n c l a y m i n e r a l s and even i n z e o l i t e s , t h e d e s c r i p t i o n and,a eht , ihoi @ .Lr

c a l c u l a t i o n o f t h e frequen-

c i e s o f t h e s e modes i s p r a c t i c a l l y u n t r a c t a b l e and anyway f a r f r o m t h e scope o f t h i s paper. The p h y s i c a l r e q u i r e m e n t f o r t h e s e modes t o be i n f r a r e d a c t i v e i s t o produce a change o f t h e d i p o l e moment. The u s u a l p r a c t i c e f o r i n t e r p r e t i n g t h e i n f r a r e d s p e c t r a o f complex m a t e r i a l s c o n s i s t s o f t h e i s o l a t i o n o f m o l e c u l a r b u i l d i n g u n i t s t o which c h a r a c t e r i s t i c bands can be assigned. F o r example,onecan a s s i g n t h e i n f r a r e d a c t i v e v3 and v modes 4 o f t e t r a h e d r a w i t h T d symmetry o r t h e s t r e t c h i n g and bending v i b r a t i o n s o f t h e OH groups.

The i n t e r a c t i o n o f i n f r a r e d r a d i a t i o n w i t h c r y s t a l s and t h e r e l a t i o n s h i p s b e t -

196 ween symmetry and c r y s t a l v i b r a t i o n s have been c l e a r l y summarized i n Farmer's book (1974) by A. Hadni (pp. 27-49), by V.C.

Farmer and A.N.

Lazarev (pp. 51-67).

I n t h e same monograph Lazarev a l s o wrote an i n t r o d u c t o r y c h a p t e r t o t h e dynamics o f c r y s t a l l a t t i c e s , (pp. 69-85). The r e a d e r , i n t e r e s t e d i n t h e t h e o r e t i c a l aspects, would f i n d i n these references t h e i n f o r m a t i o n r p l e v a n t t o t h e issues discussed here. I n p a r t i c u l a r , Lazarev l i s t e d t h e p o s s i b l e e r r o r s o f t h e molecular a p p r o x i mation i n normal c o o r d i n a t e c a l c u l a t i o n s and t h e f o r c e constants e s t i m a t i o n s o f complex anions i n c r y s t a l s , as f o l l o w s : " ( i ) t h e use o f an i d e a l i z e d c o n f i g u r a t i o n o f t h e complex anion, ( i i ) t h e n e g l e c t o f t h e mechanical c o u p l i n g between low frequency i n t e r n a l deformations o f t h e complex anion and l a t t i c e v i b r a t i o n s and ( i i i ) t h e n e g l e c t o f d i r e c t mechanical c o u p l i n g o f t h e s t r e t c h i n g v i b r a t i o n s o f t h e complex anion w i t h l a t t i c e v i b r a t i o n s " . Items ( i i ) and ( i i i ) a r e o f course o f importance f o r a s s i g n i n g s p e c t r a f e a t u res i n t h e f a r i n f r a r e d domain, as i l l u s t r a t e d by t h e same a u t h o r who compared t h e c a l c u l a t e d wave numbers o f o p t i c a l v i b r a t i o n s o f Sc2 S i 2 O7 ( t h o r t w e i t i t e ) and those o f an i d e a l i z e d c r y s t a l composed o f Sc3+ and r i g i d Si207 groupings.

I n view o f t h e d i f f i c u l t y o f t h e t a s k o f a s s i g n i n g a p r i o r i t h e bands i n t h e f a r i n f r a r e d range,the method proposed by Lazarev (1972) and c a l l e d " t h e method o f quasi i s o t o p i c s u b s t i t u t i o n " i s probably t h e b e s t adapted t o t h e s i t u a t i o n we a r e d e a l i n g w i t h here. B a s i c a l l y t h i s method i s based on t h e use o f t h e s p e c t r a o f s o l i d s o l u t i o n s between compounds Mn Xm Op and t h e isomorphous compound Mn X ' n Op, X and X I , being elements o f t h e same group o f t h e p e r i o d i c system. I n s u b s t i t u t i n g X by X ' t h e frequency s h i f t s a r e governed p r i m a r i l y by t h e change o f masses. For instance, i n c l a y m i n e r a l s o r i n z e o l i t e s , t h e s u b s t i t u t i o n o f an exchangeable c a t i o n by another o f t h e same valency should produce t h i s type o f s p e c t r a l modifications without a f f e c t i n g strongly the l a t t i c e vibrations. This i s e x a c t l y t h e type o f i n f o r m a t i o n c l a y s c i e n t i s t s can l o o k f o r . I f a d e t a i l e d v i b r a t i o n a l a n a l y s i s was p o s s i b l e , o t h e r p h y s i c a l c h a r a c t e r i s t i c s c o u l d e v e n t u a l l y be computed. For example, t h e depth o f t h e p o t e n t i a l w e l l s i n which c a t i o n s a r e l o c a t e d c o u l d be evaluated and t h e n a t u r e o f t h e b i n d i n g f o r c e b e t ween t h e exchangeable c a t i o n s and t h e s u r f a c e c o u l d be e l u c i d a t e d . I t i s a l s o p o s s i b l e t h a t isomorphic s u b s t i t u t i o n s i n t h e t e t r a h e d r a l o r octa-

hedral u n i t s produce some c h a r a c t e r i s t i c bands i n t h e f a r i n f r a r e d domain. I f we consider, i n t h e l i m i t , isomorphic s u b s t i t u e n t s as i m p u r i t i e s , i n i o n i c c r y s t a l s , i m p u r i t y - i n d u c e d a b s o r p t i o n bandscould be more d i f f i c u l t t o d i s c o v e r because o f considerable a b s o r p t i o n a t low frequencies a t room temperature. The c o o l i n g o f t h e sample t o h e l i u m temperature should help,but i n t r a n s m i s s i o n spectroscopy t h i s i s n o t easy because o f i n s t r u m e n t a l l i m i t a t i o n s e x p l a i n e d above.

197 8.5

APPLICATION OF FAR INFRARED SPECTROSCOPY TO CLAY MINERALS I s h i i and coworkers (1969)

have been among t h e f i r s t researchers t o study

a b s o r p t i o n s p e c t r a o f n a t u r a l and s y n t h e t i c micas from 1300 t o 60 cm-l. The normal c o o r d i n a t e t r e a t m e n t was a p p l i e d t o some i d e a l i z e d s t r u c t u r e o f micas and Si205 l a y e r s . On t h e b a s i s o f frequency c a l c u l a t i o n s t h e observed a b s o r p t i o n bands were assigned t o Si205 and octahedral l a y e r s . The p o t e n t i a l f u n c t i o n U used by these authors i s expressed as

u

=

u.1 n t e r l ayer

where ' i n t r a l a y e r

+

(5)

'i n t r a l ayer concerns t h e A12(Si205)2(0H)2 o r Mg3(Si205)2(OH)2 l a y e r s

i s t h e i n t e r a c t i o n p o t e n t i a l between t h e p o s i t i v e K+ c a t i o n and 'interlayer and t h e neighbouring t w e l v e oxygen atoms. I n t h e r e g i o n o f i n t e r e s t here t h e Au and Bu i n f r a r e d a c t i v e modes should appear a t t h e f o l l o w i n g frequencies :

1 M dioctahedral

Au

211 cm-'

Au

178 cm-l 1 M t r i o c t a h e d r a l

Bu

190 cm-l 1 M d i o c t a h e d r a l

Bu

159 cm-l 1 M t r i o c t a h e d r a l

I f these c a l c u l a t e d values a r e compared t o t h e observed frequencies shown i n Table 8.1,

a reasonably good agreement i s found f o r p h l o g o p i t e whereas f o r t h e

o t h e r m i n e r a l s t h e frequency s h i f t s and t h e number o f bands do n o t p e r m i t r e l i a b l e assignments. Probably t h e symmetry o f t h e s i l i c a network i s t o o i d e a l i z e d s i n c e i t i s w e l l e s t a b l i s h e d t h a t t h e "hexagonal c a v i t y " i s i n f a c t d i t r i g o n a l . The l o w e s t frequency o f t h e v i b r a t i o n i n v o l v i n g an in-phase motion o f t h e octahedral A13+ and t h e a d j a c e n t oxygen sheet which accounts f o r t h e absorption i n t h e 200 cm-1 r e g i o n should a l s o be s t r o n g l y i n f l u e n c e d by d i s t o r t i o n o f t h e octahedral s h e l l . F - i n a l l y , t h e most i n t e r e s t i n g i n f o r m a t i o n i s t h a t g i v e n by t h e l o w e s t frequency band. According t o t h e model proposed by I s h i i e t a l . ,

the

modes o f v i b r a t i o n o f t h e potassium atom a r e those shown i n F i g . 8.2. I f t h e mica f l a k e s a r e o r i e n t e d p e r p e n d i c u l a r t o t h e beam, o n l y t h e v i b r a t i o n a l modes p a r a l l e l t o t h e a b p l a n e should appear. Indeed t h e p r e d i c t e d frequencies a r e in

t h e experimental range o f t h e l o w e s t frequency band, t h e i n t e r a c t i o n force

constant b e i n g somewhat s m a l l e r f o r p h l o g o p i t e than f o r muscovite. According t o Farmer (Farmer, 1974, p.358),

t h e corresponding o u t - o f - p l a n e v i b r a t i o n s i n these

micas l i e a t 144 cm-1 and 154 cm-l, r e s p e c t i v e l y . These modes should grow i n i n t e n s i t y by t i l t i n g t h e f l a k e around t h e C a x i s . F a r i n f r a r e d spectroscopy has been proposed as a "simple" method f o r d i f f e r e n t i a t i n g v a r i o u s micas and c l a y m i n e r a l s by S. Larson e t a l . (1972). I t i s i n t e r e s t i n g t o p o i n t o u t t h a t these authors have used c l a y s molten i n petroleum j e l l y supported on a p o l y e t h y l e n e f i l m . Again t h e s p e c t r a f o r micas were o b t a i n e d w i t h

198 t h e beam p e r p e n d i c u l a r t o t h e t h i n f l a k e s . TABLE 8.1 Frequencies (cm-l) observed by I s h i i e t a l . (1969) f o r v a r i o u s l a y e r l a t t i c e minerals

.

Trioctahedral P o l y l it h i o n i t e (synthetic)

200(sh),

170 ( m ) ,

Lepidol it e (Czechoslovakia) L i fluorphlogopite (synthetic) Phlogopi t e (Japan) Talc

100 (m)

120 ( s h ) 120 (m)

95

(5)

160 ( s ) ,

150 (m)

105 ( s )

170 (sh),

155 ( s )

80 ( s )

260 ( n ) ,

230 (sh),

180 (w),

150 (m)

263*(s)

192*(m)

168*(m)

143*(m)

205 ( s )

160

215 (m),

-

Dioctahedral Muscovite Pyrophi 11it e

*

108 ( s ) 110 (5)

(5)

Frequencies r e p o r t e d by V.C. Farmer (Farmer, 1974, p. 351)

00 108cm-l

1

95cm-

00 0 fi

Fig. 8.2. Modes o f v i b r a t i o n o f t h e potassium atom between t h e s i l i c a t : : o f muscovite. C a l c u l a t e d frequencies a r e shown ( I s h i i e t a1 . ) .

layers

199 H a l l o y s i t e , k a o l i n i t e and d i c k i t e show p a i r s o f bands o f d i f f e r e n t shapes i n t h e 250 cm-l and 330 cm-I r e g i o n b u t i n absence o f an assignment and because o f considerable s c a t t e r i n g l o s s , i t i s n o t c e r t a i n t h a t these f e a t u r e s a r e a c t u a l l y representative o f characteristic vibrations. For muscovite, Larson e t a l . d e t e c t e d two i n t e n s e bands a t 105 and 161 cm-’, r e s p e c t i v e l y . The f i r s t one corresponds o b v i o u s l y t o t h e K+ i n t e r l a y e r v i b r a t i o n whereas t h e second one i s a t t h e c e n t e r o f t h e broad band w i t h 3 peaks observed by Farmer (see Table 1). I n l e p i d o l i t e t h e K+ v i b r a t i o n band s h i f t s t o 100 cm-l and i n b i o t i t e i t i s observed a t . 8 3 cm-’.

For t h e l a t t e r a broad band i s a l s o observed a t 142 cm-l.

I n t h e s t u d i e s summarized so f a r , t h e i n t e r l a y e r c a t i o n was always potassium. The p o s s i b i l i t y o f g e t t i n g auto-coherent f i l m s o f c a t i o n exchanged smectites off e r s an i n t e r e s t i n g challenge f o r a s s i g n i n g bands t o t h e i n t e r l a y e r c a t i o n v i b r a t i o n s . A f i r s t attempt i n t h a t d i r e c t i o n was made i n o u r l a b o r a t o r y by B. Chourabi (1980). Ammonium, Potassium, Cesium and Baryum Black Jack Mine B e i d e l l i t e and Camp Berteau M o n t m o r i l l o n i t e gave i d e n t i c a l bands a t t h e f o l l o w i n g frequencies. TABLE 8.2 I n t e r l a y e r c a t i o n frequencies (cm-l) observed f o r t h e Black Jack Mine B e i d e l l i t e and Camp Berteau M o n t m o r i l l o n i t e .

N H ~ 150

K+ 100

and

cs+

Ba2’

55

80

85

These bands were-observed w i t h t h e f i l m p e r p e n d i c u l a r t o t h e beam and they should r e p r e s e n t t h e r e f o r e t h e i n - p l a n e v i b r a t i o n o f t h e c a t i o n i f t h e d i s o r i e n t a t i o n degree o f t h e m i c r o c r y s t a l s i n t h e f i l m i s n o t t o o l a r g e . When t h e frequencies o f Table 8.2 were p l o t t e d a g a i n s t c a t i o n valency and M i t s

where Z i s t h e

molecular mass,an almost p e r f e c t s t r a i g h t l i n e passing

through t h e o r i g i n was o b t a i n e d i f t h e 55 cm-l frequency band observed f o r t h e Cs samples was o n l y considered ( F i g . 8.3). The t h e o r e t i c a l s i g n i f i c a n c e o f t h i s s t r a i g h t l i n e w i l l be discussed l a t e r . I n a d d i t i o n t o t h e c a t i o n v i b r a t i o n band, another band was observed f o r b o t h m i n e r a l s a t about 200 cm-l. T h i s band i s s i m i l a r t o t h a t observed by I s h i i f o r p y r o p h y l l i t e and thus t h e assignment proposed by I s h i i can be t h e same f o r d i o c t a h e d r a l smectites. As shown f o r t h e a l c a l i exchanged Black Jack Mine samples i n Fig. 8.4,

an a d d i t i o n a l band appears a t 255

cm-l. T h i s band i s absent i n t h e Camp Berteau M o n t m o r i l l o n i t e b u t i t i s observed very d i s t i n c t l y i n Muscovite. Since t h e l a t t i c e charge i n B e i d e l l i t e o r i g i n a t e s

F i g . 8.3. I n - p l a n e i n t e r l a y e r c a t i o n f r e q u e n c y observed f o r d i o c t a h e d r a l smect i t e s B l a c k Jack Mine b e i d e l l i t e and Camp B e r t e a u m o n t m o r i l l o n i t e E. (Chourabi, 1980). f r o m S i 4 + , A13+ s u b s t i t u t i o n s i n t h e t e t r a h e d r a l l a y e r (as i n m u s c o v i t e ) , t h i s mode c o u l d be r e l a t e d t o some p e r t u r b a t i o n i n t h e m o t i o n o f o c t a h e d r a l A13+ w i t h r e s p e c t t o t h e t e t r a h e d r a l oxygens i n f l u e n c e d by t h e s e s u b s t i t u t i o n s . A t t h e 6 t h I n t e r n a t i o n a l C l a y Conference i n O x f o r d , C. Roth (1978) r e p o r t e d f o r a CS'

s m e c t i t e a d o u b l e t s i m i l a r t o t h a t shown i n F i g . 8.4. These p r e l i m i n a r y o b s e r v a t i o n s prompted A l c o v e r , Gatineau and G e r v a i s (1981)

t o pursue a more e x t e n s i v e s t u d y o f c a t i o n exchanged v e r m i c u l i t e s . L l a n o vermic u l i t e was chosen f o r t h a t work because o f t h e p o s s i b i l i t y o f g e t t i n g good a u t o c o h e r e n t f i l m s i n which t h e m i c r o c r y s t a l s have a r a t h e r h i g h degree o f o r i e n t a t i o n . I n a d d i t i o n , t h e s p e c t r a were r e c o r d e d w i t h t h e beam e i t h e r p e r p e n d i c u l a r o r t i l t e d by 45°C w i t h r e s p e c t t o t h e C c r y s t a l a x i s . The r e s u l t s f o r d e h y d r a t e d samples ( h e a t e d t o 2 5 O O C ) a r e shown i n T a b l e 8.3.

I n a d d i t i o n , a s t r o n g band a t

160 cml i s observed i n a l l cases as i n p h l o g o p i t e ( s e e T a b l e 8.1). As f a r as t h e v i b r a t i o n s o f t h e i n t e r l a y e r c a t i o n a r e concerned, t h e r e i s a s h a r p d i s c o n t i n u i t y i n t h e s p e c t r a l f e a t u r e s between c a t i o n s w h i c h can w i g g l e i n t h e d i t r i g o n a l c a v i ties, the

r a d i u s o f which i s a b o u t 1.2

w,

and t h o s e f i t t i n g c l o s e l y i n t o t h e

a v a i l a b l e space. F o r t h e l a t t e r t h e i n - p l a n e v i b r a t i o n band can be s e p a r a t e d f r o m

201

F i g . 8.4. F a r i n f r a r e d t r a n s m i s s i o n s p e c t r a observed f o r t h e a l c a l i exchanged B l a c k Jack Mine b e i d e l l i t e (Chourabi, 1980). t h e o u t - o f - p l a n e v i b r a t i o n band by o b s e r v i n g t h e dichro'ic e f f e c t . One s h o u l d p o i n t o u t t h a t f o r t h e Cs'

sample t h e o u t - o f - p l a n e band i s a t about t h e same f r e -

quencyas t h e h i g h e r f r e q u e n c y band observed f o r s m e c t i t e s ( T a b l e 8.2). F o r c a t i o n s w i t h r a d i i s m a l l e r t h a n 1.2

150 cm-'

1, no

d i c h r o i c e f f e c t i s a c t u a l l y n o t i c e a b l e below

: one s i n g l e s t r u c t u r e l e s s b r o a d band between 50 and 90 cm-I appears

f o r a l l o f them. P r o b a b l y t h e s e c a t i o n s have s e v e r a l e q u i l i b r i u m p o s i t i o n s i n t h e d i t r i g o n a l c a v i t i e s and t h e r e i s a m i x t u r e o f o v e r l a p p i n g m o d e s w h i c h g i v e r i s e t o t h e broad adsorption. F i g . 8.5,which in-plane

(V

N

m.W i t h i n

s h o u l d be compared w i t h F i g . 8.3, shows t h e v a r i a t i o n o f t h e

) and o u t - o f - p l a n e (u,)

t h e l i m i t s o f t h e e x p e r i m e n t a l u n c e r t a i n t y as shown by v e r t i c a l

bars, t h e v a r i a t i o n o f ses w i t h

v i b r a t i o n frequencies w i t h respect t o

V,

i s l i n e a r whereas u

m.C o n s i d e r i n g

N

e x h i b i t s a c u r v a t u r e which i n c r e a -

t h a t an anharmonic o s c i l l a t o r can be d e s c r i b e d by a

Morse f u n c t i o n

U = A [1 - exp (TI x)12

(6)

202

- - - - II- - -

0.05

-1-

I

---

0.10

0.15

F i g . 8.5. I n - p l a n e v , and o u t - o f - p l a n e v c a t i o n v i b r a t i o n f r e q u e n c i e s o b s e r ved f o r L l a n o v e r m i c u f i t e ( A l c o v e r e t a l . 1481). TABLE 8.3 I n t e r l a y e r c a t i o n v i b r a t i o n frequencies observed by A l c o v e r e t a l . (1981) f o r c a t i o n exchanged L l a n o v e r m i c u l i t e s . Cation CS+

Rb+ K+ Ba2+ Sr2+

Ca2+ Nat Lit Mg2+

NB

*

ionicoradius (A)

1.65 1.47 1.33 1.34 1.15

In-plane f r e quency (cm-1)

59 66 80 68 80

O u t - o f - p l ane f r e q u e n c y (cm-1)

80 105 145 105 110

*

Broad band between 50-90 cm-1

0.68 0.66

no d i c h r o i c e f f e c t o b s e r v a b l e e x c e p t i n t h e 160-200 cm-l r e g i o n .

: u n c e r t a i n because o f o v e r l a p p i n g w i t h t h e 160 cm-l l a t t i c e band.

1

203

where

q

i s f u n c t i o n o f t h e c u r v a t u r e o f U a t t h e minimum, A i s t h e energy r e q u i -

r e d t o b r i n g t h e c a t i o n f r o m i t s e q u i l i b r i u m p o s i t i o n t o i n f i n i t y and x t h e v i b r a t i o n a m p l i t u d e , t h e v i b r a t i o n f r e q u e n c y can be c a l c u l a t e d as i n d i c a t e d by Landau e t a l . ( 1 9 6 6 ) . I t f o l l o w s

I f A = Z $ e, where Ze i s t h e c a t i o n charge and J, t h e e l e c t r i c a l p o t e n t i a l a t t h e c a t i o n s i t e , v i s a t f i r s t approximation a l i n e a r f u n c t i o n o f v a t i o n t h a t 'vl

>

v//

m.The

obser-

means t h a t t h e c u r v a t u r e o f t h e p o t e n t i a l energy f u n c t i o n i s

more pronounced f o r t h e p e r p e n d i c u l a r t h a n f o r t h e p a r a l l e l modes. T h i s may be expected i f e q u i p o t e n t i a l s u r f a c e s a r e normal t o t h e C a x i s . A c c o r d i n g t o equat i o n ( 7 ) t h e anharmonicity f o r the v v

I

v/,

//

mode s h o u l d be h i g h e r t h a n t h a t f o r t h e

mode i n v e r m i c u l i t e , w h e r e a s t h e a n h a r m o n i c i t y f a c t o r s h o u l d be l e s s f o r t h e i n d i o c t a h e d r a l s m e c t i t e s w i t h a l o w e r l a t t i c e charge. These f i n d i n g s d e s e r v e - f u r t h e r d e t a i l e d s t u d i e s . The p r o g r e s s i v e h y d r a t i o n o f

c a t i o n exchanged v e r m i c u l i t e has no deep e f f e c t on t h e f r e q u e n c i e s r e p o r t e d i n T a b l e 8 . 3 . A t a f i r s t g l a n c e t h i s i s unexpected b u t i n f a c t t h e f r e q u e n c y o f t h e d i f f u s i o n a l jumps,which i s i n c r e a s e d by t h e h y d r a t i o n , i s s t i l l s e v e r a l o r d e r s o f magnitude l e s s t h a n t h a t o f t h e v i b r a t i o n . However t h i s e f f e c t o f h y d r a t i o n s h o u l d be c a r e f u l l y studied,as

t h e d a t a o b t a i n e d f o r z e o l i t e s w i l l show.

The i n t e r e s t i n g o b s e r v a t i o n o f a d o u b l e t a t 55 and 85 cm-l f o r t h e Cs'

smecti-

t e s and o f a s i n g l e band f o r Cs+ v e r m i c u l i t e s ( t h e Benhavis v e r m i c u l i t e behaves s i m i l a r l y as t h e L l a n o sample) may be t e n t a t i v e l y e x p l a i n e d as f o l l o w s . Since t h e o u t - o f - p l a n e v i b r a t i o n i n v e r m i c u l i t e appears a t a b o u t 80 cm-',

by assuming a

degree o f d i s o r i e n t a t i o n i n Cs+ s m e c t i t e s much h i g h e r t h a n i n t h e c o r r e s p o n d i n g vermiculites the p r o b a b i l i t y o f f i n d i n g a noticeable f r a c t i o n o f microcrystals t i l t e d w i t h r e s p e c t t o t h e normal t o t h e f i l m , c o u l d be h i g h enough i n s m e c t i t e s t o make t h e o u t - o f - p l a n e comparing S.E.M. e t al.,

v i b r a t i o n o b s e r v a b l e . T h i s h y p o t h e s i s i s supported by

photographs o f L i '

1974). The Li'

smectite

aggregates a r e observed f o r t h e Cs'

8.6

and Cs'

s a t u r a t e d Wyoming b e n t o n i t e ( F r i p i a t

i s formed by p l a t y aggregates,whereas

granular

sample.

APPLICATION OF FAR INFRARED SPECTROSCOPY TO ZEOLITES T h i s c h a p t e r has been i n t r o d u c e d f o r t h e f o l l o w i n g reasons. I n z e o l i t e s t h e

l a t t i c e charge b a l a n c i n g c a t i o n s a r e l o c a t e d on s i t e s w i t h d i f f e r e n t environments and t h e c a t i o n i c p o p u l a t i o n s o f t h e s e s i t e s depend upon t h e h y d r a t i o n degree. T h e r e f o r e , a s t u d y o f t h e charge b a l a n c i n g c a t i o n s v i b r a t i o n s i n z e o l i t e s p r o v i des i n f o r m a t i o n

on t h e e f f e c t o f environment. T h i s paragraph w i l l deal o n l y w i t h

t h e s e c a t i o n i c v i b r a t i o n bands.

204

I.A. B r o d s k i i e t a l . (1971) were t h e f i r s t t o s t u d y t h e a b s o r p t i o n s p e c t r a o f c a t i o n exchanged Li',

Na',

Rb',

Cs'

s y n t h e t i c X z e o l i t e s a f t e r v a r i o u s dehydra-

t i o n t r e a t m e n t s . The a b s o r p t i o n bands which

were i n s e n s i t i v e t o t h e n a t u r e of

t h e c a t i o n o r t o h y d r a t i o n were c o n s i d e r e d as l a t t i c e bands.

F i g . 8.6 P r o j e c t i o n o f t e t r a h e d r a l l y a r r a n g e d s o d a l i t e u n i t s and c a t i o n i c s i t e s i n X and Y m o l e c u l a r s i e v e s . The h i g h

requency bands were a s s i g n e d t o c a t i o n s i n t h e S I and S I 1 s i t e s ,

namely those l o c a t e d i n t h e six-membered r i n g s windows opening i n t o t h e supercage ( S i t e I ) and t h e s m a l l c a v i t y ( s i t e I ) , see F i g . 8.6. Those i n s i d e t h e two b r i d g i n g f o u r r i n g s ( S i t e 111) i n t e r a c t more weakly w i t h t h e framework t h a n t h e c a t i o n s i n t h e S I and S I1 s i t e s (whose b i n d i n g e n e r g i e s a r e a l m o s t i d e n t i c a l ) and c o n s e q u e n t l y t h e i r v i b r a t i o n bands a r e a t l o w e r frequency. L a t e r on B r o d s k i i e t a l . (1974) extended t h e i r s t u d y t o z e o l i t e s w i t h d i f f e r e n t S i / A 1 r a t i o s , c o v e r i n g t h e range i n c o m p o s i t i o n between t h e X and Y members

205 o f t h e n e a r - f a u j a s i t e f a m i l y . This r a t i o i n f l u e n c e s t h e c a t i o n i c p o p u l a t i o n s i n d i f f e r e n t s i t e s . F i r s t o f a l l , as t h e Si/A1 r a t i o increases, according t o c r y s t a l l o g r a p h i c s t u d i e s t h e S I 1 1 p o p u l a t i o n decreases. The v i b r a t i o n o f charge balanc i n g c a t i o n s i n E z e o l i t e ( a s y n t h e t i c z e o l i t e w i t h c h a b a z i t e s t r u c t u r e ) was s t u d i e d by t h e same group i n 1977 ( B r o d s k i i e t a l . ,

1977). F i n a l l y i n 1980, B r o d s k i i

e t a l . presented a t t h e z e o l i t e conference a summary o f t h e i r f i n d i n g s . They a r e summarized i n Table 8.4 and they w i l l be examined l a t e r on. P a r a l l e l s t u d i e s were c a r r i e d o u t by B u t l e r e t a l . (1977) f o r s y n t h e t i c X and Y z e o l i t e s . T h e i r r e s u l t s a r e summarized i n Table 8.5 f o r t h e dehydrated s t a t e .

TABLE 8.4 I n t e r i o n i c v i b r a t i o n s (cm-l) o f c a t i o n s i n t h e framework o f X, Y and E z e o l i t e s f o r d i f f e r e n t c r y s t a l l o g r a p h i c p o s i t i o n s (dehydrated s t a t e ) , a f t e r B r o d s k i i e t a l . (1980). Site X Na' K+

S i t e 11, E

S

I' and 11" Y

E

66

88

90

188 185 155 (sh) 160

54

90

98

160

135

135

55

57

65

Rb'

40

83

cs+

33

82

Ca2+

Y

I

155

105

Ba2+

275

195

140

84

Sr2+

200

55

150 110

107

I n general t h e assignments were made according t o these main c r i t e r i a :

1. S e n s i t i v i t y t o i o n i c exchange 2. Frequency s h i f t s obeying a fVy2 3. I n t e n s i t y depending upon t h e s i t e s p o p u l a t i o n , suggested by t h e c r y s t a l l o g r a phic studies. The s i t e 11, which i s t h e most h i g h l y populated i n d r y monovalent Y z e o l i t e , should g i v e r i s e t o t h e most i n t e n s e band. A s i m i l a r band a t s l i g h t y h i g h e r f r e quency i s expected i n t h e corresponding X z e o l i t e s due t o t h e i r h i g h e r l a t t i c e charge. Since t h e s t r e n g t h o f t h e t h r e e f o l d

bonding t o a s i x - r i n g a t s i t e I' i s near-

l y i n d i s t i n g u i s h a b l e f r o m t h a t a t s i t e 11, t h e two v i b r a t i o n a l bands must o v e r l a p

considerably, as suggested by B r o d s k i i

et

a l . However t h e use of p o t e n t i a l ener-

gy c a l c u l a t i o n f o r t h e band assignment must be used w i t h c a u t i o n because t h e s t e r i c s t r a i n s must a l s o be taken i n t o account. For instance, t h e s t r u c t u r a l s t u d i e s i n d i c a t e t h a t t h e cation-oxygen d i s t a n c e a t s i t e I i s g r e a t e r than

at

206 s i t e I 1 f o r Na'

Therefore, i n s p i t e o f t h e f a c t t h a t t h e p o t e n t i a l ener-

and K'.

gy c a l c u l a t i o n s i n d i c a t e t h a t s i t e I i s a t a h i g h e r energy than s i t e 11, t h e s i t e I frequency should l i e lower than t h e s i t e I 1 frequency.

TABLE 8.5 Assignment o f t h e c a t i o n v i b r a t i o n s i n X and Y dehydrated z e o l i t e s , according t o B u t l e r e t a l . (1977). Site I11 v(cm-1) Intensity

"(cm-1)

Site I Intensity

Li+Y

Site I 1 v(cm-1) Intensity 380

sh

Na'Y

167

m sh

180

S

K'Y

107

m sh

133

S

CS+Y

30

wsh

ACJ+Y

50

W

S

m

190

ms

Na'X

67

wm

K'X

58

m

156

S

Rb+X

48

wm

108

ms

cs+x

39

wm

86

in

227

m

189

m ms

150

rn

137

S

107

S

287

wm

273

m

160

256

For d i v a l e n t c a t i o n s ,

m sh

62 82

s i t e I i s t h e most s t a b l e i n d r y z e o l i t e s and t h e

assignments i n Table 8.5 o f t h e bands observed f o r Ca2+Y and Ba2'Y

agree w i t h

t h i s i d e a . I n d i v a l e n t c a t i o n z e o l i t e s t h e band assigned t o s i t e I 1 encompasses n o t o n l y t h e c a t i o n s on I ' b u t a l s o 1 1 ' . The p a r t i c u l a r i n t e r e s t o f t h e work o f B u t l e r e t a l . i s t h e i r o b s e r v a t i o n o f t h e e f f e c t o f adsorbed molecules on t h e c a t i o n frequencies. The charge d e l o c a l i s a t i o n r e s u l t i n g from c a t i o n s o l v a t i o n i s expected t o reduce t h e v i b r a t i o n a l f r e quencies f o r n o n - s t e r i c a l l y c o n s t r a i n e d c a t i o n s . Indeed w i t h K'Xand

K+Y t h e v i b r a t i o n bands assigned t o s i t e I 1 s h i f t upon hydra-

t i o n from 156 t o 122 cm-' and from 133 t o 114 cm-l, r e s p e c t i v e l y . The s i t e I 1 1 v i b r a t i o n s observed i n X z e o l i t e s s h i f t n e g l i g i b l y i n frequency as t h e h y d r a t i o n increases b u t t h e i n t e n s i t y drops very r a p i d l y . Far i n f r a r e d a b s o r p t i o n by e x t e r n a l v i b r a t i o n s o f sorbed water has been s t u d i e d by M o e l l e r e t a l . (1971) b u t t h i s s p e c i f i c s t u d y i s beyond t h e scope o f t h i s

207

review . B r o d s k i i e t a l . (1980) p o i n t e d o u t p e r t i n e n t l y t h a t t h e assumption,that

the

c a t i o n v i b r a t i o n s a r e so weakly c o u p l e d w i t h framework v i b r a t i o n s t h a t t h e y can be c o n s i d e r e d i n d e p e n d e n t l y , i s

n o t always j u s t i f i e d .

F o r i n s t a n c e , t h e f r e q u e n c y assigned by B u t l e r e t a l . ( T a b l e 8.5) t o L i + on s i t e I 1 o f Y z e o l i t e d o e s n ' t s h i f t i n Na z e o l i t e s p a r t i a l l y s u b s t i t u t e d by 6 L i and 7 L i + c a t i o n s and t h e r e f o r e , t h e 380 cm-l band i s p r o b a b l y a l a t t i c e mode p e r t u r b e d by t h e c a t i o n . C o u p l i n g w i t h framework v i b r a t i o n s may a l s o r i s e q u e s t i o n s about assignments f o r l i g h t d i v a l e n t c a t i o n s , suchs as Mg2+

I

2

Iv

200

--_1--L-_I,J

@m 0 Butler et al. 1 @ Brodskii etal. I - - 1--1-- 1 - - 1

I

- - -I I I I

-

-17

1

F i g . 8.7 C a t i o n i c v i b r a t i o n f r e q u e n c i e s assigned t o s i t e I 1 i n X and Y z e o l i t e s and t o s i t e I 1 1 i n X z e o l i t e p l o t t e d vs

m.

F o r s i t e s 11, 1 1 ' and I ' on t h e one hand,and f o r s i t e I 1 1 on t h e o t h e r hand, t h e r e i s a r e a s o n a b l e agreement between t h e d a t a o f B u t l e r e t a l . (1977) and o f

208 B r o d s k i i e t a l . (1980). I n a d d i t i o n t h e f r e q u e n c i e s observed f o r t h e s e bands obey p r e t t y w e l l l i n e a r r e l a t i o n s h i p s w i t h r e s p e c t t o 8.7.

m,as

shown i n F i g .

I t i s w o r t h w i l e t o p o i n t o u t t h a t t h e s l o p e o f t h e l i n e a r r e l a t i o n s h i p ob-

t a i n e d f o r t h e c a t i o n s i n s i t e I 1 i s a b o u t t h e same as t h a t f o r vI i n v e r m i c u l i t e s ( F i g . 8.5). The f r e q u e n c i e s observed f o r c a t i o n s i n s i t e I 1 1 a r e more s i m i l a r t o t h o s e observed f o r v , ~

.

The f r e q u e n c i e s g i v e n f o r s i t e

I in

X z e o l i t e by B r o d s k i i e t a l . and B u t l e r

e t a l . do n o t appear t o f o l l o w such a s i m p l e r e l a t i o n s h i p and i n a d d i t i o n b o t h s e r i e s o f measurements a r e i n disagreement. These d i s c r e p a n c i e s c o u l d be due t o i n t e r a c t i o n s w i t h l a t t i c e v i b r a t i o n s f o r t h o s e c a t i o n s which a r e i n s i d e t h e hexagonal p r i s m s , l i n k i n g cubooctahedra.

8.7

CONCLUSION

I n f o c u s i n g o u r a t t e n t i o n on t h e v i b r a t i o n a l motions o f t h e charge b a l a n c i n g c a t i o n s i n c l a y s and z e o l i t e s , i t appears c l e a r l y t h a t ( i ) t h e symmetry a t t h e c a t i o n s i t e , ( i i ) t h e l a t t i c e charge and ( i i i ) e n v i r o n m e n t a l c o n s t r a i n t s , i n f l u ence s t r o n g l y t h e f r e q u e n c i e s a t which t h e v i b r a t i o n a l bands appear. What t y p e o f p h y s i c a l i n f o r m a t i o n can be o b t a i n e d f r o m t h e knowledge o f t h e s e v i b r a t i o n frequencies ?

As a l r e a d y shown by e q u a t i o n s ( 6 ) and ( 7 ) , i n f o r m a t i o n on t h e p o t e n t i a l w e l l s i n which c a t i o n s a r e l o c a t e d c o u l d be o b t a i n e d f r o m t h e s e f r e q u e n c i e s . B u t l e r e t a l . (1977) have t r i e d t h a t t y p e o f c a l c u l a t i o n u s i n g an a n a l o g y w i t h t h e models of i o n i c t r a n s p o r t i n s u p e r i o n i c c o n d u c t o r s . By thermal e x c i t a t i o n , t h e c a t i o n can be b r o u g h t i n t o energy s t a t e s above an energy gap

E~

so t h a t t h e y propagate t h r o u g h t h e l a t t i c e w i t h a v e l o c i t y

a near f r e e p a t h

$,

V, and

t o a n o t h e r l o c a l i z e d c a t i o n i c s i t e . By r e l a t i n g t h e e q u a t i o n

f o r i o n i c c o n d u c t i v i t y t o a c o n v e n t i o n a l hopping model, c 0 i s equal t o t h e a c t i v a t i o n energy, E , r e q u i r e d t o produce an i o n i c hop.

E = 1/2 M v 2 a: a.

i s t h e h o p p i n g d i s t a n c e between an o c c u p i e d and a v a c a n t s i t e . The most p r o b a b l e hopping d i s t a n c e i n Y z e o l i t e i s t h a t between s i t e s I 1 and

111 : i t v a r i e s w i t h t h e s i z e ( a n d t h u s t h e mass M) o f t h e c a t i o n i n t h e same way as t h e f o r c e c o n s t a n t and t h u s i t t e n d s t o l o w e r E. U s i n g t h i s s i n g l e r e l a t i o n s h i p , B u t l e r e t a l . f o u n d a c t i v a t i o n e n e r g i e s w h i c h were i n r e a s o n a b l e agreement w i t h t h o s e o b t a i n e d by measurements o f i o n i c c o n d u c t i v i t i e s . I n a somewhat analogous way, C a l v e t (1972) has a t t e m p t e d t o c a l c u l a t e t h e frequency o f v i b r a t i o n o f a c a t i o n w i t h respect t o i n t e r l a y e r c l a y surfaces from t h e s e l f - d i f f u s i o n c o e f f i c i e n t measurements i n a d i r e c t i o n p a r a l l e l t o t h e a b

209 plane. F o r K',

Rb'

and Cs'

m o n t m o r i l l o n i t e s t h e f r e q u e n c i e s were 105, 54 and

44 cm-l r e s p e c t i v e l y . The K'

and Cs'

r e s u l t s a r e n o t f a r from t h e i n - p l a n e v i -

b r a t i o n frequencies r e p o r t e d i n Table 8.2.

However, o u r i n t u i t i o n t e l l s us t h a t

t h e expected c o r r e l a t i o n should be w i t h t h e o u t - o f - p l a n e v i b r a t i o n . There i s no doubt t h a t c a l c u l a t i o n s along these l i n e s should l e a d t o i m p o r t a n t i n f o r m a t i o n , e s p e c i a l l y i f t h e h y d r a t i o n e f f e c t s were c a r e f u l l y c o n t r o l l e d . The comparison made between t h e v i b r a t i o n f r e q u e n c i e s observed f o r vermicul it e s and z e o l i t e s i s i n t e r e s t i n g because i t r e v e a l s s i m i l a r i t i e s when c a t i o n s a r e v i b r a t i n g i n six-membered r i n g s e.g. f o r z e o l i t e

s i t e s I 1 and vI i n v e r m i c u l i -

t e s . Because: d i c h r o f c e f f e c t s cannot be observed f o r z e o l i t e s , t h e p o s s i b i l i t y o f a s s i g n i n g some o f t h e observed v i b r a t i o n frequencies t o modes p a r a l l e l t o planes c o n t a i n i n g r i n g s has n o t been taken i n t o c o n s i d e r a t i o n by t h e authors who have s t u d i e d t h a t t y p e o f m a t e r i a l s . T h i s q u e s t i o n should o b v i o u s l y deserve more a t tention. I n t h e f u t u r e t h e f a r i n f r a r e d s t u d i e s a p p l i e d t o z e d l i t e s and t o c l a y miner a l s should be d i r e c t e d towards improving o u r understanding o f t h e e f f e c t s of c a t i o n s o l v a t a t i o n and o r complexation, and on t h e l a t t i c e v i b r a t i o n a l frequencies

i n these m a t e r i a l s . REFERENCES Alcover, J.F., Gatineau, L. and Gervais, F., 1981. Far i n f r a r e d study o f t h e v i b r a t i o n s i n v e r m i c u l i t e , i n p r e p a r a t i o n . T h i s paper was presented as a p o s t e r a t t h e European Clay Conference i n Munich, Sept. 1980. B e l l , R.J., 1972. I n t r o d u c t o r y F o u r i e r t r a n s f o r m spectroscopy. Academic Press. B r o d s k i i , I . A . , Zhdanov, S.P. and Stanevich, A.E., 1971. Far i n f r a r e d spectra o f c a t i o n - s u b s t i t u t e d t y p e X z e o l i t e s ( f a u j a s i t e s ) and t h e i r changes on dehyd r a t i o n . Opt. & Spektrosk. (USSR) p.58-62. Opt. & Spectros. (USA) 30: 30-32. Zhdanov , S.P. and Stanevich, A.E., 1974. Spectroscopic i n v e s t i Brodskii, I.A., g a t i o n s o f i n t e r i o n i c v i b r a t i o n s i n s y n t h e t i c f a u j a s i t e - t y p e z e o l i t e s . Sov. Phys. S o l i d S t a t e , 15 : 1771-1772. B r o d s k i i , I . A . , Z h d a y v , S.P., Krosavtseva and Samulevich, N.N., 1977. Longwavelength i n f r a r e d s p e c t r a o f c a t i o n - s u b s t i t u t e d c h a b a z i t e type z e o l i t e s . Sov. Phys. S o l i d S t a t e , 19: 549-550. B r o d s k i i , I . A . and Zhdanov, S.P., 1980. A p p l i c a t i o n o f f a r i n f r a r e d spectroscopy f o r a s t u d y of c a t i o n p o s i t i o n s i n z e o l i t e s . Proc. I n t . Conf. Z e o l i t e s , 5th: 234-241. B u t l e r , Wayne, M., A n g e l l , Charles, L., M c A l l i s t e r , Warren and Risen, W i l l i a m M. Jr., 1977. F a r i n f r a r e d study o f c a t i o n motion i n d r y and s o l v a t e d mono and d i v a l e n t c a t i o n c o n t a i n i n g z e o l i t e s X and Y . J . Phys. Chem., 81: 2061-2068. Calvet, R . , 1972. H y d r a t a t i o n de l a m o n t m o r i l l o n i t e e t d i f f u s i o n des i o n s compensateurs. These de d o c t o r a t , F a c u l t e des Sciences de P a r i s . Chourabi, B., 1980. C o n t r i b u t i o n a l ' e t u d e de l a s t r u c t u r e f i n e des p h y l l o s i l i cates 2 : l p a r spectroscopie I.R. 3 r d Cycle Thesis, C.R.S.O.C.I., CNRS, 45045 Or1 eans Cedex, France. Farmer, V.C., 1974. The i n f r a r e d s p e c t r a o f m i n e r a l s . M i n e r a l o g i c a l S o c i e t y Monograph 4, The M i n e r a l o g i c a l S o c i e t y , London; F r i p i a t , J . J . , Cruz, M . I . , Bohor, B.F. and Thomas, J . , 1974. I n t e r l a m e l l a r adsorpt i o n o f carbon d i o x i d e by smectites. Clays and Clay M i n e r a l s , 22: 22-30. Gervais, F., 1981. High temperature i n f r a r e d r e f l e c t i v i t y spectroscopy by scann i n g i n t e r f e r o m e t r y . I n f r a r e d and M i l l i m e t e r Waves Series, V I I , Academic Press,

210 i n press. I s h i i , M., Nakahira, M. and Takeda, H., 1969. F a r i n f r a r e d a b s o r p t i o n s p e c t r a o f micas. Proc. I n t . Clay Conf., 1969, Tokyo ( L . H e l l e r , Ed.), 1: 247-259. I s r a e l U n i v . Press, Jerusalem. Landau, L. and L i f c h i t z , E., 1967 . Mecanique q u a n t i q u e . T h @ o r i e non r e l a t i v i s t e . DeuxiPme E d i t i o n , E d i t i o n M . I . R . Moscow. Larson, S y l v i a , J., Pardoe, G.W.F., Gebbie, H,A. and Larson, E.E., 1972. Use o f f a r i n f r a r e d i n t e r f e r o m e t r i c s p e c t r o s c o p y f o r m i n e r a l i d e n t i f i c a t i o n . Amer. M i n e r a l , 57: 998-1002. Lazarev, A.N., 1972. V i b r a t i o n a l s p e c t r a and s t r u c t u r e o f s i l i c a t e s . Plenum Press, New York. M o e l l e r , K., Kunath, D. and Spangenberg, H.J., 1971. F a r i n f r a r e d a b s o r p t i o n by e x t e r n a l v i b r a t i o n o f sorbed w a t e r on t y p e A z e o l i t e s . Spectrochim. Acta, P a r t A , 27: 353-355. Roth, C . , 1978. E f f e c t s o f exchangeable c a t i o n s on t h e f a r i n f r a r e d s p e c t r a o f c l a y m i n e r a l s . P o s t e r session, Book o f summaries o f t h e 6 t h I n t e r n a t i o n a l Clay Conference, Oxford, p. 255.

211 Chapter 9

E.S.C.A.

STUDIES OF CLAY MINERALS

Paul CANESSON U n i v e r s i t i . de P o i t i e r s , L a b o r a t o i r e de Chimie X I , 40 Avenue du Recteur Pineau, 86022 - P o i t i e r s - France. INTRODUCTION

9.1

Since t h e f i r s t r e v i e w p u b l i s h e d b y K. Siegbahn e t a l . (1967) summing up t h e main r e s u l t s o b t a i n e d i n Uppsala by X-ray P h o t o e l e c t r o n Spectroscopy (X.P.S.)

a

l o t o f l a b o r a t o r i e s have been concerned w i t h t h i s t e c h n i q u e . The main f e a t u r e s of t h i s new t o o l r e s t i n i t s v e r y h i g h s e l e c t i v i t y f o r t h e s u r f a c e o f t h e sample t o be a n a l y s e d and i n i t s a b i l i t y t o d e t e c t a l l t h e elements except hydrogen. T h i s l a s t c a p a c i t y has a l l o w e d K. Siegbahn t o c a l l t h i s t e c h n i q u e E l e c t r o n Spect r o s c o p y f o r Chemical A n a l y s i s (ESCA)

.

I n a d d i t i o n t o t h e v a l e n c e e l e c t r o n s , each atom i n a compound has c o r e e l e c t r o n s t h a t a r e n o t i n v o l v e d i n t h e b o n d i n g ( e x c e p t hydrogen). The e n e r g i e s o f t h e s e e l e c t r o n s a r e c h a r a c t e r i s t i c o f t h e i n d i v i d u a l atoms; y e t , t h e y a r e a l s o r e s p o n s i v e t o changes i n t h e e l e c t r o n i c environment and t h e y may be used t o p r o v i d e i n f o r m a t i o n on bonding. S i n c e so much r e l e v a n t i n f o r m a t i o n seems t o be d i r e c t l y a v a i l a b l e f r o m t h e e l e c t r o n energy d i s t r i b u t i o n , i t i s n o t s u r p r i s i n g t h a t d i r e c t o b s e r v a t i o n o f e l e c t r o n energy l e v e l s by e l e c t r o n s p e c t r o s c o p y s h o u l d s t a n d o u t a s t h e most p o w e r f u l t o o l t o answer t h e q u e s t i o n s "What elements ? " "How many o f them ? " "What t y p e o f b o n d i n g ?". N e v e r t h e l e s s , anyone w o r k i n g w i t h an ESCA system must keep i n mind t h a t , i n most cases, t h e r e s u l t s o b t a i n e d f r o m t h i s t e c h n i q u e can be o f i n t e r e s t o n l y by comparison w i t h t h o s e o b t a i n e d b y any o t h e r means t h a t we know o f . Moreover, i n o r d e r t o o b t a i n good r e s u l t s and because o f i t s h i g h s e l e c t i v i t y f o r t h e s u r f a c e , i t i s necessary t h a t X.P.S.

(e.g.

induced problems s h o u l d be s a t i s f a c t o r i l y s o l v e d

c h o i c e o f a s u p p o r t , c h a r g i n g e f f e c t and c h o i c e o f a r e f e r e n c e , carbon

c o n t a m i n a t i o n o v e r l a y e r and i t s e f f e c t s . . . ) . B e a r i n g i n mind t h e p a r t i c u l a r case o f c l a y s and c l a y m i n e r a l s , t h e b a s i c p r i n c i p l e s o f X.P.S.

(ESCA) and t h e p r e c a u t i o n s t o be t a k e n f o r spectrum i n t e r -

p r e t a t i o n s w i l l be summarized i n t h e f i r s t p a r t o f t h i s s t u d y , b e f o r e r e v i e w i n g t h e main r e s u l t s t h a t have a l r e a d y been o b t a i n e d i n o u r p a r t i c u l a r t o p i c o f interest.

212

B A S I C PRINCIPLES OF PHOTOELECTRON SPECTROSCOPY.

9.2 9.2.1

Fundamental r e l a t i o n and consequences.

The fundamental proce'ss o c c u r i n g i n X . P . S .

( o r ESCA) i s a p h o t o e l e c t r i c e f f e c t .

An X-ray o f known energy, hv, impinges on a sample, e j e c t i n g e l e c t r o n s f r o m i t , w i t h k i n e t i c e n e r g i e s (EK) g i v e n by t h e fundamental r e l a t i o n : EK = hv - E l B

where E l B i s t h e energy t o be s u p p l i e d t o a p a r t i c u l a r atom i n o r d e r t o b r i n g o u t a p a r t i c u l a r e l e c t r o n f r o m i t s fundamental l e v e l i n s i d e t h e sample t o t h e s t a t e o f a s i n g l e e l e c t r o n w i t h a k i n e t i c energy equal t o z e r o under vacuum. I f t h e Fermi l e v e l i s t a k e n as r e f e r e n c e f o r t h e b i n d i n g energy s c a l e , E l B i s t h e sum o f two terms, namely EB, t r u e b i n d i n g energy o f t h e e l e c t r o n o f i n t e r e s t , and @ which r e p r e s e n t s t h e work f u n c t i o n o f t h e sample. The k i n e t i c energy o f t h e e j e c t e d e l e c t r o n can be w r i t t e n :

An a c c u r a t e measurement o f EK i s o b t a i n e d w i t h an e l e c t r o n s p e c t r o m e t e r and d e t a i l e d r e v i e w s o f i n s t r u m e n t a t i o n may be f o u n d elsewhere (Siegbahn, 1967; J e n k i n , 1977). An X.P.S.

spectrum r e p r e s e n t s a number o f p h o t o e j e c t e d e l e c t r o n s f r o m t h e

sample versus t h e i r k i n e t i c energy o r t h e i r b i n d i n g energy. A s h a r p l i n e appears f o r e v e r y e l e c t r o n i c l e v e l (with, o f course, a b i n d i n g energy l o w e r t h a n h v ) o f a l l t h e elements p r e s e n t i n t h e a n a l y z e d p r o d u c t . X.P.S.

i s sensitive t o a l l the

elements w i t h an a t o m i c number g r e a t e r t h a n 2 and t h e t a b l e o f b i n d i n g e n e r g i e s g i v e n by Siegbahn e t a l . (1967) i s always c o n v e n i e n t f o r t h e a t t r i b u t i o n o f t h e observed peaks i n a spectrum, t h u s X.P.S. The e x a c t p r o f i l e o f an X.P.S.

appears as an a n a l y t i c a l t o o l .

l i n e i s a complicated convolution product bet-

ween t h e n a t u r a l p r o f i l e o f t h e l e v e l o f i n t e r e s t , t h e r e s o l u t i o n o f t h e a n a l y z i n g energy system and t h e p r o f i l e o f t h e e x c i t i n g r a d i a t i o n . Working w i t h v e r y narrow X-ray l i n e s as a s o u r c e w i l l enhance r e s o l u t i o n . F o r t h i s reason, A1 Ka ( h v = 1486.6 eV)

and Mg Ka (1353.6 eV) l i n e s a r e c u r r e n t l y used as X-ray

sources. This basic p r i n c i p l e reveals t h e f i r s t l i m i t a t i o n o f e l e c t r o n spectroscopy. A g o o d d e t e r m i n a t i o n o f t h e k i n e t i c e n e r g y o f t h e e j e c t e d e l e c t r o n s c a n o n l y b e achieved

i f t h e i r mean f r e e p a t h i n s i d e t h e s p e c t r o m e t e r i s g r e a t e r t h a n t h e l e n g t h t h e y have t o t r a v e l between t h e sample and t h e d e t e c t o r ; p r e s e n t l y s p e c t r o m e t e r s a r e o p e r a t e d under vacuum b e t t e r t h a n 10-8

-

10-9 t o r r .

Under t h e s e c o n d i t i o n s t h e u s e r must make s u r e t h a t t h e s u r f a c e o f t h e sample does n o t undergo any t r a n s f o r m a t i o n under u l t r a h i g h vacuum. S i n c e t h e r e i s no

213 t e m p e r a t u r e l i m i t a t i o n , i t i s always p o s s i b l e t o a v o i d a m o d i f i c a t i o n o f t h e s u r f a c e by c o o l i n g t h e sample a t temperatures as l o w as t h a t o f l i q u i d n i t r o g e n . Moreover, t h e vapour p r e s s u r e o f t h e sample must be l o w e r t h a n t h e r e s i d u a l p r e s sure i n s i d e t h e spectrometer, p r a c t i c a l l y l i m i t i n g e l e c t r o n spectroscopy t o t h e study o f s o l i d s . Working w i t h c l a y s , t h i s l i m i t a t i o n may be a s o u r c e o f d i f f i c u l t i e s s i n c e t h e s e m i n e r a l s most o f t e n e x i s t w i t h a h i g h w a t e r c o n t e n t and keeping c l a y m i n e r a l s under vacuum can i n d u c e d e h y d r a t i o n r e a c t i o n s t h a t may change t h e s u r f a c e p r o p e r t i e s . F o r example, when m e s o - t e t r a p h e n y l p o r p h y r i n (TPP) i s adsorbed i n t h e i n t e r l a y e r s p a c e , o f m o n t m o r i l l o n i t e , t h e X.P.S.

spectrum o f t h e N 1 s l e v e l i s s u e d

from t h e adsorbed m o l e c u l e suggests t h a t t h e r e e x i s t s some e x t r a p r o t o n a t i o n of t h e TPP m o l e c u l e (Canesson e t a l . ,

1978). Vacuum b e t t e r t h a n lo-* t o r r and X-ray

bombardment would i m p l y a d e s o r p t i o n o f adsorbed w a t e r , and i t i s w e l l known t h a t c l a y s w i t h a v e r y l o w w a t e r c o n t e n t have t h e i r a c i d i t y enhanced ( F r i p i a t e t al.,

1965; B a i l e y e t a l . ,

1976). T h e r e f o r e i n s i d e t h e s p e c t r o m e t e r t h e c l a y

s u r f a c e a c t s as a super a c i d medium f o r t h e adsorbed TPP. 9.2.2

R e l a x a t i o n process.

I f a vacancy i s c r e a t e d i n an i n n e r e l e c t r o n s h e l l b y X-ray i r r a d i a t i o n , t h e

e x c i t e d atom w i l l r e v e r t t o t h e ground s t a t e by e i t h e r e m i t t i n g c h a r a c t e r i s t i c X-ray r a d i a t i o n o r by r a d i a t i o n l e s s t r a n s i t i o n s , t h e s o - c a l l e d Auger t r a n s i t i o n s . I n t h e X-ray case, t h e i n n e r vacancy i s f i l l e d w i t h an e l e c t r o n f r o m an o u t e r s h e l l and t h e r e l e a s e d energy i s e m i t t e d as e l e c t r o m a g n e t i c r a d i a t i o n i n an X-ray quantum. I n t h i s case, t h e r e l a x a t i o n process i s t h e s o - c a l l e d X-ray f l u o r e s c e n c e . I n t h e Auger case, t h e r e l e a s e d energy i s i n s t e a d t r a n s f e r r e d t o a n o t h e r e l e c t r o n i n one o f t h e o u t e r s h e l l s . T h i s e l e c t r o n i s t h e n r e l e a s e d and l e a v e s t h e atom. The Auger process can be c o n s i d e r e d as t h e sum o f X-ray f l u o r e s c e n c e and t h e p r i m b y process o f X.P.S..

A c o m p l i c a t i o n i n X.P.S.

spectroscopy i s t h a t

Auger e l e c t r o n s e m i t t e d f r o m t h e sample a r e a l s o a n a l y z e d i n t h e s p e c t r o m e t e r and appear i n t h e X.P.S.

s p e c t r a . S i n c e t h e Auger process i n v o l v e s t h r e e d i f f e -

r e n t l e v e l s i n s i d e one atom, t h e c o r r e s p o n d i n g Auger peaks a r e r a t h e r broad w i t h r e s p e c t t o t h e p r i m a r y X.P.S. s c a l e o f a s o - c a l l e d X.P.S.

e l e c t r o n s . Ift h e p o s i t i o n on t h e k i n e t i c energy e l e c t r o n depends on t h e n a t u r e o f t h e anode o f t h e

X-rays source, Auger e l e c t r o n s do n o t , because t h e i r k i n e t i c energy i s o n l y dependent on t h e n a t u r e o f t h e c o n s i d e r e d atom. 9.2.3

X.P.S.

as a s u r f a c e t e c h n i q u e

The e x c i t i n g r a d i a t i o n used i s a b f e t o e j e c t e l e c t r o n s f r o m atoms a t r a t h e r i m p o r t a n t depths, b u t i t i s w e l l known t h a t e l e c t r o n s w i t h e n e r g i e s o f a few keV o r l e s s p e n e t r a t e o n l y v e r y t h i n l a y e r s o f s o l i d m a t t e r . P h o t o e l e c t r o n s

2 14 produced by a Mg K a l i n e f o r example emerge f r o m a s u r f a c e l a y e r c o n t a i n i n g some hundred a t o m i c l a y e r s . The f r a c t i o n t h a t emerges w i t h o u t energy l o s s becomes e x c e e d i n g l y s m a l l as t h i s l i m i t i s approached. The average d e p t h a t w h i c h t h o s e e l e c t r o n s observed i n t h e e l e c t r o n l i n e s a r e produced may be o n l y a few t e n s o f an

i.The

s o - c a l l e d escape d e p t h o f p h o t o e l e c t r o n s depends on s e v e r a l f a c t o r s ,

e.g. t h e n a t u r e , d e n s i t y and c r y s t a l s t r u c t u r e o f t h e sample m a t e r i a l , t h e energy o f the radiation involved f o r e x c i t a t i o n

... .

I t i s w e l l known t h a t t h e escape

depth i s s u b s t a n t i a l l y g r e a t e r i n s i d e an o x i d e t h a n i n t h e c o r r e s p o n d i n g m e t a l , and t h e more e n e r g e t i c t h e X-ray s o u r c e i s , t h e g r e a t e r t h e escape depth. The v a r i a t i o n s o f t h e mean f r e e p a t h o f e l e c t r o n s versus t h e i r k i n e t i c energy must be t a k e n i n t o account f o r q u a n t i t a t i v e i n t e r p r e t a t i o n s o f X.P.S. 9.2.4

spectra.

Choice o f a r e f e r e n c e l i n e

We have seen t h a t X.P.S.

can be used as a q u a l i t a t i v e a n a l y t i c a l t e c h n i q u e .

L e t us now emphasize t h a t t h e e x a c t p o s i t i o n on t h e b i n d i n g energy s c a l e o f a p a r t i c u l a r l e v e l does n o t depend o n l y upon t h e n a t u r e o f t h i s l e v e l o f t h e c o n s i dered element b u t a l s o on i t s chemical environment. O x i d a t i o n s t a t e , t y p e o f bonding, n a t u r e o f t h e n e a r e s t neighbour have an i n f l u e n c e on t h e k i n e t i c energy o f t h e p h o t o e j e c t e d e l e c t r o n s , and i n my o p i n i o n one o f t h e p r i n c i p a l advantages o f X.P.S.

r e s t s here. One can e a s i l y a d m i t t h a t t h e more p o s i t i v e an element i n

a sample i s , t h e more d i f f i c u l t a f u r t h e r i o n i s a t i o n w i l l be, even i n c o r e l e v e l s . T h i s f a c t induces chemical s h i f t s i n t h e p o s i t i o n o f t h e observed l e v e l . T h i s s h i f t i s always l o w (e.g.

5 eV f r o m m e t a l l i c molybdenum t o molybdenum t r i o x i d e

(Cimino and De A n g e l i s , 1975) b u t i t c a r r i e s o u t some fundamental i n f o r m a t i o n . I t f o l l o w s t h a t t h e e x a c t p o s i t i o n o f a g i v e n l e v e l can be s a i d t o be o n l y depen-

d e n t on t h e n a t u r e o f t h i s l e v e l and t h e r e a l charge o f t h e element o f i n t e r e s t i n s i d e t h e sample. I n o r d e r t o a v o i d any u n c e r t a i n t y i n t h e i n t e r p r e t a t i o n o f s p e c t r a , t h e b i n d i n g e n e r g i e s d e t e r m i n a t i o n s must be as a c c u r a t e as p o s s i b l e . T h i s r a i s e s one o f t h e m a j o r problems w i t h X.P.S.:

any l a c k o f p r e c i s i o n i n

t h e d e t e r m i n a t i o n o f k i n e t i c energy o f p h o t o e j e c t e d e l e c t r o n s w i l l i n d u c e t h e same i n a c c u r a c y f o r b i n d i n g e n e r g i e s . A c c o r d i n g t o t h e b a s i c p r i n c i p l e o f X.P.S., some e l e c t r o n s a r e e j e c t e d f r o m t h e sample, t h e r e s u l t o f w h i c h i s a n e t p o s i t i v e charge on t h e a n a l y z e d s u r f a c e , even i f t h e sample i s i n good e l e c t r i c c o n t a c t w i t h t h e s p e c t r o m e t e r ; w o r k i n g w i t h i n s u l a t o r s o r semi-conductors does n o t cancel t h i s charging e f f e c t . I n a s t a t i o n a r y s t a t e , a c o n s t a n t p o s i t i v e charge remains on t h e s u r f a c e , which i n t u r n c r e a t e s a r e t a r d i n g e l e c t r i c f i e l d f o r t h e e l e c t r o n s l e a v i n g t h e sample. I t can be e a s i l y understood t h a t i t i s o f p r i m e i m p o r t a n c e t o a c c o u n t f o r t h i s effect,

and t h e b i n d i n g energy s c a l e must be c a l i b r a t e d f o r e v e r y sample.

Various methods have been proposed i n t h e l i t t e r a t u r e f o r t h e c a l i b r a t i o n o f X.P.S. s p e c t r a (Ebel, 1974; Hnatowich e t a l . ,

1971; O g i l v i e and Wolberg, 1972).

215 One o f t h e f i r s t methods c o n s i s t s i n u s i n g t h e C 1 s l e v e l o r i g i n a t i n g f r o m a carbon c o n t a m i n a t i o n o v e r l a y e r . E s p e c i a l l y w i t h t h e f i r s t commercial X.P.S. systems, t h e vacuum was n o t o f v e r y good q u a l i t y and t h e samples were c o a t e d w i t h a l a y e r o f carbon c o n t a i n i n g p r o d u c t s ( p r o b a b l y e s s e n t i a l l y hydrocarbons f r o m pump o i l s ) . T h i s carbon c o n t a m i n a t i o n has been used w i d e l y f o r c a l i b r a t i o n purpose, t h e b i n d i n g energy o f t h e C 1 s l e v e l b e i n g a r b i t r a r i l y f i x e d between 283.5 eV and 285.5 eV, depending on t h e a u t h o r s . P r e c i s i o n i n b i n d i n g energy d e t e r m i n a t i o n s has been e s t i m a t e d a t 0.2 eV i n t h i s case (Ebel, 1974; Contour and Mouvier, 1975). T h i s method

i s o b v i o u s l y t h e e a s i e s t t o use, b u t i t f a i l s

now w i t h new ESCA systems because t h e carbon c o n t a m i n a t i o n o v e r l a y e r i s t o o small, i f n o t absent. I t does n o t work e i t h e r when t h e r e a r e more t h a n one carbon spe-

c i e s i n t h e spectrum o f t h e C 1 s l i n e ; t h i s i s t h e case f o r s t u d i e s o f adsorbed carbon c o n t a i n i n g molecules (Defosse and Canesson, 1976). The second method c o n s i s t s i n e v a p o r a t i n g a g o l d l a y e r as t h i n as p o s s i b l e o n t o t h e s u r f a c e o f t h e sample, t h e 4 f d o u b l e t o f g o l d b e i n g used as a standard. T h i s t e c h n i q u e has been much d e s c r i b e d (Urch and Weber, 1974); n e v e r t h e l e s s , i t can be used p r o v i d e d some p r e c a u t i o n s a r e t a k e n . The g o l d l a y e r i s n o t evenly spread on t h e s u r f a c e b u t i t shows i s l a n d s (Brunner and Zogg, 1974) and t h e e l e c t r i c c o n t a c t between t h e sample and t h e s e i s l a n d s may n o t be as good as expected. Gold i s a b l e t o r e a c t w i t h some compounds, e s p e c i a l l y m e t a l l i c species, l e a d i n g t o t h e f o r m a t i o n o f a l l o y s which, i n t u r n , i n d u c e a s h i f t o f t h e Au 4 f l e v e l s (Friedman e t a l . ,

1973 a; Friedman e t a l . ,

1973 b ) . Gold e v a p o r a t i o n can

a c t as argon bombardment and a m o d i f i c a t i o n o f t h e s u r f a c e may occur, o r some s u p e r f i c i a l r e a c t i o n s such as r e d u c t i o n can show up. B e f o r e u s i n g t h i s g o l d d e c o r a t i o n t e c h n i q u e f o r t h e b i n d i n g energy s c a l e c a l i b r a t i o n , i t i s o f t h e g r e a t e s t i m p o r t a n c e t o r e c o r d a l l X.P.S.

s p e c t r a f o r q u a n t i t a t i v e purposes b e f o r e

and a f t e r g o l d e v a p o r a t i o n and t h i s p r a c t i c a l l y m u l t i p l i e s a p p r e c i a b l y t h e t i m e f o r r e c o r d i n g t h e s p e c t r a f o r one sample. Whenever p o s s i b l e , t h e l a s t method i s t h e b e s t i n my o p i n i o n .

A w e l l defined

l e v e l o f one element p r e s e n t i n t h e sample i s t a k e n as r e f e r e n c e and i t s b i n d i n g energy v a l u e i s a r b i t r a r i l y f i x e d . T h i s method i m p l i c i t e l y assumes t h a t , when s t u d y i n g v a r i o u s samples, one element i s p r e s e n t i n a l l o f them and t h e b i n d i n g energy v a l u e o f i t s c o r e l e v e l s does n o t b r i n g a b o u t any m o d i f i c a t i o n , which means t h a t t h e chemical environment o f t h i s element i s t h e same f r o m one sample t o an o t h e r . Working w i t h c l a y s i s an enormous advantage, s i n c e p r a c t i c a l l y a l l c l a y m i n e r a l s c o n t a i n s i l i c o n i n an oxygen t e t r a h e d r a l environment, and one can assume t h a t t h e b i n d i n g energy o f t h e S i 2p l e v e l i s p r a c t i c a l l y t h e same f o r a l l samples. I n t h i s case, p r e c i s i o n ’ i n b i n d i n g energy d e t e r m i n a t i o n s can be e s t i m a t e d a t 0.1 eV ( O g i l v i e and W o l b e r t , 1972). Moreover, when adsorbed compounds a r e i n v e s t i g a t e d , i t has been demonstrated t h a t a d s o r p t i o n does n o t b r i n g about any s h i f t i n t h e b i n d i n g e n e r g i e s o f t h e adsorbant l e v e l s (Brundle, 1974).

216 I n c o n c l u s i o n , i f t h e b i n d i n g energy s c a l e c a l i b r a t i o n remains a problem i n

X.P.S.,

w o r k i n g w i t h c l a y s p e r m i t s t h e use o f an i n t e r n a l s t a n d a r d and t h i s i s

t h e b e s t method f o r t h i s purpose. 9.2.5

P o s s i b i l i t i e s o f quantitative analysis.

B e f o r e c o n s i d e r i n g t h e v a r i o u s f a c t o r s t h a t a r e i n v o l v e d i n t h e i n t e n s i t y of peak, l e t us f i r s t examine t h e s e n s i t i v i t y of t h i s t e c h n i q u e . To b e g i n

an X.P.S.

w i t h , i t must be p o i n t e d o u t t h a t X.P.S.

i s n o t a b l e t o d e t e c t t r a c e s . The extreme

d e t e c t i o n l i m i t i s about 0 . 1 % o f t h e a n a l y z e d l a y e r , b u t , i n o r d e r t o deduce some i n f o r m a t i o n from an X.P.S.

spectrum, t,he s i g n a l - t o - n o i s e r a t i o must n o t

be t o o bad and i t i s r a t h e r d i f f i c u l t t o p u t f o r w a r d any c o n c l u s i o n s on t h e chemical environment o f s p e c i e s w i t h a c o n t e n t l o w e r t h a n 0.3 analyzed l a y e r . N e v e r t h e l e s s when w o r k i n g w i t h X.P.S.,

-

0.5

I

i n the

a d i s t i n c t i o n must be

made between t h e b u l k c o m p o s i t i o n and what i s s t u d i e d , namely t h e s u r f a c e compos i t i o n . I f t h e r e i s some s u p e r f i c i a l e n r i c h m e n t i n t h e elements o f i n t e r e s t , t h e d e t e c t i o n l i m i t can be as l o w as p.p.b. e t al.,

expressed as b u l k c o n t e n t r a t i o s ( H e r c u l e s

1973).

L e t us now examine what i n f l u e n c e s i n t e n s i t y , expressed as t h e i n t e g r a t e d s u r face a r e a of an X.P.S.

-

peak. T h i s i n t e n s i t y i s p r o p o r t i o n a l t o :

t h e p h o t o e l e c t r o n i c c r o s s s e c t i o n ux o f t h e c o n s i d e r e d l e v e l t h e mean f r e e p a t h Ax o f t h e e j e c t e d p h o t o e l e c t r o n s i n s i d e t h e sample t h e s o - c a l l e d l u m i n o s i t y Lx o f t h e system f o r t h e l i n e o f i n t e r e s t t h e c o n c e n t r a t i o n Cx o f t h e c o n s i d e r e d element i n s i d e t h e a n a l y z e d l a y e r of

t h e sample. The i n t e n s i t y Ix o f a l i n e x can b e w r i t t e n :

Ix = k ox X, Lx C x

k b e i n g a p r o p o r t i o n a l i t y c o n s t a n t , depending on t h e s p e c t r o m e t e r dhd i t s w o r k i n g conditions.

I f t h e various

CT

v a l u e s can be c a l c u l a t e d f o r t h e X-ray s o u r c e o f i n t e r e s t

( S c o t f i e l d , 1976), t h e v a l u e s o f k and Lx cannot be e a s i l y determined. I n t e n s i t i e s a r e used as i n t e n s i t y r a t i o s between t h e peak o f i n t e r e s t and a peak o f reference,

n o t e d r. The r a t i o R,

between t h e s e two v a l u e s i s equal t o :

I n t h i s e q u a t i o n , X i s a f u n c t i o n o f t h e k i n e t i c energy o f t h e c o n s i d e r e d electrons. I n t h e f i r s t approximation, Ax i s p r o p o r t i o n a l t o

(Carter e t a l . KX

217 1975), w h i c h means t h a t R x can be w r i t t e n as

L x a l s o depends upon t h e E

KX

value; from t h i s c o r r e l a t i o n , i t i s n o t possible

t o o b t a i n any i n f o r m a t i o n because t h e c o r r e c t r e l a t i o n s h i p i s f u n c t i o n o f t h e w o r k i n g mode o f t h e c o n s i d e r e d ESCA system. Over t h e r e c e n t y e a r s , some compilat i o n s have been made, which a r e v e r y u s e f u l . Wagner (1972) compared t h e i n t e n s i t y r a t i o s o f t h e l e v e l s issued from various s a l t s , t h e i n t e n s i t y o f the

F 1s

l i n e b e i n g t a k e n a r b i t r a r i l y as u n i t y and he has d e t e r m i n e d t h e r e l a t i v e u n i t a r y i n t e n s i t i e s o f v a r i o u s l e v e l s o f most c u r r e n t l y s t u d i e d elements. The same k i n d of work was a l s o done by Nefedov e t a l . ,

(1973, 1975, 1977). These a u t h o r s d i d

use t h e Na 1 s l e v e l s as r e f e r e n c e f o r t h e r e l a t i v e u n i t a r y i n t e n s i t i e s s c a l e . I n a l l t h e s e r e f e r e n c e s t h e e f f e c t o f t h e carbon c o n t a m i n a t i o n o v e r l a y e r was n e g l e c t e d . I f t h e a n a l y z e d s u r f a c e i s covered by such a l a y e r o f t h i c k n e s s d, t h e t r u e Rx r a t i o i s t r a n s f o r m e d i n t o a measured R I x r a t i o , the- r e l a t i o n s h i p between them b e i n g : Rlx=Rx

XIr

exp[

1

and A ' x b e i n g t h e mean f r e e p a t h s o f e l e c t r o n s i n s i d e t h e c o n t a m i n a t i n g

l a y e r . R e s p e c t i v e l y f r o m ( 6 ) , i t i s p o s s i b l e t o d e f i n e c l e a r l y when t h e contam i n a t i o n o v e r l a y e r can be n e g l e c t e d , namely :

-

when d i s equal t o zero, namely when t h e r e i s no c o n t a m i n a t i o n and, when 1 / X l r

-

1 / A I x i s n e a r l y equal t o z e r o , which means t h a t A\

cs

A;(.

L f one

assumes t h a t A ' v a l u e s behave i n t h e same way as r e a l A , t h e l a t t e r c o n d i t i o n b e i n g o b t a i n e d i f t h e p h o t o e j e c t e d e l e c t r o n s f r o m t h e r e f e r e n c e l i n e and t h e l i n e o f i n t e r e s t have t h e same k i n e t i c energy. T h i s case i s t h e most f a v o u r a b l e f o r q u a n t i t a t i v e a n a l y s i s purposes because i n t h i s case Ax # Ar and t h e sampling d e p t h i s t h e same f o r t h e two l i n e s o f i n t e r e s t . On t h e c o n t r a r y , e.g. EK,

if

>z E K ~ , t h e sampling d e p t h i s n o t e x a c t l y t h e same f o r t h e r and x l i n e s .

T h i s e f f e c t o f a s u p e r f i c i a l l a y e r was c l e a r l y shown by Defoss& e t a l . (1975). Working w i t h n i c k e l exchanged z e o l i t e s , t h e c r a c k i n g o f benzene on t h e s u r f a c e induces a carbon l a y e r w h i c h d i m i n i s h e s b y a f a c t o r o f 5 t h e r a t i o

R N i = ' N i 2p 3 / 2 / I S i

2p

A l l t h e f a c t o r s t h a t may i n f l u e n c e q u a n t t a t i v e d e t e r m i n a t i o n s by X.P.S.

were

s t u d i e d i n d e t a i l by Wagner ( 1 9 7 7 ) . A f t e r t h e s e g e n e r a l c o n s i d e r a t i o n s a b o u t t h e X.P.S.

t e c h n i q u e , l e t us now

2 18 examine what i n f o r m a t i o n has a l r e a d y been o b t a i n e d w i t h t h i s p o w e r f u l and v e r s a t i l e technique. 9.3

APPLICATIONS OF X.P.S. Since X.P.S.

TO CLAYS AND CLAY MINERALS.

can be used as an a n a l y t i c a l t o o l f o r q u a n t i t a t i v e s u r f a c e ana-

l y s i s as w e l l as f o r d e t e r m i n a t i o n o f t y p e o f bonding, i t i s d i f f i c u l t t o a r b i t r a r i l y separate t h e r e s u l t s obtained r e s p e c t i v e l y . A l l i n f o r m a t i o n i s contained i n t h e same spectrum and I have chosen t o examine s e p a r a t e l y s t a n d a r d a n a l y s i s o f b u l k m i n e r a l s and what i s t y p i c a l l y s u r f a c e a n a l y s i s , namely a d s o r p t i o n s t u d i e s . 9.3.1

Minerals analvsis

A t t e n t i o n must be drawn on t h e f a c t t h a t X.P.S.

does n o t e n a b l e us t o d i s t i n -

g u i s h between two c o o r d i n a t i o n numbers f o r t h e same c a t i o n w i t h t h e same n a t u r e o f l i g a n d s i n t h e c o o r d i n a t i o n sphere. I n s p i t e o f r e s u l t s p u b l i s h e d i n t h e e a r l y t i m e s of X.P.S.

(Nicholls e t al.,

1974; L i n d s a y e t a l . ,

1972), s e v e r a l a u t h o r s (Anderson and Swartz,

1973) have shown t h a t t h e r e i s no d i f f e r e n c e i n t h e b i n d i n g

energy v a l u e o f t h e A1 2p l e v e l i n an oxygen environment whatever i t s c o o r d i n a t i o n number : e i t h e r f o u r o r s i x . Keeping i n mind t h a t t h e e x a c t p o s i t i o n on t h e b i n d i n g energy s c a l e o f a p a r t i c u l a r l e v e l i s o n l y dependent on t h e r e a l charge o f t h e element o f i n t e r e s t , such r e s u l t i s n o t s u r p r i z i n g . I n c r e a s i n g t h e c o o r d i n a t i o n number o f A13+ f r o m f o u r t o s i x w i l l i n d u c e an i n c r e a s e i n t h e mean A1-0 bond l e n g t h and so t h e s t a t i s t i c a l e f f e c t o f t h e oxygen environment on t h e r e a l charge o f t h e c e n t r a l c a t i o n w i l l be t h e same. The r e s u l t s o b t a i n e d by Nicholls

e t a l . (1972) a r e r a t h e r n i c e l y e x p l a i n e d by L i n d s a y e t a l . (1973).

The d i f f e r e n c e i n b i n d i n g energy o f t h e A1 2p l e v e l ( 1 . 4 eV) between m i c r o l i n e (aluminum i n f o u r f o l d c o o r d i n a t i o n ) and A1203 (aluminum i n s i x f o l d c o o r d i n a t i o n ) i s a consequence o f t h e presence o f potassium i n m i c r o l i n e ; i t reduces t h e e l e c tron

-

a t t r a c t i n g a b i l i t y o f t h e oxygen atoms, so t h e r e a l charge o f A13+ i o n s

i s lowered w i t h r e s p e c t t o t h a t i n A1203 and t h e b i n d i n g energy o f t h e A1 2p l e v e l i s a l s o lowered i n m i c r o l i n e .

I f t h e c o o r d i n a t i o n number has no i n f l u e n c e on t h e b i n d i n g energy, on t h e c o n t r a r y , Urch and Murphy (1974) have i n t e r p r e t e d t h e d i f f e r e n c e s i n b i n d i n g e n e r g i e s o f A1 2p and A1 2s c o r e l e v e l s o f a s e r i e s o f a l u m i n o s i l i c a t e s m i n e r a l s as i n f l u e n c e d by t h e A1-0 bond l e n g t h s . These a u t h o r s concluded t h a t b i n d i n g energy i n c r e a s e s w i t h t h e A1-0 d i s t a n c e . When w o r k i n g w i t h ESCA on a l u m i n o s i l i c a t e s m i n e r a l s , one can g e n e r a l l y a d m i t t h a t t h e b i n d i n g e n e r g i e s o f c o r e l e v e l s o f t h e main c o n s t i t u e n t s , namely S i , A1 and 0 do n o t v a r y t o o much f r o m one sample t o an o t h e r . T h i s assumption was v e r i f i e d many t i m e s i n t h e l i t e r a t u r e b y s e v e r a l a u t h o r s on a l o t o f m i n e r a l s l i k e k y a n i t e , s i l l i m a n i t e and m u l l i t e (Anderson e t a l . ,

1974), k a o l i n i t e , c h l o r i t e

219 and i l l i t e (Koppelman and D i l l a r d , 1975) and 12 n a t u r a l s i l i c a t e s and a l u m i n o s i l i c a t e s (Carriere e t al.,

1977). T a k i n g t h e S i 2p l i n e as a s t a n d a r d , t h e A1 2s

and 0 1s l i n e s o f m o n t m o r i l l o n i t e do n o t show any m o d i f i c a t i o n i n b i n d i n g energy and i n t e n s i t y , whatever t h e charge b a l a n c i n g c a t i o n : Na+, Fez+, N i 2 + , Mn2+, Cu2+, Co2+, Sn4+. These r e s u l t s were o b t a i n e d f r o m a s t u d y o f c a t i o n exchanged montm o r i l l o n i t e s (Canesson e t a l . ,

1978).

About t h e b u l k chemical c o m p o s i t i o n o f s e v e r a l m i n e r a l s , Adams e t a l . , have demonstrated t h a t X.P.S.

(1977)

can p r o v i d e b u l k q u a n t i t a t i v e c o m p o s i t i o n o f a i r -

s t a b l e homogeneous s o l i d s w i t h an accuracy o f c.a.

5 %. From t h e i r c o m p i l a t i o n

o f a l o t o f p o l y c r i s t a l l i n e and m o n o c r i s t a l l i n e samples, t h e y concluded t h a t t h e r e i s no s u p e r f i c i a l s e g r e g a t i o n o f any o f t h e c o n s t i t u e n t s o f k a o l i n i t e , t a l c , m o n t m o r i l l o n i t e , l a p o n i t e , l e p i d o l i t e , c r y o l i t e and z e o l i t e . The o n l y dev i a t i o n f r o m b u l k c o m p o s i t i o n i s observed f o r m o n o c r i s t a l l i n e p h l o g o p i t e , which e x h i b i t s a h i g h e r c o n t e n t i n aluminum i o n s i n t h e analyzed r e g i o n t h a n i n t h e b u l k , even a f t e r r e p e a t e d c l e a v a g e s . The reason would be t h a t t h i s m i n e r a l cleaves i n r e g i o n s w i t h a h i g h aluminum c o n t e n t . More s u r p r i s i n g i s t h e l a c k o f d i s c r e pancy f o r Y z e o l i t e s , s i n c e , as i t was shown by s e v e r a l a u t h o r s , a d e p l e t i o n i n aluminum i o n s i s observed i n t h e f i r s t a t o m i c l a y e r s o f s y n t h e t i c Y z e o l i t e s (Tempere e t a1 . , 1977; Defosse e t a1

. , 1977).

S i m i l a r r e s u l t s were p u b l i s h e d by Koppelman and D i l l a r d ( 1 9 7 8 ) . These a u t h o r s found t h a t t h e S i / A l s u r f a c e r a t i o i s t h e same as i n t h e b u l k f o r k a o l i n i t e and c h l o r i t e . F o r i l l i t e , t h e y n o t e d a d e p l e t i o n i n potassium i n t h e analyzed l a y e r s i n c e t h e S i / K r a t i o deduced f r o m X.P.S.

i n t e n s i t i e s i s l o w e r t h a n t h a t determined

by more c l a s s i c a l a n a l y t i c a l methods. F o r t h e o t h e r elements (Mg, A1 and Fe), which a r e c o n s t i t u t i v e o f t h e t e t r a h e d r a l and o c t a h e d r a l l a y e r s , t h e S i / M e t a l r a t i o s were w i t h i n 6 % o f t h e b u l k chemical c o m p o s i t i o n . I t was suggested t h a t t h e observed d i s c r e p a n c i e s a r o s e because no c o r r e c t i o n f o r e l e m e n t a l depth i n t h e sample was made..Potassium i n t h e i n t e r l a y e r space i n i l l i t e i s a t a depth g r e a t e r t h a n o t h e r exposed c a t i o n s , r e d u c i n g t h e S i / K X.P.S. The c a t i o n t h a t has focused most X.P.S.

intensity ratio.

workers e f f o r t s i n t h e f i e l d o f n a t u r a l

m i n e r a l s , i s u n d o u b t e d l y i r o n . I t s chemical s t a t e (Fez+ o r Fe3+) was t e n t a t i v e l y s t u d i e d by Adams e t a l . (1972); t h e y were u n a b l e t o d i s t i n g u i s h between f e r r i c and f e r r o u s i r o n s p e c i e s i n t h e m i n e r a l s t h e y examined. T h i s i s n o t s u r p r i s i n g s i n c e t h e Fe 2p 3/2 l e v e l i s r a t h e r b r o a d and t h e i r o n c o n t e n t , e x c e p t i n some m i n e r a l s o f t h e mica f a m i l y i s r a t h e r l o w i n a l u m i n o s i l i c a t e s . The c o r r e s p o n d i n g X.P.S.

peaks a r e ill d e f i n e d , w i t h a l o w s i g n a l t o n o i s e r a t i o . Moreover, t h e

observed s h i f t s between Fez+ and Fe3+ i n p u r e i r o n o x i d e s a r e r a t h e r l o w (Mac I n t y r e and Z e t a r u k , 1977; Asami and Hashimoto, 1977). N e v e r t h e l e s s , Koppelman and D i l l a r d (1975) were a b l e t o d i s t i n g u i s h between Fez+ and Fe3+ i n i l l i t e . The comparison of t h e Fe 2p 3/2 l i n e i s s u e d f r o m n o n t r o n i t e (Fez+ s p e c i e s o n l y ) ,

220 c h l o r i t e (Fez+ s p e c i e s o n l y ) and i11 it e , a1 lowed t h e a u t h o r s t o decompose t h e Fe 2p 3/2 X.P.S.

p r o f i l e o f i l l i t e i n t o two i r o n s p e c i e s , namely Fe2+ and Fe3+.

The r e l a t i v e abundance o f Fe3+ as determined by t h e r e l a t i v e s u r f a c e a r e a o f t h e c o r r e s p o n d i n g X.P.S. mineral .

peak i s i n good agreement w i t h Mossbauer d a t a on t h e same

S t u c k i e t a l . ( 1 9 7 6 ) , s t u d y i n g n o n t r o n i t e and b i o t i t e , a l s o observed a s h i f t between t h e Fe 2p 3 / 2 s p e c t r a o f t h e s e m i n e r a l s . Upon r e d u c t i o n by e i t h e r h y d r a z i n e o r d i t h i o n i t e t h e Fe 2p l e v e l s o f n o n t r o n i t e e x h i b i t a broadening t o g e t h e r w i t h a s h i f t towards

l o w e r b i n d i n g e n e r g i e s . On t h e c o n t r a r y , o x i d a t i o n o f b i o -

t i t e by bromine induces a s h i f t towards h i g h e r b i n d i n g e n e r g i e s . W i t h such compound, t h e observed s p e c t r a a r e o f r a t h e r p o o r q u a l i t y and any c o n c l u s i o n on t h e r e l a t i v e q u a n t i t i e s o f t h e v a r i o u s i r o n s p e c i e s would be r a t h e r s p e c u l a t i v e .

Si2

P

C r u d e kaalinitc

___.x 'I i n c r e a s i n g

1

:time I I

I

0.8

I I

deferrating treatment

/

0.6

0.4 0.2 0.1

0:2

013

014

015'

si2p

F i g . 9.1. Behaviour o f t h e Fe 2p/Si 2p r a t i o v e r s u s t h e A 1 2p/Si 2p r a t i o f o r t h e YBi k a o l i n i t e a f t e r v a r i o u s c o n t a c t t i m e s w i t h ammonium o x a l a t e s o l u t i o n .

221 I r o n i n k a o l i n i t e i s a n o t h e r problem many c l a y r e s e a r c h e r s have t a c k l e d (e.g. see Angel and V i n c e n t (1978); Angel e t a l . (1977); H e r b i l l o n e t a l . (1976) and r e f e r e n c e s t h e r e i n ) . Working w i t h X.P.S.

on a p a r t i c u l a r t r o p i c a l s o i l k a o l i n i t e ,

namely t h e Y B i sample d e s c r i b e d by H e r b i l l o n e t a l . (1976), i t i s p o s s i b l e t o s t u d y t h e Fe 2p l e v e l s i n c e t h e i r o n c o n t e n t o f t h i s p a r t i c u l a r c l a y i s h i g h . Moreover, t h i s k a o l i n i t e can be c l e a n e d t o some e x t e n t by e x t r a c t i n g t h e c o a t i n g i r o n o x i d e by ammonium o x a l a t e , as a l r e a d y mentioned by H e r b i l l o n e t a l . (1976). On F i g u r e 9.1 t h e e v o l u t i o n o f t h e Fe 2p/Si 2p r a t i o i s p l o t t e d a g a i n s t t h e A1 2p/Si 2p r a t i o f o r v a r i o u s c o n t a c t t i m e s between t h e c l a y and t h e s o l u t i o n . The c u r v e c l e a r l y shows two d i s t i n c t p a r t s . I n t h e i n i t i a l s t a g e s o f t h e a t t a c k , i t can be n o t e d t h a t t h e Fe 2p/Si 2p r a t i o decreases, t h e A1 2p/Si 2p r a t i o r e -

m a i n i n g p r a c t i c a l l y c o n s t a n t . F o r l o n g e r c o n t a c t t i m e s , t h e two r a t i o s behave i n t h e same way. The c r u d e k a o l i n i t e p a r t i c l e s seem c o a t e d w i t h a h i g h i r o n and a l o w aluminum c o n t e n t g e l . A f t e r t h i s c o a t i n g has been d i s s o l v e d , t h e k a o l i n i t e c r i s t a l i s a t t a c k e d by t h e complexing s o l u t i o n and i r o n and aluminum i o n s a r e s i m u l t a n e o u s l y removed. These r e s u l t s c o n f i r m t h e f a c t a l r e a d y p o i n t e d o u t by E.P.R.,

t h a t a f r a c t i o n o f the t o t a l i r o n content o f kaolinites i s inside the

s t r u c t u r e o f t h e m i n e r a l , p r o b a b l y s u b s t i t u t i n g A13+ i o n s . From t h i s study, i t was n o t p o s s i b l e t o draw any c o n c l u s i o n a b o u t t h e o x i d a t i o n s t a t e o f i r o n s i n c e t h e X.P.5.

Fig.9.2.

peaks were t o o broad.

Geometrical c o n s i d e r a t i o n s f o r X-rays p h o t o e l e c t r o n s d i f f r a c t i o n .

222 W i t h p o l y c r i s t a l l i n e samples, as i t has been a l r e a d y p o i n t e d o u t , X.P.S.

i s not

a b l e t o d i s t i n g u i s h between t h e v a r i o u s p o s s i b l e environments o f a g i v e n i o n . Working w i t h s i n g l e c r y s t a l s , Evans e t a l .

(1979) have developed a method which a l l o w s some

comparisons between t h e v a r i o u s l o c a t i o n s i n s i d e t h e s t r u c t u r e . The s o - c a l l e d X-rays p h o t o e l e c t r o n s d i f f r a c t i o n i s based on t h e a n g u l a r d e p e n d e n c e o f t h e v a r i o u s i n t e n s i t y r a t i o s o f a w e l l o r i e n t e d sample. By r o t a t i n g t h e s i i l g l e c r y s t a l a l o n g t h e c a x i s (see f i g u r e 9 . 2 ) ,

the angle

Q

between t h e e x c i t i n g r a d i a t i o n a n d t h e a n a l y z e d

electrons being f i x e d b y t h e geometryof t h e spectrometer, i t i s p o s s i b l e t o vary the

e a n g l e c o n t i n u o u s l y f r o m 0 t o 90". The o u t g o i n g p h o t o e l e c t r o n s c a n b e d i f f r a c -

t e d by t h e atoms s u r r o u n d i n g t h e e m i t t i n g one and t h e n u m b e r o f c o l l e c t e d p h o t o e l e c t r o n s a t t h e e n t r a n c e s l i t o f t h e s p e c t r o m e t e r depends on t h a t a n g l e . I f two elements A and B a r e l o c a t e d i n s i d e t h e c r y s t a l l a t t i c e o n t h e same k i n d o f s i t e s , i . e . w i t h

t h e same number o f t h e same s u r r o u n d i n g i o n s , t h e A / B X.P.S. w i l l n o t v a r y when

e

peaks i n t e n s i t y r a t i o

changes. I f t h e two s i t e s a r e n o t e q u i v a l e n t , a dependence

oftheintensity ratio with

e w i l l be observed. I f , f o r example, t h e A e l e m e n t i s

d i s t r i b u t e d between two k i n d s o f s i t e s , one b e i n g e q u i v a l e n t t o t h e B environment, w h i l e t h e o t h e r i s n o t , an i n t e r m e d i a t e s i t u a t i o n w i l l be observed and a c c o r d i n g l y the i n t e n s i t y r a t i o w i l l bemodified. T a k i n g t h e i n t e n s i t y o f t h e S i 2p l i n e as r e f e r e n c e , Evans e t a l . (1979) s t u d i e d t h e l o c a t i o n o f t h e v a r i o u s c a t i o n s i n muscovite, l e p i d o l i t e , p h l o g o p i t e and b o t h n a t u r a l and Pb exchanged v e r m i c u l i t e . R e s u l t s o b t a i n e d by t h e X-rays p h o t o e l e c t r o n d i f f r a c t i o n patterns confirm those p r e d i c t e d by t h e s t r u c t u r a l determination

o f t h e isomorphous s u b s t i t u t i o n s i n m u s c o v i t e and l e p i d o l i t e . They a l s o

c o n f i r m t h e s u r f a c e e n r i c h m e n t i n A13+ i o n s i n t h e r e g i o n s o f f a c i l e cleavage f o r p h l o g o p i t e and v e r m i c u l i t e , aluminum b e i n g a l m o s t e x c l u s i v e l y i n f o u r f o l d coordination i n these regions. A s i m i l a r s t u d y on t i t a n i u m c o n t a i n i n g b i o t i t e and p h l o g o p i t e (Evans and

R a f t e r y , 1980) has shown t h a t t i t a n i u m i s more l i k e l y T i 3 + t h a n T i 4 + i n t h e s e micas. Moreover, a l l t h e t i t a n i u m i o n s a r e l o c a t e d i n o c t a h e d r a l s i t e s , n o t e x a c t l y e q u i v a l e n t t o t h o s e o f Mg and Fe i o n s s i n c e t h e a u t h o r s observed a var i a t i o n with the

e a n g l e o f t h e T i 2p/Mg

2s i n t e n s i t y r a t i o .

T h i s a p p l i c a t i o n i s , i n my o p i n i o n , one o f t h e most p o w e r f u l i n X.P.S.. U n f o r t u n a t e l y , i t i s l i m i t e d t o t h e s t u d y o f s i n g l e c r y s t a l s , o r , perhaps, i n t h e most f a v o u r a b l e cases, t o w e l l o r i e n t e d p o l y c r i s t a l l i n e samples. 9.3.2

AdsorDtion studies

Because i t s h i g h s e l e c t i v i t y f o r t h e s u r f a c e a n a l y s i s , X.P.S.

i s undoubtedly

an i d e a l t o o l f o r t h i s k i n d o f s t u d i e s . Working w i t h t h r e e c l a y m i n e r a l s , namely k a o l i n i t e , i l l i t e and c h l o r i t e , Koppelman and D i l l a r d have s t u d i e s t h e adsorpt i o n o f Fe3+ and C r 3 + (1975), N i 2 + and Cu2'

(1977),

Cr3+

(1980 a ) , two complexes

o f C r 3 + i o n s (1980 b ) and v a r i o u s c o b a l t i o n s (1978 b ) . The comparison o f b i n d i n g

223 e n e r g i e s between an adsorbed i o n c o r e l e v e l and t h e same i o n c o r e l e v e l i n s i d e t h e s t r u c t u r e , r e v e a l e d a g e n e r a l l o w e r i n g i n b i n d i n g energy o f t h e photopeak i s s u e d f r o m adsorbed s p e c i e s , i f a d s o r p t i o n i s performed a t l o w pH v a l u e s .

The r e l a t i v e

l o w e r i n g o f t h e b i n d i n g energy f o r adsorbed i o n s w i t h r e s p e c t t o t h e same l a t t i c e i o n s i s i n t e r p r e t e d as an i n c r e a s e i n e l e c t r o n d e n s i t y on t h e adsorbed metal due t o the negative surface p o t e n t i a l o f t h e various clays. For adsorption a t higher pH values, t h e X.P.S.

photopeaks o f adsorbed s p e c i e s a r e s i m i l a r i n b i n d i n g ener-

g i e s t o t h o s e o f t h e c o r r e s p o n d i n g h y d r o x i d e s . The q u a n t i t y o f adsorbed i o n s was found t o v a r y always i n t h e f o l l o w i n g o r d e r : c h l o r i t e > i l l i t e > k a o l i n i t e . The b e h a v i o u r o f Cu2+ i s q u i t e d i f f e r e n t . I n t h e adsorbed s t a t e , Koppelman and D i l l a r d (1977) n o t e d an i n c r e a s e i n t h e b i n d i n g energy o f t h e Cu 2p 3/2 l e v e l w i t h r e f e r e n c e t o t h e same l e v e l i n d i o p t a s e . T h i s i n c r e a s e i s a t t r i b u t e d t o t h e f o r m a t i o n o f Cu(OH)+ s u p e r f i c i a l s p e c i e s . R e s u l t s c o n c e r n i n g a d s o r p t i o n o f some m i n e r a l complexes such as [ C O ( H ~ O ) ~ I ~ + and [ C O ( N H ~ ) ~ ]on ~ +c h l o r i t e (Koppelman and D i l l a r d , 1978 b ) and [Cr(NH3)513+ and [ C r ( e t h y l e n e diamine) 313'

on c h l o r i t e , i l l i t e and k a o l i n i t e (Koppelman and

D i l l a r d , 1980 b ) l e a d t o t h e same c o n c l u s i o n t h a t t h e adsorbed s p e c i e s behave as h y d r a t e d c a t i o n s . C o n t a c t i n g c h l o r i t e w i t h s o l u t i o n o f [ C O ( N H ~ ) ~3' ] induces an i n c r e a s e i n t h e pH o f t h e s o l u t i o n t o g e t h e r w i t h a r e d u c t i o n o f C03'

species i n t o

h y d r a t e d Co2+ s p e c i e s . The l o s s o f n i t r o g e n can be p u t f o r w a r d by a decrease o f t h e

N ls/Co 2p 3 / 2 r a t i o . A d s o r p t i o n o f chromium complexes i s a l s o c h a r a c t e r i z e d by an i n c r e a s e i n t h e pH o f t h e s o l u t i o n and a c l a y c a t a l y z e d h y d r o l y s i s o f t h e amine complexes. S t u d y i n g t h e exchange p r o p e r t i e s o f b e i d e l l i t e , Adams and Evans (1979) deduced t h e c a t i o n exchange c a p a c i t i e s o f t h a t c l a y f r o m X.P.S.

i n t e n s i t i e s measurements.

The r e s u l t s t h u s o b t a i n e d a r e i n c l o s e r e l a t i o n w i t h t h e C . E . C . by chemical methods f o r Na+ and Ca2+ c a t i o n s . F o r K', an a p p a r e n t excess o f t h e C.E.C.

values determined

Pb2+ and Ba2+ t h e y n o t e d

w i t h r e s p e c t t o t h e chemical v a l u e . T h i s excess

i s i n t e r p r e t e d as a consequence o f a s t r o n g a d s o r p t i o n o f t h e s e c a t i o n s on t h e edges o r on t h e e x t e r n a l s u r f a c e o f t h e c l a y p a r t i c l e s . Working w i t h l a r g e o r g a n i c molecules such as p o r p h y r i n s , Canesson e t a l . (1978) were a b l e t o s t u d y t h e r e a c t i v i t y o f t h e s e molecules i n t h e i n t e r l a m e l l a r space o f m o n t m o r i l l o n i t e . Meso-tetraphenyl p o r p h y r i n (TPP) and m e s o - t e t r a p y r i d y l p h y r i n (TPyP) can undergo w i t h i n

por-

t h e c l a y s t r u c t u r e e i t h e r p r o t o n a t i o n o r com-

p l e x a t i o n . S i n c e t h e p o s i t i o n on t h e b i n d i n g energy s c a l e o f t h e N Is l e v e l i s s u e d from adsorbed molecules i s s e n s i t i v e t o t h e s u r f a c e environment (Defosse and Canesson,

1976), i t i s p o s s i b l e t o d i f f e r e n c i a t e p y r o l i t i c from t h e aza n i t r o g e n

atoms. Moreover, t h e i n t e n s i t y r a t i o between t h e two N 1s l e v e l s p e r m i t s a quant i t a t i v e d e t e r m i n a t i o n o f t h e v a r i o u s s p e c i e s . I t was concluded t h a t TPP undergoes p r o t o n a t i o n and t h a t t h e e x t e n t o f m e t a l l a t i o n i n t h e i n t e r l a m e l l a r space depends upon t h e n a t u r e o f t h e exchangeable c a t i o n , t h e s t a b i l i t y o r d e r b e i n g

224 Cu2+ > Co2+, Sn4+ > Fez+, Mn2'.

The s i t u a t i o n i s q u i t e d i f f e r e n t w i t h TPyP s i n c e

t h i s molecule can undergo p r o t o n a t i o n on t h e p y r i d y l s u b s t i t u e n t s c o m p a t i b l e w i t h m e t a l l a t i o n o f t h e p o r p h i n r i n g . T h i s i s observed w i t h Co2' and Cu2+ i n t e r l a y e r as c a t i o n s . W i t h o t h e r c a t i o n s , c o m p l e x a t i o n i s n o t complete and r e s u l t s o b t a i n e d by X.P.S. al.,

c o n f i r m t h o s e o b t a i n e d by U . V .

and v i s i b l e s p e c t r o s c o p y (Van Damme e t

1978).

From t h i s survey o f X.P.S. concluded t h a t X.P.S.

t e c h n i q u e s a p p l i e d on c l a y

m i n e r a l s , i t can be

emerges as a p r o m i s i n g t o o l .

Since s u r f a c e p r o p e r t i e s o f c l a y s and c l a y m i n e r a l s a r e o f m a j o r i m p o r t a n c e

X.P.S.

cannot f a i l t o occupy a more p r o m i n e n t p l a c e i n t h e f u t u r e f o r a b e t t e r un-

derstanding o f t h e various i n t e r a c t i o n s o f these minerals w i t h t h e external environment. The use o f X.P.S.

i s t h u s e x p e c t e d t o spread more b r o a d l y , n o t o n l y i n

t h e p a r t i c u l a r t o p i c o f c l a y m i n e r a l s , b u t a l s o i n a l l f i e l d s o f s c i e n c e f o r which surface p r o p e r t i e s a r e o f major i n t e r e s t .

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227 SUBJECT I N D E X A

164-166, 168

Absorption c o e f f i c i e n t

163

A b s o r p t i o n edge Accessory m i n e r a l s

A c i d base r e a c t i o n s

B

12

A c t i v a t i o n energy b y T.G.

B a c k s c a t t e r geometry i n Mossbauer

207

A c t i v a t i o n energy f o r i o n i c hop Adsorbed w a t e r i n . z e o l i t e s , external vibrations

206

frequencies i n Beidellite

222, 223

-

176 124, 125 174

106

13

Amphiboles

130

A n t i ferromagnetic o r d e r i n g

124

A r c h a e l o g i c a l samples, Wjssbauer spectra

199, 200

Benzidine

-

176, 177

Fe3+ i n t e r a c t i o n

181 180, 181

214 Biotite 123, 130, 132, 133, 180 B i o t i t e - Ht and F- NMR 102, 103 2 , 2 ' - b i p y r i d i n e complexes 176 Boehmite 20 Boehmite - NMR second moment 106 B r i 11ia n t green (dye a d s o r p t i o n ) 182 Brucite 22 i n ESCA measurements

176, 179, 180

A l u m i n o s i l i c a t e s - Na and A1 NFlR Alunite

...

alkylamnonium

B i n d i n g energy s c a l e c a l i b r a t i o n

106

NFlR second moment

A1 on c l a y s

-

Benzo-acetophenone ( i n d i c a t o r )

5, 16, 73

Allophane Allophane

n

Benzene - copper complexes

164

Aggregation o f c l a y p a r t i c l e s

Alkylammonium exchange

-

107

NMR

A d s o r p t i o n processes s t u d i e d

Akaganei t e , B-Fe203

21

Beidellite, interlayer cation

169, 171, 175

polyamines

125, 126

spectroscopy Bauxite

A d s o r p t i o n o f w a t e r and a l i p h a t i c

Ag on c l a y s

176

A u ( I I 1 ) on c l a y s

180

A c i d i t y , Lewis

213

Auger r e l a x a t i o n process

180

A c i d i t y , Bransted

by ESCA

31

microscopy)

165, 166 11, 14 164, 181

Absorption o f l i g h t

Atomic r e s o l u t i o n ( e l e c t r o n

132

B u l k chemical c o m p o s i t i o n of

219

minerals

Area o f peaks i n M s s b a u e r

120, 132 Arene r a d i c a l c a t i o n s 154, 155 Asbestos 17

Calcite

Assignments o f exchangeable c a t i o n s

Ca on c l a y s

spectroscopy

v i b r a t i o n a l modes

205

AtmosFhere c o n t r o l i n t h e r m a l analysis

16

C

Carbonates

13

176

11, 13, 16, 17

Carbon c o n t a m i n a t i o n o v e r l a y e r C a t i o n exchange c a p a c i t i e s

223

217

228 139, 141, 143, 144

Cations v i b r a t i o n i n

C o p p e r ( I 1 ) ESR

199, 200, 201, 202 Ceramics 18, 19, 21 Chamosi t e 123 Charge d e n s i t y d i s t r i b u t i o n 172

Copper e t h y l e n e d i a m i n e

smectites

Charge t r a n s f e r , LMCT band,

163, 169, 174, 176, 177,

MLCT band

178, 179 Chemical t r a n s p o r t

15

214

Chemical s h i f t s i n X.P.S. Chemical s t a t e o f i r o n

219, 220

i n minerals Chernykhite

- H+

Chloramben Chlorite

NMR

105

Crystal f i e l d s t a b i l i z a t i o n

182 132, 180

energy

163

45

microscopy)

C r y s t a l imaging ( e l e c t r o n

182

C h r y s o i d i n e (dyes on c l a y s )

microscopy)

17

Chrysoti l e

172

C r y s t a l growth ( e l e c t r o n

46, 47

C h l o r i t e ( e l e c t r o n microscopy) Chromophore

149, 172, 173, 174 Copper p y r i d i n e complexes 149 C u ( I 1 ) on c l a y s 169, 171, 175, 176, 177 Cu2+-hectorite 140, 155 C u z + - v e r m i c u l i t e 140 Cu(H20)E+ on c l a y s 169 Cr(bip):+ on c l a y s 178 C r on c l a y s 176 C r i t e r i a f o r thermal a n a l y s i s 6 comp 1exes

Cummingtonite

C l a s s i f i c a t i o n o f thermal analysis

Curie p o i n t

37 131 23

6, 8

techniques

C l a y - o r g a n i c complexes

17

0

164 Clay s u r f a c e 169, 173, 174 Cold neutrons 52 Colloidal particles 53, 54, 72 Combination band o f w a t e r 164, 168 C o ( I 1 ) on c l a y s 169, 170, 176

163, 169, 172, 177, 178, 179 20 Defect s t r u c t u r e 152 D e f i n i t i o n s i n thermal analysis 6, 8 D e h y d r a t i o n curves 5, 9

Complexation w i t h t r a n s i t i o n

D e r i v a t i v e thermodilatometry

Clay p a r t i c l e

metals Complexes

181

D e r i v a t i v e thermogravimetry

169, 171, 174, 183

Coordination

C o o r d i n a t i o n number by Plossbauer spectroscopy Coordination s h e l l

127-129 163

Copper d i m e t h y l s u l f o x i d e complexes

Diamagnetism

150

174

Dichroic e f f e c t (general )

125 164, 169

C o o r d i n a t i o n numbers and X.P.S.

18

7, 9, 11,

13

Conversion e l e c t r o n s i n M6ssbauer spectroscopy

d-d band

Decrepitation

195

Dichroism o f i n t e r l a y e r c a t i o n

218

200, 201, 202 Dicinnamalacetone ( i n d i c a t o r ) 180, 181 Dickite 17, 18 D i e l e c t r i c constant 22 Diethylenetriamine (dien) 175, 176 D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y 7, 17, 18 v i b r a t i o n a l bands

2 2 9

5, 7,

D i f f e r e n t i a l thermal a n a l y s i s 15-17, 20, 21, 22

D i f f r a c t i o n , k i n e t i c measurements

70

Ethylenediamine (en)

171, 172, 174

Evolved gas a n a l y s i s

7, 8, 12, 13,

14, 15

Diffraction o f light

164

Evolved gas d e t e c t i o n

Diffraction,

51, 53, 71, 72

Exchangeable c a t i o n s l o c a t i o n s

neutron

D i f f r a c t i o n o f X-rays

53, 56

D i f f u s e r e f l e c t a n c e spectrum

i n zeolites 164,

206

60, 63, 64, 68

F

63, 64, 68

Diffusion, rotational Diffusion,

water

60, 6 1

Diffusion coefficients D i f f u s i o n , jump

60, 64

t r a c e r measurements

Factors i n f l u e n c i n g i n t e n s i t y o f 68

D i f f u s i o n , two-dimensional

an X.P.S.

68

line

Fe on c l a y s

180, 181

Fe3+ on c l a y s

d(0Ol) Distance

171

6-Fe ODH

181, 182

Dynamics o f c r y s t a l l a t t i c e

196

216

176, 178, 179, 183

Diphenylcarbinol Dyes

203

E x t e r n a l v i b r a t i o n o f adsorbed

165, 166 Diffusion, f i c k i a n

7, 12, 13

151 124'

F e r r i anni t e

133

F e r r ih y d r i t e

23

Ferromagnetic o r d e r i n g F-factor

E

174

interferometry

59, 60, 63, 67-69

E l e c t r i c f i e l d gradient

58, 59, 61, 62

F o u r i e r t r a n s f o r m scanning

E l a s t i c incoherent structure factor

120, 122, 123, 126

Fourier transform

E f f e c t i v e n u c l e a r charge

124

192

Framework paramagnetic c e n t e r s

116, 117, 118

Free energy of exchange

151

172, 173

129, 130, 133 E l e c t r i c quadrupole i n t e r a c t i o n

114

G

E l e c t r i c quadrupole i n t e r a c t i o n i n Fe2+

117

Gibbsite

E l e c t r i c quadrupole i n t e r a c t i o n i n Fe3+

Gibbsite

117

NMR second moment

Glasses

Electron-beam s e n s i t i v i t y

44

E l e c t r o n b i n d i n g energy measurement E l e c t r o n escape depth

214

E l e c t r o n mean f r e e p a t h

216

E l e c t r o n microscope imaging 73

E l e c t r o n s p i n resonance

139

22

Glauconite

21

-

123, 130 H+

NMR

Goethite, a-Fe DOH

105 16, 18, 22, 124,

125, 126, 127, 129

181

Emanation thermal a n a l y s i s

Glauconite

106

31

E 1e c t r o n m i c r o s copy

Electron transfer

22

-

H

Hal 1o y s i t e 7, 14

5, 18

H a l l o y s i t e ( e l e c t r o n microscopy)

45

2 3 0 Halloysite

-

water NMR

H-bonding H-clay

94

I n f r a r e d spectroscopy

182, 183

microscopy)

180

Heating curves Hectorite

8

reflectance)

70, 178

-

Hectorite - fluoro

-

Hectorite

Ag

-

Na NMR

peaks

Benzene NNX

107

89, 93

Hematite, a-Fe203

23, 124, 125,

126, 127, 129 169

High-flux reactor

51

microscopy

205, 206 149

164, 169, 171

178, 183 Interlayer cation vibration

38

frequencies

107

202

I n t e r s t r a t i f i c a t i o n (electron

Hydrated metal i o n s Hydrobiotite

139

microscopy)

10

55

169

I r o n ESR

150, 151, 152

I r o n oxides

14, 15, 17, 23, 124, 125

126, 127, 129

I d e n t i f i c a t i o n o f Fe-containing

I r o n oxyhydroxides

129- 13 1 123-126

I d e n t i f i c a t i o n o f oxidation states of Fe by Mossbauer spectroscopy

I s o b a r i c mass-change d e t e r m i n a t i o n Isomer s h i f t 114 Isomer s h i f t and c o o r d i n a t i o n number

’

121-123 123

Isomer s h i f t i n Fez+

121-123

Isomer s h i f t i n Fe3+

121-123

Isomorphic s u b s t i t u t i o n

water NMR

8

I n v e r s e h e a t i n g - r a t e curves

I d e n t i f i c a t i o n o f m i n e r a l species by Mijssbauer spectroscopy

124, 125, 126,

127, 129

s i t e s i n s i l i c a t e s by Mossbauer

89

Mossbauer s p e c t r a

Image processing ( e l e c t r o n

128

164 125, 129

Isothermal mass-change d e t e r m i n a t i o n

40

Incoherent s c a t t e r i n g f u n c t i o n

58-60

J

180, 181

I n f r a r e d a c t i v e modes

6, 7, 9

Isomorphous s u b s t i t u t i o n , e f f e c t on

23

microscopy)

17

I n t r a l i g a n d t r a n s i t i o n band

I

spectroscopy

45

I n t e r s t r a t i f i e d minerals

Hydrogen neutron c r o s s - s e c t i o n

Indicators

K, Y and E z e o l i t e s

I n t e r l a m e l l a r space

31

microscopy

-

193 192

in

High v o l t a g e e l e c t r o n

Illite

Interferogram Int e r f e rome t e r

51, 71, 72

I n t e r l a m e l l a r metal complexes

High-resolution-electron

Ilmenite

117, 118

I n t e r c a l a t i o n complexes

I n t e r i o n i c vibrations o f cations

Hexaquo complexes

Illite

165

I n t e n s i t i e s o f Mossbauer a b s o r p t i o n

H e c t o r i t e - water NMR

Humin

40

I n t e g r a t i o n sphere ( d i f f u s e

5, 7, 15

H e a t i n g - r a t e curves

54

Instrumental resolution (electron

195

Jahn-Teller e f f e c t

169, 170, 180

9

2 3 1 Janus green (dye)

182

Plet eor it e s

21

Method o f q u a s i - i s o t o p i c s u b s t i t u t i o n 196

K

Kaolinite

Mg on c l a y s

5, 17, 18, 123, 152,

Mica

Mica ( e l e c t r o n microscopy)

178, 180

-

Kaolinite

NMR

106

M i ca-montmori 11o n i t e

K a o l i n i t e - water NMR Kerogen

179

10, 17, 20

89

Mn2+-hectorite

22

Kimberl it e

145, 148

Mn2+-montmori 1 l o n i t e

Keying i n phenomenon

Mn2+ m o b i l i t y

178

23

145

Mn on c l a y s

5, 10, 18, 123, 130

132, 180

178

M o n t m o r i l l o n i t e , Ca2+

L a t t i c e t r a n s i t i o n metal i o n s

178

L a t t i c e v i b r a t i o n s i n micas L e p i d o c r o c i t e , y-Fe203

198 124, 125

170, 183

Ligand f i e l d s t r e n g t h

66, 69, 71

Montmori 1 l o n i t e , i n t e r 1 ayer c a t i o n frequency i n

...

200

Montmorillonite, Li+

169, 173, 175, 176, 177

Limestone

145

Montmorillonite 169

Lattice sites

Ligand

176

Mn2+-vermicul it e

L a t t i c e oxygen

Ligand f i e l d

145

147

Mn*+-nontronite

L

41, 43, 46

17

172

70, 72

Montmori 1l o n i t e , Mg2+

66, 67

P l o n t m o r i l l o n i t e , Na+

70-72

Montmori 1 l o n i t e , Na+-deuteropyridine

21

M o n t m o r i l l o n i t e , p y r i d i n e on,

L i n e a r image approximation ( e l e c t r o n microscopy)

Montmori 1 l o n i t e - w a t e r NMR 36

L i n e shapes o f Mossbauer s p e c t r a L i n e w i d t h i n Nossbauer spectroscopy

113 - 137

Mdssbauer spectroscopy and mineral a l t e r a t i o n reactions

114, 119, 120, 121

L o r e n t z i a n l i n e shape

119

72 89, 94

Mossbauer spectroscopy 119

131-132

Mossbauer spectroscopy i n q u a n t i t a t i v e analysis

126, 127

M o b i l i t y o f i n t e r l a y e r metal i o n s

Loss tangent

19, 22

Luminescence

165, 178, 180

Mobility o f nitroxides

156

Mu 1t i s 1ice met hod ( e l e c t r o n microscopy)

ri

Muscovite

41 123, 179, 180

Magnetic h y p e r f i n e i n t e r a c t i o n 114, 118, 124 Magnetite, Fe304 Manganese ESR Manganese oxides Marble

N

23, 124 145, 148

Na on c l a y s

Maxwell i a n energy spectrum

176

Neutron d i f f r a c t o m e t e r

17

21

Neutron d i f f r a c t i o n 52

70

56-58 164

145

2 3 2 ldeutron f l u x d i s t r i b u t i o n s Neutron

-

52

nucleus i n t e r a c t i o n

52

51 - 75

Neutron s c a t t e r i n g

: k u t r o n s c a t t e r i n g coherent

51, 53

Neutron s c a t t e r i n g e l a s t i c

53,

Organic m a t t e r

14

Order o f r e a c t i o n

12

Organic r a d i c a l ESR

154

Overtone ( v i b r a t i o n )

164, 168

56-58, 63 Neutron s c a t t e r i n g i n c o h e r e n t

53, 56

Neutron s c a t t e r i n g i n e l a s t i c

53, 57

58 Neutron s c a t t e r i n g q u a s i - e l a s t i c 54, 58-60,

51, 51, 53

169, 174, 176, 184

Ni(en)3+ on c l a y s

154, 155, 156

Nomenclature i n thermal a n a l y s i s

6-8

197

125

-

N.M.R.

- C13 spectra

N.M.R.

-

N.M.R.

- Electron-nucleus c o u p l i n g

C o r r e l a t i o n times

85, 88,

89, 94 107

Doublet s p l i t t i n g

82, 88,

Phlogopite

complexes

H+ and

Phonons

F-

183 183

88, 89, 93,

178, 180

P h o t o e l e c t r o n i c cross s e c t i o n

216

Polyamines

169, 171

182, 183, 223, 224

Portlandit e

22

77, 8 1

Proton a s s o c i a t i o n t i m e Proton t u n n e l i n g 85,

Pyrophyllite

174

51

chromatography-mass

14

spectrometry Pyroxenes

70

13

Pyrolysis-gas 0

213

22

Pulsed neutron sources Pyrite

89, 94, 107

46

53, 54, 72

Pressure l i m i t a t i o n o f X.P.S.

Spin-lattice relaxation

180

Photo-chemical s u b s t i t u t i o n

81, 106

18

168 164, 166, 167

Photo-chemical chemistry

Principles

Optical electronegativity

102, 103

Photo-chemical physics

Second moment

Opal

NMR

54

P o r p h y r i ns

94, 102

19, 21, 22

176

128, 132

-

Pores s i z e s

84, 102

O i l shale

180, 181

180, 182

Polytypes ( e l e c t r o n microscopy)

93, 102

Local o r d e r

Pesticides

Photo-chemical c a t a l y t i c p r o p e r t i e s

54, 68, 70, 164 N.M.R.

N.M.R.

e f f e c t on

Photoacoustic spectroscopy

Nuclear magnetic resonance (N.M.R.)

-

-

Mossbauer spectrum

Photoacoustic spectrometer

123, 128, 130, 174

Normal coordinates i n micas

N.M.R.

71

Phlogopite

157

N.M.R.

51

Particles orientation

1,lO-Phenanthroline

174, 175

N i t r o x i d e s p i n probes

-

5, 10, 17, 180

P-Dimethylamino-azobenzene

72, 73 N i ( I 1 ) on c l a y s

N.M.R.

Palygorskit e

P a r t i c l e s accelerators Particle size

66-71

Neutron scattering, small-angle

Nontronite

P

5 130, 131, 132, 133

233

Q Quadrupole s p l i t t i n g

114, 116-118

132, 133 Quadrupole s p l i t t i n g and coordination

number 128 Quadrupole s p l i t t i n g in Fez+

117,

1 2 2 , 132 Quadrupole s p l i t t i n g in Fe3+

117,

1 2 2 , 132 Quadrupole s p l i t t i n g v a r i a t i o n s with temperature 132 Quantum y i e l d 178 Quartz 14, 16 Quartzite 20 Quasi-elastic broadening curve 60 Quenching (luminescence) 178

R Racah's i n t e r e l e c t r o n i c repulsion parameter 174, 184 Raman spectroscopy Random walk method Reaction k i n e t i c s

54 62 11, 17

21 Refractories Reciprocal l a t t i c e vector 56, 57 Reciprocal space 56 Recoil-free f r a c t i o n 120, 122, 123 126 Redox reactions

164, 181 Reflection, specular r e f l e c t i o n 164, 165 Refraction of l i g h t 164 Relationship between a c t i v a t i o n energy f o r c a t i o n i c hop and vibration frequency 207 Relationship between cation s e l f d i f f u s i o n and vibration 208, 209 Relaxation e f f e c t s 119-120 Residence t i me 68, 69

Resolution function (neutron scattering) 58 Rhodamine B (dye) 182 Rotational c o r r e l a t i o n time 60 Ru(bip)i+ on clays 176, 178

S

Safranine (dye)

182

Sample preparation f o r f a r infrared spectroscopy 194 Sampling depth (ESCA) 214, 217 Scattered wave amplitude 52, 53 S c a t t e r i n g cross-section coherent 54,55 S c a t t e r i n g cross-section incoherent 54-56 S c a t t e r i n g length coherent 54, 55 S c a t t e r i n g length incoherent 54, 55 S c a t t e r i n g vector 56 Scherzer defocus ( e l e c t r o n microscopy 37 Schists 13 Schuster-Kubel ka-Munck equation 166 Selection r u l e s (neutron s c a t t e r i n g ) 54 S e l f - c o r r e l a t i o n function (incoherent neutron s c a t t e r i n g ) 58, 59, 62 S e n s i t i v i t y of X.P.S. 216 Sepiolite 10 S e p i o l i t e ( e l e c t r o n microscopy) 45 Serpentine ( e l e c t r o n microscopy) 45 Shales 13 Shear modulus 19 Silica 164 Simultaneous techniques i n thermal 8 , 9 , 10, 12, 13, 14, ana 1ys i s 15, 16, 18, 19, 21, 22 Small-angle s c a t t e r i n g (neutron) 51, 53, 72, 73 Small-angle s c a t t e r i n g (X-rays) 72 Smectite 18, 21, 68, 151, 163, 178

2 3 4 Smectites, redox r e a c t i o n s Soils

131, 132

Thermodilatometry Thermoelectric power

23

22

Thermoelectrometry

6, 7, 22, 23

10, 14

Solid-state transitions Solid-state reactions

Specimens i n e l e c t r o n microscopy Spin magnetic moment

6, 7, 8, 9-12,

Thermogravimetry

17

11, 17

Soluble s a l t s

7, 18, 19

42

Thermoluminescence

21, 23

Thermomagnetometry

23

Thermomechanical measurement

53

Spin-orbit coupling

170

Thermomicroscopy

Spin p a i r i n g energy

174

Thermoparticulate a n a l y s i s

microscopy) Stereospecifi c i ty

8

21 7, 20

T h i n s e c t i o n n i n g ( e l e c t r o n microscope

Structure resolution (electron microscopy)

7, 21

Thermorefractometry Thermosonimetry

178

7, 14, 15 21

T hermopt omet r y

45

S t a n d a r d i z a t i o n i n thermal a n a l y s i s

7, 19

21

Thermophotometry

Stacking sequences ( e l e c t r o n

13, 18

spec imen )

38

44

Study o f surfaces

125, 126

T i m e - o f - f l i g h t spectrometers

57, 58, 66

Surface a c i d i t y

180

Torsional b r a i d analysis

19

Surface charging e f f e c t Surface oxygen

Transfer function (electron

214

microscopy)

164, 170, 172, 181, 183

~i

Surface p r o p e r t i e s ( X . P . S . )

214

Symbols i n thermal a n a l y s i s

8

-

TI*

35

transition

164, 169, 176, 177, 183

n

-f

n* t r a n s i t i o n

165, 169, 183

T r a n s i t i o n metals

163, 164, 169, 171, 178, 183

T

Triphenylcarbinol

Temperature programmed d e s o r p t i o n

14

Tetraethylenepentamine ( t e t r e n )

T r i p l e - a x i s spectrometers ( n e u t r o n )

U

172, 174

T h e o r e t i c a l expression f o r i n t e r l a y e r c a t i o n frequency

203

Thermal gas t i t r i m e t r y

13

Thermal neutrons

52

58

175,

176 Tetragonal d i s t o r t i o n

180, 181

U l t r a v i o l e t and v i s i b l e l i g h t spectroscopy UV-vis-N.I.R.

Thermally s t i m u l a t e d c o n d u c t i v i t y

163- 189

spectroscopy

163, 164,

178

22, 23 Thermally s t i m u l a t e d c u r r e n t

22

v

Thermally s t i m u l a t e d d e p o l a r i z a t i o n 22

Vanadium (neutron c r o s s - s e c t i o n )

Thermal v o l a t i l i z a t i o n a n a l y s i s Thermoacousti metry

7, 20, 2 1

12

Vanadyl ESR

55

142, 146, 147, 148

Vanadyl i o n m o b i l i t y

147

2 3 5 V02+-hectori t e

142, 146, 148

V a r i a t i o n s w i t h temperature o f Mossbauer s p e c t r a

132

Vermiculite

132, 179, 180

V e r m i c u l i t e Ca2+

66, 67, 69

V e r m i c u l i t e Co2+

72

Vermiculite, i n t e r l a y e r cation frequencies

200, 201

Vermicul ite-water N M R

89, 93, 106

V i b r a t i o n a l modes (neutron

52

scattering)

V i b r a t i o n a l modes o f potassium i n micas

197, 198

w 51, 61,

Water (neutron s c a t t e r i n g ) 66-71 Water (UV and v i s i b l e l i g h t spectroscopy)

168, 169, 171,

172, 114, 175, 178 Weakly s c a t t e r i n g o b j e c t ( e l e c t r o n microscopy)

33

X X.P.S.

fundamental r e l a t i o n s h i p s

212

X-rays fluorescence process

213

X-rays p h o t o e l e c t r o n s d i f f r a c t i o n

222

Z

Zeol it e s

14, 170

Z e o l i t e , exchangeable c a t i o n s locations i n

...

203

Z e o l i t e , s i t e I1 c a t i o n frequency

207

Z e o l i t e , s i t e I11 c a t i o n frequency 207

Zn on c l a y s

176