High-Resolution Transmission Electron Microscopy Applied to Clay

Chapter 4 High-Resolution Transmission Electron M i c r o s c o p y A p p l i e d to C l a y M i n e r a l s George D. Guthrie, Jr., and David R. Veblen

Downloaded by MONASH UNIV on November 26, 2015 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch004

Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218 The computer image simulation technique was used to elucidate problems concerning the interpretation of high-resolution transmission electron microscopy images of clay minerals (serpentine, kaolinite, muscovite, biotite, chlorite, illite/smectite, and disordered intergrowths). One-dimensional HRTEM images of clay minerals can be interpreted successfully and can provide useful information, including layer spacing, layer sequence, and compositional periodicity. However, the images vary with specimen orientation and microscope focus, so ambiguities may be present in the interpretation. The transmission electron microscope (TEM) is a unique instrument for studying chemical and structural relationships among solid phases. Its analytical capabilities allow chemical analysis of volumes as small as 1000 nm (e.g., see Chapter by Mackinnon), and its imaging capabilities allow resolution of structural detail less than 1 nm. Yet, with point-to-point resolutions greater than the size of most atoms or chemical bonds, the TEM is generally incapable of resolving individual atoms directly; hence, detailed structural interpretation of high-resolution TEM (HRTEM) images is not always straightforward but commonly requires computer simulation of the image. Some microscopes with relatively high electron accelerating potentials (e.g., £400 keV) theoretically are capable of resolving most atoms (e.g., 1); however, other factors, such as beam damage, can degrade resolution significantly, so that detailed interpretation of an image taken with an atomic resolution microscope often requires computer simulation to infer atomic-scale information. Furthermore, HRTEM images vary greatly as a function of crystal orientation and optical parameters, and simulations are necessary for assessing these effects. This chapter will discuss HRTEM techniques and interpretation of HRTEM images of clay minerals using computer image simulations. The process of computer image simulation will also be discussed. Though the images in this chapter were taken with or calculated for a specific microscope, the results are also applicable quantitatively to microscopes with similar optical characteristics and qualitatively to most microscopes. We have calculated images for microscopes with much 3

0097-6156/90/0415-O075$O6.00/0 © 1990 American Chemical Society

In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

poorer resolutions ( i . e . , a point-to-point resolution of 0.53 nm) and obtained s i m i l a r r e s u l t s .


Introduction to TEM An electron microscope uses a focussed beam of electrons to illuminate a specimen. Electrons are accelerated along the microscope column from a source toward the specimen, and magnetic lenses focus the electrons onto the specimen. In a transmission electron microscope, electrons that pass through the specimen are focussed by a d d i t i o n a l magnetic lenses to enlarge the electron d i f f r a c t i o n pattern that forms on the back f o c a l plane of the objective lens or to project an enlarged image of the specimen that forms on the image plane. Thus, e i t h e r the image plane or the back f o c a l plane may be projected on to a viewing screen, a sheet of f i l m , or some other recording device. Veblen ( 2 ) , Spence Q ) , Cowley (A), and McLaren (ϋ) discuss TEM imaging p r i n c i p l e s i n greater d e t a i l . Low-Resolution Methods. By placing an aperture i n one of the image planes below the specimen, a d i f f r a c t i o n pattern may be obtained from a s p e c i f i c region of the specimen. Conversely, by p l a c i n g an aperture i n the back f o c a l plane of the objective lens, an image may be formed from s p e c i f i c d i f f r a c t e d beams. With an appropriate placement of the aperture, images i n the TEM can be formed by using only the "undiffracted" c e n t r a l beam, one or more d i f f r a c t e d beams, or a combination of the two. In conventional b r i g h t - f i e l d TEM, images are formed from the c e n t r a l beam to produce high-contrast, low-resolution images: Strongly d i f f r a c t i n g c r y s t a l s w i l l appear dark, while c r y s t a l s that are not strongly d i f f r a c t i n g w i l l appear brighter. In contrast, d a r k - f i e l d TEM images are formed from a d i f f r a c t e d beam, causing c r y s t a l s that are strongly d i f f r a c t i n g electrons into that beam to appear bright compared to other parts of the specimen. Both conventional b r i g h t - f i e l d and d a r k - f i e l d TEM are simple techniques capable of producing images that show large-scale features, such as the relationships between c r y s t a l s . These imaging techniques also have been used extensively to study c r y s t a l defects (e.g., JL. £z£) . High-Resolution Methods. Conventional b r i g h t - and d a r k - f i e l d TEM produce image contrast based on variations i n the amplitude across the specimen of the image-forming beam. In HRTEM, images generally are formed using the c e n t r a l beam and a number of surrounding d i f f r a c t e d beams, and contrast i s produced by differences i n both the amplitudes and the phases of the image-forming beams. Hence, t h i s technique i s sometimes referred to as phase-contrast imaging. Veblen (2) gives a r e l a t i v e l y non-technical review of HRTEM imaging and Spence Q) and Cowley (4.) provide more rigorous accounts of the method. Many factors a f f e c t the d e t a i l s of images produced by HRTEM, two of which are focus conditions and c r y s t a l o r i e n t a t i o n . The c r i t i c a l lens used to image the specimen i s the objective lens, and i t s focus has a profound influence on the image produced. When the microscope i s focussed exactly on the Gaussian image plane (the f o c a l plane on which electrons passing exactly along the microscope's c e n t r a l axis are i n focus), a r e l a t i v e l y low-contrast image i s formed. To improve contrast and to p a r t i a l l y compensate for lens d e f i c i e n c i e s (such as spherical aberration), the objective



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lens i s generally weakened to focus on a plane s l i g h t l y above the specimen (3.7): t h i s focus condition i s termed underfocus. Conversely, overfocus refers to conditions when the objective lens i s focussed above the Gaussian image plane. The optimum degree of underfocus i s a function of the accelerating voltage of the electrons and the spherical aberration c o e f f i c i e n t of the objective lens; however, objective lenses normally are underfocussed between 50 and 250 nm. Various c r i t e r i a may be used to choose the optimum focus condition. The focus condition f o r the most common set of c r i t e r i a i s termed the Scherzer focus (2). Under i d e a l conditions, extreme care can be taken to orient a specimen p e r f e c t l y i n the electron microscope. In t h i s case, HRTEM images may contain d e t a i l e d information i n two dimensions. Such images, when formed at the optimum value of underfocus, commonly r e f l e c t variations i n the e l e c t r i c a l p o t e n t i a l of the structure and may be interpreted d i r e c t l y : Dark areas i n the image correspond to regions of high p o t e n t i a l , and l i g h t areas correspond t o regions of low p o t e n t i a l . These types of images are sometimes referred to as structure images, and they are e a s i l y obtainable when problems such as s t r u c t u r a l damage by incident electrons and large variations i n c r y s t a l orientation are not s i g n i f i c a n t . Under non-ideal conditions, i t i s not always possible to orient a specimen exactly or to focus the objective lens optimally. In these cases, i t may be possible to orient the specimen p a r t i a l l y i n only one d i r e c t i o n , thereby forming a one-dimensional image. Images under non-ideal conditions may not be interpreted d i r e c t l y , and computer image simulation of the s p e c i f i c structure and imaging conditions i s required to understand such images f u l l y . Even s l i g h t misorientations of the specimen and changes i n focus can a f f e c t the nature of the image, for instance, by changing the character of the dark and l i g h t areas or by s h i f t i n g t h e i r positions r e l a t i v e to the r e a l c r y s t a l structure. I t i s the i n t e r p r e t a t i o n of these types of images that w i l l be addressed below.

Computer image Simulation The point-to-point resolution of most TEMs i s larger than most chemical bond lengths; hence, atomic resolution i s normally not d i r e c t l y possible. However, atomic-level d e t a i l can be obtained i n some cases from HRTEM images when computer image simulation techniques are used. Furthermore, computer image simulation may allow the interpretation of images obtained under non-ideal imaging conditions. In computer image simulation, images of a given s t r u c t u r a l model are calculated for a set of imaging conditions; computed and experimental images are then compared to determine the correspondence between the s t r u c t u r a l model and the sample. Several d e t a i l e d discussions address the theory and practice of computer image simulation (e.g., 8-10). so only a b r i e f discussion w i l l be presented here. Computer image simulation may be achieved by several d i f f e r e n t techniques (lû) but the most common method i s the m u l t i s l i c e c a l c u l a t i o n . In t h i s method, input consists of a s t r u c t u r a l model, the projection d i r e c t i o n through the structure, and parameters describing the electron microscope. S l i g h t misorientations of the structure from the i d e a l projection d i r e c t i o n may also be simulated. In the computation, the c r y s t a l i s sectioned into s l i c e s perpendicular to the electron beam, and the e l e c t r i c a l p o t e n t i a l of the structure within each s l i c e i s projected onto a plane, forming a r



78


"phase-grating" at the bottom of the s l i c e (1Û). The electron beam i s allowed to propagate through one slice-thickness of vacuum p r i o r to each phase-grating, and the e f f e c t s of Fresnel d i f f r a c t i o n are calculated. The electron beam then interacts with the phasegrating, and phase s h i f t s and amplitude changes due to t h i s i n t e r a c t i o n are calculated. This process i s i t e r a t e d and continues u n t i l the desired sample thickness i s reached, whereupon the e f f e c t s of the electron microscope optics are considered. In t h i s step, parameters describing spherical and chromatic aberration, voltage and current fluctuations i n the electron beam and magnetic lenses, defect of focus, and aperture sizes may a l l be varied. The spherical and chromatic aberration c o e f f i c i e n t s , voltage and current fluctuations, and apertures are calculated or obtained f o r a given microscope Q ) ; the defect of focus can be varied to simulate d i f f e r e n t experimental focus conditions. The simulations presented i n t h i s chapter were c a l c u l a t e d using the SHRLI computer programs (11), version 80F. O p t i c a l parameters c h a r a c t e r i s t i c of a P h i l i p s 420T electron microscope were used; d e t a i l s of the calculations are given i n Guthrie and Veblen (12).

Introduction to TEM of Clay Minerals Clay minerals are fine-grained sheet s i l i c a t e s that exhibit a large range of structures and compositions. They t y p i c a l l y occur as breakdown products from the hydrothermal a l t e r a t i o n or weathering of many common rock-forming minerals, and hence they constitute a major portion of s o i l s . The large surface-to-volume r a t i o s r e s u l t i n g from t h e i r small sizes provide large surface areas to which water and other substances commonly adsorb. As such, clay mineral surfaces are e f f e c t i v e s i t e s f o r c a t a l y s i s (12). Because of t h e i r chemical and s t r u c t u r a l complexities at a small scale, clay minerals and t h e i r large-grained equivalents, the m a c r o s c o p i c a l l y - c r y s t a l l i z e d sheet s i l i c a t e s , are commonly studied by electron microscopy. Yet, these minerals t y p i c a l l y s u f f e r rapid r a d i a t i o n damage i n the electron beam. Furthermore, clay mineral specimens have a small p a r t i c l e size and t y p i c a l l y exhibit large v a r i a t i o n s i n c r y s t a l orientation. These two problems cause the HRTEM imaging of clay minerals to be plagued by the problem of noni d e a l imaging conditions as discussed above; i t i s generally not possible to orient the specimen or focus the microscope as accurately as i s possible with other materials. Thus, clay minerals provide an excellent opportunity to consider the i n t e r p r e t a t i o n of HRTEM images, since both i d e a l and non-ideal imaging conditions may be addressed. Images of large-grained sheet s i l i c a t e s w i l l be used to exemplify i d e a l imaging conditions, whereas images of f i n e grained clay minerals w i l l be used to discuss the non-ideal case.

Structures of Clay Minerals. The term "clay minerals" generally refers to fine-grained (< lum) sheet s i l i c a t e s . Many d e t a i l e d discussions of the structures and compositions of c l a y minerals e x i s t (e.g., 14-16), and t h e i r i n t e r e s t i n g chemical properties have been reviewed previously (13). However, a short introduction to t h e i r structures i s necessary to understand t h e i r HRTEM images. Clay minerals belong to a s t r u c t u r a l family of minerals known variously as sheet s i l i c a t e s , layer s i l i c a t e s , or p h y l l o s i l i c a t e s . Sheet s i l i c a t e s contain Si04 tetrahedra that are polymerized i n two


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dimensions to form sheets. The sheet s i l i c a t e s (and hydroxides) also contain octahedrally-coordinated cations such as A l , Mg, and Fe. Table I l i s t s the structures presented i n t h i s discussion. An octahedral sheet (O) i s defined as a two-dimensional array of cations (usually, A l , Mg, Fe) octahedrally coordinated by oxygen and/or OH. The i n d i v i d u a l octahedra share edges, and the composition of the sheet can vary from Μ (0,ΟΗ)β ( a l l t r i v a l e n t cations and termed dioctahedral) to Μ3(0,ΟΗ)β ( a l l divalent cations and termed t r i o c t a h e d r a l ) , where M s i g n i f i e s the cation. The octahedral sheet forms the basis of the layer-hydroxide minerals, gibbsite (M - Al) and brucite (M * Mg). A T-0 layer (also termed a 1:1 layer) i s composed of an octahedral sheet joined to a tetrahedral sheet consisting of cornersharing ( S i 0 ) ~ tetrahedra. The unshared a p i c a l oxygens of the tetrahedral sheet replace 2/3 of the hydroxyl groups from one side of the octahedral sheet, thereby giving a s t r u c t u r a l formula of M T 05(0H) to M T 0 (0H)4. This s t r u c t u r a l unit forms the basis of the 0.7-nm sheet s i l i c a t e s , the k a o l i n i t e (M = Al) and serpentine (M = Mg) groups. In a T-0-T layer (also termed a 2:1 layer), tetrahedral sheets attach to both sides of an octahedral sheet, thereby giving a s t r u c t u r a l formula of M T O (OH) to M T O (OH) . The T-0-T layer forms the basis of the sheet s i l i c a t e s p y r o p h y l l i t e (M - Al) and t a l c (M = Mg) . 2

4


4

2

2

4

3

2

5

2

Table I.

4

10

4

3

4

10

4

Fundamental Structural Subunits of Sheet Silicates

Structure

Composition

Octahedral Layer

M (OH)

T-0 Layer

M T 0 (OH)

T-O-T-0 Layer

2[M T 0 (OH) ]

2

2

2

to

6

5

2

2

Thickness M (OH) 3

to

4

5

4

0..48 -0..49

6

M T 0 (OH) 3

to

2

(nm)

5

0..71 -0..73

4

2[M T 0 (OH) ] 3

2

5

4

1..40 -1..41

T-O-T

Layer (A= • )

Y

AM T O (OH)

2

to

AM T O (OH)

2

0..91 -0..94

T-O-T

Layer (A=K,Na)

AM T O (OH)

2

to

AM T O (OH)

2

0,.96 -1..01

2

2

4

4

10

10

3

3

4

4

10

10

M: A l , Mg, Fe; T: S i , A l ; A: K, Na Approximate thickness. Ranges correspond to the thinner aluminum end-members and thicker iron/magnesium endmembers . Thicknesses were obtained from values given i n Ref. 2Λ. The symbol • s i g n i f i e s a vacancy: A- • indicates an unoccupied Α-site. The thicknesses given f o r the case of the occupied Α-site are f o r the true micas.

The region between adjacent T-O-T layers i s termed the i n t e r l a y e r and can contain large c a v i t i e s formed by opposing rings i n the tetrahedral sheets. When the tetrahedral sheet i s e n t i r e l y


80


occupied by s i l i c o n , the T-O-T layer i s charge balanced, and the i n t e r l a y e r i s unoccupied. However, aluminum may replace up to 1/2 of the s i l i c o n atoms i n the tetrahedral sheet, thereby creating a charge on the T-O-T layer. This charge can be compensated by cations (A), such as potassium, sodium, and calcium, occupying the i n t e r l a y e r s i t e s , thus giving a s t r u c t u r a l formula of AM T O (° ) 4 to AM T4O (OH) 4. The r e s u l t i n g minerals are c a l l e d the micas (A = Na or K; T « 3Si + Al) and b r i t t l e micas (A - Ca; T - 2Si + 2A1). The above s t r u c t u r a l subunits may be combined to form various structures. The micas, muscovite and b i o t i t e , consist of a l t e r n a t i n g T-O-T layers and f i l l e d i n t e r l a y e r s i t e s . The c h l o r i t e s consist of a l t e r n a t i n g T-O-T layers and octahedral layers. Disordered intergrowths of c h l o r i t e s , micas, 0.7-nm sheet s i l i c a t e s , and layer-hydroxides have also been observed (e.g., 18-21). H

2

3

4


4

10

10

4

HRTEM Simulation and Imaging of Clay Minerals A large v a r i e t y of sheet s i l i c a t e s may be formed by appropriate combinations of the above s t r u c t u r a l subunits (e.g., 2Λ f o r micas and 22 f o r other sheet s i l i c a t e s ) . In t h i s paper we present both experimentally-derived images and computer-simulated images f o r a range of these structures: serpentine/kaolinite (T-0), c h l o r i t e (TO-T-O), biotite/muscovite (T-O-T-I), and intergrowths of these minerals. Images from w e l l - c r y s t a l l i z e d specimens of these materials w i l l be used to i l l u s t r a t e the types of images that can be expected under i d e a l imaging conditions. Experimentally-derived and computer-simulated images f o r intergrowths of the clay minerals i l l i t e and smectite are also presented. These minerals are s t r u c t u r a l l y s i m i l a r to muscovite (T-O-T-I), but whereas the i n t e r l a y e r for muscovite i s almost completely f i l l e d with K, i l l i t e and smectite have incompletely f i l l e d i n t e r l a y e r s . The i n t e r l a y e r i n i l l i t e i s commonly approximately 75-percent occupied and the i n t e r l a y e r i n smectite i s approximately 20- to 60-percent occupied ( 1 £ ) . Computer-simulated images of intergrown i l l i t e and smectite w i l l be used to i l l u s t r a t e the types of images that can be obtained under non-ideal conditions. Interpretation of two-dimensional images of sheet s i l i c a t e s has previously been addressed. Computer-simulated images of c h l o r i t e (22) and b i o t i t e and muscovite (24.) show that two-dimensional images of these sheet s i l i c a t e s allow determination of the basal spacing (spacing perpendicular to the sheets) and stacking order ( r e l a t i v e rotations and s h i f t s between sheets). Furthermore, these calculations show that at the Scherzer (optimum) focus, many, but not a l l , aspects of the structures may be interpreted d i r e c t l y . Oftentimes, images of sheet s i l i c a t e s are one-dimensional, containing information pertaining to the basal spacing only. Since these types of images are commonly obtained i n studies of clay minerals, only one-dimensional image i n t e r p r e t a t i o n w i l l be addressed here. Interpretation of these types of images f o r mixedlayer i l l i t e / s m e c t i t e has previously been discussed (12). This discussion extends the interpretation to a wider range of sheet s i l i c a t e s and then reviews the results f o r i l l i t e / s m e c t i t e .

Microscope Parameters.

The computer-simulated images presented here were calculated f o r the P h i l i p s 42OT electron microscope (120 kV; C = 2.0 mm; C = 2.0 mm), and the conclusions are generally applicable to electron microscopes with s i m i l a r o p t i c s . In fact, s

c


4. GUTHRIE & VEBLEN

81


some of the experimental images were obtained on a P h i l i p s 400T, yet the differences between these images and images obtained from s i m i l a r structures on the P h i l i p s 420T are p r a c t i c a l l y indistinguishable. Calculations f o r JEOL 100C optics (12) show that i n t e r p r e t a t i o n of sheet s i l i c a t e images f o r electron microscopes with extremely d i f f e r e n t o p t i c a l parameters i s generally s i m i l a r , but d e t a i l s of the i n t e r p r e t a t i o n may vary.


Model Input Structures.

Input structures f o r the computer-simulated images were obtained by s l i g h t a l t e r a t i o n of published structures. The input structures f o r serpentine and k a o l i n i t e were derived from the l i z a r d i t e IX X-ray structure refinement given by M e l l i n i (23.) . Since the calculations were f o r one-dimensional images of the basal spacings, only the .^-coordinates f o r the atoms were input with the 2ς- and y.-coordinates set t o zero. This procedure was used f o r a l l of the c a l c u l a t i o n s presented here. A layer spacing of exactly 0.7 nm was used to allow comparison between 0.7-nm and 1.4-nm structures. For the serpentine simulations, magnesium and s i l i c o n were assumed to be the only cations occupying octahedral and tetrahedral s i t e s , respectively. For the k a o l i n i t e c a l c u l a t i o n s , aluminum and s i l i c o n were assumed to be the only cations occupying the octahedral and tetrahedral s i t e s , respectively. Hydrogen atoms were not included i n the c a l c u l a t i o n s . The input structure f o r c h l o r i t e was derived from the atomic coordinates given i n Brown and Bailey (26.) . Aluminum was assumed to substitute f o r s i l i c o n i n one-quarter of the tetrahedral s i t e s ; t h i s tetrahedral sheet charge was compensated by f e r r i c i r o n s u b s t i t u t i n g for one-third of the magnesium i n the octahedral layer, g i v i n g the formula Mg FeSi A10 (OH) . Hydrogen atoms were not included i n the c a l c u l a t i o n s . A layer spacing of exactly 1.4 nm was used to allow d i r e c t comparison between 0.7-nm and 1.4-nm structures. The input structure f o r b i o t i t e was derived from the refinement reported by Hazen and Burnham (22). Potassium and magnesium were assumed t o be the only cations occupying the i n t e r l a y e r and octahedral s i t e s , respectively. Aluminum was assumed to occupy 1/4 of the tetrahedral s i t e s . A layer spacing of exactly 1.0 nm was used to allow comparison to other 1.0-nm structures. The input structures f o r muscovite, smectite, and i l l i t e were derived from the refinement of a muscovite reported by Richardson and Richardson (2S.) . One layer from t h i s two-layer structure was used to obtain the atomic coordinates. The d e t a i l s of the procedure are presented by Guthrie and Veblen (12). In the muscovite structure, aluminum was assumed to occupy 1/4 of the tetrahedral s i t e s , and potassium was assumed to e n t i r e l y f i l l the i n t e r l a y e r s i t e s . In the smectite structure, aluminum was assumed to occupy 1/20 of the tetrahedral s i t e s , and potassium was assumed to f i l l 1/5 of the i n t e r l a y e r s i t e s , with vacancies f i l l i n g the remaining 4/5; the s t r u c t u r a l formula f o r t h i s smectite i s Ko.2Al2Si3.sAlo.2O10 * ) 2 · In the i l l i t e structure, aluminum was assumed to occupy 3/16 of the tetrahedral s i t e s , and potassium was assumed to f i l l 3/4 of the i n t e r l a y e r s i t e s , with vacancies f i l l i n g the remaining 1/4; the s t r u c t u r a l formula for t h i s i l l i t e i s K 75Al2Si3.25 0.75 10 (° ) 2 · The mixed-layer structure of i l l i t e and smectite was obtained by a l t e r n a t i n g layers of i l l i t e and smectite. To allow comparison to other 1.0-nm structures, a layer spacing of exactly 1.0 nm was used for muscovite and b i o t i t e , and a layer spacing of exactly 2.0 nm was used f o r the 2-layer i l l i t e / s m e c t i t e structure. 5

3

10

8

0H

Al

o

H

0#


82


HRTEM Images Serpentine and Kaolinite.

Serpentine and k a o l i n i t e are T-0 sheet s i l i c a t e s with a basal spacing of about 0.7 nm. Serpentine group minerals have magnesium or iron i n the octahedral s i t e s , g i v i n g a formula of (Mg,Fe) Si 0 (OH) . Kaolinite has aluminum i n the octahedral s i t e s , giving a formula of A l S i 0 ( O H ) . Both mineral groups exhibit a wide range of sheet morphologies: planar structures ( l i z a r d i t e and k a o l i n i t e ) , tubular structures (chrysotile and tubular h a l l o y s i t e ) , bulbous structures (some h a l l o y s i t e , a k a o l i n i t e mineral), and corrugated structures ( a n t i g o r i t e ) . Both groups are common products of the hydrous a l t e r a t i o n or weathering of common rock-forming minerals and, as such, have been studied extensively with HRTEM imaging (e.g., 29-32). 3

2

5

4


2

2

5

4

Published images of serpentine and k a o l i n i t e group minerals c h a r a c t e r i s t i c a l l y consist of one dark-and-light-stripe p a i r per 0.7 nm, with dark and l i g h t bands of approximately equal thickness. Since no computer-simulated images of these structures have been published, i n t e r p r e t a t i o n of these images has been purely i n t u i t i v e , based on the 0.7-nm p e r i o d i c i t y . Furthermore, the correspondence between the dark and l i g h t bands and the underlying structure i s not known, although i t has been assumed that, at optimum defocus, the dark fringes correspond to the T-0 layers. Figure 1 shows an experimental image of a T-0 layer s i l i c a t e ( l i z a r d i t e ) . The 0.7-nm p e r i o d i c i t y i s c l e a r l y apparent, and no s u b p e r i o d i c i t i e s (0.7 nm) can be detected. This image was obtained at or near Scherzer focus, and the c r y s t a l was oriented such that the basal planes were p e r f e c t l y p a r a l l e l to the electron beam, as determined by electron d i f f r a c t i o n patterns of the areas from which the images were obtained. Thus, t h i s image was taken under nearly i d e a l conditions i n that both the focus conditions and orientation were c a r e f u l l y c o n t r o l l e d . Interpretation of t h i s image can be made by comparing Figures 1 and 2. Figure 2a shows a computer-simulated image and the corresponding structure f o r l i z a r d i t e at Scherzer focus. The experimental and computer images match c l o s e l y , and the computer images show that l i g h t fringes i n the images are centered over the space between the T-0 layers and dark fringes are centered over the T-0 layers. This correspondence, however, i s only true f o r these s p e c i f i c imaging conditions: perfect orientation with the c_*-axis normal to the electron beam ( i . e . , the electron beam must be p e r f e c t l y p a r a l l e l to the sheets) and Scherzer focus. Figure 2b shows a computer-simulated image f o r an overfocus condition, i n which the correspondence between image and structure i s reversed: l i g h t fringes i n the images are centered over T-0 layers and dark fringes are centered near the spacing between the T-0 layers. Figure 3 shows that the same interpretations are v a l i d f o r kaolinite.

Chlorite.

Figure 4 shows an experimental image f o r c h l o r i t e obtained near the Scherzer focus and with the £*-axis p e r f e c t l y perpendicular to the electron beam. The image i s c h a r a c t e r i s t i c of published images of c h l o r i t e (e.g., 33-36) which commonly show a repeating sequence of approximately 1.4 nm consisting of a thick dark fringe/a t h i n l i g h t fringe/a t h i n dark fringe/a t h i n l i g h t f r i n g e . Figure 5 shows a computer-simulated image and the associated structure under the same conditions. Comparison of the P



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Figure 1. Experimental HRTEM image of a T-0 layer s i l i c a t e ( l i z a r d i t e ) . Microscope was focussed near the Scherzer focus.

a

b

Figure 2. Computer-simulated image of l i z a r d i t e . focus, b) +50 nm overfocus.

a

a)

Scherzer

a)

Scherzer

b

Figure 3. Computer-simulated image of k a o l i n i t e . focus, b) +50 nm overfocus.




Figure 4. Experimental HRTEM image of c h l o r i t e . Microscope was focussed near the Scherzer focus. A b i o t i t e layer (arrowed) i s intergrown.

Figure 5. chlorite.

Computer-simulated

image at

Scherzer focus of


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85

two images shows a good match between experimental and simulated images and allows a determination of the correspondence between image and structure. Under these conditions, the thick dark band o v e r l i e s the T-O-T layer, and the t h i n dark band o v e r l i e s the i s o l a t e d octahedral layer. These r e s u l t s confirm numerous i n t u i t i v e interpretations from the l i t e r a t u r e (e.g., 2Q 21 24 33) as well as those based on the two-dimensional simulations of Spinnler e_L SLL*. r


(21)

r

P

P

.

Micas. Figure 6 shows an experimental image of b i o t i t e ; the image was obtained near Scherzer focus and with the £*-axis p e r f e c t l y perpendicular to the electron beam. The image i s t y p i c a l of published images of micas (e.g., 33-34). consisting of l i g h t fringe/dark fringe pairs about 1.0-nm t h i c k . Figure 7a shows the respective computer-simulated image f o r s i m i l a r focus conditions. Comparison of the experimental and computer images shows that at Scherzer focus the dark fringe i s centered over the T-O-T layer, and the l i g h t fringe i s centered over the i n t e r l a y e r . Figure 7b shows a computer-simulated image f o r a focus of +50 nm. At moderate overfocus, the correspondence i s i d e n t i c a l ; however, contrast does reverse at a focus between -100 nm and +50 nm ( i . e . , near -50 nm), such that the l i g h t fringe i s centered over the T-O-T layer and the dark fringe centered over the i n t e r l a y e r . Figure 8 shows computersimulated images f o r muscovite where the correspondence between image and structure reverses i n overfocussed images. For Scherzer focus, an extra f a i n t l i g h t fringe appears centered over the i n t e r l a y e r . Contrast does not reverse f o r underfocuses between -100 nm and +50 nm. B i o t i t e / C h l o r i t e Intergrowths. Both ordered and disordered intergrowths of the various sheet s i l i c a t e s are very common, and one example i s the intergrowth of c h l o r i t e and b i o t i t e (e.g., 30 33-34 2Â). These images have been interpreted i n t u i t i v e l y , assuming that the images would resemble a composite of images from pure b i o t i t e and pure c h l o r i t e . Simulations of t h e i r intergrowth have not been published. Figure 4 shows a b i o t i t e layer (arrowed) intergrown with c h l o r i t e , and Figure 9 shows the computer-simulated image and model structure of the intergrowth. Comparison of the images shows that b i o t i t e / c h l o r i t e intergrowths may indeed be interpreted i n terms of a composite of a b i o t i t e image and c h l o r i t e image: the intergrowth of these two structures does not s i g n i f i c a n t l y a f f e c t the l o c a l d e t a i l s of an HRTEM image. P

I l l i t e / S m e c t i t e . Another common intergrowth of sheet s i l i c a t e s i s the mixed-layering of i l l i t e and smectite. As discussed above, i l l i t e and smectite are clay minerals whose basic structures resemble the mica muscovite. Their compositions may d i f f e r s i g n i f i c a n t l y from muscovite, but they generally have a lower occupancy of the i n t e r l a y e r s i t e s than mica. Numerous other compositional differences are possible f o r smectite; however, t h i s discussion w i l l be r e s t r i c t e d to a dioctahedral i l l i t e and a dioctahedral smectite containing potassium and vacancies i n the i n t e r l a y e r s i t e s as given above. Figures 10a and 10b show experimental images of a mixed-layer i l l i t e / s m e c t i t e obtained with the c*-axis perpendicular to the electron beam; Figure 10a was obtained with the objective lens near Scherzer focus, and Figure 10b was obtained from the same area with


r



Figure 6. Experimental HRTEM image of b i o t i t e . focussed near the Scherzer focus.

a

Microscope

was

b

Figure 7. Computer-simulated image of b i o t i t e . focus, b) +50 nm overfocus.

a)

Scherzer




GUTHRIE & VEBLEN

Figure 9. Computer-simulated image at Scherzer focus of b i o t i t e intergrown with c h l o r i t e .



88

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES the objective lens overfocussed by about 100 nm. This sample has been studied extensively by X-ray d i f f r a c t i o n (21) and i s known to contain 83-percent i l l i t e and 17-percent smectite layers (R3ordered, from Zemplini, Hungary). The exact sequences of i l l i t e layers and smectite layers can not be determined by X-ray techniques; however, the smectite layers are believed to be rather evenly d i s t r i b u t e d throughout the sample, implying an average sequence of approximately i l l i t e - i l l i t e - i l l i t e - i l l i t e - s m e c t i t e . In p r i n c i p l e , actual sequences of i l l i t e and smectite layers can be observed d i r e c t l y with HRTEM imaging. However, since the i l l i t e and smectite layers d i f f e r only s l i g h t l y i n t h e i r compositions (subtle differences i n K, A l , and S i ) , they may be d i f f i c u l t to d i f f e r e n t i a t e i n HRTEM images. Indeed, the two d i f f e r e n t layer types are barely distinguishable near the Scherzer focus (Figure 10a), but the two layers are c l e a r l y v i s i b l e f o r overfocus conditions (Figure 10b). This image allows d i r e c t observation of the sequence S-I-S-I-I-I-S-I-I-I-I-I-S-I from top to bottom. Figures 11a and l i b show calculated images f o r the structure IS-I-S-I, at Scherzer focus and overfocus. These calculated images compare well with the experimental images i n Figure 10. At Scherzer focus, l i g h t bands o v e r l i e both the i n t e r l a y e r s i t e s and the octahedral s i t e s within the T-O-T layer; thus, two l i g h t bands and two dark bands occur every 1.0 nm. The compositional p e r i o d i c i t y ( i . e . , the smectite layers) can not be distinguished. However, at overfocus, dark bands o v e r l i e the i n t e r l a y e r s i t e s , and only one l i g h t band and one dark band occur per 1.0 nm. The same e f f e c t i s seen i n the experimental images. Though the computer-simulated images were calculated f o r a two-layer repeat (I-S), calculations for a four-layer-repeat (I-I-I-S) demonstrate that the conclusions may be extended to the larger repeats (12) seen i n Figure 10. Thus, the image simulations show the conditions under which i l l i t e / s m e c t i t e mixed-layer clays can be imaged, and they allow much more rigorous i n t e r p r e t a t i o n of ordering patterns than purely i n t u i t i v e image i n t e r p r e t a t i o n .

Discussion As stated and shown above, HRTEM images are complex functions of many f a c t o r s . Only under very r e s t r i c t e d circumstances do l i g h t and dark fringes i n an HRTEM image exactly o v e r l i e or even resemble regions of low and high electron p o t e n t i a l i n the specimen. Two requirements f o r t h i s correspondence are having the c r y s t a l i n perfect orientation and having the microscope underfocussed to the Scherzer focus. Any deviation from these conditions reduces the degree of c o r r e l a t i o n between image and structure. Even f o r the simple one-dimensional images presented here, c e r t a i n aspects of an image can not be interpreted l i t e r a l l y unless orientation and focus are rigorously c o n t r o l l e d . Notwithstanding, information can be obtained from these one-dimensional images, provided ambiguities i n i n t e r p r e t a t i o n are considered.

Apparent Layer Thickness. Many images of sheet s i l i c a t e s and clay minerals exhibit layers of d i f f e r e n t thicknesses. These thickness variations commonly are measured from micrographs and interpreted as representing true differences i n thickness. However, simulations have shown that imperfect orientation and focus can cause the



4. GUTHRIE & VEBLEN


Figure 10. Experimental HRTEM image of i l l i t e / s m e c t i t e . Smectite i n t e r l a y e r s are arrowed, a) Scherzer focus, b) ~+50 nm overfocus.

a

1 nm Figure 11. Computer-simulated image of Rl-ordered i l l i t e / s m e c t i t e . Smectite i n t e r l a y e r s are arrowed, a) Scherzer focus, b) +50 nm overfocus.


89


90


12.6 7.4

Figure 12. Computer-simulated image of Rl-ordered i l l i t e / s m e c t i t e f o r a JEOL 100C operated at a defocus of -135 nm (see 12 f o r d e t a i l s ) . Smectite layers appear t h i c k e r than i l l i t e layers; however, the model structure assumed a 1.0-nm basal spacing f o r both.

Figure 13. Computer-simulated image of Rl-ordered i l l i t e / s m e c t i t e . The fringes were s h i f t e d r e l a t i v e to the structure when the c r y s t a l was t i l t e d out of perfect o r i e n t a t i o n about 2° (see 12 f o r d e t a i l s ) .



4. GUTHRIE & VEBLEN


apparent layer thicknesses indicated by fringe spacing to deviate from the layer thicknesses in the imaged structure. Intergrowths of illite and smectite represent a good example of this phenomenon. Typically, images of these intergrowths show layers of two apparently different thicknesses (i.e., Figure 10b). The apparently thicker layers (arrowed) frequently are thicker by up to a few tenths of a nanometer; thus, they commonly are interpreted as layers of expanded smectite. The computer-simulated images for illite/smectite clearly demonstrate that apparent increases in layer thickness can result from only chemical differences between adjacent layers. Figure 12 illustrates this apparent difference in layer thickness for a mixed-layer illite/smectite. The structural model had a uniform 1.0-nm spacing between layers, yet the image shows layers of 0.7 nm and 1.3 nm. However, the overall periodicity in the image is 2.0 nm. Correspondence Between Image and Structure. As indicated above, intuitive interpretation of one-dimensional images generally has assumed that, when the microscope is underfocussed to the Scherzer focus, dark fringes in an image correspond to layers of high electrical potential in the structure. For instance, the dark fringes in images of T-O-T (2:1) layer silicates are assumed to overlie the T-O-T layer, whereas the light fringes are assumed to overlie the interlayers. While the computer image simulations have shown this interpretation to be accurate, they have also shown that the position of the fringes is a strong function of the orientation and focus. In other words, deviations from perfect orientation or focus cause the fringes to shift relative to the underlying structure. Figure 13 shows that even slight misorientation can shift the fringes by tenths of nanometers (12). The computer-simulated images presented above and in Guthrie and Veblen (12) have shown that one-dimensional images of clay minerals and sheet silicates can be interpreted and can provide important information. The simulations have also shown, however, that even these simple images must be interpreted carefully. Problems such as fine grain size and rapid electron beam damage can preclude the extreme care in orienting and focussing that is required for images to be interpreted directly (without computer image simulation). Since apparent compositional periodicity, apparent layer thickness, and correspondence between image and structure are each functions of orientation and focus, some ambiguities may be present in any interpretations based on onedimensional HRTEM images of clay minerals. Literature Cited 1. Meike, Α.; Wenk, H.-R.; O'Keefe, Μ. Α.; Gronsky, R. Phys. Chem. Minerals, 1988, 15, 427-437. 2. Veblen, D. R. In Applications of Electron Microscopy in the Earth Sciences; White, J. C., Ed.; Mineralogical Association of Canada: Toronto, Canada, 1985; Chapter 3. 3. Spence, J. C. H. Experimental High-Resolution Electron Microscopy; second edition; Oxford University Press: New York, 1988. 4. Cowley, J. M. In Principles of Analytical Electron Microscopy; Joy, D. C., Romig, A. D., Jr., and Goldstein, J. I., Eds.; Plenum Press: New York, 1986; Chapter 3.


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5. McLaren, A. C. In Applications of Electron Microscopy in the Earth Sciences; White, J. C., Ed.; Mineralogical Association of Canada: Toronto, Canada, 1985; Chapter 1. 6. Hirsch, P.; Howie, Α.; Nicholson, R. B.; Pashley, D. W.; Whelan, M. J. Electron Microscopy of Thin Crystals; second edition; Robert E. Krieger Publishing Co.: Malabar, Florida, 1977. 7. Scherzer, O. J. Appl. Phys., 1949, 20, 20-29. 8. Cowley, J.; Moodie, A. F. Acta Cryst., 1957, 10, 609-619. 9. O'Keefe, M. A. In Electron Optical Systems; Hren, J. J.; Lenz, F. Α.; Munro, E.; Sewell, P. B., Eds.; AMF Ο'Hare: Chicago, 1984; p 209-220. 10. Self, P. G. and O'Keefe, M. A. In High-Resolution Tansmission Electron Microscopy; Buseck, P. R.; Cowley, J. M.; Eyring, L., Eds.; Oxford University Press: New York, 1988 (in press). 11. O'Keefe, Μ. Α.; Buseck, P. R.; Iijima, S. Nature, 1978, 274, 322-324. 12. Guthrie, G. D., Jr.; Veblen, D. R. Clays and Clay Minerals, 1989, 37, 1-11. 13. Barrer, R. M. Zeolites and Clay Minerals as Sorents and Molecular Sieves; Academic Press: London, 1978; Chapter 8. 14. Bailey, S. W. In Micas; Bailey, S. W., Eds; Mineralogical Society of America: Washington, D. C., 1984; Chapter 1. 15. Bailey, S. W., In Crystal Structures of Clay Minerals and their X-ray Identification; Brindley, G. W. and Brown, G., Eds; Mineralogical Society: London, England, 1980; Chapter 1. 16. Brindley, G. W., In Clays and the Resource Geologist; Longstaffe, F. J., Ed.; Mineralogical Association of Canada: Toronto, Canada, 1981; Chapter 1. 17. Deer, W. Α.; Howie, R. Α.; Zussman, J. An Introduction to the Rock-Forming Minerals; Longman: London, 1980. 18. Amouric, M.; Gianetto, I.; Proust, D. Bull. Minéral., 1988, 111, 29-37. 19. Livi, K. J. T. and Veblen, D. R. Am. Mineral., 1987, 72, 113-125. 20. Maresch, W. V.; Massonne, H. J.; Czank, M. Neues Jahrbuch für Mineralogie, 1985, 152, 79-100. 21. Veblen, D. R. Am. Mineral., 1983, 68, 566-580. 22. Hydrous Phyllosilicates; Bailey, S. W., Ed.; Mineralogical Society of America: Washington, D. C. 1988. 23. Spinnler, G. E.; Self, P. G.; Iijima, S.; Buseck, P. R. Am. Mineral., 1984, 69, 252-263. 24. Amouric, M.; Mercuriot, G.; Baronnet, A. Bull. Minéral., 1981, 104, 298-313. 25. Mellini, M. Am. Mineral., 1982, 67, 587-598. 26. Brown, Β. E.; Bailey, S. W. Am. Mineral., 1962, 47, 819-850. 27. Hazen, R. M.; Burnham, C. W. Am. Mineral., 1973, 58, 889900. 28. Richardson, S. M.; Richardon, J. W., Jr. Am. Mineral., 1982, 67, 69-75. 29. Veblen, D. R.; Buseck, P. R. Science, 1979, 206, 1398-1400. 30. Veblen, D. R. Am. Mineral., 1980, 65, 1075-1086. 31. Ahn, J. H.; Peacor, D. R. Am. Mineral., 1987, 72, 353-356. 32. Ahn, J. H.; Peacor, D. R. Proc. Int. Clay Conf., 1987, p. 353356.


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33. Veblen, D. R.; Ferry, J. M. Am. Mineral., 1983, 68, 1160-1168. 34. Banfield, J.; Eggleton, R. A. Am. Mineral., 1985, 70, 902-910. 35. Lee, J. H.; Ahn, J. H.; Peacor, D. R. J. Sed. Pet., 1985, 55, 532-540. 36. Buseck, P. R.; Veblen, D. R. Bull. Minéral., 1981, 104, 249260. 37. Veblen, D. R.; Guthrie, G. D.; Livi, K. J. T. ; Reynolds, R. C. Clays and Clay Minerals, submitted. 38. This work was supported by NSF grant EAR86-09277. Figures 12 and 13 are with the permission of the Clay Minerals Society.


RECEIVED May 25, 1989


High-Resolution Transmission Electron Microscopy Applied to Clay

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