Clays and Clay Minerals, VoL 48, No. 2, 282-289, 2000.
M I X E D L A Y E R I N G OF ILLITE-SMECTITE: RESULTS F R O M
HIGH-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY
AND LATTICE-ENERGY CALCULATIONS
JUAN OLIVES, MARC AMOURIC, AND RI~GIS PERBOST
CRMC2-CNRS, Campus de Luminy, Case 913, 13288 Marseille cedex 09, France
A b s t r a c t - - M i x e d layering of illite-smectite was studied both experimentally, by using high-resolution
transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM), and theoretically,
by using lattice-energy calculations.
Samples from a hydrothermal origin show the transformation of smectite to illite with different ordering
types in the illite-smectite layer sequences. Ordering ranges from complete disordered (Reichweite, R =
0 type) in the less transformed samples to increased local order, with IS and IIS sequences (R = 1 and
R = 2, respectively; I = illite, S = smectite) in more illitized samples.
Lattice-energy calculations are used to determine the structure of the illite-smectite sequence, which
corresponds to the minimum energy. The unit layers are: O0.sTI'TO0.s (O, T, and I', respectively, denote
the octahedral and tetrahedral sheets, and the interlayer. The 0.5 signifies half of the octahedral cations.)
For example, the arrangements of the perfectly ordered . . . I S I S . . . and . . . I I S I I S . . . sequences are respectively... OM(TI'T)~OM(TI'T)s... a n d . . . Ou~(TI'T)IOI(TI'T)IOM(TI'T)s . . . (the subscripts I, S, and M,
respectively, refer to compositions of illite, smectite, and midway between at 0.5). Such arrangements
produce a polar model for TOT layers, which display a T1OMTs structure in the case of IS adjacent layers.
Furthermore, the lattice energies of . . . I S I S . . . and . . . I I S I I S . . . are found to be nearly equal to the
corresponding sums of the lattice energies of illite and smectite. This result indicates that interstratified
illite-smectite and the two-phase assemblage of illite + smectite have similar stabilities.
On the basis of the above model, the solid-state transformation of one smectite layer to one illite layer,
which produces mixed-layer sequences, involves the transformation of an O0.sTI'TO0 ~ unit of smectite
into the same corresponding unit of illite.
Key Words--HRTEM-AEM, lllite-Smectite, Lattice-Energy Calculations, Mixed Layering, Polar 2:1
Layers.
INTRODUCTION
T h e t r a n s f o r m a t i o n of s m e c t i t e to illite is a r e a c t i o n
w h i c h occurs d u r i n g d i a g e n e s i s or at h y d r o t h e r m a l
conditions. M i x e d l a y e r i n g o f illite-smectite, w h i c h results f r o m this process, h a s b e e n i n t e n s i v e l y studied
b y X - r a y d i f f r a c t i o n ( X R D ; e.g., Srodofi, 1984; I n o u e
et al., 1987; R e y n o l d s , 1992) a n d directly o b s e r v e d b y
high-resolution transmission electron microscopy
( H R T E M ) . A c c o r d i n g to c o m p u t e r - i m a g e s i m u l a t i o n s
o f G u t h r i e a n d V e b l e n (1989, 1990), illite a n d s m e c t i t e
layers c a n b e c o m m o n l y d i f f e r e n t i a t e d b y u s i n g m i c r o s c o p e o v e r f o c u s conditions. O n s u c h a basis, seq u e n c e s o f illite-smectite interstratification o f e i t h e r
d i s o r d e r e d or o r d e r e d t y p e s , w e r e o b s e r v e d b y
H R T E M (e.g., A h n a n d Peacor, 1989; V e b l e n et aL,
1990; J i a n g et al., 1990; A m o u r i c a n d 0 l i v e s , 1991;
M u r a k a r n i et al., 1993; 0 l i v e s and A m o u r i c , 1994a,
1994b; H u g g e t t , 1995). I n f o r m a t i o n a b o u t illitization
mechanisms (solid-state, dissolution-crystallization)
w a s also o b t a i n e d f r o m H R T E M
observations
( A m o u r i c a n d Olives, 1991; B u a t i e r e t al., 1992; M u r a k a m i et al., 1993).
A m o u r i c a n d O l i v e s (1991) p r e s e n t e d H R T E M obs e r v a t i o n s o n illite, smectite, a n d illite-smectite (inv o l v i n g h y d r a t e d s m e c t i t e w i t h d ( 0 0 1 ) = 1.25 n m ) in
a s m e c t i t e - r i c h s a m p l e f r o m h y d r o t h e r m a l origin a n d
Copyright 9 2000, The Clay Minerals Society
s h o w i n g a smectite-to-illite t r a n s f o r m a t i o n . T h e present p a p e r c o m p l e t e s this study w i t h t w o a d d i t i o n a l illitic samples. T h e s e three s a m p l e s are r e p r e s e n t a t i v e
o f this h y d r o t h e r m a l t r a n s f o r m a t i o n a n d s h o w various
( d i s o r d e r e d a n d ordered) illite-smectite interstratification s e q u e n c e s as o b s e r v e d b y H R T E M . In addition,
the c h e m i c a l v a r i a t i o n s o f the m i x e d - l a y e r m i n e r a l s are
characterized by analytical electron microscopy
(AEM).
A t h e o r e t i c a l a p p r o a c h o n the b a s i s o f l a t t i c e - e n e r g y
c a l c u l a t i o n s is u s e d to d e t e r m i n e the structure o f
m i x e d - l a y e r illite-smectite b y e n e r g y m i n i m i z a t i o n
m e t h o d s . T h e a p p r o a c h c o n s i d e r s various types of polar a n d n o n - p o l a r 2:1 layers (see O l i v e s a n d A m o u r i c ,
1994a, 1994b). E n e r g y c a l c u l a t i o n s also lead to a better u n d e r s t a n d i n g o f the relative stability o f t h e s e m i n erals a n d yield a d e s c r i p t i o n at the unit-cell scale o f
the solid-state illitization m e c h a n i s m .
MATERIALS AND METHODS
T h e studied m a t e r i a l s b e l o n g to a h y d r o t h e r m a l series f r o m the S h i n z a n area, A k i t a P r e f e c t u r e , Japan. In
this area, the h y d r o t h e r m a l alteration o f silicic volcanic glass ( M i o c e n e E p o c h ) p r o d u c e d a series b e t w e e n
s m e c t i t e a n d illite. T h e m i n e r a l a s s e m b l a g e c o n s i s t s o f
interstratified illite-smectite, illite, smectite, quartz,
282
Vol. 48, No. 2, 2000
Mixed layering of illite-smectite
283
beam intensity with a low-light camera to reduce possible damage caused by the electron beam. The microscope is equipped with a Tracor (TN 5502) energydispersive X-ray spectrometer using a Si(Li) detector
for AEM. Chemical microanalyses were obtained using a beam diameter of 5-20 n m at a constant beam
current. Data were processed by a program based on
the method of Cliff and Lorimer (1975). Note that possible volatility of elements is accounted for by this
method, because the elemental constituents are obtained by comparison to appropriate layer-silicate standards.
EXPERIMENTAL RESULTS
Figure 1. Lattice-fringe images of various randomly disordered mixed-layer illite-smectite sequences, from the smectite-rich sample studied here (see text). [Image (a) from
Amouric and 0lives, 1991, and (b) from Olives and Amouric,
1994a.]
In the smectite-rich sample, three minerals coexist:
smectite, illite, and interstratified illite-smectite. In the
other samples, only illite and interstratified illite-smectite were present. Owing to the experimental techniques used (ultramicrotome cutting, low electronbeam intensity), the smectite crystallites showed
d(001) values of 1.25 nm, indicating that H20 was
present in the interlayer (Amouric and Olives, 1991).
This observation was used to identify 1-nm repeat distances as illite (and 1.25-nm repeats as smectite). In
images of interlayered minerals, the positions of the
lattice fringes are sensitive to experimental conditions
(e.g., focus, crystal orientation, crystal thickness). The
simulations of Guthrie and Veblen (1990) showed that,
under special conditions (overfocus), bright lattice
fringes may overlay the octahedral sites of the 2:1 layer, thus leading to a close correspondence between the
spacings of the bright lattice fringes and layer thickness. Images of mixed-layer illite-smectite (Figures 1
and 2) were obtained with such conditions. Representative examples of these images are presented in Figure 2a and 2c which shows two sets of fringes: (1)
dark fringes of 1.25-nm thickness and (2) gray fringes
of 1-nm thickness, which correspond to smectite and
illite layers, respectively.
Mixed-layer illite-smectite
and chlorite (Inoue and Utada, 1983). The three specimens selected contain interstratified illite-smectite
with, respectively, 68, 40, and 20% smectite, as determined by XRD (Inoue et al., 1987).
The samples, reduced to a powder and embedded in
Araldite resin, were cut with a diamond knife of an
LKB Ultramicrotome. The < 5 0 - n m thick sections
were deposited on carbon-coated copper grids. This
technique better preserves smectite from dehydration
than ion milling. Bright-field images were taken with
a JEOL 2000 FX electron microscope (200-kV accelerating voltage, 1,8-mm spherical aberration coefficient) with a 50-1xm objective aperture. Focus values
were in the range +50 to +100 nm for one-dimensional images and of about - 1 0 0 n m for two-dimensional images. Specimens were observed at very low
In the smectite-rich sample, the observed sequences
of illite-smectite interstratification are disordered
(Amouric and Olives, 1991). For example, Figure 1
shows the respective interlayering sequences SIISS,
SISIIISS, and SSSISISS (S = smectite, I = illite). We
consider the sequences as alternations of packets of
smectite units and packets of illite units, with the respective numbers of units of Ns and NI, where Ns and
NI are integers. In general, the sequences of this sample may be described as randomly disordered interstratifications (Reichweite, R = 0) in which Ns and NI
are equal to 1-5 and 1-3, respectively.
Because no significant differences were found in the
interlayering sequences between the more illitic sampies, they are referred to as illite-rich samples. In these
samples, consecutive smectite layers were not ob-
284
Olives, Amouric, and Perbost
Clays and Clay Minerals
Figure 2. Lattice-fringe images of various locally ordered mixed-layer illite-smectite sequences, from the illite-rich samples
studied here (see text). Note perfect R = 1 order in (c), and R = 2 in (d). [Images (a), (c), and (d) from Olives and Amouric,
1994a.]
served, which clearly indicates an ordered interstratification, i.e., R --> 1. Such interstratifications are illustrated in Figure 2a and 2b by SIIISIISIII and IIISIISIIISIS. Thus, such sequences are described with Ns
= 1 and N~ varying from 1 to 3. Locally, perfectly
ordered R -- 1 sequences ( I S I S . . . ) or R = 2 sequences ( I I S I I S . . . ) were observed also, i n v o l v i n g a few repeat units: e.g., four consecutive IS units in Figure 2c,
and three IIS units in Figure 2d. Such a trend, f r o m
disordered interstratification sequences to locally ordered sequences, as the percentage of smectite decreases, is similar to the case of interstratified kaolinite-smectite ( A m o u r i c and Olives, 1998).
Illite
N o significant change was o b s e r v e d in the morphology and the polytypism of illite crystals a m o n g
the three samples. The previous descriptions ( A m o u r i c
and Olives, 1991) remain valid for all the samples.
Illite crystals contain 7 - 2 0 parallel layers and have a
lateral extension o f 5 0 - 2 0 0 nrn. In t w o - d i m e n s i o n a l
structure images, each interlayer region appears as a
line of white dots with 0.45-nm spacing, and the shift
b e t w e e n two consecutive such lines represents the proj e c t e d stagger b e t w e e n the two tetrahedral sheets T o f
the (2:1) T O T layer, where O represents the octahedral
sheet of the same 2:1 layer ( A m o u r i c et al., 1978,
1981; Iijima and Buseck, 1978). T h e s e images show
for all the samples that the polytype is 1M with wellordered stacking (Figure 3). A c c o r d i n g to A m o u r i c
and Olives (1991), I M illite crystals are p r o d u c e d by
a dissolution-crystallization mechanism.
Chemistry
Structural formulae of illite-smectite w e r e normalized on the basis of O10(OH)2. The related standard
Vol. 48, No. 2, 2000
Mixed layering of illite-smectite
285
Figure 3. Structure images of illite crystals, from (a) the smectite-rich sample and (b) an illite-rich sample. The illite crystals
are of a regular 1M polytype. [Image (a) from Amouric and Olives, 1991.]
deviations are 0.06 for Si and A1, 0.04 for Mg and Fe,
and 0.02 for Ca and K. On the basis of A E M microanalyses, smectite, illite, and interstratified illite-smectite are dioctahedral. By grouping Fe (considered as
Fe 3§ with octahedral A1, and Ca with K (Na being
neglected), the analyses are illustrated in the pyrophyllite
Si4A12010(OH)2-muscovite K(Si3A1)A12Ox0(OH)2-celadonite KSia(A1Mg)O~0(OH) 2 diagram of Figure 4. Mixedlayer illite-smectite shows a chemical trend from
montmorillonite to illite. The chemical reaction involves the fixation of K and (tetrahedral and octahedral) AI in montmorillonite, to produce illite and the
emission of H20, Si, Mg, and Fe. On the basis of four
tetrahedral cations, the number of Fe atoms decreases
from 0.08 in the smectite-rich sample to 0.04 in the
illite-rich samples and the number of Ca atoms is nearly constant at 0.04. This indicates a chemical modification of the interlayer, tetrahedral, and octahedral cations.
Celadonite
ELECTROSTATIC-ENERGY CALCULATIONS
For a better understanding of the structure and the
stability of interstratified illite-srnectite, electrostaticenergy calculations were performed to compare the
energies of various possible mixed-layer structures
with illite and smectite as end-members. For this case,
the energy thermodynamic function is adequate because the energy of mixing of the layers is nearly
equal to the enthalpy of mixing (where the cell volumes are assumed constant) and to the Gibbs free energy of mixing (because the configurational entropy of
mixing vanishes for the perfectly ordered sequences
studied). Vibrational aspects were not taken into account. A first approximation to the lattice energy is
given by the electrostatic Ewald energy (or Madelung
energy). Justification is presented below. The Ewald
energies are calculated from the "overlap method"
(Olives, 1986a, section 2.4.2), which is presented in
the following section. The convergence of this method
is rapid, compared to the classical methods of Ewald
(1921) or Bertaut (1952).
The overlap method
Pyrophyllite
M uscovite
Figure 4. AEM microanalyses of mixed-layer illite-smectite, from the smectite-rich sample (black circles) and the illite-rich samples (open circles) studied. Note the chemical
trend from montmorillonite (Mon) to illite (I).
The electrostatic energy of an ionic crystal in which
the ions are considered as point charges was calculated
by Madelung (1918) for the special case of cubic crystals, and then by Ewald (1921), who gave a general
formula valid for any crystal. Owing to the long-range
nature of electrostatic interactions, the electrostatic energy per volume unit (more precisely, the limit of this
energy when the crystal becomes infinitely large) depends on the atomic configuration of the surface of the
crystal. The Ewald energy represents the minimum of
this electrostatic energy per volume unit with respect
to all possible atomic configurations of the surface of
the crystal (Olives, 1986b, 1987).
286
Olives, Amouric, and Perbost
Clays and Clay Minerals
Table 1. Interstratified structures corresponding to I'TOT units, TI'TO units, etc., for the ordered layer sequences...ISIS...
a n d . . . I I S I I S . . . (I = illite, S = smectite; I', T, and O, respectively, denote the interlayer, tetrahedral, and octahedral regions).
The subscripts I, S, and M, respectively, refer to compositions of illite, smectite, and midway between at 0.5.
U n i t s i n v o l v e d in t h e
interstratification
I'TOT units
TI'TO units
I'0.sTOTI'0 ~ units
T0.5I'TOT0.5 units
O0.sTI'TO0.5 units
9 .
.(I'TOT)I(I'TOT)s...
9
9 .
9
9 .
.I'ra(TOT)II'M(TOT)s...
. .
9 .
9
9 .
.O~(TI'T)IOM(TI'T)s...
In the overlap method, the Ewald energy, E, is calculated as E = E1 - E2 + E3, in which E 1 is the reciprocal-lattice sum, E1 = (1/2~rV) "~ IF(h)~(h)12/h 2, with
h~0
F(h) = ~
I n t e r s t r a t i f i e d s t r u c t u r e s for . . . I I S I I S . . .
I n t e r s t r a t i f i e d structures for . . . I S I S . . .
quexp(2~rih.u),
u
tp(h) = 3(sin ot - a cos c 0 / a 3,
E2 = (3/5R) ~
et = 2~rRllhll;
qu2;
u
and E 3 is the direct lattice sum,
E3 = (1/2R) ~
~
Iln + u -
~ quqv
9 .
.(TI'TO)I(TI'TO)I(TI'TO)s...
.I'M(TOT)II'I(TOT)II'M(TOT)s...
.TM(I'TO)ITI(I'TO)~TM(I'TO)s...
.OM(TI'T)IOI(TI'T)IOM(TI'T)s...
the two compositions o f (dehydrated) smectite, S 1 =
(Na, K)o.asSi4(All.65Mgo.35)Olo(OH)2 (montmorillonite)
and $2 = (Na, K)o.35(Sia.65Alo.35)A12OIo(OH)2 (beidellite). F o r m a l charges, qK = qNa = 1, qsi = 4, qA1 = 3,
qMg = 2 , q n = 1 , q o = - 2 , w e r e used with m e a n
charges for the interlayer, tetrahedral, and octahedral
cations, e.g., 0.75, [(3.6 • 4) + (0.4 x 3)]/4, and
[(1.65 • 3) + (0.35 • 2)]/2, respectively, for the
above I~ composition. The E w a l d energies o f illite I1,
illite I2, smectite $1, and smectite $2 w e r e calculated
at - 8 0 . 5 8 4 2 , - 8 0 . 1 6 3 5 , - 8 1 . 9 9 6 8 , and - 8 1 . 5 2 6 1 M J
mol 1, respectively (12 o x y g e n atoms per molecule; 1
M J = 106 J). The uncertainty of these values and the
following ones are < 0 . 5 • 10 -4 M J mo1-1.
vii < 2R
Mixed-layer illite-smectite
• ( l / x - 6/5 + x2/2 - 3x3/16 + x5/160),
with
Illite and smectite h a v e the same general structure
I ' T O T I ' T O T . . . ( T O T layers separated by interlayer regions I'), but the I', T, and O regions are occupied
by different cations. A fundamental p r o b l e m concerning the nature of m i x e d - l a y e r illite-smectite is the
structure of the individual illite and smectite layers.
Possibilities include: I ' T O T units, T I ' T O units, etc. Table 1 shows the interstratified structures corresponding
to unit layers, for t w o o r d e r e d l a y e r s e q u e n c e s
...ISIS...
and . . . I I S I I S . . . . In this table, the subscript M indicates a c o m p o s i t i o n m i d w a y b e t w e e n illite and smectite at 0.5 and refers to a r a n d o m mixture
of the corresponding cations. Thus, for OM, with the
above I2 and $1 compositions, the m e a n charge, [3 +
(1.65 • 3 + 0.35 x 2)/2]/2, is used for the octahedral
cations. N o t e that, either for t h e . . . I S I S . . . or for the
. ..IISIIS...
case, the derived structures h a v e the
same first-neighbor interactions (oxygen-cation and
o x y g e n - o x y g e n ) , although the arrangements differ.
Thus, the short-range energies (van der Waals attraction, overlap repulsion) are identical for these structures, and the differences b e t w e e n the lattice energies
are only related to the long-range electrostatic energy.
The calculated E w a l d energies of the preceding
m i x e d - l a y e r structures are g i v e n in Table 2 for
...ISIS...
and in Table 3 for . . . I I S I I S . . . . for the
various compositions used. F o r all cases, the results
indicate that the m o r e stable interstratification structure ( m i n i m u m energy) is based on O0.sTI'TO0. 5 units.
...
x = IIn + u - vii/R.
In these expressions, n denotes the lattice vectors, u
or v denotes the positions of the ions o f the origin cell
n = 0, qu is the charge of the ion at position u, h
denotes the reciprocal-lattice vectors, V is the v o l u m e
of the cell, R is a length parameter of any positive
value (E does not depend on R), i = N/Z-], h. u denotes the inner product, and Ilyll denotes the length of
any vector y.
As R increases, the n u m b e r of terms in the finite
sum E3 increases but, at the same time, the series E1
converges more rapidly. In our case, a v e r y rapid conv e r g e n c e was obtained with a value of R close to the
cell dimension, e.g., at R = 1 nm.
S m e c t i t e a n d illite
Because the smectite and illite structures are not
k n o w n accurately, we have used the coordinates f r o m
Jenkins and Hartman (1979) as derived f r o m Sidore n k o et al. (1975) for an Al-rich 1M m i c a for the structure of illite, (dehydrated) smectite, and interstratified
illite-smectite. The 1M polytype was assumed for the
three minerals. The structure relaxation was neglected.
Various compositions were used, n a m e l y the two c o m positions of illite, I x = Ko.75(Si3.6Alo.4)(All.65Mgo.35 )O10(OH)2 and 12 = Ko.75(Si3.25Alo.75)A12Oxo(OH)2, and
Vol. 48, No. 2, 2000
Mixed layering of illite-smectite
287
Table 2. Calculated Ewald energies of the mixed-layer structures corresponding to I'TOT units, TI'TO units, etc. (see Table
1), for the ordered layer sequence ...ISIS .... The last column gives the Ewald energy of the two-phases illite + smectite
assemblage with the same composition (i.e., energy of illite + energy of smectite). Various compositions of illite (I t, I2) and
smectite (S~, Sz) were used (see text). Values are in MJ mol 1 = 106 I mol -t and refer to 24 oxygen atoms per molecule. The
more stable interstratification structure (minimum energy) is based on O05TI'TO05 units. The energy of the mixed-layer
. . . I S I S . , . (with O05TI'TO0. 5 units) is close to the corresponding energy of the two-phases illite + smectite assemblage.
I'TOT units
TI'TO units
lnterstratified sequence . . . I S I S . . .
I'05TOTI'0~ units
T0~I'TOT05 units
O05TI'TOo 5 units
Two-phase
illite + smectite
assemblage
I182
-162.5088
-162.0361
1281
--162.0903
I2S2
-161.6175
-162.5809
-162.0533
-162.1031
-161.6896
-162.5381
-162.0654
-162.1196
-161.6468
-162.5558
-162.0449
-162.1350
-161.6645
-162.5809
-162.0867
-162.1365
-161.6896
-162.5809
-162.1103
-162.1602
-161.6896
Compositions
1181
M o r e o v e r , this m o d e l c o r r e s p o n d s to the m i n i m u m energy in c o m p a r i s o n to the o t h e r m o d e l s b a s e d o n
OxTI'TO1 x, I'xTOTI'l-x, or T~I'TOT~ x units (0 -----x <-1). T h i s result indicates that the layers i n v o l v e d i n t h e
interstratification are p r o b a b l y O0.sTI'TO05 (Olives a n d
A m o u r i c , 1994a, 1994b). W h e n the o c t a h e d r a l c o m p o s i t i o n s o f illite a n d s m e c t i t e are identical, i.e., f o r
IiS l a n d I282, note that the interstratification structure
b a s e d o n O05TI'TO0.5 u n i t s c o i n c i d e s w i t h that b a s e d
o n T I ' T O units. T h e interstratification m a y t h e n b e described as the s u p e r p o s i t i o n o f illitic or smectitic T I ' T
units, separated b y o c t a h e d r a l sheets o f M, I, or S
c o m p o s i t i o n (for the r e s p e c t i v e cases o f IS, II, or SS
a d j a c e n t layers). F i g u r e 5 illustrates a n e x a m p l e o f the
...ISIS...
interstratification. T h e l a t t i c e - e n e r g y calc u l a t i o n s lead to a p o l a r m o d e l for T O T layers (with
t w o tetrahedral sheets o f different c o m p o s i t i o n s ; see
S
0
-
M
0
"
,', ls
M
0
M
T
7
-
e.g., Gtiven, 1991). I n d e e d , the p r e c i s e a r r a n g e m e n t is
T~OMTs for the c a s e o f IS a d j a c e n t layers. S u c h a p o l a r
m o d e l (first p r o p o s e d b y S u d o et al., 1962) is in agreem e n t w i t h e x p a n d a b i l i t y m e a s u r e m e n t s , X R D analysis,
a n d H R T E M i m a g i n g (e.g., R e y n o l d s , 1980; J i a n g et
al., 1990; V e b l e n et al., 1990), a n d is s u p p o r t e d b y
N M R s p e c t r o s c o p y ( B a r t o n et al., 1985; A l t a n e r et al.,
1988; J a k o b s e n et al., 1995). O n the b a s i s o f t h e s e
results, A l t a n e r a n d Y l a g a n ( 1 9 9 7 ) also f a v o r e d t h e
p o l a r m o d e l a n d a s s u m e d this m o d e l to c o m p a r e the
m e c h a n i s m s for s m e c t i t e illitization.
F u r t h e r m o r e , the e n e r g i e s o b t a i n e d for the m i x e d layers . . . I S I S . . . a n d . . . I I S I I S . . . ( w i t h O0.sTI'TO0s
units) are close to the c o r r e s p o n d i n g e n e r g i e s o f the
t w o - p h a s e a s s e m b l a g e o f illite + s m e c t i t e (Tables 2
a n d 3). T h u s , the e n e r g y o f m i x i n g o f the t w o t y p e s
o f layers is n e a r l y e q u a l to zero. W h e n the o c t a h e d r a l
c o m p o s i t i o n s o f illite a n d s m e c t i t e are identical, i.e.,
for I1St a n d I282, note that the e n e r g y o f t h e m i x e d l a y e r is e q u a l to that o f the t w o - p h a s e a s s e m b l a g e o f
illite + smectite. A s i m i l a r r e s u l t w a s o b t a i n e d for interstratified b i o t i t e - c h l o r i t e ( 0 l i v e s , 1985, 1986a). T h e
stability o f i n t e r l a y e r e d illite-smectite is t h e n s i m i l a r
to the stability o f an a s s e m b l a g e c o n t a i n i n g d i s c r e t e
illite a n d smectite. T h i s r e s u l t m a y e x p l a i n the c o e x i s t e n c e o f illite, smectite, a n d interstratified i l l i t e - s m e c tite, w h i c h w a s o b s e r v e d in our s m e c t i t e - r i c h s a m p l e
( A m o u r i c a n d 0 l i v e s , 1991; also e.g., D o n g et aL,
1997).
Summary of assumptions
I
Figure 5. Structure of the ordered mixed-layer illite-smect i t e . . . I S I S . . . as determined by the minimization of the lattice
energy.
The
corresponding
arrangement
is
. . . OM(TI'T)~OM(TI'T)s... (I', T, O = interlayer, tetrahedral,
and octahedral regions, respectively; I, S, M = compositions
of illite, smectite, and midway between at 0.5, respectively).
T h e electrostatic E w a l d e n e r g y was c o n s i d e r e d as a
first a p p r o x i m a t i o n to the lattice energy. E w a l d energies w e r e c a l c u l a t e d f r o m the o v e r l a p m e t h o d (Olives,
1986a, s e c t i o n 2.4.2) w h i c h p r o v i d e s r a p i d c o n v e r gence. F o r m a l c h a r g e s w e r e used. R a n d o m d i s t r i b u t i o n
o f c a t i o n s i n e a c h o c t a h e d r a l , tetrahedral, or i n t e r l a y e r
region was assumed. Identical coordinates were used
for t h e structure o f illite, ( d e h y d r a t e d ) smectite, a n d
interstratified i l l i t e - s m e c t i t e ( 1 M p o l y t y p e w a s ass u m e d for t h e t h r e e m i n e r a l s ) . S t r u c t u r e r e l a x a t i o n w a s
neglected. T w o c o m p o s i t i o n s o f illite (I1, I2) a n d t w o
c o m p o s i t i o n s o f s m e c t i t e (S1, $2) w e r e used.
288
Olives, Amouric, and Perbost
Clays and Clay Minerals
Table 3. Calculated Ewald energies of the mixed-layer structures corresponding to I'TOT units, TI'TO units, etc. (see Table
1), for the ordered layer sequence . . . I I S I I S . . . The last column gives the Ewald energy of the two-phase assemblage of illite
+ smectite with the same composition (i.e., 2 • energy of illite + energy of smectite). Various compositions of illite 01, I2)
and smectite ($1, $2) were used (see text). Values are in MJ mo1-1 = 106 J tool ~ and refer to 36 oxygen atoms per molecule.
The more stable interstratification structure (minimum energy) is based on O0.sTI'TO05 units. The energy of the mixed-layer
9
O0.sTI'TO0.5 units) is close to the corresponding energy of the two-phases illite + smectite assemblage.
Compositions
III1SI
IlI1S2
I212S1
IzIzSz
I'TOT units
TI'TO units
-243.0682
-242.5954
-242.2289
-241.7561
-243.1651
-242.6184
-242.2476
-241.8531
Interstratifiedsequence. . .IISIIS...
I'0~TOTI'o.5units
TosI'TOTo5
units
-243.1223
-242.6495
-242.2830
-241.8103
Consequences f o r the smectite-to-illite solid-state
mechanism
In A m o u r i c and 0 l i v e s (1991), t w o illitization
m e c h a n i s m s w e r e d e t e c t e d on the basis o f H R T E M
observations: (1) d i s s o l u t i o n o f s m e c t i t e and crystallization o f illite, w h i c h p r o d u c e s 1M illite crystals, and
(2) solid-state t r a n s f o r m a t i o n o f one s m e c t i t e layer to
one illite layer, w h i c h leads to m i x e d - l a y e r illite-smecrite. This latter m e c h a n i s m was also s u p p o r t e d b y the
H R T E M study o f M u r a k a m i et aL (1993). The t e r m
" s o l i d - s t a t e " is u s e d h e r e in a b r o a d sense, b e c a u s e
m a s s transport is o b v i o u s l y i n v o l v e d in such a transition. Thus, the t r a n s f o r m a t i o n occurs w i t h i n a solid, at
the unit-cell scale, at the b o u n d a r y o f the n e w l y dev e l o p i n g layer and the r e a c t i o n is p r o b a b l y facilitated
by the p r e s e n c e o f water. The m a j o r part o f the preexisting solid is p r e s e r v e d (e.g., V e b l e n and Buseck,
1980; Olives et al., 1983; E g g l e t o n and Banfield,
1985; A m o u r i c and 0 l i v e s , 1998). T h e interstratification d e t e r m i n e d b y the a b o v e calculations has important c o n s e q u e n c e s for this transformation: the m e c h a n i s m p r o b a b l y consists o f a lateral t r a n s f o r m a t i o n o f
an O0.sTI'TO0.5 unit o f s m e c t i t e into the s a m e unit o f
illite. The r e a c t i o n m o s t p r o b a b l y b e g i n s at the interlayer, I', b e c a u s e o f its h i g h ion and w a t e r - e x c h a n g e
capacity, and p r o p a g a t e s to the t w o adjacent TO0.5
units. The reaction t h e n laterally runs along the layer
so that an O0.sTI'TO0.5 unit o f smectite is p r o g r e s s i v e l y
t r a n s f o r m e d into the s a m e c o r r e s p o n d i n g unit o f illite.
ACKNOWLEDGMENTS
We thank A. Inoue for providing the samples studied. We
also acknowledge the helpful comments of S. Guggenheim,
R Heaney, R. Cygan, and H. Dong.
REFERENCES
Ahn, J.H. and Peacor, D.R. (1989) Illite/smectite from Gulf
Coast shales: A reappraisal of transmission electron microscope images. Clays and Clay Minerals, 37, 542-546.
Altaner, S.R and Ylagan, R.E (1997) Comparison of structural
models of mixed-layer illite/smectite and reaction mechanisms of smectite illitization. Clays and Clay Minerals, 45,
517 533.
Altaner, S.E, Weiss, C.A., and Kirkpatrick, R.J. (1988) Evidence from 29Si NMR for the structure of mixed-layer illite/
smectite clay minerals. Nature, 331, 699-702.
-243.1337
-242.6072
-242.2984
-241.8217
OosTI'TOo5
units
243.1651
-242.6708
242.3000
-241.8531
Two-phaseillite +
smectiteassemblage
-243.1651
-242.6944
-242.3237
-241.8531
Amouric, M. and Olives, J. (1991) Illitization of smectite as
seen by high-resolution transmission electron microscopy.
European Journal of Mineralogy, 3, 831 835.
Amouric, M. and Olives, J. (1998) Transformation mechanisms and interstratification in conversion of smectite to
kaolinite: An HRTEM study. Clays and Clay Minerals, 46,
521-527.
Amouric, M., Baronnet, A., and Finck, C. (1978) Polytypisme
et d6sordre dans les micas diocta6driques synth6tiques;
6tude par imagerie de r6seau. Materials Research Bulletin,
13, 627-634.
Amouric, M., Mercuriot, G., and Baronnet, A. (1981) On
computed and observed HRTEM images of perfect mica
polytypes. Bulletin de Min~ralogie, 104, 298-313.
Barron, RE, Slade, P., and Frost, R.L. (1985) Ordering of
aluminium in tetrahedral sites in mixed-layer 2:1 phyllosilicates by solid-state high-resolution NMR. Journal of
Physical Chemistry, 89, 3880 3885.
Bertaut, E (1952) U6nergie 61ectrostatique de r6seaux ioniques. Le Journal de Physique et le Radium, 13, 499-505.
Buatier, M.D., Peacor, D.R., and O'Neil, J.R. (1992) Smectite-illite transition in Barbados accrefionary wedge sediments: TEM and AEM evidence for dissolution/crystallization at low temperature. Clays and Clay Minerals, 40,
65-80.
Cliff, G. and Lorimer, G.W. (1975) The quantitative analysis
of thin specimens. Journal of Microscopy, 103, 203-207.
Dong, H., Peacor, D.R., and Freed, R.L. (1997) Phase relations among smectite, R1 illite-smectite, and illite. American Mineralogist, 82, 379-391.
Eggleton, R.A. and Banfield, J.E (1985) The alteration of
granitic biotite to chlorite. American Mineralogist, 70, 902
910.
Ewald, RP. (1921) Die berechnung optischer und elektrostatischer gitterpotentiale. Annalen der Physik, 64, 253-287.
Guthrie, G.D. and Veblen, D.R. (1989) High-resolution transmission electron microscopy of mixed-layer illite/smectite:
Computer simulations. Clays and Clay Minerals, 37, 1-11.
Guthrie, G.D. and Veblen. D.R. (1990) Interpreting one-dimensional high-resolution transmission electron micrographs of sheet silicates by computer simulations9 American Mineralogist, 75, 276-288.
Gtiven, N. (1991) On the definition of illite/smectite mixedlayer. Clays and Clay Minerals, 39, 661-662,
Huggett, J.M. (1995) Formation of authigenic illite in palaeocene mudrocks from the central North Sea: A study by
high resolution electron microscopy. Clays and Clay Minerals, 43, 682-692.
Iijima, S. and Buseck, P.R. (1978) Experimental study of disordered mica structures by high-resolution electron microscopy. Acta Crystallographica A, 34, 709-719.
Inoue, A. and Utada, M. (1983) Further investigations of a
conversion series of dioctahedral mica/smectites in the
Vol. 48, No. 2, 2000
Mixed layering of illite-smectite
Shinzan hydrothermal alteration area, northeast Japan.
Clays and Clay Minerals, 31, 401-412.
Inoue, A., Kohyama, N., Kitagawa, R., and Watanabe, T.
(1987) Chemical and morphological evidence for the conversion of smectite to illite. Clays and Clay Minerals, 35,
111-120.
Jakobsen, H.J., Nielsen, N.C., and Lindgreen, H. (1995) Sequences of charged sheets in rectorite. American Mineralogist, 80, 247-252.
Jenkins, H.D.B. and Hartman, P (1979) A new approach to
the calculation of electrostatic energy relations in minerals:
The dioctahedral and trioctahedral phyllosilicates. Philosophical Transactions o f the Royal Society o f London A,
293, 169-208.
Jiang, W.T., Peacor, D.R., Merriman, R.J., and Roberts, B.
(1990) Transmission and analytical electron microscopic
study of mixed-layer illite/smectite formed as an apparent
replacement product of diagenetic illite. Clays and Clay
Minerals, 38, 449-468.
Madelung, E. (1918) Das elektrische feld in systemen von
regelm~iBig angeordneten punktladungen. Physikalische
Zeitschrift, 19, 524-532.
Murakami, T., Sato, T., and Watanabe, T. (1993) Microstructure of interstratified illite/smectite at 123K: A new method
for HRTEM examination. American Mineralogist, 78, 465468.
Olives, J. (1985) Biotites and chlorites as interlayercd biotitechlorite crystals. Bulletin de Mindralogie, 108, 635-641.
Olives, J. (1986a) The Ewald energies of complex crystals:
The "biochlorites". Acta Crystallographica A, 42, 340344.
0lives, J. (1986b) The electrostatic lattice energy. Physica
Status Solidi (b), 138, 457-464.
Olives, J. (1987) The electrostatic energy of a lattice of point
charges. Annales de l'Institut Henri Poincard, Physique
Thdorique, 47, 125-184.
0lives, J. and Amouric, M. (1994a) Transformation of smecrite into illite and illite-smectite interstratification: HRTEM
observations and lattice energies calculations. In Electron
Microscopy 1994, Proceedings o f the 13th International
289
Congress on Electron Microscopy, Volume 2B, B. Jouffrey
and C. Colliex, eds., Les Editions de Physique, Paris,
1281-1282.
Olives, J. and Amouric, M. (1994b) Illite-smectite interstratified minerals in the smectite-to-illite transition. In Abstracts o f the 16th General Meeting o f the International
Mineralogical Association, S. Merlino, ed., Societa Italiana
di Mineralogia e Petrologia, Pisa, 309-310.
Olives, J., Amouric, M., de Fouquet, C., and Baronnet, A.
(1983) Interlayering and interlayer slip in biotite as seen
by HRTEM, American Mineralogist, 68, 754-758.
Reynolds, R.C. (1980) Interstratified clay minerals. In Crystal
Structures o f Clay Minerals and Their X-ray Identification,
G.W. Brindley and G. Brown, eds., Mineralogical Society,
London, 249-303.
Reynolds, R.C. (1992) X-ray diffraction studies of illite/smectite from rocks, < 1 p~m randomly oriented powders, and
<1 Ixm oriented powder aggregates: The absence of laboratory-induced artifacts. Clays and Clay Minerals, 40, 387396.
Sidorenko, O.V., Zvyagin, B.B., and Soboleva, S.V. (1975)
Crystal structure refinement for 1M dioctahedral mica. Soviet Physics, Crystallography, 20, 332-335.
Srodori, J. (1984) X-ray powder diffraction identification of
illitic materials. Clays and Clay Minerals, 32, 337-349.
Sudo, T., Hayasi, H., and Shirnoda, S. (1962) Mineralogical
problems of intermediate clay minerals. In Proceedings o f
the 9th National Conference on Clays and Clay Minerals,
Lafayette, Indiana, 1960, A. Swineford, ed., Pergamon
Press, New York, 378-388.
Veblen, D.R. and Buseck, P.R. (1980) Microstructures and
reaction mechanisms in biopyriboles. American Mineralogist, 65, 599-623.
Veblen, D.R., Guthrie, G.D., Livi, K.J.T., and Reynolds, R.C.
(1990) High-resolution transmission electron microscopy
and electron diffraction of mixed-layer illite/smectite: Experimental results, Clays and Clay Minerals, 38, 1-13.
E-mail of corresponding author: [email protected]
(Received 11 December 1998; accepted I0 December
1999; Ms. 98-138; A.E. Peter J. Heaney)