Clay Minerals (1988) 23, 133-146
H I G H - C H A R G E TO L O W - C H A R G E S M E C T I T E
REACTION IN HYDROTHERMAL
ALTERATION
PROCESSES
A. B O U C H E T ,
D. P R O U S T ,
A. M E U N I E R
AND D. B E A U F O R T
Laboratoire de POtrologie des Altkrations Hydrothermales, UniversitO de Poitiers, 40 Avenue Recteur Pineau,
86022 Poitiers Cedex, France
(Received 21 July 1987; revised 18 March 1988)
A B S T R A C T : Illite/smectite mixed-layered minerals (I/S) occurring in hydrothermally altered
dacite from the island of Martinique were studied using X-ray powder diffraction, electron
microprobe and CEC measurement techniques. Microprobe analyses and X-ray identification
of high- and low-charge smectite layers indicate that the I/S hydrothermal alteration operates
from wall-rock to hydrothermal veins, with conversion of high-charge to low-charge smectite
and formation of illite layers. The overall alteration reaction can be expressed as: mixed-layered
high-charge smectite + Si4§ + K § ~ mixed-layered low-charge smectite + illite. This reaction
requires Si and K from external sources and differs from the currently invoked mechanisms for
diagenetic illitization where Si is released. The proposed reaction appears, therefore, to be the
first step of I/S alteration under low-temperature hydrothermal conditions, prior to the
crystallization of newly-formed I/S.
The use of smectites in preference to any other clay mineral as buffer materials in the
disposal of radioactive waste is based upon their high cation exchange and swelling
capacities. Evidence from various geological environments indicates, however, that
smectites may not be stable at temperatures in excess of 75~176
and may react to form
illite through intermediate I/S mixed-layered minerals. This has been observed during
conditions of diagenesis (Dunoyer de Segonzac, 1970; Perry & Hower, 1970, 1972; Foscolos
& Kodama, 1974; Hower et al., 1976; Boles & Franks, 1979; Velde & Brusewitz, 1982;
Ramseyer & Boles, 1986), contact metamorphism (Nadeau & Reynolds, 1981 ; Pytte, 1982),
and hydrothermal alteration (Steiner, 1968 ; Inoue et al., 1978; McDowell & Elders, 1980;
Inoue & Utada, 1983).
The mechanisms invoked for such smectite illitization refer to tetrahedral A1 for Si,
and octahedral Mg for A1 ionic substitutions, and reduction of Fe 3+ to Fe 2+ within the silicate
layer (Perry & Hower, 1970; Eberl & Hower, 1976; Hower et al., 1976; Eslinger et al., 1979).
These ionic modifications create an increasing layer charge deficiency which promotes K
fixation in the interlayer sites and, hence, formation of illitic layers (Howard, 1981 ; Howard
& Roy, 1985).
Experimental hydrothermal alteration (Eberl, 1978; Eberl et al., 1978; Lahann &
Roberson, 1980; Roberson & Lahann, 1981; Howard & Roy, 1985) indicated that the
reactivity of smectites depended on their chemical composition: K-smectites react more
readily to illite than N a or Ca varieties, Al-rich smectites are more reactive than Al-poor
ones, whereas Na, Ca, and Mg interlayer ions are known to inhibit the illitization process.
9 1988 The Mineralogical Society
A. Bouchet et al.
! 34
The overall result of the smectite illitization is that swelling and cation exchange capacities
(CEC) will be reduced, altering the radionuclide sorption capacity of the barrier; moreover,
the release of interlayer water during illitization may cause primitively sorbed radionuclides
to be transported through the clay barrier.
Thus, prediction of the ultimate performance of smectite buffers, raises the question of
their long-term stability under disposal vault conditions as estimated by Johnston (1983): (i)
temperatures in the range 150~176
sustained for one to ten thousands of years
(duration of the thermal transient); (ii) pressures in the range 100-300 bars; (iii) Ca-Na-C1
brine groundwaters.
An inherent problem with experimental alteration of smectite is the limitation set by
feasible laboratory timescales (months to years) which are considerably less than the duration
of the thermal transient. Considering that smectite buffers are expected to suffer mild
hydrothermal conditions over periods which are similar to geological timescales, the purpose
of the present study was to use a natural, low-temperature hydrothermal analogue of
radioactive waste disposal to test the effective reactivity of the smectites with time.
MATERIALS AND METHODS
Material
The material studied was collected in the southwestern part of the island of Martinique
(Fig. 1) from the Trois Ilets quarry which has a 5-10 m vertical working-face into dacite
(Westercamp & Mervoyer, 1976; Westercamp & Tazieff, 1980). The original lava was
primarily argillized by fumarolic alteration, resulting in a brown clayey material which is
exploited for ceramic production. A secondary hydrothermal alteration has subsequently
developed from sub-vertical cracks cross-cutting the outcrop in the nothern part of the
quarry. Two hydrothermal veins with their related wall-rocks were sampled; the macroscopic
description and the exact location of each sample with respect to the veins are listed in
Table 1.
Sample preparation
Clay minerals from each sample were concentrated in the < 2 #m size fraction using
ultrasonic dispersion and sedimentation procedures. The CEC were measured after ion
TABLE1. Locationsand macroscopicdescriptions at the two sites.
SITE A
Sample 1: Hydrothermal vein, 10 cm wide white clays without rock relics.
Sample 2: Wall-rock, 10 cm from vein--white argillized dacite.
Sample 3: Wall-rock, 50 cm from vein---coherentdacite.
SITE B
Sample 4: Hydrothermal vein, 3 cm wide--white clays without rock relics.
Sample 5: Wall-rock, 5 cm from vein--white argillized dacite.
High-charge to low-charge smectite reaction
MARTINIQUE
TROIS-ILETS
I
O
ISLAND
QUARRY
/
,"'~'~ -~
135
4"
VEINS
'i
DAClTE
VEIN
/
VEIN
1
Brown ~, argillized
argillized ~ ,dacite "
dacite
~\
2Ocm
J
SITE
A
SITE B
FIG. l. Sketch of the "Trois Ilets" quarry from Martinique, France showing locations of the two
sites sampled (A & B) and positions of the samples studied (1-5)with respect to the hydrothermal
veins.
exchange with aqueous 1 y ammoniacal solutions at pH 7 (Jackson, 1958). The size fractions
were saturated with Ca 2+ and K + using 1 r~ aqueous CaCI2 and KC1 solutions and washed
repeatedly with distilled water until chlorides could not be detected by the AgNO3 test.
136
A. Bouchet et al.
X-ray powder diffraction
The X-ray diffraction (XRD) patterns were recorded using a Philips PW 1730 X-ray
diffractometer (40 kV, 40 mA) with Fe-filtered Co-Kc~radiation, emergence, divergence and
analytical slits of 1~ 0.1 ~ and 1~ respectively, and goniometer and chart speeds of 0.5~
(continuous scanning) and 0.5 cm/min. Samples were examined after ethylene glycol (EG)
solvation, in natural and saturated states (K,Ca); K-samples were heated to 110~ overnight
prior to EG solvation.
The illite/smectite ratio in the I/S were estimated by the peak position of the 003 and 005
reflections on glycolated samples (Srodon, 1980); this method, however, needs well-resolved
integral basal reflections for accurate measurements. Unfortunately, XRD traces of the I/S
studied displayed only a strong, asymmetric 001 reflection and a more diffuse 002; Na
saturations did not reinforce the higher-order reflections. Consequently, Srodon's method
could not be applied, and the illite/smectite ratios were obtained by the following methods:
(i) the A/B ratios (A = background intensity at 4~ B = 001 peak intensity) and 002
reflection position from experimental spectra were compared with those of calculated spectra
for variable component proportions (Rettke, 1981 ; Velde et al., 1986). The simulated spectra
were obtained using the NEWMOD program (IBM PC-AT version 1.0) available from Dr R.
C. Reynolds, Department of Earth Science, Dartmouth College, Hanover, New Hampshire;
(ii) use of chemical data specific to one component in the I/S, namely the K § content
obtained from microprobe analyses for the illite component, and CEC for the smectite
component.
Electron microprobe analyses
Microprobe analyses were made on a Cameca MS 46 electron microprobe equipped with
an EGG ORTEC energy dispersive X-ray analysis system. Analyses were obtained following
the analytical procedure of Velde (1984) for clay minerals: 120 s counting time, 15 kV
acceleration potential, 1 nA sample current, and 5/~m spot diameter.
The clay was purified by removing discrete illite and kaolinite by sedimentation
(purification was checked by XRD); small pellets were then pressed for microprobe analyses
and structural formulae calculated on Olo (OH)2 basis with total Fe expressed as Fe 3§
It should be noted that the I/S purification method did not yield sufficient quantities of clay
for Ca 2+ and K § saturations for XRD. Consequently, XRD patterns were performed on the
bulk < 2 #m fractions, whereas microprobe analyses were obtained on pure I/S.
RESULTS
X-ray powder diffraction
The XRD patterns obtained from EG-solvated samples in natural and Ca-saturated states
(Fig. 2 & 3) revealed traces of discrete kaolinite (7-19 /~ reflection) and illite (9.97 A
reflection) mixed with a major smectite component (17-10 A reflection). The 17.10/~ peak,
however, displayed a pronounced asymmetry and a high background in its low-angle tail
(4~ which is not compatible with the symmetrical, low-background 001 reflection of a pure
EG-expanded smectite; this suggested a random interstratification of the smectite with
another clay mineral (Reynolds & Hower, 1970). The identification of the second component
High-charge to low-charge smectite reaction
KAOLINITE
(01i01)
mJt 1/S
ILLITE
(Olis) 9.97
137
8.42
842
14
12
I
10
I
I
10 8
I
6
I
4
I
2
FIG. 2. XRD patterns of the < 2 #m fractions (natural-EG-state)from the Trois Ilets quarry. 1-5
are samples located in Fig. 1.
in the mixed-layered mineral was based on the position of the 002 reflection and the value of
the background/001 peak (A/B) intensities ratio (Reynolds, 1980).
The d(002) values of natural and Ca-saturated samples ranged from 8.42 A-8.46/~; the
random I/S patterns which were simulated with a double EG-layer smectite component
displayed larger d(002) values, from 8.50/~ (100~o smectite) to 9.40/~ (40~ smectite). This
discrepancy between experimental and simulated d(002) values was previously observed by
Srodon (1980) and can be explained by the occurrence of smectite layers with one or two
interlayers of EG molecules in the mixed-layered mineral stacking (Tettenhorst & Johns,
1966; Cicel & Machajdik, 1981).
Similar d(002) values in the range 8-42-8.46 /~, could also be due to a random
smectite/kaolinite mixed-layered mineral; the simulated XRD patterns, however, indicated
that such a mixed-layered mineral would have a very high smectite content (80-90~ smectite)
with A/B ratios ranging from 0.06-0.14. These A/B values do not agree with the experimental
138
A. Bouchet et al.
(ool)
~/s o
KAOLINITE (O/~2) ILLITE
t
|
J
Y/
9.97
8.42 t
8.42
J
.,J
I
14
J
12
I
10
~
I
10
I
8
I
6
I
4
I
2
Fie. 3. XRD patterns of the < 2 ktmfractions (Ca-EG-state)from the Trois/lets quarry. 1-5 are
samples located in Fig. 1.
A/B ratios which range from 0.38-0.57 for EG-natural samples, and from 0.22-0.45 for
EG-Ca-saturated samples.
Thus, A/B ratios of simulated XRD patterns eliminated the possibility of a smectite/kaolinite mixed-layered mineral, whereas experimental d(002) values are consistent with a
random I/S with two types of smectite layers and one or two interlayer E G molecules.
The XRD patterns obtained from EG-solvated samples after K-saturation and heating
overnight to ll0~ (Fig. 4) displayed a 17.10 A reflection similar to that observed from
natural and Ca-saturated samples, but the 002 reflection is shifted towards lower angles in the
range d(002) = 8.82-8.85 A. These higher d(002) values indicated that the K-110~
samples had a lower content of expanded smectite than natural or Ca-saturated samples; the
part of smectites which irreversibly collapsed to 10 A after K-exchange can be assigned to
high-charge smectite layers (Howard & Roy, 1985). In these circumstances, the K-A/B ratios
139
High-charge to low-charge smectite reaction
(ooT)
i/s
17.10~.
KAOLINITE
(002)
I/S ILLITE
9)97
|
s.;l
-
I
14
I
12
I ~174
10
.Aj
I
10
I
8
I
6
I
4
I
2
FIG. 4. XRD patterns of the < 2 gm fractions (K-110~
from the Trois Ilets quarry.
1-5 are samples located in Fig. 1.
(expandability measurement) will be used to estimate the low-charge smectite content in the
I/S, as discussed below.
Proportion of components in I/ S
The total smectite proportion in the I/S was estimated using two methods:
(i) the Ca-A/B values obtained from experimental X R D patterns were plotted vs.
expandability using diagrams obtained from N E W M O D program with simulated R = 0 I/S
for variable smectite proportion, and N = 1-6 domain size (N = number of layers in the
stacking);
(ii) the K contents obtained from the microprobe analyses of pure I/S (Table 2) were
plotted vs. expandability using regression lines from Srodon et al. (1986) for I/S with more
than 50~ expandable layers.
140
A . B o u c h e t et al.
TABLE2. Structural formulae, CEC (mEq/100 g), and expandability (%Sm) of the R = 0 I/S from the Trois
Islets quarry; n = number of analyses, a = standard deviation.
Sample
Tetrahedral
SiTM
Octahedral
A1
Fe 3+
Mg
Ti
Mn
Sum
Charge
Layer
charge
K
Ca
Na
Interlayer
charge
CEC
% Sm
% Ill
l(n = 3)
029
3"71
1.51 .,I
0.25
0.28
0-02
2(n = 6)
a
0"04
0-03
0.02
0,03
0-01
2.06
- 0.08
- 0.37
0.17
0.08
0.04
+0-37
69.20
73
27
3-61
3(n = 5)
a
0"05
0.04
1,32,)
0-46
0.29
0.02
0-03
2.07
- 0.08
0.02
0.01
0.02
3-55
- 0.47
0.12
0.08
0-14
+0.42
63.20
78
22
T
1,38 3
0-31
0.36
{3-02
4(n = 3)
a
0"02
0.03
0-02
0-04
0-01
2,07
- 0.13
0.02
0-00
0.03
- 0.58
0-08
0.13
0-20
+0-54
57.50
90
10
3"81
5(n = 3)
a
0'06
0.05
1.43J
0.19
0.40
0-02
0,03
0.05
0.02
0.03
0-02
0.04
- 0.45
0.08
0.08
0.19
+0.41
69.20
83
17
0-04
1.45.,I
0.35
0,27
0.02
0-03
0.01
2.08
- 0.04
2,04
- 0.26
0.02
0.01
0.03
a
0.04
0 9;
3"61
- 0.43
0.05
0.12
0-15
+0.44
0.00
0.03
0.02
0.03
61.40
90
10
The smectite proportions d e t e r m i n e d by the C a - A / B m e t h o d lead to the following
remarks:
(i) site A - - t h e calculated C a - A / B ratios increase from wall-rock (sample 3, A / B = 0.22) to
hydrothermal vein (sample 1, A/B = 0.45), i n d i c a t i n g that the total p r o p o r t i o n of smectite
(low- a n d high-charge layers) decreases from rock (90%) to v e i n (73%);
(ii) site B - - t h e calculated C a - A / B ratios show a slight increase from wall-rock (sample 5,
A/B = 0.23) to vein (sample 4, A/B = 0-27), i n d i c a t i n g a slight decrease in the total smectite
proportion from rock (90%) to vein (83%).
The smectite proportions d e t e r m i n e d using the K c o n t e n t of the I/S a n d the regression lines
from Srodon et al. (1986) indicated a similar evolution trend from rock to vein, the m a x i m u m
scattering values differing by no more t h a n 4% (Fig. 5).
Srodon et al. (1986) have described a third m e t h o d to estimate the e x p a n d a b i l i t y of I/S: a
plot of C E C vs. e x p a n d a b i l i t y gives a linear relationship ( C E C = 0.91% S + 15.33) which
extrapolates to 15.33 m E q / 1 0 0 g for pure illite, a n d 106 m E q / 1 0 0 g for pure smectite. The
expandabilities calculated with the m e a s u r e d C E C (Table 2) a n d the above formulae were
always lower (46%-59%) t h a n the expandabilities o b t a i n e d with the first two methods.
Moreover, the evolution trend is reversed, with the total smectite proportion increasing from
rock to vein in the two sites sampled. These discrepancies b e t w e e n C E C a n d I/S
expandabilities arise from the occurrence o f high-charge a n d low-charge layers in the
smectite c o m p o n e n t ; this will be discussed in the following section.
High-charge to low-charge smectite reaction
141
-\.
SITE A
-100
-I0
-go
9
.=,
[]
m -5(3
i,-
9
\
-80
0.1
0.2
I
I
I
I
I
i
_
N
~
-0
",,
,
I
I
I
I
0.5
I
1
I
I
I
K+/Oto (OH)2
SITE
-10
"
\
-400
~,
"
-~9-J -5( .kNtOO,~
_
-70 [ ]
0
~
,
0.1
,
0~2
,
O9
-0 []
0
I
05
I
I
I
I
I
1
I
k
i
I
K~OIo(OH)2
FIG. 5. Plots of K vs. % smectite layers for I/S using the diagram from Srodon et al. (1986).
I-A-l-site A : ~- = sample 1 (3 analyses); [] = sample 2 (6 analyses); C) = sample 3 (5 analyses).
I-B]-site A : Calculated expandabilities using the lines of Srodon et al. (1986) (5~', l-q, 9 and the
Ca-A/B ratios (~k', II, Q); the K contents are the mean values calculated from the microprobe
analyses.
[~- site B: O = sample 4 (3 analyses); /~ = sample 5 (3 analyses).
[D-l-site B: Calculated expandabilities using the lines of Srodon et al. (1986) (<>, A) and the CaA/B ratios (e, A); the K contents are the mean values calculated from microprobe analyses.
Low-charge and high-charge smectite layers
T h e identification o f low- and h i g h - c h a r g e s m e c t i t e layers is based on their ability to
e x p a n d after C a - e x c h a n g e , and K - e x c h a n g e c o m b i n e d w i t h heating to l l 0 ~
this
e x p a n d a b i l i t y appears to be controlled by the layer charge density (Srodon, 1980; Srodon et
al., 1986).
A. Bouchet et al.
142
VEIN
VEIN
DISTANCEFROM VEIN~
'
I
0
'
10
I
'
I
20
'
[
30
'
40
I
50cm
O22
t, 5cm
0
23 Ca A/B
2
HIGH-CHARGE LAYERS
l
//
/
,
y/~/'/
11 11
/
LOWsCHARGE LAYERS/"
/
/
/
/
/
/
/
SITE A
/
/
/
/
/
/0,93
SITEB
FIG. 6. Comparison between the Ca-A/B and the K-A/B ratios as a function of the distance
from veins at sites A & B. Ca-A/B = A/B ratios of Ca-EG samples; K-A/B ratios = A/B ratios
of K-110~
samples; 1-5 are samples located in Fig. 1.
Thus, I/S expandability measurements after Ca-exchange will provide the total proportion
of smectite (low- and high-charge layers) whereas K-exchange + l l0~ indicates the
proportion of low-charge smectite layers (Howard & Roy, 1985). The percentage of highcharge layers will be obtained by difference.
The A/B ratios calculated from both Ca-EG (Ca-A/B) and K-110~
(K-A/B) samples
are correlated with the distance from the hydrothermal veins in Fig. 6; this correlation leads
to the following remarks:
(i) Site A. The Ca-A/B ratios have shown that the total proportion of smectite decreased
from rock to vein. The K-A/B ratios display opposite variations, decreasing from wall-rock
(A/B = 0.93) to hydrothermal vein (A/B = 0.44), suggesting that low-charge smectite layers
increase from rock to vein.
Moreover, direct comparison between Ca-A/B and K-A/B ratios (Fig. 6) shows that the
smectites in wall-rock have markedly different Ca-A/B and K-A/B values (0-22 and 0.93),
whereas the smectites from the vein have similar A/B ratios (0-45 and 0.44). This indicates
that the smectites which were primarily composed of low- and high-charge layers in the wallrock are converted into 100~ low-charge layers in the hydrothermal vein. This result agrees
with the increasing CEC values observed from rock to vein (Table 2); the existence of highcharge layers, with lower CEC values than low-charge ones (Howard & Roy, 1985), can
explain the discrepancies between CEC and smectite proportions calculated using the linear
relationship (CEC = 0"91~o S + 15.33) from Srodon et al. (1986).
It should be noted that the enhancement, from rock to vein, of the 8.85 A reflection from Kl l0~
samples (Fig. 4) suggests increasing amounts of illitic layers, as previously
observed during experimental hydrothermal alterations (Howard & Roy, 1985).
(ii) Site B. The calculated Ca-A/B ratios show a slight increase from wall-rock (A/B =
0.23) to vein (A/B = 0.27), indicating that the total proportion of smectite (low- and highcharge layers) slightly decreases from rock to vein. This evolution trend is similar to that
High-charge to low-charge srnectite reaction
143
observed for site A. The K-A/B ratios display the opposite variation, decreasing from wallrock (A/B = 0.68) to vein (A/B = 0.59); this accounts for an increasing proportion of lowcharge smectite layers within the vein. However, comparison between Ca-A/B and K-A/B
ratios (Fig. 6) shows that both rock and vein samples have different Ca-A/B and K-A/B
values (0.23-0-68 and 0-27-0.59); these values indicate that the high-charge smectite layers in
the wall-rock are not entirely converted into low-charge layers within the vein.
As previously observed in K-110~
samples from site A, the development of a
reflection at 8-85 A in the vein sample (Fig. 4) suggests increasing amounts of illitic layers;
this is in good agreement with the illite percentages in the I/S listed in Table 2.
DISCUSSION AND CONCLUSIONS
The interpretation of XRD and CEC data obtained from the two sites leads to the I/S
evolution model proposed in Fig. 7.
SITE A
VEIN
%
ROCK
1
"'"- \
',//////]
/ /////1
73
~J;~CRHARGE
r//
/ /////
/I////,
,/IIII / I
/'
,L,~Vr~S /J
i
I Iilil l i
ILLITE
27
T
,///,',/,,
,1/ f /ll
LOWCHARGEJ
/,//
, / /
LAYERS
I
i/
///
_z//L
22
•
1
10
..1_
2
3
SITE B
VEIN
:II!
/I/i
%
ROCK
+ Iill,1
I
. . . . -~_1 / / / iL_
•
4
5
FIG. 7. Evolutionof the componentlayers in the I/S as a function of the distance from veins at
sites A & B. 1-5 are samples located in Fig. I.
144
A. Bouchet et al.
The overall result is that the hydrothermal alteration of the I/S increases the proportions of
illite layers and decreases the proportions of smectite layers in the veins. The most important
point is that the smectite component, composed of high- and low-charge layers in the wallrock, converts entirely into low-charge layers in the vein at site A. Moreover, the microprobe
analyses (Table 2) indicate that the Si content of the I/S increases with alteration, i.e. from
wall-rock to vein.
It should be noted that the percentage of illitic layers formed during I/S alteration does not
exceed 30~, whereas low-charge smectite layers can reach 70~ in the veins; if the range of Si
contents in natural smectites (3-67-3.85 atoms: Eberl et al., 1978; Nadeau & Bain, 1986) is
compared with that of illites (3.25-3.52 atoms: Nadeau & Bain, 1986), it appears that the
amount of Si released by the smectite illitization will not be sufficient to promote low-charge
smectite crystallization.
These results suggest that the high-charge to low-charge smectite reaction induces illite
layer formation in the I/S and needs external supplies of Si and K; thus, the reaction can
be formally expressed as:
Mixed-layered high-charge smectite + Si 4§ + K §
mixed-layered low-charge smectite + iUite.
This reaction is different from the currently invoked mechanisms for smectite illitization
under diagenetic conditions:
(i) solid-state smectite illitization (Hower et al., 1976) expressed as:
smectite + A13§ + K + ~ illite + Si 4+
where A1 and K are supplied by dissolution of potassic feldspar and detrital mica;
(ii) dissolution of smectite and crystallization of illite (Boles & Franks, 1979) expressed as:
smectite + K + ~ illite + Si 4+
where smectite dissolution provides the A1 required for illite crystallization; in this reaction,
there is more smectite consumed that there is illite produced (the ratio of smectite
consumed :illite produced is ~ 1.6:1).
It should be noted that these two reactions lead to release of Si, a result which is opposite to
the proposed reaction in which Si is consumed; this discrepancy between hydrothermal and
diagenetic reactions can be explained by the time duration of alteration. In the samples
studied, the reaction operates as far as hydrothermal fluids can flow in the veins, and can be
stopped before complete illitization. In diagenetic series, however, the long time duration of
alteration allows more advanced alteration of smectite into I/S, and then into iUite.
The proposed mechanism for high-charge to low-charge smectite reaction appears,
therefore, to be the first step of I/S alteration under low-temperature hydrothermal
conditions, prior to the crystallization of the I/S. Thus the mineralogical study of the natural
analogues of radioactive waste repositories appears to be very appropriate for following the
alteration of smectite in a low-temperature environment, and for testing their effective
reactivity with time.
ACKNOWLEDGMENTS
The financial support for this study was provided by the "Commissariat h l'Energie Atomique (CEA)",
Fontenay aux Roses, France. The authors are particularly indebted to Dr J. C. Petit (CEA) and Dr B. Velde
(ENS, Paris) for their critical reviews of the manuscript.
High-charge to low-charge smectite reaction
145
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