Clay Minerals (1995) 30, 45-54
CLAY
STABILITY
IN C L A Y - D O M I N A T E D
.SOIL S Y S T E M S
D . RIGHI, B . VELDE* AND A . MEUNIER
Laboratoires de Pddologie et Pdtrologie des Altdrations Hydrothermales, UA 721, CNRS, Facultd des Sciences, 86022
Poitiers Cedex, and *Ddpartement de Gdologie, Ecole Normale Supdrieure, 24 rue Lhomond, 75231 Paris Cedex 05,
France
(Received 10 April 1994; revised 7 October 1994)
A B S T R A C T : Seven samples from a chronosequence of soils developed in historically created
polders on the Atlantic coast (Marais Poitevin, Vendre, France) were investigated in order to
illustrate the rate of mineralogical change in a clay-dominated system. The oldest polder was
constructed in 1665, the last one in 1912; thus the time span of soil evolution is from 80 to 330 years.
All the samples had more than 50% clay (<2 ~tm). The most reactive, fine clay sub-fraction
(<0.1 ~tm) was investigated in detail by X-ray diffraction and chemical analysis. The observed
mineralogical changes with increasing age followed the schematic reaction:
smectite + mica = illite + mixed-layer minerals.
The progress of reaction in time appears to be non-linear. This reaction seems to occur in a
chemically constant system, and the mineralogical change can be seen as a readjustment of species to
a given chemical composition.
The rate at which soil clays form is difficult to
assess. In soil formation from glacial tills or
volcanic materials some data can be found to
show the increase of the content of clay forming
from non-clay phases (Ugolini, 1968; Protz et al.,
1984; Lowe, 1986), but the evolution of clay
minerals as a function of time in clay-rich soils
has not yet been investigated. The interest in such a
problem lies mainly in the rate at which clay
minerals can respond to new chemical conditions. It
is ultimately related to the fertility of soils.
The reaction rates in soils with a high clay
content should be the lowest due to the similarity of
the phases involved. This paper describes some
historically created soils from the west of France in
an attempt to illustrate the rates of change in a clay
dominated system.
SOIL
MATERIAL
AND
METHODS
Seven soil samples were taken at a depth of 35 cm
in polders on the French Atlantic Coast near the
village of Saint-Michel-en-l'Herm, Vendre, France.
The sector (Marais Poitevin) is well known as a
sequence of historically drained zones from which
agricultural land was reclaimed from swampy and
shallow sea bottoms on this coast (Gravier, 1949;
Bourcart, 1958). The time interval of drainage over
the entire area investigated is 330 years. The oldest
known dyke was constructed in 1665. Figure 1
shows the sample locations and the ages of the
successive dykes which made drainage and
agriculture possible. It is assumed that the
sediments of the bay were similar from one point
to another and hence the different periods of
drainage initiated soil processes at different times
with the same starting material.
The depth of sampling was that just below the
plough-pan in the sector and hence it represents the
top of an as yet undisturbed soil profile.
The silt and clay fractions were obtained from
the samples by sedimentation after destruction of
organic matter with diluted, Na-acetate buffered
H202 and dispersion at pH 9 (NaOH). The clay
fractions (<2 p.m) were divided into fine (<0.1 txm)
and coarse clay ( 0 . 1 - 2 p.m) sub-fractions using a
Beckman J2-21 centrifuge equipped with the JCF-Z
continuous-flow rotor. X-ray diffraction (XRD)
patterns were obtained from oriented specimens
using a Philips diffractometer with Fe-filtered CoK~x radiation. Pretreatment of the specimens
included Ca-saturation and solvation with ethylene
glycol, and K-saturation followed by heating to
200~ High-charge and low-charge smer
layers
were identified on the basis of re-expansion with
9 1995 The Mineralogical Society
46
D. Righi et al.
tion procedure allowed use of either Gaussian or
Lorentzian functions for the elementary computed
curves. The best fits were obtained assuming
Gaussian curves. Lanson & Velde (1992) showed
that the Gaussian shape was adequate for I-S illite
and mica-like phases. By running a large series of
tests, Lanson (1990) demonstrated that the standard
deviation for determination of curve characteristics
is induced mostly by the experimental equipment
and that the calculated characteristics are representative of the sample studied. Lanson & Besson
(1992) indicated also that a minimum difference of
0.3 and 0.2 ~ 20, respectively, on peak position and
width, is necessary for DECOMPXR to separate the
contributions of two phases. However, the
difference in one parameter may be smaller if the
other is larger.
Bulk chemical analyses were performed on the
<0.I gm sub-fractions according to the procedure
described by Jeanroy (1972). Silicon, A1, Fe, Ti,
Mg, Ca, Na and K were analysed by atomic
absorption spectroscopy (AAS). Cation exchange
capacity (CEC) was obtained by saturation with
Mg 2+, the excess of Mg salt (MgC12) being
carefully washed out with ethanol. Then Mg 2+
was exchanged by NH~ and analysed by AAS in the
exchange solution. Loss on ignition at 1100~ was
measured by thermogravimetric analysis (TGA)
(Netzsch STA 409 EP). Individual particles of
coarse clay (0.1-2 gm) and fine silt sub-fractions
(2-5 gm) were analysed using a scanning electron
microscope (Jeol JMS 6400) equipped with an EDS
analyser (Kevex).
RESULTS
FIG. 1. Study area. Age of dykes (1912....1665) and
sampling sites (#1, 2...7).
ethylene glycol after K-saturation and heating to
ll0~
The diffractograms were recorded numerically by
a DACO-MP recorder associated with a microcomputer using the Diffrac AT software
(SOCABIM, France). The XRD patterns were then
decomposed into their elementary component
curves using the least-squares computer program
(DECOMPXR) of Lanson (1993). The decomposi-
All the soil samples were heavy clays with >50% of
the particles in the <2 gm fraction (Fig. 2). They
contained calcium carbonate, in decreasing amounts
from the youngest sample (12.8%) to the oldest
(9.7%). Sample pH in H20 was ~ 8.2.
X-ray diffraction
The XRD diagrams from the 2 - 5 gm subfractions appeared to be identical for all the
samples. They exhibited the reflections of chlorite
(d = 1.425, 0.714, 0.472 and 0.354 nm), mica (d =
0.999, 0.499 and 0.335 nm) and kaolinite (d = 0.714
and 0.359 nm) (Fig. 3). Other reflections indicated
quartz ((d = 0.425 nm), calcite (d = 0.385 nm) and
Clay stability in clay-dominated soils
feldspars (d = 0.324 and 0.320 nm). The 0 . 1 - 2 gm
sub-fractions exhibited additional reflection at
d = 1.679 nm, indicating smectite. The latter was
the dominant clay species in the <0.1 gm subfraction, associated with minor amounts of mica
and/or illite, chlorite and kaolinite.
Little change was visible from sample #1 to
sample #7 in the XRD patterns of different sizefractions. As the <0.1 gm sub-fractions were
thought to be the most reactive, the curve
decomposition method was applied to these
fractions in order to identify less obvious changes
in mineralogy. The air-dried samples showed the
greatest variation as a function of the age of the
polder, and will hence form the core of the
9O
-50
12
/
1
2
5
20
50
pm
particle size
Fro. 2. Cumulative grain size distribution curve
(sample #1).
'~
~
2-51
i
~
l
~
am
I
~.
'
1
I
.~
47
~
n
I
I
J
!
0.1 - 2pro
0',
0
< 0 . 1 lam
o
!
'
"/
"
I'I
"
I'5
!
e
~
I
g
t
2'7
FIG. 3. XRD patterns for 2 - 5 , 0.1-2 and <0.1 p.m sub-fractions, sample #1. Ca-saturated, ethylene glycol
solvated. Co-Ka radiation, d-spacings in nm.
D. Righi et
48
discussion w h i c h follows. T h e decomposition
treatment was used essentially in the 4 - 1 2 ~ 20
region where the major 001 clay mineral reflections
were located (Fig. 4). As recommended by Lanson
& B e s s o n (1992), the X R D p a t t e r n s were
d e c o m p o s e d by progresssively increasing the
number of elementary curves, in order to obtain a
good fit with the smallest number of curves. The
first run gave a good fit, using only four elementary
curves, with their positions near 1.51, 1.4, 1.2 and
1.0 nm, respectively. However, the decomposition
of the XRD patterns from ethylene glycol solvated
samples, in the 10 ~ and 1 8 - 2 3 ~ 20 regions (not
shown) required sharp (width at half height 0.5 ~ 20)
/
*"
~ ~
9
.
i
v.q
i
4
~
al.
curves at 1.012 and 0.501 nm. This clearly indicates
the presence of a mica phase. To be consistent, a
second run of decomposition of the XRD patterns
f r o m the a i r - d r i e d s a m p l e s was p e r f o r m e d ,
assuming a supplementary sharp elementary curve
near 1.0 nm. This p r o c e d u r e p r o d u c e d two
elementary curves; one sharp (width at h a l f
height: 0.5 ~ 20) near 1.01 nm, and a broader one
(width at half height: 1.4 ~ 20) near 1.03 nm. Thus,
five elementary curves were needed to obtain a
good fit with the experimental patterns. The same
initial parameters in the decomposition procedure
were assumed for all the samples.
By its position and sharpness, the curve near
1.01 nm was attributed to a detrital mica phase.
This phase was also well developed in the coarser
fractions ( 2 - 5 gm, 0 . 1 - 2 gm). The band near 1.03
nm was typical of an illitic mineral, which was
thought to have a finer grain size than the mica.
This is indicated by a broader peak, with a width at
half height near 1.4 ~ 20. An interstratified mineral
was identified by the band near 1.20 nm. This band
was displaced by the ethylene glycol treatment. It
was assumed to be an illite-smectite mixed-layer
phase. The same change in band position occurred
for the 1.42 nm curve which could be attributed to
illite-smectite mixed-layers but also to vermiculite
and/or chlorite-smectite mixed-layers The curve
near 1.50 nm is that of a dominantly smectitic
mineral. The smectite was of a high-charge type as
#l
1
t~
,,
~ "-"'"
"""I
i
,~
'
,
- "' i
o20
,'\
2"
'
!
1'2
FIG. 4. Decomposed XRD diagrams. Samples #1 and
#7. Fine clay (<0.1 nm) sub-fractions. Ca-saturated,
air-dried. 9
experimental curve; . . . .
computed
elementary curve;
best-fit computed curve. Co-Kct
radiation, d-spacings in nm.
i~
.
.
.
.~
2'.3
FIG. 5. Decomposed XRD diagram in the 18-23 ~ 20
region. Sample #1. Fine clay (<0.1 nm) sub-fraction.
Ca-saturated, air-dried,
experimental curve; . . . .
computed elementary curve;
best-fit computed
curve. Co-K~ radiation, d-spacings in nm.
49
Clay stability in clay-dominated soils
eeq
9
+.
%
!
%
l
O
%
00
t~
1
i!
I*%
u
%
iI
.--..
IN
"i 6
b
16o
~ /--.,,~,,\
! "
&
|
~
[
......
9
u
t-
260
360
time of evolution (years)
"i
020
1'2
FIG. 6. Decomposed XRD diagram. Sample #1. Fine
clay (<0.1 nm) sub-fraction. K-saturated, heated to
200~
experimental curve; . . . . computed
elementary curve;
best-fit computed curve. CoKct radiation, d-spacings in nm.
shown by the absence of re-expansion after Ksaturation and heating.
Evidence for interstratified chloritic layers
associated with the mixed-layer phases was
obtained from the decomposition of the XRD
diagrams in the 18-23 ~ 20 region. An elementary
curve at 0.485 nm, characteristic of illite-chlorite
mixed-layers, was needed to fit the experimental
pattern correctly (Fig. 5). Moreover, the XRD
pattern from the K-saturated samples which were
heated to 200~ gave elementary curves at 1.185
and 1.033 nm, indicating interstratification of
thermally stable 1.40 nm layers (chlorite) with
FIG. 8. The ratio of maximum intensity of the mica
peak to maximum intensity of the illite peak vs. time
of soil evolution (years). Dashed line indicates trend of
change with time.
illite and/or collapsed vermiculite or smectite layers
(Fig. 6). From sample #1 to sample #7 the general
trend indicated by the XRD patterns was a decrease
in the proportion of chloritic layers. A more precise
identification of these interstratified minerals (which
could be three-components, illite-smectite-chlorite
mixed-layers) was beyond the scope of this study.
The <0.1 I.tm fraction shows consistent change as
a function of the age (soil development period).
Although there is not much difference, at first
glance, the decomposed spectra do show distinct
evolutions of the different peaks and hence the
different mineral categories present. The relations
are striking for almost all the phases.
Mica and illite. The peak position of the band
attributed to illite (Fig. 7) moves towards higher
1.25.
1.0S
,O
%
e~
o
%%%
99
o
!
1.00
9
x9 O 9
e~
1.1.5
16o
260
3bo
lime of evolution (years)
FIG. 7. Illite peak position (nm) vs. time of soil
evolution (years). Dashed line indicates trend of
change with time.
16o
26o
3bo
l i m e o f e v o l u t i o n (years)
FIG. 9. Mixed-layer (1.20 nm) peak position vs. time of
soil evolution (years). Dashed line indicates trend of
change with time.
D. Right et al.
50
1 . 6 0
-
1.40
-
1.20
-
t
a-t
t
t
tl
1.00
,
,
,
,
0
,
50
,
v
,
% illite
,
~ t
100
F~. 10. Peak position for random (R = 0) illitesmectite mixed-layers. Air-dried condition (two water
layers), coherent scattering domain size, N = 1 0 - 2 2
layers.
'
m
~
#
1.50
e,
0
9 ~l~'~,,~ go ~
0
.~,
~m O
1.40
,6o
time oI evolution
3 0- 0
(years)
FIG. 11. Mixed-layer (1.40 nm) peak position vs. time
of soil evolution (years). Dashed line indicates trend of
change with time.
1.55
O~'~,
4
'
~
Fie. 13. Decomposed XRD diagram. Sample #1. Fine
clay (<0.1 nm) sub-fraction. K-saturated, heated to
ll0~
ethylene glycol solvated,
experimental
curve; . . . . computed elementary curve;
best-fit
computed curve. Co-K(x radiation, d-spacings in nm.
9
E
&tO
0
r
1.45
u
i
a
1o o
200
3oo
t i m e of e v o l u t i o n ( y e a r s )
FIC. 12. Smectite peak position (nm) vs. time of s0il
evolution (years). Dashed line indicates trend of
change with time.
~ angles with sample age. The relative intensities
of the two micaceous minerals are also a function
of the age of the polder, the samples containing less
mica and more illite as age increases (Fig. 8).
Mixed-layer phases. The peak position of the
mixed-layer population (peaks between 1.25 and
1.15 nm) changes regularly with soil age as seen in
Fig. 9. The expandability of this fraction, assumed
as a first approximation to be of random ordering
type, decreases from 35 to 25% smectite layers
according to the simulations made . w i t h the
Clay stability in clay-dominated soils
M R 3+
2 R 3+
2 R 3+
M R 3+
2R 3+
51
M R 3+
Chl
3 R 2+
FIG. 14. Chemical composition of individual particles (1 = 2 - 5 i~m sub-fractions, samples #1 and #7; 2 = 0.12 ltm sub-fraction, sample #1; 3 = 0 . 1 - 2 p.m sub-fraction, sample #7) in a ternary MRa+-3R2+-2R3+ diagram
(Velde, 1985). Mu = muscovite composition, Chl = chlorite composition.
NEWMOD program (Reynolds, 1985) (Fig. 10).
This indicates a decrease in the expandability of the
minerals with increasing age of the soil. The peak
position of the other more expandable mixed-layer
phase (peaks between 1.41 and 1.45 nm) also shifts
towards higher ~
angles with time (Fig. 11).
According to the NEWMOD simulations (again
assuming a random illite-smectite mixed-layer), the
expandability of this phase decreases from 85 to
60% smectite layers.
Smectite. The peak position of the smectite phase
(Fig. 12) moves from 1.55 to 1.48 nm with time.
Moreover, the re-expansion with ethylene glycol
after K-saturation and heating is weaker as the
sample becomes older (Fig. 13). Again this seems
to indicate the lowering of expandability associated
with a higher charge on some layers in the older
polders.
All of the changes in mineralogy observed on the
XRD patterns seem to be a non-linear function of
time, being slower in the older samples.
Chemical analysis
The chemical data obtained from particles of the
2 - 5 gm fraction were plotted in a MR3+-2R 3+3R 2§ ternary system (Fig. 14). They confirm the
results of the XRD, which has shown mica and
chlorite as major phyllosilicates in that fraction.
Particles with a muscovite composition were
analysed as well as particles with a chlorite
composition. It is interesting to note that many
individual particles have an intermediate composition. As XRD does not show mica-chlorite mixedlayers in this size-fraction, it can be concluded that
these particles are made of clusters of mica or
chlorite crystals. No change was observed from one
sample to another in this fraction. In the 0 . 1 - 2 ~tm
sub-fractions the same particles were analysed, but
the particles with a chlorite or intermediate
composition were less abundant than in the
coarser fraction. Also, they were less abundant in
the older sample (#7) than in the younger (#1). This
seems to indicate the degradation of chlorite layers
with time in the 0 . 1 - 2 gm sub-fraction.
The chemical composition of the fine clay
(<0.1 p,m) sub-fraction (Table 1) was constant.
The CEC, the K and Fe content and loss on
ignition are virtually identical for the different
samples of the polders of different age and for a
sample of recent mud on the shore of the channel
(#0). The CEC reflects the high smectite content in
the fine size sub-fractions, with CEC values of
5 4 - 6 0 mEq/100g, roughly half that of a pure fully
expandable smectite. The K20 content ( ~ 2 . 5 % )
indicates ~ 25% of mica and illite layers.
DISCUSSION
Particle size distribution and chemical data show
that the source material in all the polders was, as
assumed, very homogeneous. The <0.1 Ixm subfraction is isochemical, i.e. the samples do not show
52
D. Righi et al.
TABLE 1. Bulk chemical analyses as % dry sample and CECs (mEq/100 g) of the <0.1 ~tm sub-fractions.
Sample#
#0
#1
#2
#3
#4
#5
#6
#7
SiOz
A1203
Fe203
MgO
TiO2
MnO
CaO
NazO
K20
L.O.I. (1100~
Total
44.80
21.49
11.07
2.66
0.49
0.14
0.50
0.50
2.42
14.10
98.17
46.38
21.62
11.81
2.61
0.53
0.08
0.60
0.61
2.39
12.80
99.43
45.44
21.46
12.33
2.43
0.54
0.12
0.37
0.62
2.36
13.90
99.57
44.92
21.01
11.92
2.44
0.42
0.13
0.53
0.53
2.28
15.10
99.28
45.10
21.47
ll.6t
2.33
0.49
0.14
0.54
0.51
2.31
14.30
98.80
45.81
21.53
11.59
2.27
0.45
0.11
0.44
0.60
2.34
13.40
98.54
46.94
22.47
12.38
2.48
0.53
0.12
0.40
0.41
2.46
10.40
98.59
46.64
22.10
11.58
2.33
0.56
0.09
0.20
0.42
2.28
12.60
98.80
CEC
58.50
60.10
57.20
56.40
57.90
54.50
58.00
57.50
L.O.I. = loss on ignition.
any chemical or physical transport of material from
the soil horizons investigated. There seems to be
little migration of elements into, or out of, the clay
minerals into grains of other size-fractions.
With time, the chlorite particles and the chloritic
layers in complex interstratified minerals appeared
to be weathered. This is indicated by changes in the
XRD patterns from the <0.1 ~tm sub-fractions and
the chemical analyses of particles from the 0.12 I.tm sub-fractions. The overall composition of this
sub-fraction tends to cluster in the mica (muscovite)
compositional field as the polder becomes older
(Fig. 14). The instability of chlorite minerals in soil
environments is well documented (Bain, 1977;
Righi et al., 1993).
It is apparent that the detrital mica phase of the
<0.1 ~tm portion of the sediment is degraded into an
illitic material, assumed here to be of a finer grain
size. This is indicated by the change in intensity of
the mica peak relative to the illite peak. The shift of
the mica peak position towards lower values (in
nm) as the samples grow older, indicates that either
the illite grains grow, or that the grains have fewer
smectite layers present. It is possible that both
effects operate in these samples but it is most likely
that the growth of the smaller grains is important
(Lanson & Besson, 1992). This growth of small
grains at the expense of large ones illustrates the
differences between the detrital mica (near
muscovite in K content) and the illite which have
less K and are more stable at low temperatures. The
same effect has been noted in the weathering of
granite where muscovite is degraded, supplying
material for a newly formed illite phase (Meunier &
Velde, 1976).
The decrease in the basal spacings of the smectite
and mixed-layer phases can be explained as an
evolution of a certain number of the smectite layers
into a non-expanding material.
It is evident that the changes are subtle, only
'visible' using a curve decomposition method of
analysis of XRD diagrams. Nevertheless, the
changes are important, especially where the mica
is changed into a finer grain size-fraction and the
smectite loses its expandability. These reactions
seem to occur in a chemically constant system
where there is exchange of material between the
different mineral components of the system. The
mica to illite evolution is likely to be accomplished
with the loss of some K from the mica. The stable
form of micaceous mineral at surface conditions
seems then to be illite, i.e. a slightly K-poor, silicarich form of dioctahedral, aluminous mica (Velde,
1985). As a compensatory change, the smectite
appears to have a higher charge, showing lower
average expandability in the air-dried state and after
K-saturation and ethylene glycol solvation. Mixedlayered phases also lose expandability. This
suggests a higher alumina content and probably an
increased K content. Very schematically, the
following overall reaction can be written:
mica + smectite = illite + mixed-layer minerals.
This shows a transfer of K from the micaceous
materials into the expandable minerals. At 35 cm
Clay stability in clay-dominated soils
depth, wetting and drying cycles occur regularly in
soils of the area, a process known to favour the
fixation of K in the interlayers of smectite (Mamy
& Gaultier, 1976; Eberl et al., 1986; Eberl et al.,
1993).
The striking observation is that the overall CEC
remains almost constant for the <0.1 gm subfraction. Hence, the loss of non-expanding phases
(mica, chlorite) seems to be balanced by the loss of
the expanding phases. This appears to be a general
adjustment in a basically chemically closed system,
where the clays dominate the solid phase. The
mineralogical changes can be seen as a readjustment of species to a given bulk chemical
composition. The mineralogical changes observed
through peak ratios and peak positions in
decomposed XRD patterns seem to change in a
non-linear way as a function of time. This is of
course to be expected as new material, furthest out
of equilibrium will react more rapidly than material
which has re-adjusted in part to new equilibrium
conditions. The exact relations of reaction percent
vs. time cannot be estimated with the data
presented.
CONCLUSION
The indications of clay mineral change described
are of great importance as the possibility of clay
mineral change in an essentially clay-dominated
system has not yet been demonstrated. Until now
clay reactions have been reported from sandy or
gravelly soils in which the mineralogical change is
from metamorphic or other rock fragments into a
clay fraction, a situation of high metastability. The
situation in the Marais Poitevin soils is quite the
reverse, as the soils are clay dominated. In such an
example the minerals initially present, especially
the smectite, as formed under or near-surface
conditions, could be expected to react very
slowly, if at all, during soil development. In
sedimentary burial sequences, the detrital sediments tend to react only very slowly as
temperature increases up to 5 0 - 8 0 ~
(Velde &
lijima, 1988), and this happens over a time span of
at least 1 myr. In the samples studied, the mineral
changes are apparent in less than 350 y. The
mineral changes in these soils appear to be a nonlinear function of time, the rate of alteration
decreasing with time.
53
ACKNOWLEDGMENTS
The authors thank Dr Alain Bouchet (Socitt6 ERM)
for providing the peak position curve of simulated
illite-smectite mixed-layers (Fig. 10).
REFERENCES
BA1N D.C. 0977) The weathering of chloritic minerals
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BOURCARTJ. (1958) Le littoral de la Tranche (Vendte) ~t
l'Ile Madame (Charente-Maritime). Bull Soc. Geol.
Fr. 3, 393-397.
EBERL D., SRODOr~ J. & NORTHROP H.R. (1986)
Potassium fixation in smectite by wetting and
drying. Pp. 296-326 in: Geochemical Process at
Mineral Surfaces (J. A. Davis & K. F. Hayes,
editors). American Chemical Society Symposium
Series 323.
EBERL D., VELDEB. & MCCORMICKT. (1993) Synthesis
of illite-smectite from smectite at Earth surface
temperatures and high pH. Clay Miner. 28, 49-60.
GRAVIERJ. (1949) Le Marais Poitevin. Bull. Soc. Belge
d'Etudes Geogr. XVIII, 37-55.
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R E S U M E: Sept 6chantillons d'une chronos6quence de sols de polders de la c6te Atlantique (Marais
Poitevin, Vend6e, France) ont 6t6 6tudi6s dans le but d'appr6cier la cin6tique des transformations
min6ralogiques dans des sols trbs argileux. Le polder le plus ancien est dat6 de 1665, le plus r6cent
de 1912. La dur6e d'6volution des sols s'6chelonne donc de 80 ?: 330 ann6es. Tousles 6chantillons
one une teneur en argile (<2 gm) sup6rieure ~t 50%. La fraction d'argile fine (<0.1 gm) consid6r6e
comrne la plus r6active a 6t6 6tudiEe de fagon d6taill6e par diffraction de rayons X et analyse
chimique. Avec le temps les changements min6ralogiques suivent la r6action:
smectite + mica = illite + interstratifi6s.
La progression de la r6action n'est pas lin6aire et elle semble se produire dans un systbme
chimiquement invariant. Les changements min6ralogiques peuvent ~tre consid6rEs comme un
r6ajustement des structures min6ralogiques aux conditions chimiques du syst~me.