Clays and Clay Minerals, Vol. 46, No. 2, 167-177, 1998.
P E D O G E N I C F O R M A T I O N OF H I G H - C H A R G E BEIDELLITE IN A
V E R T I S O L OF S A R D I N I A (ITALY)
DOMINIQUE RIGHI l, FABIO TERRIBILE2 AND SABINE PETIT 1
1 "Hydrog6ologie, Argiles, Sols et Alt6rations", UMR CNRS 6532, Facult6 des Sciences, 86022 Poitiers, France
2 CNR-ISPAIM, PO Box 101, 80040 San Sebastiano al Vesuvio, Napoli, Italy
A b s t r a c t - - T h e fine clay (<0.1 p~m) fraction of a clayey soil (Vertisol) from Sardinia (Italy) was studied
by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, cation exchange
capacity (CEC) and surface area measurements. Smectites were the dominant clay minerals both in the
parent material and the soil horizons. The magnitude and location of the smectites' layer-charges were
analyzed using the Hofmann & Klemen effect (suppression of the octahedral charge following lithiumsaturation and heating). The amount of montmorillonite layers was evaluated by measuring the reduction
of total surface area and CEC after suppression of octahedral charges and subsequent collapse of montmorillonite interlayers. Reduction of total surface area and CEC after fixation of K and irreversible
collapse of interlayers were used to quantify high-charge layers before and after the suppression of
octahedral charges. This allowed us to evaluate the amount of tetrahedral high-charge layers. The smectites
in the parent material were montmorillonitic-beidellitic mixed layers and exhibited a small proportion of
high-charge layers; in particular, layers with a high tetrahedral charge were a very minor component. In
the upper soil horizons, the amount of montmorillonitic layers decreased whereas the amount of smectite
layers with a high tetrahedral charge increased. FTIR spectroscopy indicated more Fe for A1 substitution
in the smectites of the soil horizons than in the smectites of the parent material. The results suggested
that octahedral charged smectite layers (montmorillonitic) were altered, whereas high-charge beidellitic
layers were formed in this soil environment characterized by rather high pH (>8.0).
Key Words--Beidellite, High-charge Smectite Layers, Illite-Smectite Mixed Layers, Italy, Smectite, Soil,
Vertisol.
INTRODUCTION
It is well e s t a b l i s h e d that beidellites are m o r e c o m m o n in soil e n v i r o n m e n t s , w h e r e a s m o n t m o r i l l o n i t e s
are m o r e typical o f g e o l o g i c materials ( W i l s o n 1987).
Moreover, h i g h - c h a r g e beidellites were r e p o r t e d as the
d o m i n a n t clay m i n e r a l in the h i g h l y smectitic soils
k n o w n as Vertisols ( R o s s i g n o l 1983; B a d r a o u i et al.
1987; B a d r a o u i a n d B l o o m 1990; B o u a b i d et al. 1996).
Soil beidellites are t h o u g h t to exist as the w e a t h e r i n g
p r o d u c t of m i c a s a n d chlorites, b e c a u s e these already
h a v e the tetrahedral substitution r e q u i r e d for the beidellite structure. T h e e v i d e n c e for the f o r m a t i o n o f
smectites f r o m soil solution is difficult to establish.
A c c o r d i n g to B o r c h a r d t (1989), beidellites w o u l d b e
c r y s t a l l i z e d f r o m soil solutions i f the p H is less t h a n
6.7, in w h i c h case e x c h a n g e a b l e A1 is present, w h e r e a s
m o n t m o r i l l o n i t e s w o u l d b e f o r m e d in b a s e - s a t u r a t e d
soils w i t h l o w organic m a t t e r c o n t e n t a n d h i g h p H
( > 6 . 7 ) , w h i c h is the case in Vertisols. H o w e v e r , Koun e t s r o n et al. (1977) s h o w e d that beidellite was f o r m e d
in a Vertisol f r o m the w e a t h e r i n g o f m i c a t h r o u g h dissolution a n d r e c r y s t a l l i z a t i o n p r o c e s s e s r a t h e r t h a n b y
s i m p l e t r a n f o r m a t i o n o f the m i c a structure.
R i g h i et al. (1995) d e s c r i b e d the p e d o g e n i c alteration o f low- to h i g h - c h a r g e beidellites in a Vertisol
f r o m S o u t h Italy. H i g h - c h a r g e beidellites were prod u c e d t h r o u g h p e d o g e n i c alteration o f l o w - c h a r g e
smectites in a soil e n v i r o n m e n t c h a r a c t e r i z e d b y h i g h
Copyright 9 1998, The Clay Minerals Society
p H (pH = 8 - 9 ) a n d soil solutions that c o n c e n t r a t e w i t h
e v a p o r a t i o n . E i t h e r solid-state r e o r g a n i z a t i o n or dissolution-recrystallization were possible mechanisms
for p r o d u c i n g m o r e A1 for Si substitutions in the tetr a h e d r a l sheet. T h e p u r p o s e o f the p r e s e n t p a p e r w a s
to p r o v i d e a d d i t i o n a l results to s u b s t a n t i a t e the s a m e
p r o c e s s in a Vertisol f r o m a n o t h e r l o c a t i o n in Italy.
T h e m a g n i t u d e a n d d i s t r i b u t i o n o f the s m e c t i t e layercharge, either in the t e t r a h e d r a l or in the o c t a h e d r a l
sheets, was i n v e s t i g a t e d to address the p r o b l e m o f
m o n t m o r i l l o n i t e a n d beidellite stability in this soil env i r o n m e n t . C l a y m i n e r a l s e x h i b i t structural p e r m a n e n t
surface c h a r g e (related to l a y e r c h a r g e ) a n d p H - d e p e n d e n t charge. F o r 2:1 layer-type clay m i n e r a l s the structural c h a r g e is the greater c o m p o n e n t o f the clay surface charge, w h i c h is p r o b a b l y the m o s t i m p o r t a n t
p r o p e r t y affecting the c h e m i c a l a n d p h y s i c a l b e h a v i o r
o f soils a n d is u l t i m a t e l y related to soil fertility.
SOIL MATERIAL
T h e selected Vertisol (Typic Pelloxerert, Soil Surv e y S t a f f 1994) is located n e a r the village o f Suelli
(Cagliari) in Sardinia, Italy. T h e geological s u b s t r a t u m
in this area is part o f a large M i o c e n e f o r m a t i o n w h i c h
consists o f marls, a r e n a c e o u s m a r l s a n d m a r l calcarenites. T h e area c o v e r s a large plateau w h i c h w a s ext e n s i v e l y eroded. T h e p r e s e n t l a n d s c a p e c o n s i s t s o f
few relicts o f the plateau a n d r o u n d e d hills, w h i c h are
167
168
Righi, Terribile and Petit
Clays and Clay Minerals
Table 1. Characteristics of the soil horizons clay (<2 p.m), CaCO3, OM (organic matter) as percent of 105 ~ dried soil.
Soil horizon
Clay
OM
CaCO~
pH (H20)
CEC
Ca % C E C
Na %CEC
Mg %CEC
K %CEC
Ap
Bssl
Bss2
Bss/Ck
Ck
62.4
61.0
61.2
63.0
48.5
2.13
1.82
1.26
0.77
0.33
0.1
0.8
1.9
3.8
39.2
8.3
8.1
8.5
8.6
8.7
33.5
32.5
33.6
28.9
13.1
74.9
71.1
59.2
47.4
37.4
4.2
5.8
7.4
8.3
9.9
17.9
20.3
31.5
42.2
51.1
3.0
2.8
1.8
2.1
1.5
CEC: cmol~ kg t; Ca, Na, Mg, K: exchangeable cations.
interconnected by means of concave slopes forming
large and small basins filled with fine colluvial materials. The study site was selected at the foot of a short
concave slope of about 150 m length with a mean
slope gradient of about 5%. In this topographic setting,
soils were the deepest and exhibited the best vertic
characteristics. A short description of the studied soil
is given below, and analytical data are presented in
Table 1. Munsell colors are for the moist sample.
0 - 4 0 cm: Ap, very dark grey (10YR3/1) clay, very
friable, fine granular and fine to coarse subangular
blocky structure, slight HCI effervescence, regular
clear boundary;
4 0 - 1 2 0 cm: Bssl, black (10YR2/1) clay, firm,
strong coarse angular blocky structure with wedgeshaped peds, large peds have slickensides, slight HC1
effervescence, smooth gradual boundary;
120-140 cm: Bss2, black (10YR2/1) clay, very
firm, very coarse angular blocky structure with wedgeshaped peds, larger peds have distinct slickensides,
slight HC1 effervescence, undulate clear boundary;
140-175 cm: Bss/Ck, very dark greyish brown
(10YR3/2) clay, very firm, very coarse angular blocky
and massive structure, few and very large slickensides,
strong HC1 effervescence, regular gradual boundary;
>175 cm: Ck, olive (5Y4/3) clay, very firm, massive structure, strong HC1 effervescence.
Two additional soil horizons (Ap, 0 - 1 0 cm and C,
> 30 cm) were sampled from a shallow eroded soil
located at the top of the slope in order to characterize
material that could be added to the studied Vertisol
through erosion and deposition by colluvial process.
METHODS
The silt and clay fractions were obtained from the
soil horizons by sedimentation after destruction of organic matter with dilute H202 buffered with Na-acetate. Dispersion was done in a NaOH solution at pH
9. No treatment for dissolution of calcium carbonate
was used to avoid possible alteration of clay minerals
induced by acidification of the medium. The fine clay
fraction (<0.1 ~Lm) was obtained from the clay ( <2
p,m) fraction using a Beckman J2-21 centrifuge equiped with the JCF-Z continuous flow rotor. X R D diagrams were obtained from parallel-oriented specimens
using a Philips diffractometer with Fe-filtered CoKc~
radiation. Pretreatment of the specimens included Ca
saturation and solvation with ethylene glycol.
The Hofmann and Klemen effect (1950), which is
the suppression of the octahedral charge in dioctahedral smectites after lithium-saturation and heating at
300 ~ was used to distinguish octahedral from tetrahedral charged smectite layers. The conventional
treatment was modified by adding a back-saturation
step with Ca after the heating sequence. In such conditions smectite layers with a minimum residual tetrahedral charge were expected to re-expand when solvated with ethylene glycol, whereas only smectite layers with no (or very low) tetrahedral charge (ideal
montmorillonite) would remain unexpandable by annulation of their octahedral charge. This procedure actually does not distinguish between montmorillonite
and beidellite (and nontronite) according to the conventional arbitrary dividing point (50% tetrahedral
charge); rather, it distinguishes between ideal montmorillonite (no tetrahedral charge) and other tetrahedrally charged dioctahedral smectites.
High-charge smectite layers were identified on the
basis of irreversible collapse of interlayers following
fixation of K. Briefly, the fine clay fraction was Ksaturated, heated overnight at 110 ~ and then backsaturated with Ca, ethylene glycol solvated and submitted to XRD. The loss of expansibility following
this treatment was attributed to irreversibly collapsed
high-charge layers. The treatment was also applied to
lithium-treated fine clay fractions. In such conditions,
the loss of expansibility was attributed to high-charge
layers with the charge located in the tetrahedral sheet
only.
The diffractograms were recorded numerically by a
D A C O - M P recorder associated with a microcomputer
using the Diffrac AT software (SOCABIM). The
N E W M O D e program (Reynolds 1985) was used to
simulate XRD patterns and to provide a basis for the
interpretation of experimental X RD patterns.
CEC was measured with the procedure of Anderson
and Sposito (1991), which distinguished between accessible structural permanent charge and variable
charge. The procedure was used before and after the
fine clay fraction was submitted to the lithium-treatment. The reduction of the CEC after the lithium-treatment was assumed to represent the octahedral charge.
Vol. 46, No. 2, 1998
Pedogenic formation of beidellite in a Vertisol
169
100-
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i i i ii::i
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100
10
i i i i!il
1000
10000
particle size (pro)
Figure 1.
Cumulative grain size distribution curves.
The C E C was also measured after the fine clay fraction
was K-saturated and heated at 110 ~ In this case, the
C E C was obtained with a conventional exchange
m e t h o d (Righi et al. 1995). The reduction of the C E C
after this treatment was assumed to represent the fraction o f layer-charge neutralized by irreversibly fixed
K. The K-saturation and heat treatment were applied
on fine clay fractions previously submitted to the lithium-treatment (suppression of the octahedral charge).
F o l l o w i n g this procedure, smectite layers with high
density of tetrahedral charges (irreversibly collapsed)
might be distinguished from smectite layers with low
density of tetrahedral charges (re-expanded upon ethylene glycol solvation).
Total chemical analyses were p e r f o r m e d on the bulk
soil ( < 2 m m ) and fine clay (<0.1 p,m) fraction according to the procedure described by Jeartroy (1972).
Silicon, A1, Fe, Ti, Mg, Ca, N a and K were analyzed
by atomic absorption spectrometry.
Total and external surface areas were measured acc o r d i n g to the e t h y l e n e g l y c o l m o n o e t h y l ether
( E G M E ) (Heilman et al. 1965) and B r u n a u e r - E m mett-Teller (BET) techniques, respectively.
F T I R spectroscopy was p e r f o r m e d on K B r disks
prepared by mixing 1 m g sample with 300 m g K B r
and pressing at 13 kg c m -2. F T I R spectra were recorded using a Nicolet 510 spectrometer.
RESULTS
Soil Analysis
The cumulative grain size distribution curves c o m puted for the 2 - 2 0 0 0 p,m portion of the 5 horizons
were v e r y similar (Figure 1). All the horizons were
heavy clays with m o r e than 60% of the particles in
the < 2 p-m fraction (Table 1). The coarse silt and sand
fractions were always m i n o r fractions, less than 15%
of the whole soil material. C a l c i u m carbonate was
present in increasing amounts from the Ap horizon
(0.1%) to the Ck horizon (39.2%). The p H in H20 of
soil horizons was about 8.2 in the 2 upper horizons
(Ap, Bssl) and higher in the deeper horizons (8.5 to
8.7). The C E C s of the soil horizons were b e t w e e n 13.1
and 33.6 cmol~ kg -~ where exchangeable M g and Na
increased and K and Ca decreased with depth.
Total chemical analysis of the bulk soil material o f
the various horizons (Table 2) gave similar values
throughout the profile, indicating chemically h o m o geneous materials in the parent material and the soil
horizons.
Table 2. Total analyses of the bulk soil samples (<2 mm) as percent of 105 ~ dried sample.
Sample
SiO2
A1203
FezO 3
MnO
MgO
CaO
Na20
K20
TiO:
LOI
Ap
Bss2
Bss/Ck
Ck
62.51
63.12
62.75
63.90
11.21
11.40
11.10
11.48
4.68
4.53
4.69
4.74
0.11
0.10
0.11
0.08
1.50
1.61
1.79
2.50
1.50
1.64
1.88
1.41
1.12
1.11
1.04
1.15
2.00
1.83
1.84
2.01
0.61
0.59
0.59
0.65
12.96
12.08
12.40
10.48
LOI: loss on ignition. (CaO percentages were corrected with reference to calcium carbonate content.)
170
Clays and Clay Minerals
Righi, Terribile and Petit
Table 3. Total analyses of the fine clay (<0.1 ixm) fraction of the studied horizons.
Sample
SiO 2
AI203
Fe20~
MnO
MgO
CaO
Na20
K20
TiO2
LOI
Ap
Bss2
Ck
51.15
51.44
50.89
18.39
17.98
18.75
9.80
9.56
8.76
0.10
0.05
0.02
2.54
2.63
3.20
0.33
0.38
0.37
0.88
0.81
0.74
2.93
2.75
2.43
0.47
0.58
0.58
13.40
13.60
14.30
LOI: loss on ignition as percent of 105 ~ dried sample.
Fine Clay Fraction Analyses
Total c h e m i c a l analysis p r o d u c e d h o m o g e n e o u s resuits; all the fine clay fractions h a d a r a t h e r c o n s t a n t
c h e m i c a l c o m p o s i t i o n (Table 3). T h e K 2 0 c o n t e n t s
c o m p r i s e d b e t w e e n 2.43 a n d 2 . 9 3 % o f the 105 ~
dried s a m p l e s , i n d i c a t i n g less t h a n 3 0 % illite or m i c a
layers, as n o K - f e l d s p a r s w e r e p r e s e n t in the s a m p l e s
(Kiely a n d J a c k s o n 1965). B e c a u s e n o t r e a t m e n t was
u s e d to r e m o v e iron oxides, F e z O 3 c o n t e n t s w e r e ass u m e d to r e p r e s e n t silicate iron plus a m o r p h o u s a n d /
or c r y s t a l l i n e f o r m s o f iron oxides 9
X-ray Diffraction
All the X R D p a t t e r n s f r o m the fine clay ( < 0 . 1 [zm)
fractions (Figure 2) e x h i b i t e d the typical diffraction
b a n d s o f smectitic minerals: i n t e n s e p e a k at a b o u t 1.52
~0.1 ~ml
Ap
|
Ck <0.1 ~ m
Figure 2.
I
,~
i
I
l
10
20
30
~
XRD patterns for fine clay (<0.1 ~m) fractions. Ca-saturated, ethylene glycol solvated, CoNa radiation; d(nm).
Vol. 46, No. 2, 1998
Pedogenic formation of beidellite in a Vertisol
171
~.S=0.6
illite/smectite mixed layers, 40% illite
1.711
0.336
/',,._
O.551
PI.s = 0.45
1.711
0.337
0.556
1.711
PLS =
0.35
0.336
0.560
~
i
I
,
5
J
8
,
I
18
,
I
~
I
12 14
,
I
,
I
,
16 18
I
,
I
20 22
,
I
,
I
24 26
,
I
~
I
28 30
,
I
i
32
Figure 3. Simulated XRD patterns using NEWMOD 9 program: A) illite-smectite, random, 40% illite; B) illite-smectite,
effect of segregation of illite (I) and smectite (s) layers shown by PI.S = 0.45 (PLS: probability of having a smectite layer
following an illite layer, l~.s = 0.6 represents random condition, smaller PI.S represents increasing segregation); C) illitesmectite, effect of segregation shown by PLS = 0.35. Coherent scattering domain size: 4 - 6 layers; d(nm).
n m (air-dry, not s h o w n ) or 1 . 7 2 - 1 . 8 5 n m ( e t h y l e n e
glycol) w i t h h i g h e r o r d e r at 0 . 8 7 - 0 . 8 8 , 0 . 5 5 8 - 0 . 5 6 3
a n d 0.335 nm. In a d d i t i o n s m a l l p e a k s at 1 . 0 1 - 1 . 0 2
n m a n d 0 . 5 0 4 - 0 . 5 0 8 n m , attributed to illitic material,
are s h o w n o n the d i a g r a m s f r o m the B s s 2 a n d A p h o rizon samples. All the fine clay p h y l l o s i l i c a t e s w e r e
d i o c t a h e d r a l m i n e r a l s , as i n d i c a t e d b y a 0 6 0 b a n d at
0 . 1 5 0 n m (not shown). T h e i n c r e a s e o f the intensity
ratio o f the 001 p e a k at a b o u t 1 . 7 0 - 1 . 8 0 n m to the
b a c k g r o u n d , d o w n a n g l e f r o m t h e 001 peak, i n d i c a t e d
i l l i t e - s m e c t i t e m i x e d layers ( I - S ) w i t h i n c r e a s i n g prop o r t i o n o f illitic layers f r o m the C k to the A p horizon.
Actually, this ratio h a s b e e n used as a r o u t i n e deterruination ( k n o w n as the " s a d d l e / 0 0 1 " m e t h o d ) o f per-
c e n t a g e o f smectite layers i n I - S m i x e d layers ( i n o u e
et al. 1989). H o w e v e r , the total surface areas a n d C E C
values (see b e l o w ) were c o n s i s t e n t a n d s u g g e s t e d the
same proportion of collapsed and expanded compon e n t s in the m i x e d layers f r o m the u p p e r soil h o r i z o n s
a n d the p a r e n t material. Therefore, for the interpretation o f X R D patterns, a s i m u l a t i o n o f the X R D diag r a m s was m a d e u s i n g the N E W M O D ~ p r o g r a m
w h i c h , a m o n g o t h e r facilities, a l l o w e d us to s i m u l a t e
the effect o f the s e g r e g a t i o n o f illite a n d smectite layers in a n I - S m i x e d layer. It was p o s s i b l e to s i m u l a t e
the e x p e r i m e n t a l p a t t e r n s for I - S m i x e d layers w i t h
6 0 % s m e c t i t e (Figure 3). T h e s i m u l a t i o n o f the X R D
d i a g r a m f r o m the C k h o r i z o n r e q u i r e d i n t r o d u c t i o n o f
172
Righi, Terribile and Petit
Clays and Clay Minerals
A p <0.I p m
B s s 2 <0.1 ~ m
B ss2 <0.1 ~tm
--4
g
e~
--4
.S
l
I
4
I
6
~
Ck<O.1 ~tm
I
8
I
I
l0
t
I
12
I
k
14
i
t
16
I
I
~
I
6
Ck <0.1 ~m
I
i
8
r
i
10
I
I
12
i
i
14
I
I
16
I
I
020
Figure 4. XRD patterns for the fine clay (<0.1 ~m) fractions after K-saturation, heating and back-saturation with Ca.
Ethylene glycol solvated, CoKa radiation; d(nm).
Figure 5. XRD patterns for the fine clay (<0.1 Ixm) fractions after lithium-saturation, heating at 300 ~ and backsaturation with Ca. Ethylene glycol solvated, CoKa radiation;
d(nm).
a rather strong segregation effect (as shown by the
probability Pl.s, of having a smectite layer following
an illite layer equal to 0.35) not needed for the simulation of the Ap horizon diagram. Thus, in agreement
with CEC values and total surface areas, all the soil
horizons would contain I-S mixed layers with identical proportion of smectite layers; only the distribution
of smectite and illite layers in the stacking sequence
would change from one horizon to another. More segregation of the collapsed and expandable layers would
be present in the clay crystals of the parent material
than in those of the upper soil horizons.
A decrease in the proportion of expandable layers
of the fine clay fractions was obtained by the K-satu-
ration and heat treatment (Figure 4), as shown by the
increase of the saddle/001 peak ratio.
Following the lithium-treatment, all the fine clay
fractions exhibited a partial re-expansion upon ethylene glycol solvation (Figure 5). In response to that
treatment, a physical mixture of beidellite and montmorillonite would produce 2 XRD peaks, one near
1.70 n m for expanded beidellite and a second one near
0.97 nm for collapsed montmorillonite. Compared to
the untreated fine clay fractions (see Figure 2), no increase of the diffracted intensity at about 0.97 nm that
would indicate discrete particles of octahedral charged
smectite layers was observed for the studied samples.
Moreover, the XRD patterns were those of collapsed
Vol. 46, No. 2, 1998
Pedogenic formation of beidellite in a Vertisol
173
( m o n t m o r i l l o n i t i c ) a n d e x p a n d e d (beidellitic) m i x e d
layers, as s h o w n b y the h i g h saddle/001 p e a k ratio.
T h u s , the s m e c t i t e c o m p o n e n t in the p a r e n t m a t e r i a l
a n d the soil h o r i z o n s e x h i b i t e d h e t e r o g e n e o u s c h a r g e
d i s t r i b u t i o n a n d c o u l d b e seen as a r a n d o m interstratification o f beidellitic a n d m o n t m o r i l l o n i t i c layers.
F o l l o w i n g the K - s a t u r a t i o n a n d heat t r e a t m e n t applied to the l i t h i u m - t r e a t e d samples, the X R D p a t t e r n
o f the fine clay f r a c t i o n f r o m the p a r e n t m a t e r i a l was
o n l y slightly affected. O n l y a s m a l l i n c r e a s e o f the
relative intensity o f the p e a k at 1.01 n m was o b s e r v e d
(Figure 6). T h e relative intensity o f the p e a k at 1.02
nm was increased and a more pronounced change was
o b s e r v e d for the X R D p a t t e r n s o f the fine clay fractions f r o m the u p p e r soil h o r i z o n s c o m p a r e d w i t h the
original l i t h i u m - t r e a t e d s a m p l e (see F i g u r e 5).
A p <0.1 [zm
B ss2 <0.1 ~ m
Surface Area Measurements
I
Ck ' ~ . 1 ~m
I
4
I
I
I
6
I
8
I
I
10
t
I
12
I
t
14
I
I
I
16
*20
Figure 6. XRD patterns for the lithium-treated fine clay
(<0.1 /xm) fractions after K-saturation, heating and back-saturation with Ca. Ethylene glycol solvated, CoKct radiation;
d(nm).
T h e e x t e r n a l surface areas (ES) of the fine clay fraction in the soil h o r i z o n s d e c r e a s e d f r o m 159 ( A p h o rizon) to 141 ( B s s 2 h o r i z o n ) a n d 102 m 2 g l ( C k h o rizon) (Table 4). T h e total surface areas (TS) w e r e relatively similar a n d c o m p r i s e d b e t w e e n 536 ( A p horiz o n ) a n d 565 ( C k h o r i z o n ) m 2 g-l. B y s u b t r a c t i o n o f
the e x t e r n a l f r o m the total surface area (TS - ES),
internal surface areas (IS) were e s t i m a t e d to b e 377
( A p h o r i z o n ) , 4 2 0 ( B s s 2 ) a n d 465 m 2 g-1 (Ck).
F o l l o w i n g the K - s a t u r a t i o n treatment, the total surface area w a s r e d u c e d to 377, 373 a n d 4 4 9 m 2 g-I for
the Ap, B s s 2 a n d C k h o r i z o n samples, r e s p e c t i v e l y
(KTS, Table 4). T h e r e d u c t i o n w a s attributed to the
collapse o f h i g h - c h a r g e smectite interlayers. T h u s , the
d i f f e r e n c e TS - K T S g i v e s the i n t e r n a l surface area
o f h i g h - c h a r g e s m e c t i t e layers (hc layers, Table 4). It
was 159 (Ap h o r i z o n ) a n d 188 m E g-1 ( B s s 2 h o r i z o n )
a n d 116 m 2 g - i ( C k h o r i z o n ) , i n d i c a t i n g m o r e coll a p s e d h i g h - c h a r g e layers in the u p p e r soil horizons.
F o l l o w i n g the l i t h i u m - t r e a t m e n t , the total surface
area w a s r e d u c e d to 4 3 1 , 4 6 6 a n d 3 4 4 m 2 g-1 for the
Ap, B s s 2 a n d C k h o r i z o n samples, r e s p e c t i v e l y (LiTS,
Table 4). T h e r e d u c t i o n w a s attributed to t h e c o l l a p s e
o f the m o n t m o r i l l o n l t i c smectite layers a n d the differe n c e TS - LiTS was a s s u m e d to r e p r e s e n t the internal
surface o f t h e s e layers ( M t layers, Table 4) w h i c h w a s
221 m 2 g-~ for the Ck, 95 m 2 g-1 for the B s s 2 a n d 105
Table 4. Surface areas (m 2 g-~) for the fine clay fraction of the studied horizons.
Horizon
TS (egme)
Ap
Bss2
Ck
536
561
565
ES (BET)
159
141
102
LiTS
KTS
K/LiTS
431
466
344
377
373
449
374
401
328
IS (TS -
377
420
463
ES)
Mt layers
hc layers
hc Bd layers
105
95
221
159
188
116
57
65
16
TS: total surface area; ES: external surface area; LiTS: total surface area of Li-treated samples; KTS: total surface area of
K-saturated and heated samples; K/LiTS: total surface area of K-saturated and heated Li-treated samples; IS: internal surface
area; Mt layers = (TS - LiTS): internal surface area of collapsed layers after Li-treatment (montmorillonitic); hc layers =
(TS - KTS): internal surface area of high charge layers; hc Bd layers = (LiTS - K/LiTS): internal surface area of layers
with high tetrahedral charge (beidellitic).
174
Righi, Terribile and Petit
Clays and Clay Minerals
Table 5. Cation exchange capacities (cmol~ kg l) of the fine clay fraction of studied horizons.
Horizon
Vertisol
Ap
Bss2
Ck
Eroded soil
Ap
C
apc
vc
53.1
51.5
52.3
18.2
18.3
16.1
m
m
m
m
CEC
Li-apc
Li-vc
Li-CEC
oct-apc
71.3
69.8
68.4
32.8
33.7
23.5
19.8
20.2
16.9
52.6
53.9
40.4
20.3
17.8
28.8
71.8
67.3
---
---
37.0
28.0
34.8
39.3
K-CEC
51.0
49.1
55.5
A CEC
20.3
20.7
12.9
m
m
m
apc: accessible permanent charge; vc: variable charge; CEC = (apc + vc); Li-apc: accessible permanent charge of the Litreated samples = accessible tetrahedral charge; Li-vc: variable charge of Li-treated samples; Li-CEC: CEC of Li-treated
samples; oct-apc: octahedral accessible permanent charge = ( a p c - Li-apc); K-CEC: CEC of K-saturated and heated samples;
A CEC = (CEC - K-CEC). - - : not measured.
m 2 g l for the A p horizon. T h u s , the p r o p o r t i o n o f
s m e c t i t e layers c o l l a p s e d b y the l i t h i u m t r e a t m e n t
( m o n t m o r i l l o n i t i c layers w i t h n o or v e r y low tetrahedral c h a r g e ) in the p a r e n t m a t e r i a l was a b o u t t w i c e the
v a l u e o f the u p p e r soil horizons.
F o l l o w i n g K - s a t u r a t i o n a n d heat t r e a t m e n t applied
o n l i t h i u m - t r e a t e d s a m p l e s , the total surface area was
r e d u c e d to 374, 401 a n d 328 m 2 g-I for the Ap, B s s 2
a n d C k h o r i z o n s a m p l e s , r e s p e c t i v e l y (K/LiTS, Table
4). T h e d i f f e r e n c e L i T S - K / L i T S was a s s u m e d to
m e a s u r e the internal surface area o f h i g h - c h a r g e s m e c tite layers w i t h the layer c h a r g e located in the tetrah e d r a l sheet o n l y (hc B d layers, Table 4). T h i s area
i n c r e a s e d f r o m 16 m 2 g ~ for the C k h o r i z o n to 65 m 2
g-~ for the B s s 2 h o r i z o n a n d 57 m 2 g ~ for the A p
h o r i z o n , i n d i c a t i n g m o r e irreversibly c o l l a p s e d layers
(layers with h i g h tetrahedral c h a r g e ) in the u p p e r soil
h o r i z o n s t h a n in the p a r e n t material.
Cation Exchange Capacity
CEC values o b t a i n e d f r o m the fine clay fractions are
g i v e n in Table 5. T h e C E C v a l u e was v e r y similar for
all the fine clay fractions (68.4 cmolc kg -~, C k horizon,
to 71.3 cmolc kg -~, A p horizon). T h e p o r t i o n o f the
C E C attributed to v a r i a b l e c h a r g e (vc) was 18.2 c m o l c
kg -~ in the A p a n d 16.0 cmolr kg -1 in the C k horizon,
w h e r e a s a n accessible structural p e r m a n e n t c h a r g e
(apc) o f a b o u t 52 cmolc kg -~ was f o u n d for the fine
clay fractions f r o m the 3 soil h o r i z o n s investigated.
T h e a c c e s s i b l e p e r m a n e n t c h a r g e located in the tetr a h e d r a l sheet was o b t a i n e d f r o m the fine clay fractions s u b j e c t e d to the l i t h i u m - t r e a t m e n t (Table 5). T h e
a c c e s s i b l e tetrahedral c h a r g e (Li-apc) w a s smaller for
the p a r e n t m a t e r i a l (23.5 c m o l c k g -1) t h a n for the u p p e r
soil h o r i z o n s (33.7 cmolc kg -1 B s s 2 h o r i z o n , 32.8
cmolc k g - i A p h o r i z o n ) , w h i l e the r e v e r s e was true for
the a c c e s s i b l e octahedral c h a r g e (oct-apc). O n l y a
s m a l l i n c r e a s e o f the f r a c t i o n o f the CEC attributed to
v a r i a b l e c h a r g e s (Table 5) was o b s e r v e d after the lithium-test, i n d i c a t i n g that e d g e s o f clay particles w e r e
n o t d r a m a t i c a l l y altered.
T h e C E C values o f the fine clay fractions w e r e red u c e d after the K - s a t u r a t i o n t r e a t m e n t (Table 5). T h e
r e d u c t i o n o f C E C was a b o u t 20 cmolc kg -1 ( 3 0 % o f
the original C E C ) for the A p a n d B s s 2 h o r i z o n fine
clay fractions a n d a b o u t 13 cmolc k g 1 ( 2 0 % o f the
original C E C ) for the C k h o r i z o n fine clay fraction.
M o r e K was fixed in the u p p e r soil h o r i z o n s t h a n in
the p a r e n t material.
FFIR Spectroscopy
T h e FlaIR spectra (Figure 7) f r o m all the fine clay
fractions e s t a b l i s h e d the a l u m i n o u s c h a r a c t e r o f the
smectites (ALOHA1 b a n d at 9 1 2 cm-~). S u b s t i t u t i o n o f
M g for A1 a n d thus o c c u r r e n c e o f c h a r g e in the octah e d r a l sheet was s h o w n b y the A 1 O H M g b a n d n e a r
840 c m -1. T h i s b a n d w a s m o r e i n t e n s e for the fine clay
fraction f r o m the p a r e n t m a t e r i a l o f the Vertisol a n d
the h o r i z o n s o f the s h a l l o w e r o d e d u p s l o p e soil. In
addition, substitution o f Fe(III) for A1 in the fine clay
fraction f r o m the 2 u p p e r soil h o r i z o n s w a s d e m o n strated b y the A1OHFe(III) b a n d at 880 c m ~. T h i s
b a n d w a s n e a r l y a b s e n t in the spectra f r o m the fine
clay fraction o f the p a r e n t m a t e r i a l o f the Vertisol a n d
the s h a l l o w e r o d e d soil. It was p r e s e n t b u t v e r y w e a k
for the s a m p l e f r o m the A p h o r i z o n o f the s h a l l o w
e r o d e d soil.
DISCUSSION
Particle size d i s t r i b u t i o n a n d total c h e m i c a l analysis
o f the b u l k soil g a v e similar results for e a c h o f the
soil horizons. T h e fine silt ( 2 - 5 }xm) fractions (not
s h o w n ) f r o m the p a r e n t m a t e r i a l a n d the u p p e r soil
h o r i z o n s c o n t a i n e d the s a m e m i n e r a l suite characterized b y m i c a (peaks at 1.010 a n d 0.501 nm), quartz
(0.427 a n d 0.335 n m ) , feldspars (0.380 a n d 0 . 3 2 0 n m )
and k a o l i n i t e (0.715 a n d 0.355 nm). T h i s s u p p o r t s the
assumption that before pedogenic alteration took
place, the soil p a r e n t m a t e r i a l was h o m o g e n e o u s , without a n y lithologic discontinuity. O n l y c a l c i u m c a r b o n ate c o n t e n t i n c r e a s e d w i t h depth, a n d m a y b e attributed
to p e d o g e n i c redistribution. A c c o r d i n g to t o p o g r a p h y ,
Vol. 46, No. 2, 1998
Pedogenic formation of beideilite in a Vertisol
175
L soil
r
%
I
Ii
+
oO
I
Ii
t" r
oo
!
Vertisol
C
Ap
Bss2
950
900
850
wave number cm 4
Figure 7.
FTIR spectra for the Vertisol and shallow eroded soil fine clay (<0.1 p.m) fractions.
the additions of material to the Vertisol from erosion
of the upper area of the hills should not be excluded.
The fine clay fractions from a shallow eroded upslope
soil exhibited similar characteristics to that of the parent material of the Vertisol; the layer charge was dominantly octahedral and the FTIR spectra did not show
the AIOHFe(III) band at 880 cm -~ which was present
in the fine clay fractions from the upper soil horizons
of the Vertisol. Consequently, the mineralogical
changes observed from the parent material to the upper soil horizons could not be attributed to contamination by colluvial material.
The XR D patterns from the fine clay fractions were
characterized by the increase of the saddle/001 peak
intensity ratio from the Ck to the Ap horizon. Based
on XR D simulated diagrams (Reynolds 1980; Inoue et
al. 1989), the increase of this ratio suggested I - S
mixed layers with decreasing proportion of smectite
layers. However, the total surface area (lower than that
attributed to pure smectite, 810 m 2 g-I, (Carter et al.
1965) and CEC values were consistent and suggested
the same proportion of collapsed and expandable components in the mixed layers from the upper soil horizons and the parent material. Moreover, the total surface area and CEC values were in good agreement
with the results of Srodorl et al. (1992) who found that
an I - S mixed layer with 60% smectite, as evaluated
from XRD, would have a total surface area of 550 m 2
g-i and a CEC of 70 cmolc kg -]. Using computed
X RD diagrams, it was possible to simulate the experimental patterns of the 3 samples with I - S mixed layers having the same proportion of smectite layers
(60%). Only more segregation of the collapsed and
expandable layers would be present in the clay crystals
of the parent material than in those of the upper soil
horizons. This suggested that the clay minerals in the
upper soil horizons did not derive simply by inheritance from the parent material, but rather, new clay
crystals with a different distribution of layers in the
stacking sequence were formed by pedogenesis.
The fine clay fractions exhibited X RD patterns typical of I - S mixed layers; however, the K20 contents
176
Righi, Terribile and Petit
Ap
Bss2
Ck
|
i
50
,
m
100
,
i
,
i
150
200
internal surface a r e a
( m 2 g-l)
,
i
250
Figure 8. Histograms of internal surface areas for layers
with (white bar) a tetrahedral charge <0.24 molJO10(OH)2
(montmorillonitic layers) and layers with (black bar) a tetrahedral charge >0.77 molflOl0(OH)2 (high-charge beidellitic
layers).
were not high enough to account for the I - S mixed
layers with 40% illite identified from the XRD patterns. Therefore, it is likely that a part of these mixed
layers were interstratifications of irreversibly collapsed
and expandable smectite layers rather than true I - S
mixed layers.
The XRD patterns, the reduction of the CEC and
the total surface area after K-saturation and heating
gave consistent results, suggesting more high-charge
smectite layers in the upper soil horizons than in the
parent material, as it was previously observed in another Vertisol (Righi et al. 1995). According to Eberl
(1980), irreversible K-ffxation requires a layer charge
of 0.77 molJO10(OH)2, but K could be fixed at lower
layer charge if the clay is oven-dry.
The lithium-treatment was used to distinguish smecrite layers exhibiting a tetrahedral charge. It was postulated that back-saturation with Ca of the lithium-saturated and heated fine clay fraction would allow any
smectite layers with a remaining tetrahedral charge to
re-expand upon ethylene glycol solvation, as demonstrated by Jaynes and Bigham (1987). Malla and
Douglas (1987) stated that, after lithium-treatment,
only smectite layers with a remaining tetrahedral
charge higher than 0.24 molflO~0(OH)2 could re-expand. Therefore, irreversibly collapsed layers, as measured by the reduction of total surface area, were assumed to be smectite layers with charges located almost exclusively in the octahedral sheet, whereas the
smectite layers which re-expanded following the lithium-treatment were assumed to have a tetrahedral
charge higher than 0.24 molc/Ol0(OH)2. The reduction
of the total surface area was consistent with the reduction of the CEC, both indicating more octahedralcharged smectite layers in the parent material than in
Clays and Clay Minerals
the upper soil horizons. Moreover, X RD patterns have
shown that tetrahedral-charged layers were interstratiffed with the octahedral-charged smectite layers and
did not exist as separate crystals as it should be expected if the beidellitic smectite layers were formed
by weathering of a mica phase.
Compared to the untreated fine clay fraction, far less
high-charge smectite layers were found after the lithium-treatment, indicating that a large portion of the
tetrahedral-charged layers also had an octahedral
charge that contributed to the high-charge behavior
and was suppressed by the lithium-treatment. The
smectite layers with a high-charge behavior that could
be attributed to the tetrahedral charge alone were
found essentially in the upper soil horizons, suggesting
that the overall increase of the layer charge was located in the tetrahedral sheet. Figure 8 shows the distribution in the different soil horizons of the smectite
layers with either a dominant octahedral or a high tetrahedral charge. This shows the mineralogical changes
in the studied soil: a decrease of the relative proportion
of octahedral-charged smectite layers and an increasing amount of layers with high density of tetrahedral
charges. Montmorillonitic smectites would be more affected by weathering than beidellitic smectites that
would be more stable in this soil environment.
In soils, high-charge beidellites were often described as the weathering product of micas (Borchardt
1989; Bouabid et al. 1996). In the studied soil, the silt
and clay fractions of the parent material and the upper
soil horizons contained micas; therefore, their contribution to the formation of smectite layers with high
tetrahedral charge should not be excluded. The total
chemical analyses of the bulk soil indicated that the
soil parent material contained very small amounts of
mica, and it is unlikely that the whole mass of tetrahedral charged smectites found in the soil horizons
was derived from mica.
The increase of the proportion of smectites with tetrahedral charges in the upper soil horizons was associated with a change of the octahedral composition, as
shown by FTIR spectroscopy. The upper soil horizons
contained more smectite layers with Fe(III) for A1 substitution than the parent material. This strongly suggested that the mineralogical change proceeds through
dissolution-recrystallization processes rather than solid-state reorganization.
CONCLUSION
Righi et al. (1995) proposed the hypothesis of formation of high-charge beidellites in Vertisols by increase of tetrahedral substitution as a pedogenic process. The theory was supported by the experimental
work of Eberl et al. (1993) that showed the formation
of high-charge smectite layers at low temperature and
alkaline environment. The present study confirmed the
phenomenon: smectites in the upper soil horizons had
Vol. 46, No. 2, 1998
Pedogenic formation of beidellite in a Vertisol
m o r e t e t r a h e d r a l s u b s t i t u t i o n s t h a n t h e s m e c t i t e s in t h e
parent material. This suggested the pedogenic format i o n o f b e i d e l l i t e s in this soil e n v i r o n m e n t .
ACKNOWLEDGMENTS
This study was part of a cooperative program supported by
CNR (Italy) and CNRS (France). Programme de Recherche
en Coop6ration sur Conventions Internationales du CNRS,
Projet #3187/1997. The authors would like to thank A. Aru,
P. Baldaccini and their collaborators for their essential support
in choosing and sampling the study site.
REFERENCES
Anderson S J, Sposito G. 1991. Cesium-adsorption method for
measuring accessible structural surface charge. Soil Sci Soc
A m J 55:1569-1576.
Borchardt G. 1989. Smectites. In: Dixon JB, Weed SB, editors. Minerals in soil environments, 2nd ed. Madison, WI:
Soil Sci Soc Am. p 675-727.
Bouabid R, Bradaoui M, Bloom PR, Daniane M. 1996. The
nature of smectites and associated interstratified minerals
in soils of the Gharb plain of Morocco. Eur J Soil Sci 47:
165-174.
Bardaoui M, Bloom PR. 1990. Iron-rich high-charge beidellite in Vertisols and Mollisols of the High Chaouia region
of Morocco. Soil Sci Soc A m J 54:267-274.
Bardaoui M, Bloom PR, Rust RH. 1987. Occurrence of highcharge beidellite in a Vertic Haplaquoll of northwestern
Minnesota. Soil Sci Soc Am J 51:813-818.
Carter DL, Heilman MD, Gonzalez CL. 1965. Ethylene glycol monoethyl ether for determining the surface area of
silicate minerals. Soil Sci 100:356-360.
Eberl DD. 1980. Alkali cation selectivity and fixation by clay
minerals. Clays Clay Miner 28:161-172.
Eberl DD, Velde B, McCormick T. 1993. Synthesis of illitesmectite from smectite at Earth surface temperatures and
high pH. Clay Miner 28:49-60.
Heilman MD, Carter DL, Gonzalez CL. 1965. The ethylene
glycol monoethyl ether (EGME) technique for determining
soil-surface area. Soil Sci 100:409-413.
177
Hofmann VU, Klemen R. 1950. Verlust der austauschfahigkeit yon lithium-ionen an bentonit durch erhitzung. Zeitung
fOr Anorganische Chemie 262:95-99.
Inoue A, Bouchet A, Velde B, Meunier A. 1989. Convenient
technique for estimating smectite layer percentage in randomly interstratified illite/smectite minerals. Clays Clay
Miner 37:227-234.
Jaynes WE Bigham JM. 1987. Charge reduction, octahedral
charge, and lithium retention in heated, Li-saturated smecrites. Clays Clay Miner 35:440-448.
Jeanroy E. 1972. Analyse totale des silicates naturels par
spectrom6trie d'absorption atomique. Application au sol et
h ses constituants. Chim Anal 54:159-166.
Kiely PV, Jackson ML. 1965. Quartz, Feldspar and Mica determination for soils by sodium pyrosulfate fusion. Soil Sci
Soc A m Proc 29:159-I63.
Kounetsron O, Robert M, Berrier J. 1977. Nouvel aspect de
la formation des smectites dans les vertisols. C R Acad Sci
Paris 284:733-736.
Malla PB, Douglas LA. 1987. Problems in identification of
montmorillonite and beidellite. Clays Clay Miner 35:232236.
Reynolds RC Jr. 1980. Interstratified clay minerals. In: Brindley GW, Brown G, editors. Crystal structures of clay minerals and their X-ray identification. London: Miner Soc. p
249-359.
Reynolds RC Jr. 1985. NEWMOD: A computer program for
the calculation of one-dimensional diffraction powders of
mixed-layer clays. 8 Brook Rd., Hanover, NH 03755: RC
Reynolds, Jr.
Righi D, Terribile E Petit S. 1995. Low-charge to high-charge
beidellite conversion in a Vertisol from South Italy. Clays
Clay Miner 43:495-502.
Rossignol JR 1983. Les Vertisols du nord de l'Uruguay. Cah
ORSTOM s6r P6dol 20:271-291.
Soil Survey Staff. 1994. Keys to soil taxonomy, 5th ed.
Blacksburg, VA: Pocahontas Pr.
Srodo6 J, Elsass F, McHardy WJ, Morgan DJ. 1992. Chemistry of illite/smectite inferred from TEM measurements of
fundamental particles. Clay Miner 27:137-158.
Wilson MJ. 1987. Soil smectites and related interstratified
minerals: Recent developments. In: Schultz LG, van O1phen H, Mumpton FA, editors. Proc Int Clay Conf; 1985;
Denver, CO. Bloomington, IN: Clay Miner Soc. p 167-173.
(Received 4 November 1996; accepted 9 August 1997; Ms.
2827)