Geoscience Reference
In-Depth Information
Pure clays end-members
Mixtures
Kaolinite/chlorite
Illite
Montmorillonite/Smectite
Shaly sands
+
Saprolites
Soils
10
3
Smectite
Figure 1.9
Specific surface area of clay
minerals
S
s
(in m
2
g
−
1
) as a function of
the cation exchange capacity (CEC) (in
meq g
−
1
with 1 meq g
−
1
= 96,320 C kg
−
1
in
SI units) for various clay minerals. The
ratio between the CEC and the specific
surface area gives the equivalent total
surface charge density of the mineral
surface. The shaded circles correspond to
generalized regions for kaolinite, illite, and
smectite. The two lines correspond to 1
1 charge nm
-2
Illite
10
2
3 charges nm
-2
3
elementary charges per unit surface area.
Data for the clay end members are from
Patchett (1975), Lipsicas (1984), Zundel
and Siffert (1985), Lockhart (1980),
Sinitsyn et al. (2000), Avena and De Pauli
(1998), Shainberg et al. (1988), Su et al.
(2000), and Ma and Eggleton (1999).
Saprolite data: Revil et al. (2013). Soil
data: Chittoori and Puppala (2011).
-
+
+
Kaolinite
and chlorite
10
1
10
-2
10
-1
10
0
Cation exchange capacity CEC (meq g
-1
)
> SiOH
1 2+
> SiO
1 2
−
+H
+
,
K
2
AlOH
2
=
Γ
1
A
C
f
H
+
K
1
φ
0
k
b
T
e
1 55
0
Γ
exp
−
1 58
O
−
−
Si < +Na
+
,
K
3
>Al
−
ONa
−
Si <
>Al
−
1 56
0
2
B
SiO
=
Γ
0
Γ
1 59
where
K
1
,
K
2
, and
K
3
are the equilibrium constants of
reactions (1.54)
0
2
B
C
f
H
+
K
2
refers to the crys-
talline framework. The surface site >Al
-
(1.56) and the sign
“
>
”
SiOH
=
Γ
e
φ
0
k
b
T
0
Γ
exp
−
1 60
-
O
-
Si< carries a
net (
1) negative charge (Avena & DePauli, 1998). We
assume that the surface complexation reactions occur
on the {010} and {110} planes of kaolinite.
The availability of the surface sites introduced by the
chemical reactions described previously at the surface of
the {010} and {110} planes can be described by the conser-
vation equations for the three types of sites (aluminol,
silanol, and >Al
−
0
3
C
AlOSi
=
Γ
Γ
1 61
0
3
C
C
f
Na
+
K
3
φ
β
k
b
T
e
AlONaSi
=
Γ
Γ
exp
−
1 62
where
A
,
B
, and
C
are given by
Si< surface sites). Solving these equa-
tions, we obtain the concentrations of the different sur-
face sites:
-
O
-
A
=1+
C
f
H
+
e
φ
0
k
b
T
K
1
exp
−
1 63
B
=1+
C
f
H
+
0
1
A
φ
0
k
b
T
e
AlOH
=
Γ
K
2
exp
−
1 64
Γ
1 57
