Geoscience Reference
In-Depth Information
Kaolinite
Smectite
001
110
Edges
010
(a)
(b)
T
O
T
O
T
Al, Mg, Fe
Si, Al
OH
- (1/2)
1:1 clay
2:1 clay
OH
+(1/2)
>Al
110
010
Faces
110
010
Faces
OH
0
>Si
OH
0
>Si
>Al
(a)
(b)
OH
+(1/2)
T
O
T
OH
0
>Si
(a) Octahedral sheet (O)
(b) Tetrahedral sheet (T)
OH
+(1/2)
Basal surface
X
-
X
-
X
-
X
-
X
-
Figure 1.8
Active surface sites at the edge of
a)
1:1 clays (kaolinite) and
b)
2:1 clays (smectite or illite). In the case of kaolinite,
the surface sites are mainly located on the edge of the mineral ({110} and {010} planes). In the case of smectite and illite and in the
pH range near neutrality (5
9), the surface sites are mainly located on the basal plane ({001} plane), and they are due to
isomorphic substitutions inside the crystalline framework (Modified from Leroy & Revil, 2004). Note also the difference in the
morphology of the clay particles. T and O represent tetrahedral and octahedral sheets, respectively.
-
the determination of the complex conductivity of these
minerals. This parameter is called the partition coefficient
f
(dimensionless). It describes the amount of counterions
in the Stern layer by comparison with the total amount
of counterions in the Stern and diffuse layers together.
We consider first a kaolinite crystal in contact with a
binary symmetric electrolyte like NaCl. We restrict our
analysis to the pH range 4
-
10, which is the pH range use-
ful for most practical applications in geophysics. In this
pH range and in the case of kaolinite, the surface mineral
reactions at the aluminol, silanol, and
>
Al
-
O
-
Si
<
surface
sites can be written as
>AlOH
2
1 2+
>AlOH
1 2
−
+H
+
,
K
1
1 54