Geoscience Reference
In-Depth Information
Fig. 1.4 Idealized structural
diagram of kaolinite layer
viewed along one axis (from
Dixon
1989
)
constituting a 0.7 nm layer in a triclinic unit cell. Two-thirds of the octahedral
positions are occupied by Al; the tetrahedral positions are occupied by Si and Al,
which are located in two rows parallel to the x-axis. Every third row of octahedral
sites is vacant. The surface plane of octahedral anions and a third layer of the inner
plane containing anions in each 0.7 nm layer is built up by hydroxyl (OH
-
)
groups. The surface hydroxyls bond through their hydrogens to the oxygen sheet of
the bordering layer. The idealized structural diagram of a kaolinite layer, as
suggested by Brindley and MacEwan (
1953
) and modified by Dixon (
1989
), is
shown in Fig.
1.4
.
Giese (
1982
) showed that the kaolinite structure exhibits covalent sharing of H,
which leads to conceptualization of a kaolinite structure with two inner surface
hydroxyls perpendicular to the layer, bonded to the O of an adjacent layer. As a
result of polarization of the Si-O and O-H groups, the surface oxygen and proton
plane become negatively or positively charged. Electrostatic attraction, in addition
to van der Waals forces, contributes to the stacking of kaolinite unit layers. The
negative charge of kaolinite was proven by demonstrating that the surface retains
Na
+
under acidic conditions (Schofield and Samson
1953
). The exchangeable
cations are bound only to the tetrahedral basal plane of kaolinite (Weiss and
Russow
1963
). Kaolinite exhibits a very low cation exchange capacity, in general
lower than 1 cmol
c
kg
-1
(centimoles per kg) at pH = 7 (Lim et al.
1980
).
Schofield and Samson (
1953
) also show that the positive charge on kaolinite
occurs on the edges of the plates, which become positive by acceptance of H
+
in
the acid pH range.
1.1.2.2 Smectites
Smectites are clay minerals with an expanding nature, a negative charge, and a
large total surface area. These properties are of major importance in controlling the
fate of chemicals in the subsurface, by affecting their retention, transport, and
persistence.
