Geology Reference
In-Depth Information
Si tet. sheet
basal spacing or
c spacing
2:1 unit layer
Al oct. sheet
Si tet. sheet
interlayer space
clay edge
clay face
Figure 5 Structure of a 2:1 aluminosilicate clay mineral
(CEC). This permanent negative charge is the major characteristic of
temperate soils, in which the aluminosilicate clays dominate the reactive
fraction. The consequences of this are discussed in relation to ion-
exchange processes in Section 5.5.
Kaolinite is the commonest 1:1 clay mineral. There is a small amount of
isomorphous substitution of Al for Si in the tetrahedral sheet, resulting in a
charge of
r
0.005 mol negative charge per unit cell. The unit layers are
bound by hydrogen bonds between the oxygen atoms on the tetrahedral
face and the hydroxyls on the adjacent octahedral face, resulting in a low
CEC and surface area (Table 6) and a fixed c spacing of 0.7 nm. Unit
layers stack up to form hexagonal crystals typically 0.05-2 mm thick. This
means that the variable, pH-dependent charge on the clay edge is relatively
more important in kaolinite than the other aluminosilicate clays.
Illite (a hydrous mica) is a 2:1 clay mineral with isomorphous
substitution occurring mainly in the tetrahedral sheet (Al
31
for Si
41
),
although a small amount also occurs in the octahedral sheet (Mg
21
or
Fe
21
for Al
31
), resulting in a charge of 1.5mol negative charge per unit
cell. Most of this charge is neutralized by K
1
ions in the interlayer space.
The geometrical arrangement of oxygen atoms on the tetrahedral face
allows the K
1
ions to sit very close to the clay surface, forming a very
strong bond between unit layers. This results in a low CEC and surface
area (Table 6) and a fixed c spacing of 1.0 nm.
Vermiculite, a 2:1 clay mineral, is also a hydrous mica, with isomor-
phous substitution in the tetrahedral sheet, resulting in a charge of
between 1.2 and 1.9 mol negative charge per unit cell. In this case the
