Geology Reference
In-Depth Information
pH-dependent charge. This results in very different properties of tropical
soils compared to temperate soils.
5.2.3.1 Aluminosilicate Clay Minerals (Phyllosilicates). The alumi-
nosilicate clay minerals are sheet silicates. They are sometimes referred
to as phyllosilicates, but strictly this term should also include the micas,
which are primary minerals.
The basic building blocks of all the aluminosilicate clay minerals are
the silica tetrahedron (Figure 3a) and alumina octahedron (Figure 3b).
These build up into tetrahedral sheets by the sharing of oxygens between
adjacent silicons, and into octahedral sheets by the sharing of oxygens or
hydroxyls between adjacent aluminiums. The sharing of oxygens be-
tween tetrahedral and octahedral sheets leads to the formation of clay
unit layers, of which there are two basic types (Table 5): 1:1, one
tetrahedral Si sheet and one octahedral Al sheet, and 2:1, two tetra-
hedral Si sheets sandwiching one octahedral Al sheet.
Unit layers are held together in various ways to produce a clay crystal.
These are regular, rigid systems and the distance between equivalent
points in adjacent unit layers, the c spacing or basal spacing, can be
measured by X-ray diffraction and is used to identify the clay minerals
(Figure 5).
Within the sheets, Si or Al may be replaced by a different element by
the process of isomorphous substitution. Common replacements in the
clay minerals are Al
31
for Si
41
in the tetrahedral sheet and Mg
21
or
Fe
21
for Al
31
in the octahedral sheet. Substitution of an element by one
of a lower valency results in a deficit of positive charge, and hence the
development of a permanent negative charge on the clay mineral. In soils,
this charge is neutralized by cations in solution being attracted to the
negatively charged surface, and is called the cation-exchange capacity
Table 5 Components of soil clays and how they combine to form a clay mineral
Basic building
blocks
Si tetrahedra, Al octahedra
Sheets
Tetrahedral sheet, made up of Si tetrahedra sharing oxygen atoms;
octahedral sheet, made up of Al octahedra sharing oxygens or
hydroxyls
Unit layers
Sheets combined in the ratio of one tetrahedral:one octahedral (1:1), or
two tetrahedral:one octahedral (2:1)
Clay crystal
Stacks of unit layers held together by various bonding mechanisms,
depending on the type of clay
