Page 88 - Soil and water contamination, 2nd edition

Solid phase constituents 75

- K Vermiculite + Al
Micas Illite Secondary Chlorite
+ K Montmorillonite - Al
Primary Chlorite
Feldspars - Si Goethite
Pyroxene -Ca, Mg, K, Na, Si Allophane, Kaolinite, Halloysite
Amphiboles Weathering products + K Illite
Olivin + Mg, Ca
Montmorillonite, Vermiculite
- Si
2:1 Clay minerals Kaolinite, Halloysite Gibbsit, Goethite, Hematite 6642 6642 6642
+ Si
Figure 4.2 Schematic overview of formation of clay minerals from primary minerals. Adapted from Scheffer and
Schachtschabel (1989).
It is present in the clay fraction of nearly all soil types. In tropical and subtropical soils,
kaolinites constitute the primary part of the clay minerals , but in soils in the temperate
climate region they exist only in small amounts, mainly derived from parent sedimentary
bedrock. Montmorillonite belongs to the smectites, which are swelling, sticky clays. It
occurs mainly in soils formed from basic igneous rocks (e.g. basalt, gabbro). In the tropics
and subtropics, montmorillonite can make up the primary clay mineral in soils formed in
calcium- and magnesium -rich parent material under poor drainage conditions. Vermiculite
occurs in slightly acid soils, but is seldom the dominant clay mineral in soils. Illite is the most
abundant clay mineral in soils of the humid temperate climate region, especially in fluvial
sediments.
Clay minerals are alumino-silicates with a so-called lattice structure consisting of alumina
octahedra joined to one or two layers of silica tetrahedra (Figure 4.3) A silica tetrahedron
2-
consists of a relatively small silicon (Si) atom bonded with four larger O atoms, three in
the base layer and one in the apex layer. In an alumina octahedron, an Al atom bonds with
-
six OH ions, three in the base layer and three in the top layer. The spacing of both the
-
2-
OH in the alumina lattice and the O apex layer of the silica lattice favours a bonding of
these two lattices. Either a two-layer (1:1 clay mineral) or a three-layer structure (2:1 clay
mineral) is formed, leading to a variety of clay minerals . As examples, Figure 4.4 shows the
lattice structures of kaolinite , illite , and montmorillonite . The clay mineral kaolinite consists
of stacked sheets of the two-layer unit, and is thus a 1:1 clay mineral. The minerals illite
and montmorillonite are both 2:1 clay minerals and consist of stacks of three-layer units.
6642

Tetrahedral
sheet

Octahedral
sheet

Si Al O OH

Figure 4.3 Arrangement of tetrahedral and octahedral sheets in a 1:1 clay mineral (left) and a 2:1 clay mineral
(right).

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