Agriculture Reference
In-Depth Information
industrial uses. All are alumino-silicate compounds comprising layers of tetrahedrally-
coordinated silicon atoms and layers of octahedrally-coordinated aluminium atoms.
These layers are arranged in various ways according to the clay mineral group
involved; in some, the vertical repeating unit (the unit layer), consists of one layer of
silica tetrahedra and one of aluminium octahedra, the so-called 1:1 minerals. The 2:1
minerals consist of two layers of silica tetrahedra with an intervening alumina layer.
In the 1:1 clay minerals, the succeeding unit layers are tightly held together by hydrogen
bonds and these minerals do not normally expand. In the 2:1 minerals, the bonds
between the layers joining the repeating units are much weaker, consisting of Van der
Waal's forces and the electrostatic attraction of the cations present. Water, other polar
solvents and a range of other materials may penetrate these layers leading to variable
spacings between the repeating layers, depending on the hydration state of the soil
and the dominant cations present.
The central aluminium atoms of the octahedral layers and the silicon atoms of
the tetrahedral layers may be replaced by other elements of appropriate size and charge,
commonly magnesium and iron. The replacing ion is usually of lower valency and this
process of isomorphous replacement leaves the sheet with a permanent net negative
charge which is normally balanced by cations in the soil solution. A further source of
charge arises from the dissociation of hydroxyl ions on the surfaces and particularly at
the edges of the clay sheets; in these cases the charge is variable, being dependent on
the pH and ionic concentration of the soil solution.
The common phyllosilicates fall into seven groups, based on the relative number of
sheets in the unit layers, the types of ions that occur between the unit layers and
the nature of the isomorphous replacement. These seven groups are (Millot, 1979):
kaolin, illite, smectite, chlorite, vermiculite, mixed-layer and sepiolite-attapulgite, of which
the first four are perhaps the most common in soils. Minerals in the sepiolite-attapulgite
group are not organised as plate-like structures, but form fibrous, needle-like crystals.
The kaolin group is most commonly represented by kaolinite, a 1:1 clay mineral
whose unit layer is made up of one tetrahedral and one octahedral sheet (Figure I.7)
(Schulze, 1989). It has a relatively small surface area because its unit layers are held
together tightly by electrostatic bonds and cannot be penetrated by polar solvents;
its internal surfaces are thus unable to contribute to its often low overall apparent surface
area (Table I.5).
Kaolinite normally has a low level of isomorphous substitution and thus little
permanent negative charge. However, crystals of this mineral from tropical soils may be
much smaller than those from kaolin deposits ( ca. in comparison to
and possess high surface areas of Schwertmann and Herbillon (1992)
showed that the isomorphous substitution of iron within the kaolinite structure gave rise
to smaller and more active crystals effecting substantial changes to such soil properties as
phosphate adsorption. In addition to its permanent charge properties, this mineral has a
pH-dependent charge based on the dissociation of hydroxyl ions. Kaolinite and halloysite,
another member of this group, are common clay minerals in the highly-weathered and
nutrient-poor soils of many tropical areas.
The illites are 1:1 clay minerals and are amongst the most common of the phyllosilicate
clay minerals. The distance between the unit layers is fixed (1.0 nm) and they are bound
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