The structures of a few important clay minerals are described below:
Kaolinite:
The kaolinite is the most common mineral of the kaolinite group of minerals. Its basic structural unit consists of an aluminium sheet (gibbsite G) combined with silica sheet S. Tips of the silica sheet and one base of the Alumina sheet from a common interface. The total thickness of the structural unit is about 7 Angstorm (`A^0`). The kaolinite mineral is formed by stacking, one over the other, several such basic structural units. Figure below shows two such units.
The structural units join together by hydrogen bond, which develops between the oxygen of silica acid and hydroxyl of aluminium sheet. As the bond is fairly strong, the mineral is stable. Moreover, water cannot easily enter between the structural units and causes expansion.
The Kaolinite mineral is electrically neutral. However in the presence of water, some hydroxyl ion dissociate and loose hydrogen and leave the Crystal with a small residual negative charge . The flat surface of the mineral attract positive ions (cation) and water. A thick layer of absorbed water is formed on the surface
The Kaolinite mineral generally have a hexagonal shape in plan, with the side of the hexagon between 0.5 to 1.0 Micron. The thickness of the mineral is about 0.05 Micron. The specific surface is about `15 M^2/g`. The most common example of the kaolinite mineral is China clay.
Halloysite is a clay mineral which has the same basic structure is kaolinite, but in which the successive structural units are more randomly packed, and are separated by a single molecular layer of water. The properties of halloysite depend upon this water layer. If the water layer is removed by drying, the properties of the mineral drastically changes.Halloysite particles are tabular in shape, in contrast to the the platy shape of kaolinite particle. The soils containing halloysite have a very low mass density.
Commercial Uses of Kaoline:
Kaolin is used in ceramics, medicine, coated paper, cosmetics and as a food additive in toothpaste, and also as a light diffusing material in white incandescent electric bulbs. It is generally the main component in porcelain.
It is also used as filler for paint, rubber, and plastics as it is relatively inert and is long lasting. However, the greatest demand for kaolinite is in the paper industry to produce a glossy paper. Natural kaolinite usually contains small amounts of uranium and thorium. Hence, it is useful in radiological dating.
Montmorillonite:
The Montmorillonite is the most common mineral of the Montmorillonite group of minerals. The basic structural unit consists of an aluminium sheet sandwiched between two silica sheets. Successive structural units are stacked on over another, like leaves of a book. Figure below shows two such structural unit. The thickness of a structural unit is about `10 A^0`.
The two successive structural units are joined together by a link between oxygen Ion of the two silica sheets. The link is due to natural attraction for the cations in the intervening space and due to Vander Waal forces. The negatively charged surface of the silica sheet attract water in the space between two structural units. This results in expansion of the mineral. It may also causes deissociation of the mineral into individual structural units of thickness 10 Angstorm. The soil containing a large amount of the mineral Montmorillonite exhibit high shrinkage and high swelling characteristics. The water in the intervening space can be removed by heating at 200 degree Celsius to 300 degree Celsius.
Montmorillonite minerals have lateral dimension of `0.1 μ` to `0.5 μ` and the thickness of `0.001 μ` to `0.005 μ`. The specific surface is about `800 M^2/(gm)`.
The gibbsite sheet in a Montmorillonite mineral may contain iron or magnesium instead of aluminium. Some of the Silicon atoms in the silica sheet may also have isomorphous substitution. This results in giving the mineral a residual negative charge. It attracts the soil water to form an observed layer, which gives plasticity characteristics to the soil.
ILLITE:
The basic structue of illite is similar to that of Montmorillonite except that there is always substantial replacement of silicones by aluminium in the tetrahedral layers and potassium are between the layers serving to balance the charge resulting from the replacement and to tie the sheet units together. The basic unit is symbolically represented as shown in the figure. The cation bond of illite is weaker than hydrogen bond of kaolinite, but is stronger than the water bond of Montmorillonite. Due to this, the illite crystal has a greater tendency to split into ultimate platelets consisting of gibbsite layer between two silica layers, than that in kaolonite. However, the illite structure does not swell because of moment of water between the sheets, as in the case of Montmorillonite. Illite clay particle may be 50 Angstorm to 500 Angstorm thick and 1000 Angstorm to 5000 angstorm in lateral dimension.The specific surface is about `80 M^2/(gm)`.
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