COMPOSITION OF CLAY MINERALS
The word 'clay' is generally understood
to refer to a material composed of a mass of small mineral particles
which, in association with certain quantities of water, exhibits the
property of plasticity.
According to the clay mineral concept,
clay materials are essentially composed of extremely small
crystalline particles of one or more members of a small group of
minerals that are commonly known as clay minerals.
These minerals are essentially hydrous
aluminum silicates, with magnesium or iron replacing wholly or in
part for the aluminum, in some minerals. Many clay materials may
contain organic material and water-soluble salts.
Organic materials occur either as
discrete particles of wood, leaf matter, spores, etc., or they may be
present as organic molecules adsorbed on the surface of the clay
mineral particles. The water-soluble salts that are present in clay
materials must have been entrapped in the clay at the time of
accumulation or may have developed subsequently as a consequence of
ground water movement and weathering or alteration processes.
Clays can be divided into three general
groups on the basis of their crystalline arrangement and it is
observed that roughly similar engineering properties are connected
with all the clay minerals belonging to the same group.
STRUCTURE OF CLAY MINERALS
Clay minerals are essentially
crystalline in nature though some clay minerals do contain material
which is non-crystalline (for example allophane). Two fundamental
building blocks are involved in the formation of clay mineral
structures.
They are:
1. Tetrahedral unit.
2. Octahedral unit.
The tetrahedral unit consists of four
oxygen atoms (or hydroxyls, if needed to balance the structure)
placed at the apices of a tetrahedron enclosing a silicon atom which
combines together to form a shell like structure with all the tips
pointing in the same direction. The oxygen at the bases of all the
units lie in a common plane.
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