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Coal
Clay
Those rock or soil particles with
a diameter of less than 0.005 millimetre. They are the smallest size of soil particle. Clay is also the name given to the mineral that makes this kind of particle.
Clay particles do not form from the wearing away of larger pieces of rock. They are only produced by chemical weathering. As chemical weathering goes on,
tiny crystals of clay are
produced, made exclusively of clay minerals. (See also: Feldspar.)
Clay particles are so small that water cannot easily get between them. That is why clay soils waterlog easily. Clay particles also stick together easily. As a result, clay soils are ‘heavy’ and are difficult to use for growing crops. They often need special kinds of drainage.
Pure deposits of clay are widely used to make pottery and bricks. (See also: Alfisol; Argillaceous; Clastic rock; Eluviation; Flocculation; Friable.)
Clay minerals
Secondary minerals, meaning that they do not occur in either igneous or metamorphic rocks, but only as a result of the weathering of minerals in these rocks. Clays are a major constituent of mudstone and shale. Crystals of clay minerals are so small that they cannot
be seen except with the most powerful electron microscope. They are plate-like in structure. Weathered feldspar is the main source of clay minerals.
Clay soil
A soil in which the percentage of clay is greater than any other size of material. Clay soils are often hard to work and they
 Coal – This is steam coal, a less carbon rich form of coal than anthracite, but a typical coal of home hearths and power plants.
waterlog
easily (they are known as heavy soils).
Cleavage
The tendency of some minerals to break along one or more lines of weakness. Some crystals have well-developed cleavage, as is the case with mica. However, in other minerals, such as quartz, there is no cleavage. (See also: Fracture.)
Coal
The carbon-rich, solid mineral derived from fossilised plant remains (fossils). It is found as bands, called seams, between other kinds of sedimentary rocks. Types of coal include bituminous, brown, lignite and anthracite. Coal is also one of the most important fossil fuels.
Coal forms when forests grow
in swampy conditions. As trees die, they topple into the swamp waters. These waters are acidic and contain little oxygen. As a result,
plants decay only slowly in them. Over
thousands of years, fallen plant
material becomes compressed by the
weight of new material falling
on old. This process does not, however, directly produce coal. For coal to form, the compressed plant remains have to be buried by many layers of rock so that they are changed by heat and pressure. First, the water is squeezed out, together with some of the materials in
the wood, leaving only the carbon behind. Then the compressed material becomes hot as chemical changes take place. The degree
to which these changes occur
is responsible for the different categories of coal. The most complete change results in the type of coal called anthracite. Anthracite contains only carbon and so burns without smoke; other coals have the remains of other materials in them and so produce smoke when they burn. (See also: Cyclothem.)
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