Coal
Coal is a brown-to-black combustible rock that originated from peat deposits in large swamp environments, through their burial to great depths and over a few hundred thousand to tens of millions of years. During burial peat is converted first into lignite, then subbituminous and bituminous coal, and, uncommonly, anthracite. Due to the loss of moisture during burial (peat has about 90 percent in its natural state, bituminous coal as little as 2 to 3 percent) and the chemical changes in the plant material that are induced by the rising temperature during burial to thousands of feet (increased carbon and decreased oxygen contents in particular), the heating value of coal increases significantly from peat to lignite and on to bituminous coal and anthracite.
The various environments that prevailed in the peat swamps (e.g., forests with large trees; marshes with sedges, grasses, and reeds) produced various kinds of peat and thus coal with significantly different properties. The major coal types—banded, nonbanded, and impure coal—are easily recognizable. Banded coals are most common. In subbituminous and bituminous coal the bands are composed of vitrain (shiny, glassy, brittle), clarain (bright luster, tough), durain (dull luster, hard), and fusain (charcoal-like, soft). Under the microscope, the so-called macerals become visible.
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