Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Energy value of coal "Q" is a complex function of the elemental composition of the coal. "Q" can be determined experimentally using calorimeters. Dulong suggests the following approximate formula for "Q" when the oxygen content is less than 10%: where "C" is the mass percent of carbon, "H" is the mass percent of hydrogen, "O" is the mass percent of oxygen, and "S" is the mass percent of sulfur in the coal. With these constants, "Q" is given in kilojoules per kilogram. Energy value of coal The energy value of coal, or the fuel content, is the amount of potential energy in coal that