Sedimentary rock

Coal: peat to anthracite

At least 50% organic matter by weight, formed mostly from plant debris preserved as peat in swamps. With burial, heat (more than pressure or time) drives off water and volatiles and raises the carbon content: peat → lignite (brown coal) → sub-bituminous → bituminous (dense, black, often with bright and dull bands; about 84% carbon) → anthracite (hard, glossy, highest carbon, conchoidal fracture). About 90% of coal beds were deposited in the Carboniferous and Permian.

In thin section

Coal contains different plant materials (macerals) that can be identified under the microscope.

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