Coal forms from organic-rich sediments typically deposited in swamps.
Two types of coals are recognized: sapropelic and humic.
Humic coals are by far the most common. They are bright, usually stratified, rich in aromatics and composed primarily of the remains of higher plants. Less common sapropelic coals are dull, rarely stratified, and derived from lipid-rich organic matter such as the remains of algae (boghead coals or torbanites) or spores (cannel coals).
The primary maceral group of humic coals is vitrinite, that of sapropelic coals is exinite.
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