anthracite - teaching hand specimen of metamorphic "hard coal" from eastern Pennsylvania
Anthracite, or metamorphic coal, is the hardest and hottest burning of all coal types. It has a glossy surface and conchoidal fracture. It burns smoke-free and is desired for both steel-making and for heating. The anthracite fields of Pennsylvania lie east of the Susquehanna River.
Plant debris-bearing strata were deposited in swampy areas of the Appalachian foreland basin at the beginning of the Appalachian orogeny, were buried by a great thickness of sediments, and were compressed and heated to alter the plant material to first form peat, then lignite, bituminous coal and finally anthracite, a sequence students should know. Metamorphism beyond anthracite results in graphite. In eastern Pennsylvania, uplift and subsequent erosion removed the overlying sediments and exposed the coal.
Bituminous coal mainly originated in the Triassic, 205 - 245 million years ago. Western U.S. lignite and pre-lignite coals are primarily Cretaceous, 70 - 140 million years in age. Pennsylvania anthracite is older, forming during the Pennsylvanian Epoch or "Upper Carboniferous," from 298 to 318 million years ago.
Anthracite is unusual, since sediments are only rarely subjected to the temperatures and pressures necessary to form anthracite. Pennsylvania anthracite forms just 1% of US coal reserves, but over 90% of known global reserves of anthracite.
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