Anthracite coal (Middle Pennsylvanian; Pioneer Tunnel Coal Mine, Ashland, Pennsylvania, USA) 1
Anthracite coal - large block on display at Pioneer Tunnel Coal Mine in the Pennsylvanian of Pennsylvania, USA.
Pioneer Tunnel in Ashland, Pennsylvania is a tourist site that allows examination of an old anthracite strip mine and underground anthracite coal workings in Mahanoy Mountain. The surface strip mine and the underground mine exploited anthracite coal beds in the Llewellyn Formation (middle Desmoinesian Series, upper Middle Pennsylvanian), as did numerous mines in eastern Pennsylvania's coal fields.
Shown above is anthracite coal, the highest-rank coal. It forms by very low-grade metamorphism (anchimetamorphism) of bituminous coal. Anthracite is always black-colored, with a glassy texture, and is harder & heavier than the other coals (although it is still relatively soft & lightweight). Unlike lignite and bituminous coal, anthracite is not sooty to the touch. Anthracite burns hotter than other coal types, due to its high carbon content (~90% C). It also is the cleanest-burning of all the coals.
Anthracite is a scarce variety of coal. The highest concentration of anthracite on Earth is in the Pennsylvanian-aged coal fields of eastern Pennsylvania, USA. There is still some uncertainty in the details about the origin of Pennsylvania anthracite coal. In Colorado, an anthracite coal deposit occurs next to an igneous intrusion - the anthracite formed by heating from contact or hydrothermal metamorphism. It's been suggested that Pennsylvania anthracite was hydrothermally metamorphosed. The anthracite in Pennsylvania was originally deposited in coal swamps that were relatively high on ancient alluvial plains - those environments are usually not preserved in mountain belts (they get uplifted and eroded). In Pennsylvania, the high alluvial plain facies were downdropped and got preserved, resulting in anthracites representing different facies from those seen in bituminous coal fields.
Info. on Pioneer Tunnel Coal Mine & its geology:
Ermert, E.R. 1994. The Story of Pioneer Tunnel Coal Mine & Steam Train. 14 pp.
Edmunds, W.E., V.W. Skema & N.K. Flint. 1999. Stratigraphy and sedimentary tectonics, Pennsylvanian. in The geology of Pennsylvania. Pennsylvania Geological Survey Special Publication 1: 148-169.