English: Bituminous coal from the Cretaceous of Utah, USA.

Coal is a carbon-rich, biogenic sedimentary rock. It forms by the burial and alteration of organic matter from fossil land plants that lived in ancient swamps. Coal starts out as peat. With increasing burial and diagenetic alteration, peat becomes lignite coal, sub-bituminous coal, and then bituminous coal. Bituminous coals tend to break and weather in a blocky fashion, are relatively sooty to the touch, and are harder and heavier than lignite coal (but still relatively soft and lightweight). Discernible plant fossil fragments may be present on bituminous coal bedding planes - sometimes in abundance. Bituminous coals commonly have irregular patches of shiny, glassy-textured organic matter (vitrain).

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Info. from public signage at Wittenberg University's Geology Department (Springfield, Ohio, USA):

Origin of Coal

Coal is formed from accumulated vegetation that grew in peat-forming swamps on broad lowlands that were near sea level. Cyclothems indicate that the land must have been at a "critical level" since the change from marine to non-marine sediments shows that the seas periodically encroached upon the land.

Formation of Coal

The change from plant debris to coal involves biochemical action producing partial decay, preserval of this material from further decay, and later dynamochemical processes. The biochemical changes involve attack by bacteria which liberate volatile constituents, and the preserval of the residual waxes and resins in the bottom of the swamps where the water is too toxic for the decay-promoting bacteria to live. The accumulated material forms "peat bogs". The dynamochemical process involves further chemical reactions produced by the increased pressure and temperature brought about by the weight of sediment that is deposited on top of it. These reactions are also ones in which the volatile constituents are driven off.

Rank of Coal

The different types of coal are commonly referred to in terms of rank. From lowest upward, they are peat (actually not a coal), lignite, bituminous, and anthracite. The rank of the coal is the result of the different amounts of pressure and time involved in producing the coal.

Bituminous

Bituminous coal is a dense, dark, brittle, banded coal that is well jointed and breaks into cubical or prismatic blocks and does not disintegrate upon exposure to air. Dull and bright bands and smooth and hackly layers are evident. It ignites easily, burns with a smoky yellow flame, has low moisture contnet, medium volatile content, and fixed carbon and heating content is high. It is the most used and most desired coal in the world for industrial uses.

In the United States, the Northern Appalachian fields lead in production, followed by the interior fields of the Midwest.

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This sample comes from Utah's Bronco Mine, which reportedly started in the 1880s. The coal ranks as high-volatile C bituminous coal, which means it gives off less heat than high-volatile A or B bituminous coals. The former gives off about 11,500 British thermal units (Btu) of heat per pound of coal. The latter two give off about 14,000 and 13,000 Btu per pound, respectively.

Stratigraphy: coal horizon in the Ferron Sandstone Member, Mancos Shale, Upper Cretaceous

Locality: Bronco Mine (= Emery Deep Mine), Emery County, central Utah, USA