Editing Coal (section)

===Types===
[[File:Sydney Mines Point Aconi Seam 038.JPG|thumb|Coastal exposure of the Point Aconi Seam in [[Nova Scotia]]]]
[[File:Coal Rank USGS.png|thumb|upright=1.35|Coal ranking system used by the [[United States Geological Survey]]]]

As geological processes apply [[pressure]] to dead [[biotic material]] over time, under suitable conditions, its [[metamorphic grade]] or rank increases successively into:
* [[Peat]], a precursor of coal
* [[Lignite]], or brown coal, the lowest rank of coal, most harmful to health when burned,<ref name=Heal/> used almost exclusively as fuel for electric power generation
* [[Sub-bituminous coal]], whose properties range between those of lignite and those of bituminous coal, is used primarily as fuel for steam-electric power generation.
* [[Bituminous coal]], a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of bright and dull material. It is used primarily as fuel in steam-electric power generation and to make [[coke (fuel)|coke]]. Known as steam coal in the UK, and historically used to raise steam in steam locomotives and ships
* [[Anthracite coal]], the highest rank of coal, is a harder, glossy black coal used primarily for residential and commercial [[space heating]].
* [[Graphite]], a difficult to ignite coal which is mostly used in pencils, or powdered for [[lubrication]].
* [[Cannel coal]] (sometimes called "candle coal"), a variety of fine-grained, high-rank coal with significant hydrogen content, which consists primarily of [[liptinite]]. It is related to boghead coal.

There are several international standards for coal.<ref>{{cite web |title=Standards catalogue 73.040 – Coals |url=https://www.iso.org/ics/73.040/x/ |publisher=[[ISO]]}}</ref> The classification of coal is generally based on the content of [[coal volatiles|volatiles]]. However the most important distinction is between thermal coal (also known as steam coal), which is burnt to generate electricity via steam; and [[metallurgical coal]] (also known as coking coal), which is burnt at high temperature to make [[steel]].

[[Hilt's law]] is a geological observation that (within a small area) the deeper the coal is found, the higher its rank (or grade). It applies if the thermal gradient is entirely vertical; however, [[metamorphism]] may cause lateral changes of rank, irrespective of depth. For example, some of the coal seams of the [[Madrid, New Mexico]] coal field were partially converted to anthracite by [[contact metamorphism]] from an igneous [[Sill (geology)|sill]] while the remainder of the seams remained as bituminous coal.<ref>{{cite journal |last1=Darton |first1=Horatio Nelson |title=Guidebook of the Western United States: Part C - The Santa Fe Route, with a side trip to Grand Canyon of the Colorado |journal=U.S. Geological Survey Bulletin |date=1916 |volume=613 |page=81 |doi=10.3133/b613|bibcode=1916usgs.rept....2D |hdl=2027/hvd.32044055492656 |hdl-access=free }}</ref>