Editing Lignite (section)

== Geology ==
[[File:Canal Run shadows (5179305812).jpg|thumb|[[Okefenokee Swamp]], a modern peat-forming swamp]]
[[File:U S Geological Survey Circular 1143 Lignite Structure.png|thumb|Partial molecular structure of a lignin-derived organic molecule in lignite]]

Lignite begins as partially decayed plant material, or peat. Peat tends to accumulate in areas with high moisture, slow land [[subsidence]], and no disturbance by rivers or oceans – under these conditions, the area remains saturated with water, which covers dead vegetation and protects it from atmospheric oxygen. Otherwise, peat swamps are found in a variety of climates and geographical settings. Anaerobic bacteria may contribute to the degradation of peat, but this process takes a long time, particularly in acidic water. Burial by other sediments further slows biological degradation, and subsequent transformations are a result of increased temperatures and pressures underground.<ref>{{cite journal |last1=Schweinfurth |first1=Stanley P. |last2=Finkelman |first2=Robert P. |title=Coal – A complex natural resource |journal=U.S. Geological Survey Circular |date=2002 |volume=1143 |doi=10.3133/cir1143|doi-access=free |hdl=2027/umn.31951d02181642b |hdl-access=free }}</ref>

Lignite forms from peat that has not been subjected to deep burial and heating. It forms at temperatures below {{convert|100|C||}},<ref name=brit/> primarily by biochemical degradation. This includes the process of humification, in which microorganisms extract hydrocarbons from peat and form humic acids, which decrease the rate of bacterial decay. In lignite, humification is partial, coming to completion only when the coal reaches sub-bituminous rank.<ref>{{cite web |title=Coal types, formation, and methods of mining |url=http://epcamr.org/home/content/reference-materials/coal-types-formation-and-methods-of-mining/ |website=Eastern Pennsylvania Coalition for Abandoned Mine Reclamation |access-date=5 May 2021 |date=2016 |archive-date=17 July 2020 |archive-url=https://web.archive.org/web/20200717042100/http://epcamr.org/home/content/reference-materials/coal-types-formation-and-methods-of-mining |url-status=live }}</ref> The most characteristic chemical change in the organic material during formation of lignite is the sharp reduction in the number of C=O and C-O-R functional groups.<ref>{{cite journal |last1=Ibarra |first1=JoséV. |last2=Muñoz |first2=Edgar |last3=Moliner |first3=Rafael |title=FTIR study of the evolution of coal structure during the coalification process |journal=Organic Geochemistry |date=June 1996 |volume=24 |issue=6–7 |pages=725–735 |doi=10.1016/0146-6380(96)00063-0|bibcode=1996OrGeo..24..725I }}</ref>

Lignite deposits are typically younger than higher-ranked coals, with the majority of them having formed during the [[Tertiary]] period.<ref name=brit/>