Editing Calcite (section)

== Natural occurrence ==
Calcite is a common constituent of [[sedimentary rock]]s, limestone in particular, much of which is formed from the shells of dead marine organisms.  Approximately 10% of sedimentary rock is limestone. It is the primary mineral in [[Metamorphic rock|metamorphic]] [[marble]]. It also occurs in deposits from [[hot spring]]s as a [[Vein (geology)|vein]] mineral; in caverns as [[stalactite]]s and [[stalactite|stalagmite]]s; and in [[volcanic rock|volcanic]] or [[mantle (geology)|mantle-derived]] rocks such as [[carbonatite]]s, [[kimberlite]]s, or rarely in [[peridotite]]s.

[[Cactus|Cacti]] contain Ca-oxalate biominerals. Their death releases these biominerals into the environment, which subsequently transform to calcite via a monohydrocalcite intermediate, [[Carbon sequestration|sequestering carbon]].<ref>{{Cite journal |last=Garvie |first=Laurence A. J. |date=2006 |title=Decay of cacti and carbon cycling |url=https://pubmed.ncbi.nlm.nih.gov/16453105/ |journal=Die Naturwissenschaften |volume=93 |issue=3 |pages=114–118 |doi=10.1007/s00114-005-0069-7 |issn=0028-1042 |pmid=16453105}}</ref><ref>{{Cite web |date=2021-10-18 |title=Carbon Sequestration and Sonoran Desert Cacti |url=https://blog.azgs.arizona.edu/index.php/blog/2021-10/carbon-sequestration-and-sonoran-desert-cacti |access-date=2024-09-23 |website=e-Magazine of the AZ Geological Survey}}</ref>

Calcite is often the primary constituent of the shells of [[Marine biology|marine organism]]s, such as [[plankton]] (such as [[coccolith]]s and planktic [[foraminifera]]), the hard parts of red [[algae]], some [[sea sponge|sponge]]s, [[brachiopod]]s, [[echinoderm]]s, some [[Serpulidae|serpulids]], most [[bryozoa]], and parts of the shells of some [[Bivalvia|bivalves]] (such as [[oyster]]s and [[rudists]]). Calcite is found in spectacular form in the [[Snowy River Cave]] of [[New Mexico]] as mentioned above, where microorganisms are credited with natural formations. [[Trilobite]]s, which became [[Permian–Triassic extinction event|extinct a quarter billion years ago]], had unique compound eyes that used clear calcite crystals to form the lenses.<ref>{{cite news |last= Angier| first= Natalie |author-link=Natalie Angier |date=3 March 2014 |title=When Trilobites Ruled the World |url= https://www.nytimes.com/2014/03/04/science/when-trilobites-ruled-the-world.html |newspaper= [[The New York Times]] |access-date=10 March 2014 }}</ref> It also forms a substantial part of birds' eggshells, and the [[δ13C|δ{{sup|13}}C]] of the diet is reflected in the δ{{sup|13}}C of the calcite of the shell.<ref name="Lynch-et-al-2007">{{cite journal | last1=Lynch | first1=Amanda H. | last2= Beringer | first2=Jason | last3=Kershaw | first3=Peter | last4=Marshall | first4=Andrew | last5=Mooney | first5=Scott | last6=Tapper | first6=Nigel | last7=Turney | first7=Chris | last8=Van Der Kaars | first8=Sander | display-authors= 3| title=Using the Paleorecord to Evaluate Climate and Fire Interactions in Australia | journal=[[Annual Review of Earth and Planetary Sciences]] | volume=35 | issue=1 | year=2007 | doi=10.1146/annurev.earth.35.092006.145055 | pages=215–239| bibcode=2007AREPS..35..215L }}</ref>

The largest documented single crystal of calcite originated from Iceland, measured {{convert|7|×|7|×|2|m|sigfig=2|abbr=on}} and {{convert|6|×|6|×|3|m|sigfig=2|abbr=on}} and weighed about 250 tons.<ref>{{cite journal| url=http://www.minsocam.org/ammin/AM66/AM66_885.pdf| access-date=2024-02-12| journal=American Mineralogist| volume=66| pages=885–907| year=1981| title=The largest crystals| last=Rickwood| first=P. C.}}</ref> Classic samples have been produced at [[Madawaska Mine]], near [[Bancroft, Ontario]].<ref>{{Cite journal |last=McDougall |first=Raymond |date=2019-09-03 |title=Mineral Highlights from the Bancroft Area, Ontario, Canada |journal=Rocks & Minerals |volume=94 |issue=5 |pages=408–419 |doi=10.1080/00357529.2019.1619134 |bibcode=2019RoMin..94..408M |s2cid=201298402}}</ref>

[[Bed (geology)|Bedding]] parallel veins of fibrous calcite, often referred to in quarrying parlance as ''beef'', occur in dark organic rich mudstones and shales, these veins are formed by increasing [[fluid pressure]] during [[diagenesis]].<ref>{{Cite journal |last1=Ravier| first1=Edouard| last2=Martinez |first2=Mathieu |last3=Pellenard |first3=Pierre |last4=Zanella |first4=Alain |last5=Tupinier |first5=Lucie| display-authors=3| date=December 2020 |title=The milankovitch fingerprint on the distribution and thickness of bedding-parallel veins (beef) in source rocks |journal=Marine and Petroleum Geology |language=en |volume=122| page=104643 |doi=10.1016/j.marpetgeo.2020.104643 |bibcode=2020MarPG.12204643R |s2cid=225177225 |url=https://hal-insu.archives-ouvertes.fr/insu-02922274/file/ravier-2020.pdf}}</ref>