Editing Calcium (section)

==Occurrence and production==
[[File:Pamukkale Hierapolis Travertine pools.JPG|thumb|[[Travertine]] terraces in [[Pamukkale]], [[Turkey]]]]
At 3%, calcium is the fifth [[Abundance of elements in Earth's crust|most abundant element in the Earth's crust]], and the third most abundant metal behind [[aluminium]] and [[iron]].{{Sfn|Greenwood|Earnshaw|1997|p=109}} It is also the fourth most abundant element in the [[lunar highlands]].{{sfn|Hluchan|Pomerantz|2005|p=483}} [[Sedimentary rocks|Sedimentary]] [[calcium carbonate]] deposits pervade the Earth's surface as fossilized remains of past marine life; they occur in two forms, the [[rhombohedral]] [[calcite]] (more common) and the [[orthorhombic]] [[aragonite]] (forming in more temperate seas). Minerals of the first type include [[limestone]], [[Dolomite (mineral)|dolomite]], [[marble]], [[chalk]], and [[iceland spar]]; aragonite beds make up the [[Bahamas]], the [[Florida Keys]], and the [[Red Sea]] basins. [[Coral]]s, [[sea shell]]s, and [[pearl]]s are mostly made up of calcium carbonate. Among the other important minerals of calcium are [[gypsum]] (CaSO<sub>4</sub>·2H<sub>2</sub>O), [[anhydrite]] (CaSO<sub>4</sub>), [[fluorite]] (CaF<sub>2</sub>), and [[apatite]] ([Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>X], X = OH, Cl, or F){{Sfn|Greenwood|Earnshaw|1997|p=108}}

The major producers of calcium are [[China]] (about 10000 to 12000 [[tonne]]s per year), [[Russia]] (about 6000 to 8000 tonnes per year), and the [[United States]] (about 2000 to 4000 tonnes per year). [[Canada]] and [[France]] are also among the minor producers. In 2005, about 24000 tonnes of calcium were produced; about half of the world's extracted calcium is used by the United States, with about 80% of the output used each year.{{sfn|Hluchan|Pomerantz|2005|p=484}}

In Russia and China, Davy's method of electrolysis is still used, but is instead applied to molten [[calcium chloride]].{{sfn|Hluchan|Pomerantz|2005|p=484}} Since calcium is less reactive than strontium or barium, the oxide–nitride coating that results in air is stable and [[lathe]] machining and other standard metallurgical techniques are suitable for calcium.{{sfn|Greenwood|Earnshaw|1997|p = 110}} 

In the United States and Canada, calcium is instead produced by reducing [[Lime (material)|lime]] with aluminium at high temperatures.{{sfn|Hluchan|Pomerantz|2005|p=484}} In this process, powdered high-calcium lime and powdered aluminum are mixed and compacted into [[Briquette|briquettes]] for a high degree of contact, which are then placed in a sealed [[retort]] which has been [[Vacuum|evacuated]] and heated to ~1200 C.{{sfn|Hluchan|Pomerantz|2005|p=484}} The briquettes release calcium vapor into the vacuum for about 8 hours, which then condenses in the cooled ends of the retorts to form 24-34 kg pieces of calcium metal, as well as some residue of [[Calcium aluminates|calcium aluminate]].{{sfn|Hluchan|Pomerantz|2005|p=484}} High-purity calcium can be obtained by [[Distillation|distilling]] low-purity calcium at high temperatures.{{sfn|Hluchan|Pomerantz|2005|p=484}}

===Geochemical cycling===
{{Main|Carbonate–silicate cycle}}
[[Calcium cycle|Calcium cycling]] provides a link between [[tectonics]], [[climate]], and the [[carbon cycle]]. In the simplest terms, mountain-building exposes calcium-bearing rocks such as [[basalt]] and [[granodiorite]] to chemical weathering and releases Ca<sup>2+</sup> into surface water. These ions are transported to the ocean where they react with dissolved CO<sub>2</sub> to form [[limestone]] ({{chem|CaCO|3}}), which in turn settles to the sea floor where it is incorporated into new rocks. Dissolved CO<sub>2</sub>, along with [[carbonate]] and [[bicarbonate]] ions, are termed "[[Total inorganic carbon|dissolved inorganic carbon]]" (DIC).<ref name="Berner" />

The actual reaction is more complicated and involves the bicarbonate ion (HCO{{su|b=3|p=−}}) that forms when CO<sub>2</sub> reacts with water at seawater [[pH]]:

:{{chem2 | Ca(2+) + 2 HCO3- -> CaCO3↓ + CO2 + H2O }}

At seawater pH, most of the dissolved CO<sub>2</sub> is immediately converted back into {{chem|HCO|3|-}}. The reaction results in a net transport of one molecule of CO<sub>2</sub> from the ocean/atmosphere into the [[lithosphere]].<ref>{{cite web |author=Zeebe |date=2006 |title=Marine carbonate chemistry |url=https://editors.eol.org/eoearth/wiki/Marine_carbonate_chemistry |access-date=2010-03-13 |publisher=National Council for Science and the Environment}}</ref> The result is that each Ca<sup>2+</sup> ion released by chemical weathering ultimately removes one CO<sub>2</sub> molecule from the surficial system (atmosphere, ocean, soils and living organisms), storing it in carbonate rocks where it is likely to stay for hundreds of millions of years. The weathering of calcium from rocks thus scrubs CO<sub>2</sub> from the ocean and atmosphere, exerting a strong long-term effect on climate.<ref name="Berner">{{Cite journal|last1=Berner|first1=Robert|title= The long-term carbon cycle, fossil fuels and atmospheric composition |journal=Nature|date=2003|volume=426|pages= 323–26|doi=10.1038/nature02131|pmid=14628061|issue=6964|bibcode = 2003Natur.426..323B |s2cid=4420185}}</ref><ref>{{Cite journal|title = A negative feedback mechanism for the long-term stabilization of Earth's surface temperature|journal = Journal of Geophysical Research: Oceans|date = 1981-10-20|pages = 9776–82|volume = 86|issue = C10|doi = 10.1029/JC086iC10p09776|first1 = James C. G.|last1 = Walker|first2 = P. B.|last2 = Hays|first3 = J. F.|last3 = Kasting|bibcode=1981JGR....86.9776W}}</ref>