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[[File:Pollucite-RoyalOntarioMuseum-Jan18-09.jpg|thumb|Pollucite, a caesium mineral|alt=A white mineral, from which white and pale pink crystals protrude]]
{{See also|:Category:Caesium minerals|l1=Caesium minerals}}
Caesium is a relatively rare element, estimated to average 3 [[parts per million]] in the [[abundance of elements in Earth's crust|Earth's crust]].<ref>{{cite journal |last1=Turekian |first1=K. K. |last2=Wedepohl |first2=K. H. |title=Distribution of the elements in some major units of the Earth's crust |journal=Geological Society of America Bulletin |volume=72 |issue=2 |pages=175–192 |doi=10.1130/0016-7606(1961)72[175:DOTEIS]2.0.CO;2 |issn=0016-7606 |bibcode=1961GSAB...72..175T |year=1961 |doi-access=free}}</ref> It is the 45th most abundant element and the 36th among the metals. Nevertheless, it is more abundant than such elements as [[antimony]], [[cadmium]], [[tin]], and [[tungsten]], and two orders of magnitude more abundant than mercury and [[silver]]; it is 3.3% as abundant as [[rubidium]], with which it is closely associated, chemically.<ref name="USGS"/>
Due to its large [[ionic radius]], caesium is one of the "[[incompatible element]]s".<ref>{{cite web |url=http://www.asi.org/adb/02/13/02/cesium-occurrence-uses.html |title=Cesium as a Raw Material: Occurrence and Uses |first=Simon |last=Rowland |publisher=Artemis Society International |date=4 July 1998 |access-date=15 February 2010 |archive-date=8 July 2021 |archive-url=https://web.archive.org/web/20210708104437/http://www.asi.org/adb/02/13/02/cesium-occurrence-uses.html |url-status=dead }}</ref> During [[fractional crystallization (geology)|magma crystallization]], caesium is concentrated in the liquid phase and crystallizes last. Therefore, the largest deposits of caesium are zone [[pegmatite]] ore bodies formed by this enrichment process. Because caesium does not substitute for [[potassium]] as readily as rubidium does, the alkali evaporite minerals [[sylvite]] (KCl) and [[carnallite]] ({{chem|KMgCl|3|·6H|2|O}}) may contain only 0.002% caesium. Consequently, caesium is found in few minerals. Percentage amounts of caesium may be found in [[beryl]] ({{chem|Be|3|Al|2|(SiO|3|)|6}}) and [[avogadrite]] ({{chem|(K,Cs)BF|4}}), up to 15 wt% Cs<sub>2</sub>O in the closely related mineral [[pezzottaite]] ({{chem|Cs|(Be|2|Li)|Al|2|Si|6|O|18}}), up to 8.4 wt% Cs<sub>2</sub>O in the rare mineral [[londonite]] ({{chem|(Cs,K)Al|4|Be|4|(B,Be)|12|O|28}}), and less in the more widespread [[rhodizite]].<ref name="USGS"/> The only economically important ore for caesium is [[pollucite]] {{chem|Cs(AlSi|2|O|6|)}}, which is found in a few places around the world in zoned pegmatites, associated with the more commercially important [[lithium]] minerals, [[lepidolite]] and [[petalite]]. Within the pegmatites, the large grain size and the strong separation of the minerals results in high-grade ore for mining.<ref name="Cerny">{{cite journal |title=The Tanco Pegmatite at Bernic Lake, Manitoba: X. Pollucite |first1=Petr |last1=Černý |author-link1=Petr Černý |first2=F. M. |last2=Simpson |journal=Canadian Mineralogist |volume=16 |pages=325–333 |date=1978 |url=http://rruff.geo.arizona.edu/doclib/cm/vol38/CM38_877.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://rruff.geo.arizona.edu/doclib/cm/vol38/CM38_877.pdf |archive-date=10 October 2022 |url-status=live |access-date=26 September 2010}}</ref>
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