Editing Selenium (section)

==Occurrence==
{{Category see also|Selenide minerals}}
[[File:Selenium in sandstone Westwater Canyon Section 23 Mine Grants, New Mexico.jpg|thumb|Native selenium in sandstone, from a uranium mine near [[Grants, New Mexico]]]]

Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals.<ref>{{cite web |publisher=Webminerals |url=http://www.galleries.com/minerals/elements/selenium/selenium.htm |title=Native Selenium |access-date=2009-06-06 |df=dmy-all}}</ref> Isolation of selenium is often complicated by the presence of other compounds and elements.

Selenium occurs naturally in a number of inorganic forms, including [[selenide]], [[selenate]], and [[Selenite (ion)|selenite]], but these minerals are rare. The common mineral [[selenite (mineral)|selenite]] is not a selenium mineral, and contains no [[Selenite (ion)|selenite ion]], but is rather a type of [[gypsum]] (calcium sulfate hydrate) named like selenium for the moon well before the discovery of selenium. Selenium is most commonly found as an impurity, replacing a small part of the sulfur in sulfide ores of many metals.<ref name="geosel1">{{cite journal |pmid=9726787 |year=1998 |last=Kabata-Pendias |first=A. |author-link=Alina Kabata-Pendias |title=Geochemistry of selenium |volume=17 |issue=3–4 |pages=173–177 |journal= Journal of Environmental Pathology, Toxicology and Oncology}}</ref><ref name="geosel2">{{cite journal |doi=10.1579/0044-7447(2007)36[94:SGAH]2.0.CO;2 |volume=36 |pages=94–97 |title=Selenium Geochemistry and Health |last1=Fordyce |first1=Fiona |journal=Ambio: A Journal of the Human Environment |year=2007|issue=1 |pmid=17408199 |s2cid=18925825 |url=http://nora.nerc.ac.uk/id/eprint/19045/1/AMBIO_Fordycefinal.pdf }}</ref>

In living systems, selenium is found in the amino acids [[selenomethionine]], [[selenocysteine]], and [[methylselenocysteine]]. In these compounds, selenium plays a role analogous to that of sulfur. Another naturally occurring [[organoselenium compound]] is [[dimethyl selenide]].<ref name="biol1">{{cite journal |doi=10.1515/BC.2007.138 |title=Selenium in chemistry and biochemistry in comparison to sulfur |year=2007 |last1=Wessjohann |first1=Ludger A. |last2=Schneider |first2=Alex |last3=Abbas |first3=Muhammad |last4=Brandt |first4=Wolfgang |journal=Biological Chemistry |volume=388 |issue=10 |pages=997–1006 |pmid=17937613|s2cid=34918691 }}</ref><ref name="biol1a">{{cite journal |doi=10.1039/B205802M |title=Trends in selenium biochemistry |date=2002 |last1=Birringer |first1=Marc |last2=Pilawa |first2=Sandra |last3=Flohé |first3=Leopold |journal=Natural Product Reports |volume=19 |issue=6 |pages=693–718 |pmid=12521265}}</ref>

Certain soils are selenium-rich, and selenium can be [[bioaccumulation|bioconcentrated]] by some plants. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.<ref name="geosel1" /><ref name="geosel2" /> Ocean water contains significant amounts of selenium.<ref>{{cite journal |doi=10.1016/S0012-821X(01)00370-3 |title=Role of oceans as biogenic sources of selenium |year=2001 |last1=Amouroux |first1=David |last2=Liss |first2=Peter S. |last3=Tessier |first3=Emmanuel |last4=Hamren-Larsson |first4=Marie |last5=Donard |first5=Olivier F.X. |journal=Earth and Planetary Science Letters |volume=189 |issue=3–4 |pages=277–283 |bibcode=2001E&PSL.189..277A |display-authors=3}}</ref><ref>{{cite journal |doi=10.1080/08910600701698986 |title=How to use the world's scarce selenium resources efficiently to increase the selenium concentration in food |year=2007 |last1=Haug |first1=Anna |last2=Graham |first2=Robin D. |last3=Christophersen |first3=Olav A. |last4=Lyons |first4=Graham H. |journal=Microbial Ecology in Health and Disease |volume=19 |issue=4 |pages=209–228 |pmid=18833333 |pmc=2556185}}</ref>

Typical background concentrations of selenium do not exceed 1&nbsp;ng/m<sup>3</sup> in the atmosphere; 1&nbsp;mg/kg in soil and vegetation and 0.5&nbsp;μg/L in freshwater and seawater.<ref>{{Cite book|last=Rieuwerts|first=John|url=https://www.worldcat.org/oclc/886492996|title=The Elements of Environmental Pollution|publisher=Earthscan Routledge|year=2015|isbn=978-0-415-85919-6|location=London and New York|pages=262|oclc=886492996}}</ref>

Anthropogenic sources of selenium include coal burning, and the mining and smelting of sulfide ores.<ref>{{cite web |url=http://www.atsdr.cdc.gov/toxprofiles/tp92-c1.pdf |publisher=Agency for Toxic Substances and Disease Registry |title=Public Health Statement: Selenium |access-date=2009-01-05 |df=dmy-all}}</ref>