Editing Chromite (section)

== Properties ==
Chromite minerals are mainly found in mafic-ultramafic [[Intrusive rock|igneous intrusions]] and are also sometimes found in [[metamorphic rock]]s. The chromite minerals occur in layered formations that can be hundreds of kilometres long and a few meters thick.<ref>{{Cite journal|last1=Latypov|first1=Rais|last2=Costin|first2=Gelu|last3=Chistyakova|first3=Sofya|last4=Hunt|first4=Emma J.|last5=Mukherjee|first5=Ria|last6=Naldrett|first6=Tony|date=2018-01-31|title=Platinum-bearing chromite layers are caused by pressure reduction during magma ascent|journal=Nature Communications|volume=9|issue=1|pages=462|doi=10.1038/s41467-017-02773-w|pmid=29386509|pmc=5792441|issn=2041-1723|bibcode=2018NatCo...9..462L}}</ref> Chromite is also common in [[iron meteorite]]s and form in association with [[silicate]]s and [[troilite]] minerals.<ref name="Fehr-2004">{{Cite journal|last1=Fehr|first1=Karl Thomas|last2=Carion|first2=Alain|s2cid=55658406|date=2004|title=Unusual large chromite crystals in the Saint Aubin iron meteorite|journal=Meteoritics & Planetary Science|volume=39|issue=S8|pages=A139–A141|doi=10.1111/j.1945-5100.2004.tb00349.x|issn=1086-9379|bibcode=2004M&PS...39..139F|doi-access=free}}</ref>

=== Crystal structure ===
The chemical composition of chromite can be represented as FeCr<sub>2</sub>O<sub>4</sub>, with the iron in the +2 [[oxidation state]] and the chromium in the +3 oxidation state.<ref name=Mindat /> [[Bauxite]], when presented as an [[ore]], or in massive form, forms as fine granular aggregates. The structure of the [[ore]] can be seen as platy, with breakages along planes of weakness. Chromite can also be presented in a thin section. The grains seen in thin sections are disseminated with crystals that are [[Euhedral and anhedral|euhedral]] to [[Euhedral and anhedral|subhedral]].<ref>{{Cite journal|last=Fortier|first=Y.|date=1941|title=Geology of Chromite|url=http://digitool.library.mcgill.ca/R/-?func=dbin-jump-full&amp;current_base=GEN01&amp;object_id=129707|journal=McGill University}}</ref>

Chromite contains Mg, ferrous iron [Fe(II)], Al and trace amounts of [[Titanium|Ti]].<ref name=Mindat /> Chromite can change into different minerals based on the amounts of each element in the mineral. Chromite is a part of the [[spinel group]], which means that it is able to form a complete [[solid solution]] series with other members in the same group. These include minerals such as chenmingite (FeCr<sub>2</sub>O<sub>4</sub>), [[xieite]] (FeCr<sub>2</sub>O<sub>4</sub>), magnesiochromite (MgCr<sub>2</sub>O<sub>4</sub>) and [[magnetite]] (Fe<sup>2+</sup>Fe<sup>3+</sup><sub>2</sub>O<sub>4</sub>). Chenmingite and xieite are [[Polymorphism (materials science)|polymorphs]] of chromite while magnesiochromite and magnetite are [[isostructural]] with chromite.<ref name="Mindat" />

=== Crystal size and morphology ===
Chromite occurs as massive and granular crystals and very rarely as [[Octahedron|octahedral]] crystals. [[Crystal twinning|Twinning]] for this mineral occurs on the {III} plane as described by the [[Crystal twinning#Common twin laws|spinel law]].<ref name=Mindat />

Grains of minerals are generally small in size. However, chromite grains up to 3&nbsp;cm have been found. These grains are seen to crystallize from the liquid of a [[meteorite]] body where there are low amounts of chromium and oxygen. The large grains are associated with stable [[Supersaturation|supersaturated]] conditions seen from the meteorite body.<ref name="Fehr-2004" />

=== Reactions ===
Chromite is an important mineral in helping to determine the conditions that rocks form. It can have reactions with various gases such as [[Carbon monoxide|CO]] and [[Carbon dioxide|CO<sub>2</sub>]]. The reaction between these gases and the solid chromite grains results in the reduction of the chromite and allows for the formation of iron and chromium [[alloy]]s. There could also be a formation of metal [[carbide]]s from the interaction with chromite and the gases.<ref>{{Citation|last=Eric|first=Rauf Hurman|chapter=Production of Ferroalloys|date=2014|pages=477–532|publisher=Elsevier|language=en|doi=10.1016/b978-0-08-096988-6.00005-5|isbn=9780080969886|title=Treatise on Process Metallurgy}}</ref>

Chromite is seen to form early in the [[crystallization]] process. This allows for chromite to be resistant to the alteration effects of high temperatures and pressures seen in the [[Metamorphic rock|metamorphic]] series. It is able to progress through the [[Metamorphic rock|metamorphic]] series unaltered. Other minerals with a lower resistance are seen to alter in this series to minerals such as [[Serpentine subgroup|serpentine]], [[biotite]] and [[garnet]].<ref>{{Cite web|url=https://www.galleries.com/minerals/oxides/chromite/chromite.htm|title=CHROMITE (Iron Chromium Oxide)|website=www.galleries.com |url-status=live |archive-url=https://web.archive.org/web/20111017013049/https://www.galleries.com/minerals/oxides/chromite/chromite.htm |archive-date=October 17, 2011 |access-date=2019-03-17}}</ref>