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== Distribution of deposits ==
== Distribution of deposits ==
[[File:Yukon chromite prospect.jpg|thumb|left|A chromite prospect in [[Yukon]]. The black bands are chromite, which also carries [[platinum group metals]]. Gray rock is bleached [[ultramafic]]s.]]
[[File:Yukon chromite prospect.jpg|thumb|left|A chromite prospect in [[Yukon]]. The black bands are chromite, which also carries [[platinum group metals]]. Gray rock is bleached [[ultramafic]]s.|244x244px]]
Chromite is found as orthocumulate lenses of [[chromitite]] in [[peridotite]] from the Earth's [[mantle (geology)|mantle]]. It also occurs in [[Layered intrusion|layered]] [[ultramafic]] intrusive rocks.<ref>{{cite journal | doi = 10.1016/0892-6875(88)90045-3 | title = Chromite- mineralogy and processing | year = 1988 | author = Gu, F | journal = Minerals Engineering | volume = 1 | pages = 235 | last2 = Wills | first2 = B | issue = 3}}</ref> In addition, it is found in metamorphic rocks such as some [[serpentinite]]s. [[Ore]] deposits of chromite form as early magmatic differentiates. It is commonly associated with [[olivine]], [[magnetite]], [[Serpentine group|serpentine]], and [[corundum]]. The vast [[Bushveld igneous complex]] of [[South Africa]] is a large layered [[mafic]] to [[ultramafic]] igneous body with some layers consisting of 90% chromite making the rare rock type, [[chromitite]].<ref>Guilbert, John M., and Park, Charles F., Jr. (1986) ''The Geology of Ore Deposits,'' Freeman, {{ISBN|0-7167-1456-6}}</ref> The [[Stillwater igneous complex]] in [[Montana]] also contains significant chromite.<ref name=Klein/>
Chromite is found as orthocumulate lenses of [[chromitite]] in [[peridotite]] from the Earth's [[mantle (geology)|mantle]]. It also occurs in [[Layered intrusion|layered]] [[ultramafic]] intrusive rocks.<ref>{{cite journal | doi = 10.1016/0892-6875(88)90045-3 | title = Chromite- mineralogy and processing | year = 1988 | author = Gu, F | journal = Minerals Engineering | volume = 1 | pages = 235 | last2 = Wills | first2 = B | issue = 3}}</ref> In addition, it is found in metamorphic rocks such as some [[serpentinite]]s. [[Ore]] deposits of chromite form as early magmatic differentiates. It is commonly associated with [[olivine]], [[magnetite]], [[Serpentine group|serpentine]], and [[corundum]]. The vast [[Bushveld igneous complex]] of [[South Africa]] is a large layered [[mafic]] to [[ultramafic]] igneous body with some layers consisting of 90% chromite making the rare rock type, [[chromitite]].<ref>Guilbert, John M., and Park, Charles F., Jr. (1986) ''The Geology of Ore Deposits,'' Freeman, {{ISBN|0-7167-1456-6}}</ref> The [[Stillwater igneous complex]] in [[Montana]] also contains significant chromite.<ref name=Klein/>


Chromite is found in large quantities that is available for commercial mining. The chromite minerals are found in 2 main deposits, which are [[stratiform]] deposits and podiform deposits. Stratiform deposits in layered intrusions are the main source of chromite resources and are seen in countries such as [[South Africa]], [[Canada]], [[Finland]], and [[Madagascar]]. Chromite resources from podiform deposits are mainly found in [[Kazakhstan]], [[Turkey]], and [[Albania]]. [[Zimbabwe]] is the only country that can obtain chromite resources from both stratiform and podiform deposits.<ref>{{Cite book|url=https://www.worldcat.org/oclc/947118220|title=Environmental materials and waste : resource recovery and pollution prevention|others=Prasad, M. N. V. (Majeti Narasimha Vara), 1953-, Shih, Kaimin,|isbn=9780128039069|location=London|oclc=947118220}}</ref>
Chromite is found in large quantities that is available for commercial mining. The chromite minerals are found in 2 main deposits, which are [[stratiform]] deposits and podiform deposits. Stratiform deposits in layered intrusions are the main source of chromite resources and are seen in countries such as [[South Africa]], [[Canada]], [[Finland]], and [[Madagascar]]. Chromite resources from podiform deposits are mainly found in [[Kazakhstan]], [[Turkey]], and [[Albania]]. [[Zimbabwe]] is the only country that can obtain chromite resources from both stratiform and podiform deposits.<ref>{{Cite book|url=https://www.worldcat.org/oclc/947118220|title=Environmental materials and waste : resource recovery and pollution prevention|others=Prasad, M. N. V. (Majeti Narasimha Vara), 1953-, Shih, Kaimin,|isbn=9780128039069|location=London|oclc=947118220}}</ref>

=== Stratiform deposits ===
[[Stratiform]] deposits are formed as large sheet-like bodies, usually formed in layered [[mafic]] to [[Ultramafic rock|ultramafic]] [[Igneous rock|igneous]] complexes. This type of deposit is used to obtain 98% of the worldwide chromite reserves.<ref name=":2">{{Cite book|url=http://worldcat.org/oclc/191989186|title=Ore deposit models 7 : magmatic segregation deposits of chromite|last=M.|first=Duke, J.|oclc=191989186}}</ref>

[[Stratiform]] deposits are typically seen to be of [[Precambrian]] in age and are found in [[Craton|cratons]]. The [[mafic]] to [[Ultramafic rock|ultramafic]] [[Igneous rock|igneous]] provinces that these deposits are formed in were likely intruded into [[Continental crust|continental crust]], which may have contained [[Granite|granites]] or [[Gneiss|gneisses]]. The shapes of these intrusions are described as tabular or funnel-shaped. The tabular intrusions were placed in the form of [[Sill (geology)|sills]] with the layering of these intrusions being parallel. Examples of these tabular intrusions can be seen in the [[Stillwater igneous complex|Stillwater Igneous Complex]] and [[Bird River greenstone belt|Bird River]]. The funnel-shaped intrusions are seen to be dipping towards the center of the intrusion. This gives the layers in this intrusion a [[syncline]] formation. Examples of this type of intrusion can be seen in the [[Bushveld Igneous Complex]] and the [[Great Dyke]].<ref name=":2" />

Chromite can be seen in [[stratiform]] deposits as multiple layers which consist of [[chromitite]]. Thicknesses for these layers range between 1 cm to 1 m. Lateral depths can reach lengths of 70 km. Chromitite is the main rock in these layers, with 50-95% of it being made of chromite and the rest being composed of [[olivine]], [[Pyroxene|orthopyroxene]], [[plagioclase]], [[Pyroxene|clinopyroxene]], and the various alteration products of these minerals. An indication of water in the magma is by the presence of brown [[mica]].<ref name=":2" />


==Applications==
==Applications==

Revision as of 19:43, 17 March 2019

This article is about the mineral. For the chromium(III) anion and its salts, see Chromite (compound).
Chromite
Chromite from Zimbabwe
General
CategoryOxide minerals
Spinel group
Spinel structural group
Formula
(repeating unit)		(Fe, Mg)Cr2O4
Strunz classification4.BB.05
Crystal systemIsometric
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m
Unit cella = 8.344 Å; Z = 8
Identification
ColorBlack to brownish black; brown to brownish black on thin edges in transmitted light
Crystal habitOctahedral rare; massive to granular
TwinningSpinel law on {Ill}
CleavageNone, parting may develop along {III}
FractureUneven
TenacityBrittle
Mohs scale hardness5.5
LusterSubmetallic
StreakBrown
DiaphaneityTranslucent to opaque.
Specific gravity4.5 - 4.8
Optical propertiesIsotropic
Refractive indexn = 2.08-2.16
Other characteristicsWeakly magnetic
References[1][2][3][4]

Chromite is a mineral and can be described as an iron chromium oxide, with a chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MgCr2O4).[5] A substitution of the element aluminium can also occur, leading to hercynite (FeAl2O4).[6] Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.[7]

Chromite grains are commonly found in large mafic igneous intrusions such as the Bushveld in South Africa and Travancore in India. Chromite is iron-black in color with a metallic luster, a dark brown streak and a Moh's hardness on the scale of 5.5.[8]

Properties

Chromite minerals are mainly found in mafic-ultramafic igneous intrusions and are also sometimes found in metamorphic rocks. The chromite minerals occur in layered formations that can be hundreds of kilometres long and a few meters thick.[9] Chromite is also common in iron meteorites and form in association with silicates and troilite minerals.[10]

Crystal structure

The chemical composition of chromite is Fe2+Cr3+2O4.[11] Chromite, 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 to subhedral.[12]

Chromite contains Mg, ferric iron [Fe(III)], Al and trace amounts of Ti.[11] 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 (FeCr2O4), xieite (FeCr2O4), magnesiochromite (MgCr2O4) and magnetite (Fe2+Fe3+2O4). Chenmingite and xieite are polymorphs of chromite while magnesiochromite and magnetite are isostructural with chromite.[11]

Crystal size and morphology

Chromite occurs as massive and granular crystals and very rarely as octahedral crystals. Twinning for this mineral occurs on the {III} plane as described by the spinel law.[11]

Grains of minerals are generally small in size. However, chromite grains up to 3 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 supersaturated conditions seen from the meteorite body.[10]

Reactions

Chromite is an important mineral in helping to determine the conditions that rocks form. It can have reactions with various gases such as CO and CO2. 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 alloys. There could also be a formation of metal carbides from the interaction with chromite and the gases.[13]

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 series. It is able to progress through the metamorphic series unaltered. Other minerals with a lower resistance are seen to alter in this series to minerals such as serpentine, biotite and garnet.[14]

Distribution of deposits

A chromite prospect in Yukon. The black bands are chromite, which also carries platinum group metals. Gray rock is bleached ultramafics.

Chromite is found as orthocumulate lenses of chromitite in peridotite from the Earth's mantle. It also occurs in layered ultramafic intrusive rocks.[15] In addition, it is found in metamorphic rocks such as some serpentinites. Ore deposits of chromite form as early magmatic differentiates. It is commonly associated with olivine, magnetite, serpentine, and corundum. The vast Bushveld igneous complex of South Africa is a large layered mafic to ultramafic igneous body with some layers consisting of 90% chromite making the rare rock type, chromitite.[16] The Stillwater igneous complex in Montana also contains significant chromite.[2]

Chromite is found in large quantities that is available for commercial mining. The chromite minerals are found in 2 main deposits, which are stratiform deposits and podiform deposits. Stratiform deposits in layered intrusions are the main source of chromite resources and are seen in countries such as South Africa, Canada, Finland, and Madagascar. Chromite resources from podiform deposits are mainly found in Kazakhstan, Turkey, and Albania. Zimbabwe is the only country that can obtain chromite resources from both stratiform and podiform deposits.[17]

Stratiform deposits

Stratiform deposits are formed as large sheet-like bodies, usually formed in layered mafic to ultramafic igneous complexes. This type of deposit is used to obtain 98% of the worldwide chromite reserves.[18]

Stratiform deposits are typically seen to be of Precambrian in age and are found in cratons. The mafic to ultramafic igneous provinces that these deposits are formed in were likely intruded into continental crust, which may have contained granites or gneisses. The shapes of these intrusions are described as tabular or funnel-shaped. The tabular intrusions were placed in the form of sills with the layering of these intrusions being parallel. Examples of these tabular intrusions can be seen in the Stillwater Igneous Complex and Bird River. The funnel-shaped intrusions are seen to be dipping towards the center of the intrusion. This gives the layers in this intrusion a syncline formation. Examples of this type of intrusion can be seen in the Bushveld Igneous Complex and the Great Dyke.[18]

Chromite can be seen in stratiform deposits as multiple layers which consist of chromitite. Thicknesses for these layers range between 1 cm to 1 m. Lateral depths can reach lengths of 70 km. Chromitite is the main rock in these layers, with 50-95% of it being made of chromite and the rest being composed of olivine, orthopyroxene, plagioclase, clinopyroxene, and the various alteration products of these minerals. An indication of water in the magma is by the presence of brown mica.[18]

Applications

The only ores of chromium are the minerals chromite and magnesiochromite. Most of the time, economic geology names chromite the whole chromite-magnesiochromite series: FeCr2O4, (Fe,Mg)Cr2O4, (Mg,Fe)Cr2O4 and MgCr2O4.[4] The two main products of chromite refining are ferrochromium and metallic chromium; for those products the ore smelter process differs considerably. For the production of ferrochromium the chromite ore (FeCr2O4) is reduced with either aluminium or silicon in an aluminothermic reaction and for the production of pure chromium the iron has to be separated from the chromium in a two step roasting and leaching process.[19] Chromite is also used as a refractory material, because it has a high heat stability.[20]

The chromium extracted from chromite is used in chrome plating and alloying for production of corrosion resistant superalloys, nichrome, and stainless steel. Chromium is used as a pigment for glass, glazes, and paint, and as an oxidizing agent for tanning leather.[21] It is also sometimes used as a gemstone.[22]

Heating element with nichrome wires


Gallery of chromite mineral specimens

See also

References

  1. ^ Handbook of Mineralogy
  2. ^ a b Klein, Corneis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., pp. 312–313. ISBN 0-471-80580-7.
  3. ^ Webmineral data
  4. ^ a b Mindat.org
  5. ^ http://www.mindat.org/min-8675.html Mindat
  6. ^ http://www.mindat.org/min-8674.html Mindat
  7. ^ "Potential Toxic Effects of Chromium, Chromite Mining and Ferrochrome Production: A Literature Review" (PDF). May 2012. Retrieved March 15, 2019. {{cite web}}: Cite has empty unknown parameter: |dead-url= (help)
  8. ^ 1906-2005., Hurlbut, Cornelius S. (Cornelius Searle), (1998). Dana's minerals and how to study them. Sharp, W. Edwin., Dana, Edward Salisbury, 1849-1935. (4th ed. ed.). New York: Wiley. ISBN 0471156779. OCLC 36969745. {{cite book}}: |edition= has extra text (help); |last= has numeric name (help)CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  9. ^ Latypov, Rais; Costin, Gelu; Chistyakova, Sofya; Hunt, Emma J.; Mukherjee, Ria; Naldrett, Tony (2018-01-31). "Platinum-bearing chromite layers are caused by pressure reduction during magma ascent". Nature Communications. 9 (1). doi:10.1038/s41467-017-02773-w. ISSN 2041-1723.
  10. ^ a b Fehr, Karl Thomas; Carion, Alain (2004). "Unusual large chromite crystals in the Saint Aubin iron meteorite". Meteoritics & Planetary Science. 39 (S8): A139–A141. doi:10.1111/j.1945-5100.2004.tb00349.x. ISSN 1086-9379.
  11. ^ a b c d "Chromite: Mineral information, data and localities". www.mindat.org. Retrieved 2019-03-16.
  12. ^ Fortier, Y. (1941). "Geology of Chromite". McGill University.
  13. ^ Eric, Rauf Hurman (2014), "Production of Ferroalloys", Treatise on Process Metallurgy, Elsevier, pp. 477–532, doi:10.1016/b978-0-08-096988-6.00005-5, ISBN 9780080969886, retrieved 2019-03-17
  14. ^ "CHROMITE (Iron Chromium Oxide)". www.galleries.com. Retrieved 2019-03-17.
  15. ^ Gu, F; Wills, B (1988). "Chromite- mineralogy and processing". Minerals Engineering. 1 (3): 235. doi:10.1016/0892-6875(88)90045-3.
  16. ^ Guilbert, John M., and Park, Charles F., Jr. (1986) The Geology of Ore Deposits, Freeman, ISBN 0-7167-1456-6
  17. ^ Environmental materials and waste : resource recovery and pollution prevention. Prasad, M. N. V. (Majeti Narasimha Vara), 1953-, Shih, Kaimin,. London. ISBN 9780128039069. OCLC 947118220.{{cite book}}: CS1 maint: extra punctuation (link) CS1 maint: others (link)
  18. ^ a b c M., Duke, J. Ore deposit models 7 : magmatic segregation deposits of chromite. OCLC 191989186.{{cite book}}: CS1 maint: multiple names: authors list (link)
  19. ^ Papp, John F.; Lipin Bruce R. (2006). "Chromite". Industrial Minerals & Rocks: Commodities, Markets, and Uses (7th ed.). SME. ISBN 978-0-87335-233-8.
  20. ^ Routschka, Gerald (2008). Pocket Manual Refractory Materials: Structure - Properties - Verification. Vulkan-Verlag. ISBN 978-3-8027-3158-7.
  21. ^ http://www.mineralszone.com/minerals/chromite.html
  22. ^ Tables of Gemstone Identification By Roger Dedeyne, Ivo Quintens, p.189

External links

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