chromite

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The only ore mineral of chromium. Chromite is jet black to brownish black, has a submetallic luster. It belongs to the spinel group of minerals and has cubic symmetry. Naturally occurring chromite has the general formula (Mg,Fe²⁺)(Cr,Al,Fe³⁺)₂O₄ and ranges from 15 to 64 wt % Cr₂O₃, with minor amounts of nickel, titanium, zinc, cobalt, and manganese. The specific gravity of chromite ranges from 4 to 5, depending on its composition. Pure chromite, Fe²⁺Cr₂O₄, is extremely rare in nature and has been found only in meteorites.

Chromite has a variety of uses. Chromium is extracted from it to make stainless steel and other alloys for which resistance to oxidation and corrosion is important. Chromium is also used as a plating and tanning agent. The mineral chromite is made into refractory lining for steel-making furnaces. See also Chromium; Spinel; Stainless steel.

Environment

Magmatic segregations and in basic rocks.

Octahedral crystals usually small and inconspicuous. Generally massive and granular.

Black. Luster submetallic; hardness 5Ɖ; specific gravity 4.1-4.9; streak brown; fracture uneven; cleavage none. Brittle; sometimes slightly magnetic.

A ferrous chromic oxide (68.0% Cr ₂ O ₃ , 32.0% FeO).

Infusible on charcoal, but gives green color to cooled borax beads.

Distinguished from magnetite by its weak magnetism and from spinel by its dark streak and lesser hardness. Commonly associated with green minerals (uvarovite garnet) and the purple chlorite kaemmererite (H ₈ [Mg,Fe,Cr] ₅ Al ₂ Si ₃ O ₁₈ ).

Chromite is the only ore of chromium, and a valuable refractory. Sometimes found as isolated crystals in veins in or scattered through serpentine; but the economically important occurrences are in more or less pure lenses, perhaps magmatic segregation lenses in altered basic rocks. It is also sparsely disseminated through basic rocks as an accessory mineral. Crystals almost 1 in. (2 cm) long have been found in Sierra Leone and lately in Brazil.

Minute crystals are found in the serpentines near New York City (Hoboken and Staten I.), in Maryland, and embedded in pyrrhotite at Outokumpu, Finland, accompanied by the rare (found only there) rhombohedral equivalent of hematite, eskolaite (Cr ₂ O ₃ ). Small economically workable deposits have been found in Maryland, North Carolina, and California. Russia, India, Africa, Turkey, Brazil, Cuba, and New Caledonia have commercially important deposits that can be worked for ore.

Although today the U.S. produces very little chromite, for many years a mine in Maryland was the world's only producing locality. At that time it was used solely as a pigment and for tanning. To date there is no such thing as a really good mineral specimen of chromite, relatively abundant though it is.

Chromite
General
Category	Mineral
Chemical formula	iron magnesium chromium oxide: (Fe, Mg)Cr2O4
Identification
Color	Black to brownish black
Crystal habit	Octahedral rare; massive to granular
Crystal system	Isometric; hexoctahedral
Cleavage	absent
Fracture	Conchoidal
Mohs scale hardness	5.5
Luster	Submetallic
Streak	Dark brown
Specific gravity	4.5 - 4.8
Refractive index	Subtranslucent to opaque
Fusibility	Infusible
Other characteristics	Weakly magnetic
Major varieties

Chromite is iron magnesium chromium oxide: (Fe, Mg)Cr₂O₄. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts; also, aluminium and ferric iron commonly substitute for chromium.

Contents 1 Occurrence 2 Uses 3 Mining 3.1 Minor production 4 References 5 External links

Occurrence

Chromite is found in peridotite from the Earth's mantle. It also occurs in layered ultramafic intrusive rocks.^[1] 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.

Uses

Chromite from Albania

Chromite is also used as a refractory material, because it has a high heat stability.^[2]

The only ore of chromium is the mineral chromite. 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 a 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.^[3]

Mining

Chromium ore output in 2002

In 2002 14,600,000 metric tons of chromite have been mined. The largest producers have been South Africa (44%) India (18%), Kazakhstan (16%) Zimbabwe (5%), Finland (4%) Iran (4%) and Brazil (2%) with several other countries producing the rest of less than 10% of the world production. ^[4][5]

Minor production

In Pakistan, Chromite is mined from the ultramafic rocks in mainly the Muslim Bagh area of Zhob District of Balochistan. Most of the chromite is of metallurgical grade with Cr₂O₃ averaging 40% and a chrome to iron ratio of 2.6:1. Afghanistan has significant deposits of high grade Chromite ore.

Recently, the biggest user of Chromite ore has been China, importing large quantities from South Africa, Pakistan and other countries. The concentrate is used to make ferrochromium, which is in turn used to make stainless steel and some others alloys.

References

1. ^ Gu, F (1988). "Chromite- mineralogy and processing". Minerals Engineering 1: 235. doi:10.1016/0892-6875(88)90045-3. 
2. ^ Routschka, Gerald (2008). Pocket Manual Refractory Materials: Structure - Properties - Verification. Vulkan-Verlag. ISBN 9783802731587. http://books.google.de/books?id=0ugkNtlWKWEC&pg=PA205. 
3. ^ Papp, John F.; Lipin Bruce R. (2006). "Chromite". Industrial Minerals & Rocks: Commodities, Markets, and Uses (7th ed.). SME. ISBN 9780873352338. http://books.google.de/books?id=zNicdkuulE4C&pg=PA309. 
4. ^ Papp, John F.. "Mineral Commodity Summary 2006: Chromium". United States Geological Survey. http://minerals.usgs.gov/minerals/pubs/commodity/chromium/chrommcs06.pdf. Retrieved 2009-02-24. 
5. ^ Papp, John F.. "Minerals Yearbook 2006: Chromium". United States Geological Survey. http://minerals.usgs.gov/minerals/pubs/commodity/chromium/myb1-2006-chrom.pdf. Retrieved 2009-02-24. 

• Dana's Manual of Mineralogy ISBN 0-471-03288-3
• Guilbert, John M., and Park, Charles F., Jr. (1986) The Geology of Ore Deposits, Freeman, ISBN 0-7167-1456-6

External links

• Mineral Galleries: Chromite
• Webmineral.com
• Minerals.net
• USGS info.

Please help improve this article by expanding it. Further information might be found on the talk page. (September 2008)

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