Chromite

Chromite
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	(Fe, Mg)Cr2O4
Strunz classification	04.BB.05
Crystal symmetry	Isometric hexoctahedral H-M symbol: (4/m3 2/m) Space group: F d3m
Unit cell	a = 8.344 Å; Z = 8
Identification
Color	Black to brownish black; brown to brownish black on thin edges in transmitted light
Crystal habit	Octahedral rare; massive to granular
Crystal system	Isometric
Twinning	Spinel law on {1ll}
Cleavage	None, parting may develop along {111}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5.5
Luster	Submetallic
Streak	Brown
Diaphaneity	Translucent to opaque.
Specific gravity	4.5 - 4.8
Optical properties	Isotropic
Refractive index	n = 2.08-2.16
Other characteristics	Weakly magnetic
References	[1][2][3][4]

This article is about the mineral. For the chromium(III) anion and its salts, see Chromite (compound).

Chromium ore output in 2002

Chromite is an iron chromium oxide: FeCr₂O₄. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MgCr₂O₄);^[5] substitution of aluminium occurs leading to hercynite (FeAl₂O₄).^[6]
It is an industrially important mineral for the production of metallic chromium, used as an alloying ingredient in stainless and tool steels.

Contents

• 1 Occurrence
• 2 Usage
• 3 Mining
  • 3.1 Minor production
• 4 References
• 5 External links

Occurrence

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

Chromite is found in peridotite from the Earth's mantle. It also occurs in layered ultramafic intrusive rocks.^[7] 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.^[8] The Stillwater igneous complex in Montana also contains significant chromite.^[2]

Usage

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.^[9]
Chromite is also used as a refractory material, because it has a high heat stability.^[10]

Mining

In 2002 14,600,000 metric tons of chromite were mined. The largest producers were South Africa (44%) India (18%),^[11] 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.^[12][13]

Minor production

Afghanistan has significant deposits of high grade chromite ore, which is mined illegally in Khost Province and then smuggled out of the country.^[14]
In Pakistan, chromite is mined from the ultramafic rocks in mainly the khanozai area of Pishine District of Balochistan. Most of the chromite is of metallurgical grade with Cr₂O₃ averaging 54% and a chrome to iron ratio of 2.6:1.
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 other alloys.^[15]
In April 2010 the Government of Ontario announced^[16] that they would be opening up a large chromite deposit to development in the northern part of Ontario known as the Ring of Fire.
Australia has a single working chromite mine in the Pilbara region of Western Australia, near the Indigenous community of Jigalong. The mine produces high grade lump chromite in the region of 300,000 tonnes per year.

References

1. Jump up ^ http://www.handbookofmineralogy.com/pdfs/chromite.pdf Handbook of Mineralogy
2. ^ Jump up to: ^{a b} Klein, Corneis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 20th ed., pp. 312-313 ISBN 0-471-80580-7
3. Jump up ^ http://webmineral.com/data/Chromite.shtml Webmineral data
4. ^ Jump up to: ^{a b} http://www.mindat.org/min-1036.html Mindat.org
5. Jump up ^ http://www.mindat.org/min-8675.html Mindat
6. Jump up ^ http://www.mindat.org/min-8674.html Mindat
7. Jump up ^ Gu, F; Wills, B (1988). "Chromite- mineralogy and processing". Minerals Engineering 1 (3): 235. doi:10.1016/0892-6875(88)90045-3.
8. Jump up ^ Guilbert, John M., and Park, Charles F., Jr. (1986) The Geology of Ore Deposits, Freeman, ISBN 0-7167-1456-6
9. Jump up ^ Papp, John F.; Lipin Bruce R. (2006). "Chromite". Industrial Minerals & Rocks: Commodities, Markets, and Uses (7th ed.). SME. ISBN 978-0-87335-233-8.
10. Jump up ^ Routschka, Gerald (2008). Pocket Manual Refractory Materials: Structure - Properties - Verification. Vulkan-Verlag. ISBN 978-3-8027-3158-7.
11. Jump up ^ "Chromites of India".
12. Jump up ^ Papp, John F. "Mineral Commodity Summary 2006: Chromium". United States Geological Survey. Retrieved 2009-02-24.
13. Jump up ^ Papp, John F. "Minerals Yearbook 2006: Chromium". United States Geological Survey. Retrieved 2009-02-24.
14. Jump up ^ http://www.nytimes.com/2012/09/09/world/asia/afghans-wary-as-efforts-pick-up-to-tap-mineral-riches.html?_r=1&nl=todaysheadlines&emc=edit_th_20120909
15. Jump up ^ "How Products are Made, Vol. 1". Retrieved 29 Dec., 2010.
16. Jump up ^ "YouTube - premierofontario's Channel". Youtube. Retrieved 2010-04-13.