Chromite

Chromite
Octahedral chromite crystal from the Freetown Layered Complex in Sierra Leone, Africa (size: 1.3 x 1.2 x 1.2 cm)
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	(Fe, Mg)Cr2O4
IMA symbol	Chr[1]
Strunz classification	4.BB.05
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m (no. 227)
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
Twinning	Spinel law on {Ill}
Cleavage	None, parting may develop along {III}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5.5
Luster	Resinous, Greasy, Metallic, Sub-Metallic, Dull
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	[2][3][4][5]

Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the 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).^[6] A substitution of the element aluminium can also occur, leading to hercynite (FeAl₂O₄).^[7] Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.^[8]

Chromite grains are commonly found in large mafic igneous intrusions such as the Bushveld in South Africa and India. Chromite is iron-black in color with a metallic luster, a dark brown streak and a hardness on the Mohs scale of 5.5.^[9]