Chromite
| Chromite | |
|---|---|
 |
|
| General | |
| Category |
Oxide minerals Spinel group Spinel structural group |
|
Formula (repeating unit) |
(Fe, Mg)Cr2O4 |
| Strunz classification | 4.BB.05 |
| Crystal system | Isometric |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m) |
| Space group | Fd3m |
| 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 {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 | |
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); substitution of aluminium occurs leading to hercynite (FeAl2O4).
It is by far the most industrially important mineral for the production of metallic chromium, used as an alloying ingredient in stainless and tool steels.
Chromite is found as orthocumulate lenses of chromitite in peridotite from the Earth's mantle. It also occurs in layered ultramafic intrusive rocks. 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. The Stillwater igneous complex in Montana also contains significant chromite.
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