Chromite
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). A substitution of the element aluminium can also occur, leading to hercynite (FeAl2O4). Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.
| 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 |
| 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 | |
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.