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Crystalline mineral
the mineral

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 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.

Chromite minerals are mainly found in mafic ultramafic igneous intrusions and are also sometimes found in metamorphic rocks. The chromite minerals occur in layered formations that can be hundreds of kilometres long and a few meters thick. Chromite is also common in iron meteorites and form in association with silicates and troilite minerals.

The chemical composition of chromite can be represented as FeCr2O4, with the iron in the +2 oxidation state and the chromium in the +3 oxidation state. Chromite contains Mg, ferrous iron [Fe(II)], Al and trace amounts of Ti. Chromite is seen to form early in the crystallization process. This allows for chromite to be resistant to the alteration effects of high temperatures and pressures seen in the metamorphic series. It is able to progress through the metamorphic series unaltered. Other minerals with a lower resistance are seen to alter in this series to minerals such as serpentine, biotite and garnet.

Chromite is found as orthocumulate lenses 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, forming the rare rock type chromitite (cf. chromite the mineral and chromitite, a rock containing chromite). The Stillwater Igneous Complex in Montana also contains significant chromite.