Chromite

Chromite is a mineral from the spinel group, made up mostly of iron(II) oxide and chromium(III) oxide, with the formula FeCr2O4. Magnesium can replace iron in varying amounts, forming a solid solution with magnesiochromite (MgCr2O4). Aluminium can also substitute for chromium, producing hercynite (FeAl2O4). Today, chromite is mainly mined to create stainless steel, via the production of ferrochrome (FeCr), an iron-chromium alloy.

Chromite grains are typically found in large mafic igneous intrusions, such as the Bushveld in South Africa and deposits in India. The mineral is iron-black, has a metallic luster, a dark brown streak, and a Mohs hardness of 5.5.

**Properties** Chromite occurs mostly in mafic-ultramafic igneous intrusions and sometimes in metamorphic rocks. It forms layered deposits that can stretch hundreds of kilometers long but are only a few meters thick. Chromite is also common in iron meteorites, where it appears alongside silicates and troilite.

**Crystal structure** The composition FeCr2O4 has iron in the +2 oxidation state and chromium in the +3 state. The mineral’s structure is platy, with breakage along weak planes. In thin sections, chromite appears as disseminated, euhedral to subhedral crystals. It contains magnesium, ferrous iron, aluminium, and trace titanium. Because chromite belongs to the spinel group, it forms complete solid solution series with other group members, including chenmingite (FeCr2O4), xieite (FeCr2O4), magnesiochromite (MgCr2O4), and magnetite (Fe²⁺Fe³⁺₂O₄). Chenmingite and xieite are polymorphs of chromite, while magnesiochromite and magnetite are isostructural with it.

**Crystal size and morphology** Chromite appears as massive and granular crystals; octahedral crystals are very rare. Twinning follows the spinel law on the {III} plane. Grains are usually small, but some up to 3 cm have been found. These large grains crystallize from the liquid of a meteorite body under low chromium and oxygen conditions, associated with stable supersaturation.

**Reactions** Chromite helps determine rock formation conditions. It reacts with gases like CO and CO₂, reducing the chromite and forming iron and chromium alloys, as well as possible metal carbides. Chromite crystallizes early in the cooling process, making it resistant to alteration from high temperatures and pressures during metamorphism. It can pass through the metamorphic series unchanged, while less resistant minerals alter to serpentine, biotite, or garnet.

**Distribution of deposits** Chromite occurs as orthocumulate lenses in peridotite from Earth’s mantle, in layered ultramafic intrusive rocks, and in metamorphic rocks like some serpentinites. Ore deposits form as early magmatic differentiates, often associated with olivine, magnetite, serpentine, and corundum. The Bushveld Igneous Complex in South Africa has layers up to 90% chromite, forming the rare rock chromitite. The Stillwater Igneous Complex in Montana also holds significant chromite. Commercially viable deposits are limited to a few large sites. Two main types exist: stratiform deposits (in layered intrusions, providing 98% of global reserves) found in South Africa, Canada, Finland, and Madagascar; and podiform deposits found in Kazakhstan, Turkey, and Albania. Zimbabwe is the only country with notable reserves of both types.

**Stratiform deposits** These form as large sheet-like bodies in layered mafic to ultramafic igneous complexes, typically Precambrian in age and located in cratons. The intrusions likely entered continental crust containing granites or gneisses. They are tabular or funnel-shaped. Tabular intrusions (e.g., Stillwater and Bird River) occur as sills with parallel layering. Funnel-shaped intrusions (e.g., Bushveld and Great Dyke) dip toward the center, forming synclines. Chromite appears in multiple layers of chromitite, each 1 cm to 1 m thick, extending laterally up to 70 km. Chromitite makes up 50–95% of these layers.