Chromite

Chromite is an iron chromium oxide and the world's sole commercial source of chromium metal. A member of the spinel group, it crystallizes in the cubic system, though well-formed octahedral crystals are rare; it usually occurs as granular masses or disseminated grains.

It is opaque, black to brownish-black, with a metallic to submetallic luster and a distinctive brown streak. Unlike many black oxides, chromite is only weakly magnetic to non-magnetic, which helps distinguish it from magnetite.

Chromite is economically vital, underpinning stainless steel production and the chrome plating, refractory, and pigment industries.

Chromite is a high-temperature mineral that crystallizes early from cooling mafic and ultramafic magmas. As one of the first minerals to solidify, dense chromite grains settle and concentrate at the base of layered intrusions, forming distinct chromitite layers known as stratiform deposits.

The most important examples are the Bushveld Complex of South Africa and the Great Dyke of Zimbabwe. Chromite also forms podiform (pod-shaped) bodies in ophiolite sequences — slices of oceanic crust and mantle thrust onto land — found in Kazakhstan, Turkey, the Philippines, and Albania. Because it is hard and chemically resistant, chromite also accumulates in placer (alluvial) sands.

Chromite is black to brownish-black, opaque, with a metallic to submetallic luster and a diagnostic brown to dark-brown streak (magnetite gives a black streak). It is hard at about 5.5 and feels heavy.

It is at most weakly magnetic, distinguishing it from strongly magnetic magnetite and lodestone. Its association with green ultramafic host rocks such as serpentinite and dunite is a strong contextual clue. Confirming the brown streak on unglazed porcelain is the simplest reliable field test versus magnetite, ilmenite, and hematite.

Chromite is mined almost entirely for chromium, the essential alloying element that makes stainless steel corrosion-resistant. Ferrochrome smelted from chromite goes into a vast range of steels and superalloys.

Beyond metallurgy, high-grade chromite serves as a refractory material for furnace linings because of its high melting point, as a foundry sand for casting, and as the raw material for chromium chemicals and pigments. It has essentially no gemstone or jewelry use, though it is the indirect source of the chromium that colors rubies, emeralds, and chrome diopside.