Chromite, iron magnesium chromium oxide: (Fe,Mg)Cr2O4, is an oxide mineral belonging to the spinel group. Magnesium is always present in variable amounts, also aluminium and iron substitute for chromium. Chromite is found in peridotite and other layered ultramafic intrusive rocks and also found in metamorphic rocks such as serpentinites. Ore deposits of chromite form as early magmatic differentiates. It is commonly associated with olivine, magnetite, serpentine, and corundum.
Chromium is a steel-grey, lustrous, hard metal that is hard, brittle, and corrosion resistant, and takes a high polish. Chromium is mined as chromite (FeCr2O44) ore. Roughly half the chromite ore in the world is produced in South Africa. Substantial deposits are also found in Kazakhstan, Zimbabwe, Russia, New Zealand, Turkey, Iran, Albania, Finland, Democratic Republic of Madagascar and the Philippines. The metal is usually produced by reducing the oxide with aluminium. Chromium is used to manufacture stainless steel, harden steel, and form many useful alloys. It is mostly used in plating to produce a hard, beautiful surface and to prevent corrosion. Chromium gives glass an emerald green colour and is widely used as a catalyst. The refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stability of crystalline structure.
All compounds of chromium are coloured. The most important chromates are those of sodium and potassium, the dichromates, and the potassium and ammonium chrome alums. The dichromates are used as oxidizing agents in quantitative analysis, also in tanning leather. Other compounds are of industrial value; lead chromate is chrome yellow, a valued pigment. Chromium compounds are used in the textile industry as mordants, and by the aircraft and other industries for anodizing aluminium. Rubies and emeralds also owe their colours to chromium compounds. Potassium dichromate (K2Cr2O7) is used in the tanning of leather while other chromium compounds are used as mordants, materials which permanently fix dyes to fabrics. Chromium compounds are also used to anodize aluminium, a process which coats aluminium with a thick, protective layer of oxide. Chromite, chromium's primary ore, is used to make moulds for the firing of bricks because of its high melting point, moderate thermal expansion and stable crystal structure.
