Chromite - Encyclopedia

    Class : Oxides and hydroxides
    Subclass : Oxides
    Crystal system : Cubic
    Chemistry : FeCr2O4
    Rarity : Common


Chromite is a mineral from the spinel group whose composition is variable, with iron and aluminum being able to replace chromium. It is an accessory mineral of peridotites and serpentinites which derive from them, independent rocks (ophiolites), or constituting beds within stratified mafic massifs. The chromite forms stratiform disseminations and lenticular clusters formed by magmatic differentiation during crystallization : the very dense chromite (nearly 5), "sinks" and is deposited at the bottom of the magma chamber, notably in the company of olivine, and sometimes platinum. Chromite, unalterable, also concentrates in detrital deposits of the placer type. It owes its name to its chemical composition. Chromite usually occurs in black xenomorphic grains, with a metallic luster, forming compact granular masses ; the crystals, octahedra or sometimes truncated cubes, are very rare. Its use will depend on its composition : a Cr/Fe ratio greater than 1 and the chromite will be of metallurgical quality and will then constitute the only chrome ore, a high Al content and the chromite will be unsuitable for metallurgy but used as a refractory.

Main photo : Chromite from Hangha, Kenema District, Eastern Province, Sierra Leone © Rob Lavinsky

Chromite in the World

The Scottish islands of Shetland have produced crystals larger than a centimeter; centimeter-sized crystals are also found in Mutorashanga (Zimbabwe). Octahedra of 2 cm are known from Hangha (Sierra Leone) while millimetric crystals are found in many localities.

Chromite in France

The largest chromite crystals reach 5 cm and come from Tiebaghi (New Caledonia). Millimeter crystals are reported at Peumerit (Finistère).

Right photo : Chromite from Pic du Champ-de-Bataille, New Caledonia © Cédric Gineste

Twinning

Twins are known on [111].

Fakes and treatments

No fake identified for this mineral species.



Hardness : 5.5
Density : 4.5 to 4.8
Fracture : Irregular
Streak : Brown



TP : Translucent to opaque
RI : 2.08 to 2.16
Birefringence : 0
Optical character : None
Pleochroism : None
Fluorescence : None


Solubility : Insoluble


Magnetism : Paramagnetic
Radioactivity : None

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