Chromite is a black chromium-bearing mineral, FeCr2O4. Chromite is a principal ore of chromium.
Chromite is a member of the spinel group. These minerals share the same structure but differ in composition. Al and Mg in spinel are replaced by chromium and iron respectively in the crystal structure of chromite.
The appearance of chromite may be similar to magnetite (which also belongs to the spinel group but has slightly different structure). Both are opaque and occur in octahedral crystals (double pyramids with eight crystal faces). However, both minerals occur more often as anhedral or granular masses. It isn’t hard to distinguish chromite from magnetite because chromite may be only weakly magnetic while magnetite shows very strong affinity to magnets. Pure chromite is rare. It usually contains magnesium which replaces iron. Other elements like trivalent iron, Zn, Mn, and Al may be present as well. Chromite that contains more magnesium than iron is named magnesiochromite. There is continuous solid solution between the two and most commonly natural chromites are somewhere in the middle between the two endmembers but more on magnesiochromite side. Hence, most chromites are probably not chromites sensu stricto.
Chromite is relatively resistant to weathering. It may be important constituent of sand but not everywhere. Chromite is found in mafic and ultramafic rocks like pyroxenite and peridotite (including dunite). These rocks are widespread deep below in the mantle but they are not common rocks near the surface. Best chances to find chromite in sand is near ophiolites. These are assemblages of rocks which once formed ocean floor but are now tectonically pushed on top of the continental landmasses. Chromite is also found in some meteorites.
Chromite in igneous rocks may form bands or layers which are very important mineral resources. These layers are the result of a crystal settling in a magma chamber. Such chromite deposits are for example Bushveld complex in South Africa and the Great Dike in Zimbabwe. Chromite is among the first minerals to crystallize out of magma. Important chromite mining countries are Kazakhstan, South Africa, Zimbabwe, and India. Chrome is needed for a variety of purposes but it is mostly used to make stainless steel.
Chromite crystals picked out of a chromite concentrate from the Rustenburg mine, Western Bushveld, South Africa. The width of the view is 5 mm.
Serpentinized chromite ore from The Zhob Valley, Pakistan. Green is serpentine, black is chromite. This is a chromite rich cumulate rock associated with ultramafic rocks. The width of the hand sample is 6 cm. The hand sample belongs to the Museum of Geology of the University of Tartu.
You can take a much closer look into similar rock from the Stillwater complex in Montana. Here are thin section images and gigapans by Ron Schott.
