1911 Encyclopædia Britannica/Chromite
CHROMITE, a member of the spinel group of minerals; an oxide of chromium and ferrous iron, FeCr2O4. It is also known as chromic iron or as chrome-iron-ore, and is the chief commercial source of chromium and its compounds. It crystallizes in regular octahedra, but is usually found as grains or as granular to compact masses. In its iron-black colour with submetallic lustre and absence of cleavage it resembles magnetite (magnetic iron-ore) in appearance, but differs from this in being only slightly if at all magnetic and in the brown colour of its powder. The hardness is 5½; specific gravity 4.5. The theoretical formula FeCr2O4 corresponds with chromic oxide (Cr2O3) 68%, and ferrous oxide 32%; the ferrous oxide is, however, usually partly replaced by magnesia, and the chromic oxide by alumina and ferric oxide, so that there may be a gradual passage to picotite or chromespinel. Much of the material mined as ore does not contain more than 40 to 50% of chromic oxide. In the form of isolated grains the mineral is a characteristic constituent of ultrabasic igneous rocks, namely the peridotites and the serpentines which have resulted from their alteration. It is also found under similar conditions in meteoric stones and irons. Often these rocks enclose large segregated masses of granular chromite. The earliest worked deposits were those in the serpentine of the Bare Hills near Baltimore, Maryland, U.S.A.; it was also formerly extensively mined in Lancaster county, Pennsylvania, and is now mined in California, as well as in Turkey, the Urals, Dun Mountain near Nelson in New Zealand, and Unst in the Shetlands.
Chrome-iron-ore is largely used in the preparation of chromium compounds for use as pigments (chrome-yellow, &c.) and in calico-printing; it is also used in the manufacture of chrome-steel. (L. J. S.)