The mineral called chromite is a member of the spinellid group of minerals which crystallize in the isometric system. The spinellids have the general formula R″O · R‴2O3. Chromite, however, is seldom found as a pure double oxide. Replacement of FeO by MgO and Cr2O3 by either or both A12O3 and Fe2O3 gives rise to varietal types and chief among these are chrompicotite, chrom-hercynite, magnesiochromite (mitchellite), picotite and chrom-iferous spinels. The mineral called chromite can seldom be expressed by a definite formula. It is usually a mixture of the varietal types cited. There is a strong probability that the spinellids, magnetite, magnesioferrite and others enter into its chemical makeup.