Chromite investigations of the U. S. Geological Survey since 1939 indicate that economic deposits of chromite rich in normative spinel ((Mg, Fe)Al ₂ O ₄ ) occur in peridotites which are closely associated with gabbro; high-chrome chromites occur in feldspar-free peridotites; and chromites rich in both normative chromite ((Mg, Fe)Cr ₂ O ₄ ) and magnetite (FeFe ₂ O ₄ ) are found in the pyroxene-rich stratiform complexes. Reaction of high-alumina chromite with diopsidic gabbro to form anorthite, olivine, and enstatite is described, and some probable equilibria between chromite and silicate minerals are discussed.

A plot of 63 complete analyses on a triangular prism of composition shows that chromites from different geologic provinces in the Western Hemisphere vary consistently and fall in overlapping, but distinct, fields. The chromites of the Caribbean Province, which occur in dunite and troctolite, have the general formula Cr (sub 25-80) Al (sub 15-75) Fe (sub 1-5) (Mg (sub 65-75) ) and average about Cr ₅₀ Al ₄₇ (Mg ₇₀ ). The chromites of the Pacific Coast, which occur in dunite and saxonite, average about Cr ₇₀ Al ₂₄ (Mg ₆₀ ) in the range Cr (sub 60-77) Al (sub 16-35) Fe (sub 3-12) (Mg (sub 45-75) ).

The stratiform deposits of the Stillwater and Bushveld complexes occur in harzburgite and bronzitite, and are very much alike, averaging Cr ₆₀ Al ₃₂ (Mg ₅₂ ) and Cr ₅₈ Al ₃₄ (Mg ₄₄ ) respectively. Chromites from gabbroic facies of the Bushveld complex are much richer in normative magnetite, presumably reflecting a higher CaO: Al ₂ O ₃ ratio in the silicate environment.