ABSTRACT
Chromium is not a very plentiful element in the Earth’s crust, but it is nevertheless fairly widespread. Its average crustal abundance of 122 ppm (Fortescue, 1980) is less than that of manganese. Average concentrations in rocks range from 4 to 90 mg kg−1, in soil ∼70 mg kg−1, in freshwater ∼1 µg kg−1, and in seawater ∼0.3 µg kg−1 (Bowen, 1979). Its chief mineral occurrence is chromite, in which the chromium has an oxidation state of +3. It is a spinel whose end members are MgCr2O4 and FeCr2O4. The chromium in this mineral can be partially replaced by Al or Fe. Chromite is of igneous origin. Other chromium minerals of minor occurrence include eskolite (Cr2O3), daubréelite (FeS · Cr2S3), crocoite (PbCrO4), uvarovite, which is also known as garnet [Ca3Cr2(SiO4)3], and others (Smith, 1972).
