Detrital chrome-spinel grains in heavy-mineral sand deposits from southeast Africa

Detrital chrome-spinels are contaminant grains within ilmenite concentrates produced from heavy-mineral deposits along the coast of southeast Africa. The presence of even minor levels of chromia in the predominantly ilmenite-rich concentrates, downgrades their market value as potential feedstocks for the production of titania pigment. An understanding of their composition can assist in their removal from the ilmenite concentrates. Compositions from a database of close to 900 chrome-spinel analyses shows the major element components and their ranges (in wt.%) are: Cr: 0.4-45.3, Al: 0.0-31.0, Fe: 8.5-69.6 and Mg: 0.0-12.2. Minor components include Ti: 0.1-11.4 and Zn: 0.0-13.7. The chrome-spinel data fall into two compositionally distinct groups. The first group of spinels is dominated by a strong trend reflecting the mutual substitution between Al3+ and Cr3+ in the spinet structure. The second group of spinels is characterized by compositions containing abundant Fe3O4 magnetite component. The clear division between chrome-spinel compositional types indicates the grains are derived from at least two chemically dissimilar provenances. The compositional differences between the chronic-spinet groups has a positive impact on subsequent ilmenite upgrading treatments as the spinels which contain the highest magnetite component are easily removed via low-intensity magnetic separation procedures.