Abstract: Chromites from two horizons of the Sukinda area (India) marked as ‘grey ore’ and ‘brown ore’ zones have been studied by ⁵⁷Fe Mössbauer spectroscopy, which revealed that both chromite types are oxidised and have a type of disordered spinel structure in which octahedral sites are occupied by Fe²⁺ ions.

The spectra of the grey ore sample can be fitted to three doublets corresponding to Fe²⁺ (A), Fe³⁺ (A) and Fe²⁺ (B) sites. This sample is less oxidised than the brown ore, in which progressive oxidation in the magmatic (?) stage led to the complete conversion of Fe²⁺ in A sites to Fe³⁺. The spectra of the brown ore are characterised by two doublets correpsonding to two tetrahedral (A) sites of Fe³⁺ with different next-nearest neighbour configurations and a third doublet for Fe²⁺ at the B site. The brown ores have higher chromium and Fe³⁺ content and have lesser amounts of Ni and Al in comparison to the grey ores. Megascopically, the former shows larger crystal sizes. The high Fe³⁺ content in the brown ore suggests that this type of chromite was formed in a region of high ƒ_O2 in the magmatic environment. This perhaps occurred at the part of the mantle where the temperature was higher and the rate of cooling was slower than that of the grey ores which crystallised in the magmatic melt.