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Mineral chemistry and petrological data of chromites from chromitite bands in the N–S trending schist belt of Nuggihalli (southern Karnataka, India), belonging to the Dharwar craton of South India, are presented in this paper. Crystal chemical data indicate a komatiitic affinity of the chromitite. P–T calculations of the chromite-hosting peridotites yielded a pressure range of 13 to 28 kbar and temperatures ranging from 775 to 1080 °C; the oxygen fugacity (log fO2) varies from +0.5 to +1.6 above the QFM buffer. The P, T and fO2 data indicate that Nuggihalli chromitites crystallized in an environment akin to the upper mantle. The studied samples also show partial resetting; the lower temperatures ranging from 515 to 680 °C are ascribed to subsequent metamorphism of the area.

In the Nuggihalli schist belt (India), chromite bodies were affected by intense serpentinization followed by weathering. In spite of the strong oxidation of the chromite grains, some unaltered cores were preserved, and they were characterised using electron probe microanalysis, single-crystal X-ray diffraction and low temperature 57Fe Mossbauer spectroscopy. Results of such investigations revealed that chromite cores from the Nuggihalli schist belt are effectively not oxidised, and their study revealed that chromite was not affected by greenschist-facies metamorphism. The apparently highly ordered cation distribution of the chromites, which would suggest low equilibration temperature, is just caused by the very high Cr contents, which prevent Mg–Al exchange between T and M sites. Using Fabries (1979) geothermometer a temperature of about 1180 °C was retrieved (Mitra and Bidyananda, 2003), which therefore corresponds to the quenched-in igneous equilibrium.

Several samples of Cr-bearing spinel from Indian ophiolites have been studied by X-ray single crystal diffraction and structure refinement, electron probe microanalyses and Mössbauer spectroscopy. Differences between samples coming from massive chromitite bands and those coming from podiform chromitite deposits have been evidenced: the former have (Mg,Fe)-chromite component ≥ 75%, the latter from 69 to 74%. In both cases the complementary components are aluminate (mainly spinel sensu stricto MgAl2O4). As magnesiochromite (MgCr2O4) is always dominant with respect to chromite (FeCr2O4), the studied samples are classified as magnesiochromite. The multi-analytical approach has revealed that some of the studied spinels are non-stoichiometric (due to Fe2+ oxidation and vacancy formation), with those coming from massive chromitite being more oxidised than those from the pods. Comparison between our results and recent literature suggested that spinel oxidation is more common than usually believed: it is not restricted to ophiolites but may occur in different geological settings and may help reconstructing the complex thermo-oxidative history of the host rock. Consequently, the proposed multi-analytical approach is crucial for an accurate crystal chemical characterisation of spinels and other Fe-bearing mineral phases, especially when geothermobarometric calculations have to be performed.