Mn-olivine from Gangpur Group of rocks, Orissa

Mn-olivine (tephroite) bearing assemblage is often recorded in manganese silicate-carbonate-oxide rocks metamorphosed to green schist facies condition in Precambrian Gangpur Group, Orissa. Divalent Mn and Fe dominate this assemblage with most phases having high Mn/Fe ratios. Minerals in the assemblage include near end - member tephroite (Tp(90.33-90.95)Fa(4.76-8.40)Fo(0.55-4.71)), rhodochrosite, pyroxmangite, jacobsite, spessartine dominated garnet with minor quartz, apatite and barite. Tephroite is sometimes altered to either Mn-rich minnesotaite or to a sepiolite related mineral that resembles serpentine in Mg-olivine rocks. This assemblage is very likely evolved through decarbonation-oxidation processes of a carbonatic precursor under a relatively low oxygen fugacity condition.