Geochemistry of braunite and associated phases in metamorphosed non-calcareous manganese ores of India

Abstract

In the metamorphosed manganese oxide ores of India, braunite is ubiquitous in all assemblages from chlorite to sillimanite grades. Chemical analyses of braunite from different prograde assemblages confirm the presence of a fixed R⁺² (=Mn⁺²+Mg+Ca) SiO₃ molecule in the mineral. Element partitioning between coexisting braunite and bixbyite indicates a near-ideal mixing of Fe⁺³/ -Mn⁺³ in the phases. This also indicates that braunite became relatively ferrian while equilibrating with associated phases such as bixbyite, hollandite and jacobsite during prograde reactions. Petrogenetic studies show that as a general trend, prograde lower oxide phases appeared by deoxidation of higher oxide phases. But braunite, a more reduced phase than bixbyite, appeared early from deoxidation of pyrolusite in presence of quartz. Bixbyite could appear later from the reacting pyrolusite-braunite-quartz assemblage. Inferred mineral reaction paths and the general trend of pro-grade deoxidation reactions suggest that the composition of ambient fluid phase was internally buffered during metamorphism.