Abstract
The reaction MnCO3 + SiO2 = MnSiO3 + CO2 was investigated at 1-kbar pressure at temperatures between 390 and 485 °C by using hydrothermal techniques in a CO2-H2O mixture. Equilibrium conditions for the reaction, determined from reversed experiments, were used to derive an entropy 113 ± 6 J/(mol K) and a Gibbs free energy of formation of −1237 ± 3 kj/mol for the manganese silicate phase MnSiO3 (pyroxenoid). Metastable pyroxmangite was obtained in the decarbonation reaction. Shape and surface textures of experimental products suggest a dissolution-(re)precipitation mechanism for the reaction. The obtained T-XCO2 curve indicates nonideal-mixing behavior of the CO2-H2O fluid at the experimental P-T conditions.
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