DETERMINATION OF GIBBS FREE-ENERGIES OF FORMATION FOR THE SILICATES MNSIO3, MN2SIO4 AND MN7SIO12 IN THE TEMPERATURE-RANGE 1000-1350-K BY SOLID-STATE EMF-MEASUREMENTS

Standard Gibbs free energies of formation (DELTA(F)G0) for the minerals rhodonite (MnSiO3), tephroite (Mn2SiO4) and braunite (Mn7SiO12) have been determined by measuring the oxygen fugacities of the following redox equilibria: 3Mn2SiO4(s) + 1/2O2(g) reversible 3MnSiO3(s) + Mn3O4(s) MnSiO3(s) + 2Mn3O4(s) + 1/2O2(g) reversible Mn7SiO12(s) and 7MnSiO3(s) + 3/2O2(g) reversible Mn7SiO12(s) + 6SiO2(s) The measurements were performed by means of the solid-state emf technique involving calcia-stabilized zirconia as electrolyte material. The temperature range covered was 1000-1350 K. The results obtained were used to construct a diagram at 1200 K with the axes lgf(O2)-molar ratio Si/(Si + Mn) to indicate the stability ranges of the manganese silicates.