A thermodynamic interretation of the zonal arrangement of minerals in the bedded manganese deposits of the noda-tamagawa mine, Japan

Abstract

Calculated phase relations in the system MnOSi0₂-C02-0₂ were used to propose a thermodynamic explanation for the thermal metamorphism of rhodochrosite beds lying between chert strata. The metamorphic MnOSi0₂ minerals are arranged in order quartz(chert), rhodonite. tephroite and manganosite-hausmannite-pyrochroite rhodonite across the ore bed. The calculation covered temperatures up to 1000 K and pressures up to 5 kb. The zoning was interpreted as the result of a continuous rise in metamorphic temperature. The equilibrium partner of rhodochrosite changed from rhodonite through manganosite. Across the ore bed there are gradients in the chemical potential of MnO and SiO₂ but fugacities of volatlle components such as C0₂. 0₂ and H₂0 were probably uniform at any given time and location during formation of the zones. Assuming that the total pressure and the fugacity of C0₂ were at 1.4 kb and 1.01 b. respectively. rhodonite. tephroite and manganosite would have formed at 472. 478 and 629 K.