Thermodynamic Optimization of the MgO-MnO-Mn2O3-SiO2 System

English Abstract:

The MgO-MnO-Mn₂O₃-SiO₂ system is of great importance in metallurgy and geochemistry. It contains many important minerals of geological interest such as the clino-, ortho-, and proto-pyroxenes (MgSiO₃), the pyroxenoids rhodonite (MnSiO₃) and pyroxmangite (MgSiO₃-MnSiO₃), olivine (Mg₂SiO₄-Mn₂SiO₄) and braunite (Mn₇SiO₁₂), some of which being major constituents of the Earth’s mantle. In order to develop a self-consistent thermodynamic database for the above system at 1 atm total pressure, all phase diagram (e.g. Fig 1a) and thermodynamic data (e.g. Fig 1b) available in the literature were critically evaluated. The Gibbs energies of liquid and solid solutions were described using thermodynamic models reflecting the structures of solutions. The liquid solutions were described using the Modified Quasichemical Model (MQM) while the solid solutions, such as the pyroxenes and olivine, were parametrized using the Compound Energy Formalism (CEF). For rhodonite, pyroxmangite and monoxide solid solutions, a simple Bragg Williams random mixing polynomial model was used to describe the solutions. The complex phase relationships in the system have been elucidated, and discrepancies among the data have been resolved. The database of the model parameters can be used along with Gibbs energy minimization software in order to calculate any phase diagram section or thermodynamic property.