A complete literature review, critical evaluation, and thermodynamic optimization of phase diagrams and thermodynamic properties of the MgO-MnO-Mn2O3-SiO2 system at 1atm pressure are presented. The molten oxide phase was described by the Modified Quasichemical Model considering the short-range ordering in molten oxide, and the Gibbs energies of solid solutions were described using the Compound Energy Formalism considering the crystal structure of each solid solution. A set of optimized model parameters of all phases was obtained which reproduces all available and reliable thermodynamic data and phase diagrams within experimental error limits from 25 degrees C to above the liquidus temperatures over the entire range of composition under the oxygen partial pressures from metallic saturation to 1atm. The database of the model parameters can be used along with software for the Gibbs energy minimization to calculate any phase diagram section and thermodynamic property within the present system.