A coupled key phase diagram study and critical evaluation and optimization of all available experimental data of the Li2O-MgO-SiO2 system was performed to obtain a set of Gibbs energy functions to reproduce all the reliable phase equilibria and thermodynamic data. Differential scanning calorimetry measurements and equilibration/quenching experiments were performed in the Li2SiO3-MgO and Li4SiO4-Mg2SiO4 sections, respectively, using sealed Pt capsules to prevent the volatile loss of Li. According to the present experimental results, Li2MgSiO4 is the only compound present in the Li4SiO4-Mg2SiO4 isopleth, which shows a peritectic melting at 1465 +/- 6 degrees C (1738 +/- 6 K). The Modified Quasichemical Model, which considers short-range ordering in the melt, was employed to describe the thermodynamic properties of the liquid phase. The Li4SiO4-Li2MgSiO4 and Mg2SiO4-rich solid solutions were modeled using the Compound Energy Formalism.