Abstract Enthalpies of solution of synthetic enstatite (Mg 2Si 2O 6), forsterite (Mg 2SiO 4), akermanite (Ca 2MgSi 2O 7), monticellite (CaMgSiO 4), and merwinite (Ca 3MgSi 2O 8) and their component oxides were determined in eutectic (Li, Na)BO 2 at 1073 K. Resulting enthalpies of formation at 1073 are enstatite: −8.10 ± 0.42 kcal ; forsterite: −14.23 ± 0.45 kcal ; akermanite: −42.60 ± 0.39 kcal ; monticellite: −25.05 ± 0.41 kcal ; and merwinite: −51.10 ± 0.49 kcal . The value for the synthetic monticellite of composition Mo .965Fo .035 was corrected slightly for non-stoichiometry based on experimental monticellite-forsterite phase equilibrium relations. The enthalpies of formation of enstatite and forsterite are somewhat less negative than yielded by several other solution calorimetric studies but are in good agreement with the recent Pb 2B 2O 5 solution calorimetry of Kiseleva et al. (1979), and are in good agreement with values to be derived from reliable phase equilibrium data in the system MgO-Al 2O 3-SiO 2. The enthalpies of formation of akermanite, monticellite and merwinite are all much less negative than values tabulated by robie et al. (1978) and helgeson et al. (1978) but are shown to be compatible with reliable phase equilibrium data for the system CaO-MgO-SiO 2, whereas the tabulated values are not. Several methods of analysis yield an entropy of monticellite at 1000 K of 69.9 ± 0.2 cal/K.