Enthalpy of formation of forsterite,enstatite, akermanite,monticellite and merwinite at 1073 K determined by alkali borate solution calorimetry

Enthalpies of solution of synthetic enstatite (Mg₂Si₂O₆), forsterite (Mg₂SiO₄), akermanite (Ca₂MgSi₂O₇), monticellite (CaMgSiO₄), and merwinite (Ca₃MgSi₂O₈) and their component oxides were determined in eutectic (Li, Na)BO₂ at 1073 K. Resulting enthalpies of formation at 1073 are enstatite: $\text{?8.10 ± 0.42}\text{kcal}$; forsterite: $\text{?14.23 ± 0.45}\text{kcal}$; akermanite: $\text{?42.60 ± 0.39}\text{kcal}$; monticellite: $\text{?25.05 ± 0.41}\text{kcal}$; and merwinite: $\text{?51.10 ± 0.49}\text{kcal}$. 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 Pb₂B₂O₅ solution calorimetry of Kiselevaet al. (1979), and are in good agreement with values to be derived from reliable phase equilibrium data in the system MgO-Al₂O₃-SiO₂. The enthalpies of formation of akermanite, monticellite and merwinite are all much less negative than values tabulated by robieet al. (1978) and helgesonet al. (1978) but are shown to be compatible with reliable phase equilibrium data for the system CaO-MgO-SiO₂, whereas the tabulated values are not. Several methods of analysis yield an entropy of monticellite at 1000 K of $\text{69.9 ± 0.2 cal/K}$.