A thermochemical calculation of the pyroxene saturation surface in the system diopside-albite-anorthite

The pyroxene saturation surface in the system diopside-albite-anorthite may be calculated to ±10°C from thermochemical data over most of its composition range. The thermochemical data used are the experimentally determined enthalpies of mixing of the ternary liquids and the enthalpy of fusion of diopside. These are combined with a mixing model for the configurational entropy in the melt and the activity of CaMgSi₂O₆ in the clinopyroxene, which is less than unity due to departures from CaMgSi₂O₆ stoichiometry. The 'two-lattice' melt model appears to work satisfactorily throughout the pyroxene primary phase field but probably needs modification at more anorthite-rich compositions.