A thermodynamic model for subsolidus equilibria in the system CaOMgOAl 2O 3SiO 2

Abstract

This study integrates phase equilibrium and calorimetric data for the following phases in the CaOMgOAl 2O 3SiO 2 system: clinopyroxene, orthopyroxene, garnet, fomente, anorthite, spinel, kyanite, sillimanite, corundum and quartz. The internally consistent data set includes mixing properties for clinopyroxene, orthopyroxene, and garnet solid solutions and enables calculation of any equilibrium involving the components of the above 10 phases under any desired P, T conditions. Major conclusions of the study (apart from the data set) are: 1. (1) Spinel appears to have an enthalpy of Mg-Al exchange on the order of 20 Kcal or more, substantially above the calorimetrically measured value. 2. (2) CaAl 2SiO 6 pyroxene has virtually complete Al-Si disorder. 3. (3) Enstatite has an enthalpy of formation from oxides at 1000 K of about −16840 cal/mole, substantially greater than the calorimetric value of −17620 ± 340 cal. 4. (4) Anorthite has about 0.6 cal/K entropy of Al-Si disorder. 5. (5) CMAS garnets of between Pyrope 100 and Pyrope 80Grossular 20 composition have almost zero excess entropy. 6. (6) Nonstoichiometry in clinopyroxene has a moderate but predictable influence on reaction boundaries in SiO 2-saturated compositions.