Thermochemistry of some pyroxenes and related compounds

Abstract

The enthalpies of formation of a number of crystalline silicates from the oxides at 986 K were determined by oxide melt solution calorimetry. The values of ΔH° f, 986 , in kcal/mol, are as follows: MgCaSi 2O 6, − 34.3 ± 0.4; CoCaSi 2O 6, − 26.7 ± 0.5; NiCaSi 2O 6, − 27.1 ± 0.5; MnSiO 3, − 6.3 ± 0.3; Mn 2SiO 4, − 12.2 ± 0.3. In addition, for MnSiO 3 (rhodonite)→ MnSiO 3 (pyroxmangite), ΔH° 986 = + 0.06 ± 0.3 3 kcal/ mol and for MgCaSi 2O 6 (diopside) = MgCaSi 2O 6 (glass), ΔH° 986 = + 21.0 ± 0.3 kcal/ mol. For hedenbergite, FeCaSi 2O 6, ΔG° 1350 = −25.6 ± 1.5 kcal/ mol. In terms of pyroxene phase equilibria and crystal chemistry, our thermochemical data support the generally accepted crystallographic arguments that (a) the C2/c clinopyroxene structure increases in stability with decreasing size of the ion occupying the Ml site in the MCaSi 2O 6 series, and (b) the energy (and enthalpy) differences between orthopyroxene, clinopyroxene, and pyroxenoid structures are generally quite small and often less than 500 cal/mol in magnitude.