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: MgCaSi2O6, - 34.3 ± 0.4; CoCaSi2O6, - 26.7 ± 0.5; NiCaSi2O6, - 27.1 ± 0.5; MnSiO3, - 6.3 ± 0.3; Mn2SiO4, - 12.2 ± 0.3. In addition, for MnSiO3 (rhodonite)→ MnSiO3 (pyroxmangite), ΔH°986 = + 0.06 ± 0.33 kcal/mol and for MgCaSi2O6 (diopside) = MgCaSi2O6 (glass), ΔH°986 = + 21.0 ± 0.3 kcal/ mol. For hedenbergite, FeCaSi2O6, 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 MCaSi2O6 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.
ASJC Scopus subject areas
- Geochemistry and Petrology