Thermochemistry of some pyroxenes and related compounds

Details Thermochemistry of some pyroxenes and related compounds Thermochemistry of some pyroxenes and related compounds

Oct 1976

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976gecoa..40.1281n&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 40, Issue 10, pp.1281-1288

Physics

22

Scientific paper

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 H ° f , 986 , in kcal/mol, are as follows: MgCaSi 2 O 6 , - 34.3 ± 0.4; CoCaSi 2 O 6 , - 26.7 ± 0.5; NiCaSi 2 O 6 , - 27.1 ± 0.5; MnSiO 3 , - 6.3 ± 0.3; Mn 2 SiO 4 , - 12.2 ± 0.3. In addition, for MnSiO 3 (rhodonite) MnSiO 3 (pyroxmangite), H ° 986 = + 0.06 ± 0.3 3 kcal / mol and for MgCaSi 2 O 6 (diopside) = MgCaSi 2 O 6 (glass), H ° 986 = + 21.0 ± 0.3 kcal / mol . For hedenbergite, FeCaSi 2 O 6 , G ° 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 2 O 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.

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