Calorimetric studies of crystalline and glassy high pressure phases.

The enthalpies of phase transformation can be combined with free energies obtained from high pressure experiments to obtain accurate estimates of entropies of transition and independent assessment of pressure-temperature slopes. Transitions in the systems CoO- SiO₂, 'FeO'-SiO₂, and MgO-GeO₂ have been studied by calorimetry, and the thermochemical and phase equilibrium data are in good agreement for the olivine-spinel transition, and stability fields of alpha , beta , and gamma -Co₂SiO₄, and the formation and breakdown of pyroxene. However, some discrepancies exist between calculated and experimental P-T slopes of spinel disproportionation reactions. Conclusions concerning crystal chemical systematics are drawn from NaC1-CsC1, DELTA S^oC is not constant but varies strongly with DELTA V^oC. The calorimetric data argue against any major changes in glass structure (coordination number) in these high pressure glasses.-Authors