Thermodynamics of polymorphic transformations in silica. Thermal properties from 5 to 1070° K and pressure-temperature stability fields for coesite and stishovite
Cryogenic heat-capacity measurements on coesite and stishovite provide thermo-dynamic properties from 5 to 350°K. The heat capacities ( Cp), entropies ( S°), and Gibbs energy functions [ -(G° - H° 0) /T] are 10.85, 9.65, 4.124, and 10.27, 6.64, 2.362 at 298.15°K for coesite and stishovite, respectively, in cal/(mole °K). Enthalpies of transition for phase changes were determined by solution calorimetry in a lead-cadmium-borate solvent at 697°C. The following values (in kcal/mole) were obtained upon adjustment to 298.15°K: quartz → coesite (1.2 1 ± 0.15), quartz → cristobalite (0.6 4 ± 0.15), quartz → silica glass (2.1 5 ± 0.15), and cristobalite → silica glass (1.5 1 ± 0.15). The enthalpy of coesite to 1070°K and the enthalpy of transformation for stishovite-silica glass were provided by means of a novel technique, "transposed-temperature" drop calorimetry. These data permit the delineation of P-T field phase boundaries for the stable equilibria coesite-quartz and coesite - stishovite, as well as for the metastable quartz-stishovite boundary, which are in reasonable agreement with the results of recent equilibrium and formation studies.