Thermodynamics of polymorphic transformations in silica. Thermal properties from 5 to 1070[deg] K and pressure-temperature stability fields for coesite and stishovite

Abstract

Cryogenic heat-capacity measurements on coesite and stishovite provide thermo-dynamic properties from 5 to 350[deg]K. The heat capacities (Cp), entropies (S[deg]), and Gibbs energy functions [-(G[deg] - H[deg]0)/T] are 10.85, 9.65, 4.124, and 10.27, 6.64, 2.362 at 298.15[deg]K for coesite and stishovite, respectively, in cal/(mole [deg]K). Enthalpies of transition for phase changes were determined by solution calorimetry in a lead-cadmium-borate solvent at 697[deg]C. The following values (in kcal/mole) were obtained upon adjustment to 298.15[deg]K: quartz --> coesite (1.21 +/- 0.15), quartz --> cristobalite (0.64 +/- 0.15), quartz --> silica glass (2.15 +/- 0.15), and cristobalite --> silica glass (1.51 +/- 0.15). The enthalpy of coesite to 1070[deg]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.