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 thermodynamic properties from 5 to 350°K. The heat capacities (C,), entropies (SO), and Gibbs energy functions [-(Go H”,)/T] are 10.85, 9.65, 4.124, and 10.27, 6.64, 2.362 at 298015°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, f 0*15), quartz + cristobalite (0.6, & 0*15), quartz +silica glass (2.1, f 0*15), and cristobalite + silica glass (1.5, & 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.