The quartz-coesite-stishovite transformations: new calorimetric measurements and calculation of phase diagrams

High temperature solution calorimetry of synthetic quartz, coesite and stishovite provides enthalpies of transition. ΔH₉₇₅⁰ for quartz-coesite and ΔH₂₉₈⁰ for coesite-stishovite transition are 320 ± 70 and 11700 ± 410 cal mol^?1, respectively. The present transformation enthalpy data represent a small but significant revision of those of Holm et al. Using the published phase equilibrium data, thermal expansivity, compressibility and heat capacity data, ΔS₉₇₅⁰ for the quartz-coesite and ΔS₂₉₈⁰ for the coesite-stishovite transition are ?1.2 ± 0.1 and ?1.0 ± 0.4 cal K^?1 mol^?1, respectively. These thermochemical data are used to calculate phase boundaries of the transitions. The calculated quartz-coesite transition boundary agrees well with the one determined experimentally by Bohlen and Boettcher. The calculated coesite-stishovite boundary is generally consistent with data by Yagi and Akimoto and by Suito.