CALORIMETRIC STUDY OF THE COESITE STISHOVITE TRANSFORMATION AND CALCULATION OF THE PHASE-BOUNDARY

High temperature drop-solution calorimetry in molten 2 PbO . B2O3 at 1 044 K for coesite and stishovite polymorphs of silica was carried out to determine the enthalpy of the coesite-stishovite transition. These experiments were performed on high-purity, single-phase samples of coe site and stishovite. Our new value for the enthalpy of the coesite-sti shovite transition (Delta H-298(0)) is 29.85+/-0.78 kJ/mol, which is a bout 35% lower than previously reported by Akaogi and Navrotsky (1984) and Helm et al. (1967), but which compares well with new measurements by Akaogi et al. (1994 b). Using these new data, we have calculated t he equilibrium phase boundary between coesite and stishovite and obtai ned a slope, dP/dT=0.0031 (2) GPa/K. This calculated slope is in good agreement with that determined [0.0026 (2) GPa/K] from the in-situ X-r ay diffraction study of Zhang et al. (1996).