High-pressure transitions of diopside and wollastonite: phase equilibria and thermochemistry of CaMgSi2O6, CaSiO3 and CaSi2O5–CaTiSiO5 system

Phase transitions of CaMgSi2O6 diopside and CaSiO3 wollastonite were examined at pressures to 23 GPa and temperatures to 2000 °C, using a Kawai-type multiavil apparatus. Enthalpies of high-pressure phases in CaSiO3 and in the CaSi2O5–CaTiSiO5 system were also measured by high-temperature calorimetry. At 17–18 GPa, diopside dissociates to CaSiO3-rich perovskite + Mg-rich (Mg,Ca)SiO3 tetragonal garnet (Gt) above about 1400 °C. The solubilities of CaSiO3 in garnet and MgSiO3 in perovskite increase with temperature. At 17–18 GPa below about 1400 °C, diopside dissociates to Ca-perovskite + β-Mg2SiO4 + stishovite. The Mg, Si-phases coexisting with Ca-perovskite change to γ-Mg2SiO4 + stishovite, to ilmenite, and finally to Mg-perovskite with increasing pressure. CaSiO3 wollastonite transforms to the walstromite structure, and further dissociates to Ca2SiO4 larnite + CaSi2O5 titanite. The latter transition occurs at 9–11 GPa with a positive Clapeyron slope. At 1600 °C, larnite + titanite transform to CaSiO3 perovskite at 14.6±0.6 GPa, calibrated against the α–β transition pressure of Mg2SiO4. The enthalpies of formation of CaSiO3 walstromite and CaSi2O5 titanite from the mixture of CaO and SiO2 quartz at 298 K have been determined as −76.1±2.8, and −27.8±2.1 kJ/mol, respectively. The latter was estimated from enthalpy measurements of titanite solid solutions in the system CaSi2O5–CaTiSiO5, because CaSi2O5 titanite transforms to a triclinic phase upon decompression. The enthalpy difference between titanite and the triclinic phase is only 1.5±4.8 kJ/mol. Using these enthalpies of formation and those of larnite and CaSiO3 perovskite, the transition boundaries in CaSiO3 have been calculated. The calculated boundaries for the wollastonite–walstromite–larnite + titanite transitions are consistent with the experimental determinations within the errors. The calculated boundary between larnite + titanite and Ca-perovskite has a slope of 1.3–1.8(±0.4) MPa/K, and is located at a pressure about 2 GPa higher than that determined by [Am. Mineral. 79 (1994) 1219].