Negative pressure-temperature slopes for reactions forming MgSiO ₃ perovskite from calorimetry

A new and sensitive differential drop solution calorimetric technique was developed for very small samples. A single experiment using one 5.18-milligram sample of perovskite, synthesized at 25 gigapascals and 1873 Kelvin, gave 110.1±4.1 kilojoules per mole for the enthalpy of the ilmenite-perovskite transition in MgSiO₃. The thermodynamics of the reaction of MgSiO₃ (ilmenite) to MgSiO₃ (perovskite) and of Mg ₂SiO₄ (spinel) to MgSiO₃ (perovskite) and MgO (periclase) were assessed. Despite uncertainties in heat capacity and molar volume at high pressure and temperature, both reactions clearly have negative pressure-temperature slopes, -0.005±0.002 and -0.004±0.002 gigapascals per Kelvin, respectively. The latter may be insufficiently negative to preclude whole-mantle convection.