The postspinel phase boundary in Mg₂SiO₄ determined by in situ X-ray diffraction

The phase boundary between spinel (γ phase) and MgSiO₃ perovskite + MgO periclase in Mg₂SiO₄ was determined by in situ x-ray measurements by a combination of the synchrotron radiation source (SPring-8) and a large multianvil high-pressure apparatus. The boundary was determined at temperatures between 1400°to 1800°C, demonstrating that the postspinel phase boundary has a negative Clapeyron slope as estimated by quench experiments and thermodynamic analyses. The boundary was located at 21.1 (± 0.2) gigapascals, at 1600°C, which is ~2 gigapascals lower than earlier estimates based on other high-pressure studies.