The phase boundary between spinel (γ phase) and MgSiO ₃ perovskite + MgO periclase in Mg ₂ SiO ₄ 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.

The stability of Mg ₂ SiO ₄ , a major constituent in the Earth's mantle, has been investigated experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800° to 1600°C. The phase stability of α-Mg ₂ SiO ₄ and β-Mg ₂ SiO ₄ was determined by taking account of the kinetic behavior of transition. The phase boundary between α-Mg ₂ SiO ₄ and β-Mg ₂ SiO ₄ is approximated by the linear expression P = (9.3 ± 0.1) + (0.0036 ± 0.0002) T where P is pressure in gigapascals and T is temperature in degrees Celsius.

In situ observations of the ilmenite‐perovskite transition in MgSiO₃ were carried out in a multianvil high‐pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300–1600 K was determined to be P (GPa) = 28.4(±0.4) ‐ 0.0029(± 0.0020)T (K) based on Jamieson's equation of state of gold [Jamieson et al., 1982] and P (GPa) = 27.3(±0.4) ‐ 0.0035(±0.0024)T (K) based on Anderson's equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamieson's equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Anderson's equation of state implies that the depth of the 660‐km seismic discontinuity does not match the pressure of this transition.