In situ high P‐T X ray diffraction studies on three polymorphs (α, β, γ) of Mg2SiO4

Unit cell volumes of the beta (β) and spinel (γ) phases (Mg2SiO4) have been measured under simultaneous high pressure and high temperature using synchrotron X ray radiation, in a cubic anvil apparatus. With volume‐temperature data at constant pressure, we determine the average volume thermal expansion coefficients of the β phase from 724 to 872 K at 7.6 GPa to be 2.28(±0.45)×10−5/K and of the γ phase from 759 to 962 K at 9.8 GPa to be 1.71(±0.14)×10−5/K. Thermodynamic relations are used to constrain the temperature derivative of the isothermal bulk modulus (KT) from the high‐pressure thermal expansion data: (∂KT/∂T)P is found to be −2.7(±0.5)×10−2 GPa/K for the β phase and −2.8(±0.3)×10−2 GPs/K for the γ phase. Unit cell volumes of the olivine (α) phase, back‐transformed from the β and γ phases at high temperatures, have been measured under pressure at temperatures above the Debye temperature; using thermal pressure equations, we find (∂KT/∂T)P for the α phase to be −2.1(±0.2)×10−2 GPa/K. These new data on the temperature derivatives of the bulk modulus for the three phases are consistent with an upper mantle containing 60 to 65% olivine and the absence of a velocity signature for the β to γ phase transition near 520 km depth.