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

Abstract

Unit cell volumes of the beta (β) and spinel (γ) phases (Mg 2 SiO 4 ) 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 ( K T ) from the high‐pressure thermal expansion data: (∂ K T /∂ 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 (∂ K T /∂ 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.