Effect of pressure and temperature on the molar volumes of Wüstite and of three (Fe, Mg)2SiO4spinel solid solutions

Abstract

The effect of pressure on the molar volumes of wüstite (Fe0.924O) and three spinel phases of Fe2SiO4, Fe1.8Mg0.2SiO4 and Fe1.6Mg0.4SiO4 has been determined by means of the X-ray diffraction method at pressures up to 255 kb at room temperatures. The isothermal bulk moduli for these minerals have been evaluated at zero pressure from the volume data by a least-squares fit to the Birch equation. The value for wüstite thus determined is 1.42±0.10 Mb. The values for the spinel phases of olivines range between 1.96 and 2.12±0.10 Mb and are indistinguishable from each other. The mean volume thermal expansivity for the spinel phase of Fe2SiO4 has been determined to be (23±1)×10−6 °C−1 at a pressure of 1 bar and a temperature range of 8°–396°C by means of the X-ray diffraction technique. On the basis of these data, the stability boundary for an assumed reaction, Fe2SiO4 (spinel) = 2 FeO(wüstite) + SiO2(stishovite), has been estimated to be P (kb) = 227.5 - 0.022 T(°C).