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).
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