Thermal expansivity of mantle relevant magnesium silicates derived from vibrational. spectroscopy at high pressure

Thermal expansivities for the MgSiO3 phases of orthoenstatite, high clinoenstatite, ilmenite, and majorite; and for stishovite were estimated using the thermodynamic Maxwell relation (partial derivative S/partial derivative P)(T) = -(partial derivative V/partial derivative T)(P), where the entropies at high pressures were derived using a statistical method and spectroscopic data. The spectroscopically determined thermal expansivities for ail minerals are in excellent agreement with previously determined volumetric data, where available. A value of 3.25(10) x 10(-5) K-1 for orthoenstatite at room temperature was obtained; this value is situated in the middle of the large spread of reported values and is in excellent agreement with the two latest volumetric determinations. For high clinoenstatite, cc at room Tis estimated as 2.56(9) x 10(-5) K-1. This method provides good high temperature estimates of cc for the high-pressure polymorphs, where data are scanty or unavailable. Included in this report are previous data for the Mg2SiO4 phases and MgO for completeness. The following equations may be used to extrapolate a to higher temperatures at 1 arm in 10(-5) K-1: alpha(majorite) = 2.95 + 0.000521x; alpha(gamma-Mg2SiO4) = 2.70 + 0.000648x; alpha(ilmenite) = 2.64+ 0.000537x-; alpha(perovskite) = 2.51 + 0.000805x; and alpha(stishovite) = 2.19 + 0.000485x, where x is (T/K-750).