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

Thermal expansivities for the MgSiO 3 phases of orthoenstatite, high clinoenstatite, ilmenite, and majorite; and for stishovite were estimated using the thermodynamic Maxwell relation (∂S/∂P)T = -(∂V/∂T) P where the entropies at high pressures were derived using a statistical method and spectroscopic data. The spectroscopically determined thermal expansivities for all minerals are in excellent agreement with previously determined volumetric data, where available. A value of 3.25(10) × 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, α at room T is estimated as 2.56(9) × 10 -5 K -1 . This method provides good high temperature estimates of α for the high-pressure polymorphs, where data are scanty or unavailable. Included in this report are previous data for the Mg 2 SiO 4 phases and MgO for completeness. The following equations may be used to extrapolate a to higher temperatures at 1 atm in 10 -5 K -1 : a(majorite) = 2.95 + 0.000521x; α(γ-Mg 2 SiO 4 ) = 2.70 + 0.000648x; α(ilmenite) = 2.64+ 0.000537x; α(perovskite) = 2.51 + 0.000805x; and α(stishovite) = 2.19 + 0.000485x, where x is (T/K-750)