Abstract

Thermal expansivities for the MgSiO3 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 Mg2SiO4 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: α(majorite) = 2.95 + 0.000521x; α(γ-Mg2SiO4) = 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).

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