Anharmonicity of three minerals at high temperature: Forsterite, fayalite, and periclase

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Abstract
Recent data on Ks (the adiabatic bulk modulus) and α (the volume coefficient of thermal expansion) versus T (temperature) at high temperatures (500°C < T < 1000°C) have been published or are in press. These data, taken at ambient pressure, extend the measurement of single‐crystal elastic constants for forsterite, fayalite and periclase to record temperatures. The high temperature anharmonic properties of forsterite and fayalite are presented for the first time in this paper, and they are compared with similar previously published data for MgO. The anharmonic properties referred to above concern the dependence of γ (the Grüneisen ratio), PTH (the thermal pressure), and Cv (the specific heat) with T. If γ (at constant V) is independent of T at high T, the anharmonicity in γ is said to be nil; similarly, for Cv. If PTH at constant V is proportional to T at high T, then the anharmonicity in PTH is said to be nil. The anharmonicity determined by these experiments indicates that the minerals are not alike with regard to their properties γ, PTH, and Cv. The γ versus T at constant V indicates that there is anharmonicity for all three minerals, but the effects are opposite in fayalite and forsterite in such a way that anharmonicity should be absent in olivine. For PTH at 1 bar, anharmonicity is detectable and positive in forsterite, absent in fayalite, and detectable and negative in periclase. It would be slight in olivine. In all three solids, anharmonicity in Cv is pronounced and positive.