Anharmonicity of forsterite and the thermal pressure of insulators

A recent paper [ Bouhifd et al., 1996] has given the measured thermal expansivity, α, of forsterite up to 1900 K, which is 2.5 times the Debye temperature, Θ. Using the measured α, Bouhifd et al. [1996] found that the α K T curve (where K T is the isothermal bulk modulus) is parallel to the T axis, showing that (∂(α K T )/∂ T ) P = 0. Using these results, I show that (∂ C v /∂ T ) P > 0 at high T , requiring C v to be anharmonic, is in agreement with other recent work. Further, I show that Bouhifd et al. 's [1996] work requires that all vestiges of anharmonicity be absent in the thermal pressure and the thermal expansivity of forsterite. Thus it is proven that forsterite is a solid that is both anharmonic in C v and quasiharmonic in the thermal pressure in the high‐temperature region.