High‐Temperature Vibrational Spectra Between Mg (OH)₂ and Mg (OD)₂: Anharmonic Contribution to Thermodynamics and D/H Fractionation for Brucite

Abstract

Abstract Brucite, Mg (OH) 2 , is an important analog for studying the thermodynamics of hydrous silicate minerals in the deep Earth, as well as H/D isotope fractionation between minerals and water. In this study, we measured in situ Raman and Fourier transform infrared spectra for the natural and deuterated brucite samples, at high temperatures to 650 K, just before the dehydration of brucite at ambient pressure. All of the optical modes systematically shift to lower frequencies at elevated temperature, while deuterium substitution reduces the magnitudes of the temperature dependence. The isobaric mode Grüneisen parameters ( γ iP ), as well as the intrinsic anharmonic parameters ( a i ), have been evaluated for the vibrational modes between Mg (OH) 2 and Mg (OD) 2 . The anharmonic contribution to the thermodynamic properties (such as internal energy, isochoric and isothermal heat capacities, and entropy) is positive and severe at high temperature. The difference in the heat capacity is up to ~7% at 700 K due to the anharmonic effect. The deuterium isotopic effect on the thermodynamics is positive, and the magnitude of the isotopic effect is comparable to that from the anharmonic effect. On the other hand, the anharmonicity significantly decreases the magnitude of the positive pressure dependence of the D/H fractionation β factor for brucite, and this correction could be more important at elevated temperature. At the temperature of 800 K, 10 3 ·(∂ln β /∂ P ) T decreases from +0.23 GPa −1 (for quasi‐harmonic approximation) to approaching zero, due to the anharmonic correction.