Equilibrium hydrogen isotope fractionation between portlandite/brucite and water: Implication from the vibrational spectra at elevated temperature and pressure

Minerals in brucite-type structure, including brucite (Mg(OH)2) and portlandite (Ca(OH)2), have been studied as important analogs of hydrous silicate minerals for their thermodynamic properties, including equilibrium hydrogen and oxygen isotope fractionations between minerals and water at elevated temperatures and pressures. In this study, Raman and Fourier transform infrared (FTIR) spectra were collected at high-temperature (T) and high-pressure (P) conditions on synthetic hydrogenated and deuterated portlandite samples (i.e., Ca(OH)2 and Ca(OD)2), and their isobaric and isothermal mode Grüneisen parameters, as well as anharmonic parameters, were then evaluated. These high-precision vibrational spectra enable the evaluation of thermodynamic properties of portlandite (ΔU, CV, CP and ΔS) up to 427 °C, to which anharmonicity has positive contributions. More importantly, they help to determine the isobaric (at P = 1 bar) β factors of brucite and portlandite for equilibrium D/H fractionation considering anharmonic effects, and the D/H fractionation factor (103∙lnα) between brucite/portlandite and water at high-P,T conditions. The calculated equilibrium fractionation factor (lnαbrucite-water) agrees with experimental results in the temperature range from 300 to 647 K (∼27 to ∼374 °C, the critical point of water) within analytical uncertainties estimated using Monte Carlo method. The Ab initio calculation using DFT theory and VASP program was also carried out to obtain phonon spectra for brucite and portlandite with three-split hydrogen sites and to evaluate the dispersion effect, which is found to be smaller than the statistical uncertainty at high temperatures. Our results show that the internal OH-stretching modes in brucite/portlandite play a dominant role in determining the β values, and the anharmonic OH-stretching potential (xi parameter) contributes significantly to the D/H fractionation factor. Because previous studies show that OH-stretching frequencies generally decrease with increasing mass of cation in the metallic hydroxide having similar crystal structure, our results imply that the deuterium isotope may be preferentially enriched in the hydroxide phase with the light cation bonded to the hydroxyl group, which is also consistent with the observed D/H fractionations among hydrous silicates.