Vibrational properties of H-D substituted brucite from experiments and ab initio calculations: a step towards the prediction of D/H fractionation for complex structures

D/H partitioning between water and minerals are often not well constrained from equilibrium experiments, except for simple hydroxides such as brucite. Vibrational models and experiments can be used to constrain more accurately isotopic fractionation factors, provided they are first calibrated and tested on relatively simple systems such as MgO-SiO2-H2O (MSH) starting with brucite Mg(OH)2, then going towards increasing complexity in minerals like talc Mg3Si4O10(OH)2 and antigorite Mg48Si34O85(OH)62. We present here first results from combining Raman and IR spectroscopic and ab initio calculations on brucite. Raman measurements were carried out on significantly isotopically doped phases, with several intermediate compositions between end-members. Various mode behaviors are observed, with low frequency modes shifting smoothly in frequency with increasing isotopic substitution while high frequency modes present a two-mode behavior. The good agreement between predicted and observed frequencies allows to identify the modes corresponding to those associated to OH-OD vibrations. Ab initio calculations can thus be used as a guide for deciphering the more complex vibrational spectra such as those obtained on antigorite; and will help constructing reliable vibrational models of D/H partitioning.