Computer modelling of the energies and vibrational properties of hydroxyl groups in a- and B-Mg2SiO4
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The structure, formation energies and infrared (IR) active vibrational modes of hydrous defects in the iron free end members of two of the most important minerals of the Earth's mantle, a- and B-Mg2SiO4, are studied by atomic-scale computational modelling in order to identify the hydrogen incorporation mechanism observed in experiment. Two computational methods are used; calculations based on inter-atomic potentials provide information on all defect configuration in the two minerals, and a combined quantum mechanical/molecular mechanics embedded cluster method is used to validate selected results. For forsterite (B-Mg2SiO4), the results suggest that IR bands at low frequencies (wavenumbers 3000-3250 cm-1) are related to protons populating M1 vacancies. Despite the unfavourable creation of silicon vacancies, calculated medium- and high-frequency IR bands are linked to protons occupying vacant Si sites. For iron-freewadsleyite (B-Mg2SiO4) IR frequencies for hydrated cation vacancies have been calculated for the first time. The main doublet at 3360-3326 cm-1 is attributed to two OH groups located in a vacant M3 site. IR bands at higher wavenumber such as the anisotropic doublet at 3615-3580 cm-1 appear to be linked to OH in vacant Si sites. Low accuracy on the calculated frequencies does not permit a strict and rigorous assignment of each individual IR band observed in hydrous forsterite and wadsleyite. However, it does allow the identification of the most favourable site for protonation and provides a useful approximation to the corresponding IR stretching frequencies for a given hydrogen incorporation mechanisms in these nominally anhydrous silicate structures.
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