Cation vacancy and possible hydrogen positions in hydrous forsterite, Mg_1.985Si_0.993H_0.06O₄, synthesized at 13.5 GPa and 1300 °C

The crystal structure of a single crystal of synthetic hydrous forsterite, (Mg_1.985Si_0.993H_0.06O4), synthesized at T= l300 °C and P = 13.5 GPa has been investigated using X-ray diffractometry and unpolarized infrared absorption spectroscopy. The data show an Mg atom occupancy of 98.6(2)% at the M(1) site and 99.9(2)% at the M(2) site, indicating that the cation vacancies predominantly occur at the M(1) site. A comparison of the calculated bond distances shows that the most probable H atom locations are at the O(2) and O(3) sites, where H may be located in association with the vacancy At the M(1) or the T site, forming hydrogen bonds between two oxygen pairs, O(3)-(1) and O(2)-O(1), which correspond to the OH stretching vibrations at 3566 cm^-1 and 3612 cm^-1 observed in the IR spectra.