Cation vacancies and possible hydrogen atom positions in Fe-bearing hydrous forsterite

The crystal structure of a single crystal of synthetic Fe-bearing hydrous forsterite (Mg1.85Fe0.14Si0.99H0.06O4) synthesized at 13.5 GPa and 1400 °C has been investigated using X-ray diffractometry. The data show a site occupancy of 96(3)% Mg + 4(1)% Fe + 0(4)% at the M1 site and a site occupancy of 89(3)% Mg + 8(1)% Fe + 3(4)% at the M2 site, indicating that the cation vacancies at the octahedral M sites predominantly occur at the M2 site. The occupancy at the T site was 99.0% Si (fixed) + 0(1)% Fe + 1(1)% , indicating a small number of vacancies with no detectable Fe occupation. A comparison of the calculated bond distances and reported stretching frequencies of the polarized IR spectra shows that the most probable H atom locations are near to the edges of the MO6 octahedra for both M-site and T- site vacancies.