Cation vacancies and possible hydrogen atom positions in Fe-bearing hydrous forsterite, Mg_1.85Fe_0.14Si_0.99H_0.06O₄, synthesized at 13.5 GPa and 1400 °C

Abstract

The crystal structure of a single crystal of synthetic Fe-bearing hydrous forsterite (Mg_1.85Fe_0.14Si_0.99H_0.06O₄) 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.