Structure and cation disorder of hydrous ringwoodite, γ-Mg_1.89Si_0.98H_0.30O₄

The structure of a single crystal hydrous ringwoodite, Mg_1.89Si_0.98H_0.30O₄ synthesized at conditions of 1300 °C and 20 GPa has been analyzed. Crystallographic data for hydrous ringwoodite obtained are; Cubic with Space group: Fd3m (no. 227). a = 8.0693(5) Å, V = 526.41(9) ų, Z = 8, D(calc) = 3.48 g cm^-3. The results of site occupancy refinement using higher angle reflections showed the existence of a small degree of Mg²⁺ -Si⁴⁺ disorder in the structure such as (Mg_1.84Si_0.05[]_0.11)(Si_0.93Mg_0.05[]_0.02)H_0.30O₄. The IR and Raman spectra were measured and OH vibration spectra were observed. A broad absorption band was observed in the IR spectrum and the maxima were observed at 3160 cm^-1 in the IR and at 3165 cm^-1 and 3685 cm^-1 in relatively sharp Raman spectra, which suggest that locations between O-O pairs around the octahedral 16c and 16d sites are possible sites for hydrogen.