Formation of hydrous stishovite from coesite in high-pressure hydrothermal environments

In low-Temperature, high-pressure hydrothermal environments coesite transforms into hydrous forms of stishovite. We studied hydrous stishovite produced from hydrothermal treatment of silica glass as initial SiO₂ source at temperatures of 350-550 °C and pressures around 10 GPa. The P-T quenched samples were analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermal analysis, and IR and magic-Angle spinning (MAS) NMR spectroscopy. The presence of significant amounts of H₂O (ranging from 0.5 to 3 wt%) is shown from thermogravimetric measurements. PXRD reveals that at temperatures below 400 °C, hydrous stishovite is obtained as two distinct phases that may relate to the solid ice-VII environment present at prevailing P-T conditions. Initially formed hydrous stishovite is metastable and dehydrates over time in the low-Temperature, high-pressure hydrothermal environment. The primary mechanism of H incorporation in stishovite is a direct substitution of 4H₊ for Si₄₊ yielding unique octahedral hydrogarnet defects. In IR spectra this defect manifests itself by two broad but distinct bands at 2650 and 2900 cm_-1, indicating strong hydrogen bonding. These bands are shifted in the deuteride to 2029 and 2163 cm_-1, respectively. Protons of the octahedral hydrogarnet defect produce