Crystal chemistry and high-temperature behavior of Al-bearing stishovite and Al-rich phase D: Implications for water storage in the deep mantle Available to Purchase

High-quality single crystals of Al-bearing stishovite and Al-rich phase D were analyzed using X-ray diffraction, high-temperature Raman and FTIR spectroscopy. The samples were synthesized in the MgO─SiO₂─Al₂O₃─H₂O system at 25 GPa, 1350–1600 °C. The results demonstrate that Al-bearing SiO₂ glass formed by the amorphization of stishovite under high-temperature conditions can incorporate water in the form of hydroxyl groups, with a concentration of at least several hundred ppm H₂O by weight. Al³⁺ prefers entering into the crystal structure of phase D by substitution for Si⁴⁺ in the M2 site. Since Al³⁺ can be charge balanced by H⁺, Al-rich phase D may have a higher water storage capacity than pure Mg-phase D. Raman and infrared spectral features of Al-rich phase D at ambient temperature can still be observed up to 350 °C. This indicates that, compared to pure Mg-phase D, Al-rich phase D maintains stability at higher temperatures. Al-rich silicate glass formed by the amorphization of phase D under high-temperature conditions can also retain water in the form of hydroxyl groups. In the deep mantle, Al-rich silicate glasses and Al-bearing SiO₂ glasses may serve as potential reservoirs of water (hydrogen) in high-temperature regions beyond the ideal stability fields of dense hydrous magnesium silicates and high-pressure SiO₂ phases.