Synthesis, crystal structure, and phase relations of AlSiO3OH, a high-pressure hydrous phase

Phase egg, first described by Eggleton et al. (1978), was synthesized and its composition determined to be AlSiO3OH. The crystal structure of AlSiO3OH, including the position of the hydrogen, has been solved and refined from high-resolution X-ray powder diffraction. The resulting lattice constants are a = 7.14409(2) Å, b = 4.33462(1) Å c = 6.95253(2) Å, and β = 98.396(1)°. The space group is P21ln; Z = 4, V0 = 212.99(1) Å3, and the zero pressure density is 3.74 g/cm3. This phase, which has features in common with the stishovite structure, occurs above 11 GPa and 700°C. AlSiO3OH forms topaz-OH with increasing pressure and persists to more than 17.7 GPa and 1300°C. From a Schreinemaker analysis, we predicted that phase egg decomposes with pressure to an unknown, possibly hydrous aluminosilicate. Potentially, phase egg could replace topaz-OH or kyanite in subducted crustal bulk compositions and may transport some water into the deep Earth.