Synthesis, crystal structure, and phase relations of AlSiO ₃ OH, a high-pressure hydrous phase

Phase egg, first described by The crystal structure of AlSiO 3 OH, 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) A , b 4.33462(1) A , c 6.95253(2) A , and 98.396(1). The space group is P2 1 /n; Z 4, V 0 212.99(1) A 3, and the zero pressure density is 3.74 g/cm 3 . This phase, which has features in common with the stishovite structure, occurs above 11 GPa and 700 C. AlSiO 3 OH forms from 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.