Phase egg, first described by Eggleton et al. (1978), was synthesized and its composition determined to be AlSiO 3 OH. 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) Aa, b = 4.33462(1) Aa c = 6.95253(2) Aa, and beta = 98.396(1) degrees . The space group is P2 1 ln; Z = 4, V 0 = 212.99(1) Aa 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 degrees C. AlSiO 3 OH forms topaz-OH with increasing pressure and persists to more than 17.7 GPa and 1300 degrees 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.