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 o f AlSiO3OH, including the position of the hydrogen, has been solved an d refined from high-resolution X-ray powder diffraction. The resulting lattice constants are a = 7.14409(2) Angstrom, b = 4.33462(1) Angstro m, c = 6.95253(2) Angstrom, and beta = 98.396(1)degrees. The space gro up is P2(1)/n; Z = 4, V-0 = 212.99(1) Angstrom(3), and the zero pressu re density is 3.74 g/cm(3). This phase, which has features in common w ith the stishovite structure, occurs above II GPa and 700 degrees C. A lSiO3OH forms from topaz-OH with increasing pressure and persists to m ore than 17.7 GPa and 1300 degrees C. From a Schreinemaker analysis, w e predicted that phase egg decomposes with pressure to an unknown, pos sibly hydrous aluminosilicate. Potentially, phase egg could replace to paz-OH or kyanite in subducted crustal bulk compositions and may trans port some water into the deep Earth.