SYNTHESIS, PROPERTIES AND STABILITY OF AL3SI2O7(OH)3 (PHASE PI), A HYDROUS HIGH - PRESSURE PHASE IN THE SYSTEM AL2O3 - SIO2 - H2O (ASH)

A crystalline phase with the composition Al3Si2O7(OH)3, here called ph ase Pi, has been synthesized at pressures of 40-55 kbar and temperatur es between 500-650-degrees-C from gels of various compositions. It is identical with the synthetic phase first obtained and named piezotite by Coes (1962), who, however, attributed a wrong composition to it. Ph ase Pi forms euhedral triclinic crystals elongated parallel to [110], exhibiting 26 pedial faces. The refined triclinic cell-dimensions (Z = 2, S.G. P1) are a = 6.0885(6) angstrom, b = 7.2832(9) angstrom, c = 7 .7234(9) angstrom, alpha = 115.71(1)-degrees, beta = 88.85(1)-degrees, gamma = 92.89(1)-degrees, thus differing from those given by Coes (19 62) and Vaughan & Berman (1963). Optical properties are n(x) = 1.665(1 ), n(y) = 1.681(1), n(z) = 1.698(1), 2V(x) = 86-degrees. Heating at 53 0-degrees-C, 1 atm leads to the breakdown of phase Pi into a yet unide ntified metastable phase. The stability field of phase Pi is limited b y the following five reaction curves: Towards lower temperatures: 1 ph ase Pi <-- --> 2 coesite + 3 diaspore, and 1 phase Pi <-- --> 2 quartz + 3 diaspore; towards higher temperatures: 2 phase Pi <-- --> 1 quart z + 3 kyanite + 3 water, and 2 phase Pi <-- --> 1 coesite + 3 kyanite + 3 water; towards higher pressures: 2 phase Pi <-- --> 1 coesite + 3 ''topaz - OH''. The low-pressure limit lies near 22 kbar at about 430- degrees-C. In nature, phase Pi may form in cold subduction zones at de pths greater than about 70 km.