Original paper

Synthesis, properties and stability of Al3Si2O7(OH)3 (phase Pi), a hydrous high - pressure phase in the system Al2O3 - SiO2 - H2O (ASH)

Wunder, Bernd; Medenbach, Olaf; Krause, Werner; Schreyer, Werner

European Journal of Mineralogy Volume 5 Number 4 (1993), p. 637 - 650

19 references

published: Jul 22, 1993
manuscript accepted: Mar 5, 1993
manuscript received: Nov 3, 1992

DOI: 10.1127/ejm/5/4/0637

BibTeX file

ArtNo. ESP147050504001, Price: 29.00 €

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Abstract

Abstract A crystalline phase with the composition Al3Si2O7(OH)3, here called phase Pi, has been synthesized at pressures of 40-55 kbar and temperatures between 500-650°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. Phase Pi forms euhedral triclinic crystals elongated parallel to [110], exhibiting 26 pedial faces. The refined triclinic cell-dimensions (Z = 2, S.G. PI) are a = 6.0885(6) Å, b = 7.2832(9) Å, c = 7.7234(9) Å, α = 115.71(1)°, ß = 88.85(1)°, γ = 92.89(1)°, thus differing from those given by Coes (1962) and Vaughan & Berman (1963). Optical properties are nx = 1.665(1), ny = 1.681(1), nz = 1.698(1), 2VX = 86°. Heating at 530°C, 1 atm leads to the breakdown of phase Pi into a yet unidentified metastable phase. The stability field of phase Pi is limited by the following five reaction curves: Towards lower temperatures: 1 phase Pi 2 coesite + 3 diaspore, and 1 phase Pi 2 quartz + 3 diaspore; towards higher temperatures: 2 phase Pi 1 quartz + 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°C. In nature, phase Pi may form in cold subduction zones at depths greater than about 70 km.

Keywords

high pressurehydrous Al-silicatecell dimensionsoptical datamorphologythermal expansionstability fieldphase Pi