Al3Si2O7(OH)3, phase Pi (formerly piezotite): Crystal structure of a synthetic high-pressure silicate rediscovered
Daniels, Peter; Wunder, Bernd
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1283 - 1292
published: Jan 8, 1997
manuscript accepted: Jul 18, 1996
manuscript received: Aug 11, 1995
ArtNo. ESP147050806007, Price: 29.00 €
Abstract The crystal structure of the triclinic high-pressure phase with the chemical composition Al3Si2O7(OH)3 (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)°; space group P1, Z = 2), here named phase P1, is based on a cubic closest packing of oxygen atoms. The characteristic pattern of occupied polyhedra produces layers of interconnected AlO6-octahedra and single SiO4-tetrahedra. The most probable location of hydrogen atoms, not found in the X-ray single-crystal analysis, is derived from calculated bond valence sums. Comparison with other structures in the system Al2O3-SiO2-H2O relates phase Pi to phyllosilicates and other neso-subsilicates with similar chemical composition. Structural comparisons also show that, in this system, there are trends in the development of polyhedral topologies and oxygen packing with increasing pressure.