AL3SI2O7(OH)(3), PHASE PI (FORMERLY PIEZOTITE) - CRYSTAL-STRUCTURE OFA SYNTHETIC HIGH-PRESSURE SILICATE REDISCOVERED

The crystal structure of the triclinic high-pressure phase with the ch emical composition Al3Si2O7(OH)(3) (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; space group P1, Z = 2), her e named phase Pi, is based on a cubic closest packing of oxygen atoms. The characteristic pattern of occupied polyhedra produces layers of i nterconnected AlO6-octahedra and single SiO4-tetrahedra. The most prob able location of hydrogen atoms, not found in the X-ray single-crystal analysis, is derived from calculated bond valence sums. Comparison wi th other structures in the system Al2O3-SiO2-H2O relates phase Pi to p hyllosilicates and other neso-subsilicates with similar chemical compo sition. Structural comparisons also show that, in this system, there a re trends in the development of polyhedral topologies and oxygen packi ng with increasing pressure.