Al (sub 3) Si (sub 2) O (sub 7) (OH) (sub 3) , phase Pi (formerly piezotite); crystal structure of a synthetic high-pressure silicate rediscovered

The crystal structure of the triclinic high-P phase with this composition (a 6.0885, b 7.2832, c 7.7234 Aa, alpha 115.71, beta 88.85, Gamma 92.89 degrees , space group P1, Z = 2), here named phase Pi, is based on a cubic closest packing of oxygen atoms. The characteristic pattern of occupied polyhedra produces layers of interconnected AlO (sub 6) octahedra and single SiO (sub 4) tetrahedra. The most probable location of H atoms (not found in the X-ray single-crystal analysis) is derived from calculated bond valence sums. Comparison with other structures in the system Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O relates phase Pi to phyllosilicates and other neso-subsilicates with similar chemical composition; structure comparisons also show that in this system there are trends in the development of polyhedral topologies and oxygen packing with increasing P.