Systematics of High-Pressure Silicate Structures

Abstract

Mineral-like phases quenched from high-pressure synthesis experiments, as well as minerals formed in natural high-pressure environments, reveal crystal chemical principles that govern the formation of dense oxide and silicate structures. A systematic survey of known high-pressure silicate structures that incorporate octahedrally-coordinated silicon (VISi) is thus appropriate for this volume on the variation of crystal structures with temperature and pressure. The pace of discovery of new high-pressure silicates is rapid. A decade ago, Finger and Hazen (1991) summarized the twelve known high-density structural topologies with VISi. By late 1999, twice that number had been described. Of the original dozen structure types, seven (stishovite, perovskite, ilmenite, hollandite, calcium ferrite, pyrochlore, and K2NiF4) contain only six-coordinated silicon. The five additional high-pressure silicates known at that time, including the garnet, pyroxene, wadeite, anhydrous phase B, and phase B structures, contain both IVSi and VISi. Finger and Hazen (1991) used these twelve structure types to identify five systematic trends related to structural topology and isomorphous substitutions that might point to the existence of other high-pressure silicate structures. Two of these trends systematize groups of structurally related phases: 1. the structures of rutile, hollandite and calcium ferrite form from edge-sharing chains of silicon octahedra; and, 2. homologous structures in the system Mg-Si-O-H, including phase B and anhydrous phase B, feature edge-sharing clusters of twelve magnesium octahedra surrounding a silicon octahedron (Finger and Prewitt 1989). The other three criteria recognize similarities between high-pressure silicates and room-pressure isomorphs. High-pressure silicates often have structure types that were first observed in: