SYSTEMATIC CRYSTAL CHEMISTRY OF HIGH-PRESSURE SILICATES: AN INTERACTIVE GRAPHICS DEMONSTRATION R. M. Hazen and R. T. Downs, Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington D.C. 20015, phone (202) 696-2410 x2470, e-mail: hazen@gl.ciw.edu

Approximately two dozen structure types with six-coordinated silicon have been synthesized at the high pressures and temperatures characteristic of the earth's deep interior. A new interactive graphics program, XtalDraw, facilitates comparison of these structures and ellucidates their crystal chemical behavior. These investigations reveal recurrent patterns of polyhedral linkages that can be used to classify known phases and to predict new ones. Known phases can be classified according to the arangement of six-coordinated silicon, which occurs as isolated octahedra (in Phase B and majorite garnet), as corner-sharing chains (in titanite-type CaSi₂O₅), as corner-sharing layers (in Ca₂SiO₄), as corner-sharing frameworks (in silicate perovskites), or as edge-sharing chains (in stishovite and hollandite). Silicate octahedra also combine with silicate tetrahedra to form a new class of dense alkali or alkaline earth framework silicates.