The High Pressure Behavior of Cristobalite

The most abundant rock-forming minerals in the eanh's crust are framework silicates with complex crystal structures. These materials displaya remarkable chemical and structUral diversity. In addition,~structUral phase transitions involving displacive distonions are panicularly common, leading to yet more structural variation. There have been suggestions that the distonive behavior of the framework 'skeleton' can be interpreted in terms of a rigid unit model. The flexible Si-O-Si linkage permits crumpling or shearing of frameworks, with little or no distonion of the polyhedra themselves. In essence, the bridging oxygens act as 'universal joints' . Yet at a practical level, understanding the precise role of the framework topology in many minerals is complicated by the effects of ionic substitUtions, the presence of cavity-filling cations, non-stoichiometry, and order-disorder amongst the tetrahedral sites. Attention has therefore focused on the Si02 polymorphs, as most clearly demonstrating the intrinsic behavior of such linkages. In many ways, cristobalite is the archetypal framework mineral. The cristobalite structure comprises a three-dimensional framework of corner-linked Si04 tetrahedra, arranged in sixfold and four-fold rings. At high. temperatures cristobalite is cubic, and the structural topology is analogous to cubic-close-packing (c.f. high tridymite). Below T c 490 K, there is a .displacive transition to a tetragonal phase with spacegroup P4]2]2. This transition involves a large volume change and is strongly first order in character. The high pressure behavior of cristobalite has attracted much interest recently, with the reponing of a possible phase transition at moderate pressures (Yeganeh-Haeri etaL, 1991). It was the purpose of this work to follow the structUral behavior of cristo bal ite on increasing pressure, working with high-quality specimens of natUral cristobalite, under hydrostatic conditions.