Stishovite, SiO

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

Stishovite is a dense, meiastable polymorph of SiO with a C.N. = 6 for silicon. It forms at pressures above 8.5 GPa. In the meteoritic impact vs. vulcanism controversy over the nonconformity at the Cretaceous-Tertiary boundary ( What killed the dinosaurs ), the presence of stishovite at the K/T boundary has been used as an argument in favor of meteoritic impact rather than volcanic activity (See McHone, J. F. Nieman. R. A. Lewis. C. F. Yates, A. M. Science 1989. 243, 1182-1184). Discuss the possible changes involved in the quartz-to-stishoviic-" phase transitions in terms of heat and pressure, and how they relate to meteorites vs. volcanoes. (Sec also Sigurdsson, H. D Hondt. S. Arthur. M. A. Bralower, T. J. Zachos. J. C. van Fossen. M. Channcll, J. E. T. Nature, 1991. 349, 482-487.)... 

In its simple molecules and ions Si does not exhibit a covalency of less than four except (possibly) in the silyl ion (see later). Unlike carbon it does not form a small number of multiple bonds, as does carbon in CO, COj, -CN, etc. Only one oxide of Si is stable at ordinary temperatures, and this exists in a number of crystalline forms, in all of which-with the exception of stishovite (p. 804)-there is a tetrahedral arrangement of four bonds from each Si atom, and in every case the Si and 0 atoms form an infinite 3D network. The lower oxide SiO has nothing in common with the gaseous CO. It is produced by heating SiOj with Si at... 

In stishovite, each oxide anion is bonded to three Si cations and lies in the plane defined by the cations. The anion was found to be coordinated by five well-defined domains of electron localization (Gibbs et al., unpublished data). Of these, three were found to be displaced slightly off the SiO bond vectors toward the exterior of the shared edges as observed in deformation maps (Spackman et al. 1987) The remaining two were... 

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