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HR: 14:00h
AN: T22E-02
TI: Packing Systematics of Stishovite
AU: * Thompson, R M
EM: thompson@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721-0077 United States
AU: Downs, R T
EM: downs@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721-0077 United States
AU: Teter, D M
EM: dmteter@sandia.gov
AF: Geochemistry Department, Sandia National Laboratories, Albuquerque, NM 87185-0750 United States
AB: Thompson and Downs (2001) devised an algorithm to quantify the distortion of the anion skeleton in a crystal structure from ideal closest-packing. Application of this algorithm to pyroxenes, olivines, spinels, wadsleyites, and kyanite shows that they all become more efficiently packed with pressure, and less so with temperature. In spite of Megaw's (1973) assertion that the rutile structure is not based on the closest-packing of anions, we find that it can be interpreted as a distorted HCP structure. However, the observed structure of stishovite becomes more distorted with increasing pressure. Based upon structures determined by theory, this trend is reversed above 70 GPa. Theoretical determinations in the calcium-chloride structure show that silica is slightly more efficiently packed than stishovite at the transition pressure and becomes rapidly more closest-packed as pressure increases. Theoretical high pressure polymorphs of silica, such as the $\alpha$-PbO$_2$ structure, are all close to ideal closest-packed. It has been observed that rutile-structured compounds do not exhibit the usual pressure-temperature inverse relationship (Hazen and Finger, 1981). This is also true of the packing efficiency of rutile and SnO$_2$, as they become more distorted with both pressure and temperature. Stishovite, however, becomes less distorted with temperature. This presentation will shed light on anion-anion interactions in the stishovite structure. Megaw, H.D. (1973) Crystal Structures: A Working Approach. W.B. Saunders Company, Philadelphia. Thompson, R.M. and Downs, R.T. (2001) Quantifying distortion from ideal closest-packing in a crystal structure with analysis and application. Acta Crystallographica B57, 119-127. Hazen, R.M. and Finger, L.W. (1981) Bulk moduli and high-pressure crystal structures of rutile-type compounds. The Journal of the Physical Chemistry of Solids, 42, 143-151.
DE: 1000 GEOCHEMISTRY
DE: 3620 Crystal chemistry
DE: 3924 High-pressure behavior
SC: T
MN: 2001 AGU Fall Meeting