Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.t22e..11s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #T22E-11
Physics
3600 Mineralogy And Petrology, 3662 Meteorites, 3954 X Ray, Neutron, And Electron Spectroscopy And Diffraction
Scientific paper
Stishovite, the rutile-structured polymorph of SiO2, is an important index mineral for demonstrating shock metamorphism of rocks that contain SiO2 minerals. The structures and stabilities of post-stishovite SiO2 phases, polymorphs that are more dense than stishovite, have been debated in the literature because of their potential occurence in Earth's deep lower mantle. Recently, several high-pressure SiO2 polymorphs have been identified in the SNC meteorite Shergotty, including an orthorhombic phase similar to α -PbO2 (1) and a monoclinic baddeleyite-structured phase (2). Recent DAC experiments have shown that compression of cristobalite can produce α -PbO2-structured SiO2 that can be quenched (3, 4). The recovered material, space group Pnc2, is said to be the same as the α -PbO2-like mineral described by Sharp et al. (1). TEM imaging and electron diffraction of the SiO2 polymorphs in Shergotty shows complex intergrowths of four SiO2 polymorphs. The most abundant is the α -PbO2-like phase (a = 4.16Å, b = 5.11 Å, c = 4.55 Å) which has electron-diffraction symmetry consistent with the Pbcn space group. Orthogonal intergrowths of stishovite and cubic cristobalite are also observed. The cristobalite is likely a quench product from the phase XI of Tsuchida and Yagi (5) which was intergrown with stishovite or CaCl2-structured SiO2 at high pressure. The fourth polymorph, which is too irradiation sensitive to be characterized by electron diffraction, may be the baddeleyite-structured SiO2 described by El Goresey et al. (2). The SiO2 polymorphs in Shergotty represent the quench products from a mixture of post-stishovite structures, stishovite and phase XI that formed from the transformation of tridymite. This mixture of SiO2 structures suggests a modest shock pressure of around 45 GPa, the pressure at which silica Phase XI transforms into the α -PbO2 structure. (1) T. G. Sharp, A. El Goresy, B. Wopenka, M. Chen, Sciecne 284 (1999). (2) A. El Goresy, L. Dubrovinsky, T. G. Sharp, S. K. Saxena, M. Chen, Science 288, 1632-1634 (2000). (3) L. Dubrovinsky et al., Chemical Physics Letters 333, 264-270 (2001). (4) N. A. Dubrovinskaia et al., Eur. J. Mineral. 13, 479-483 (2001). (5) Y. Tsuchida, T. Yagi, Nature 347, 267-269 (1990).
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