Stishovite and post-stishovite polymorphs of silica in the shergotty meteorite: Their nature, petrographic settings versus theoretical predictions and relevance to Earth's mantle

The Shergotty meteorite contains three dense silica polymorphs in distinct petrographic settings: (1) two post-stishovite SiO₂ polymorphs in individual multiphase grains coexisting with glass with nearly labradorite composition, and (2) large individual stishovite grains in shock-melt pockets which also contain the new CAS phase (Calcium-aluminosilicate; CaAl ₄Si₂O₁₁; [Phys Earth Planet Interiors 97 (1996) 97; Geophys Abstract 5(2003)] and hollandite structured plagioclase composition. Prismatic and wedge-shaped grains of the original accessory tridymite (or cristobalite) in the Shergotty meteorite were densified during a major impact event on the Shergottite-Nakhlite-Chaissingite (SNC) parent body and inverted either to (1) multiphase assemblages of several post-stishovite polymorphs depicting prominent tweed pattern or to (2) Large homogeneous stishovite grains in melt pockets. In the first setting we identified an orthorhombic and a monoclinic post-stishovite silica polymorph, respectively. TEM investigations of a grain containing the orthorhombic polymorph revealed an α-PbO₂ like phase that could be assigned to either Pnc2 (with the cell parameters: a=4.55±0.01Å, b=4.16±0.03Å, c=5.11±0,04Å), or Pbcn space group and dense SiO₂ glass. The X-ray diffraction pattern from a second grain revealed a polymorph with a monoclinic lattice with the space group P21/c, that is related to the baddeleyite (ZrO₂) structure with the cell parameters: a=4.375(1)Å, b=4.584(1)Å, c=4.708(1)Å, β=99.97(3), ρ=4.30(2)g/cm³. TEM-SAED pattern of this grain revealed the presence of the α-PbO₂-like SiO₂ polymorph, stishovite, secondary cristobalite, and dense silica glass. The coexistence of several high-density polymorphs and dense silica glass in the same grain suggests that several post-stishovite phases were formed during the shock event in Shergotty. Some of these polymorphs were highly unstable and vitrified, presumably in the decompression stage. Based on diamond anvil experiments on cristobalite a peak shock pressure in excess of 40 GPa could be deduced. The petrographic setting and texture of the single stishovite grains in the melt pockets is different. The mono-phase individual grains occur exclusively as large (>10 μm) rounded objects inside melt pockets together with hollandite structured plagioclase composition and the new CAS phase [1]. Stishovite in melt pockets is barren of any sign of a tweed pattern and contains no silica glass. This suggests that the mechanisms of phase transitions were different in the two lithologies. Stishovite in the melt pockets probably did not form by a retrograde transformation from a post-stishovite polymorph.