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Silicon is tetrahedrally coordinated by oxygen in the low-pressure SiO2 polymorphs; quartz, tridymite, cristobalite, and in its high-pressure polymorph coesite. Silicon is coordinated by six oxygens in the high-pressure SiO2 polymorph stishovite. The synthesis of stishovite and its subsequent discovery in naturally shocked rocks in Meteor Crater, Arizona and the Ries Crater, Germany, has revolutionised the study of shock metamorphism and impact cratering by providing an index mineral, in addition to coesite, that can be used as proof of shock metamorphism.
Synthesis of stishovite also ignited considerable interest the existence and stabilities of other dense polymorphs with octahedrally coordinated silicon. SiO2 polymorphs that are more dense than stishovite are important in determining the stability of perovskite in the Earth's lower mantle. Such "post-stishovite" polymorphs of silica are also of eminent importance in understanding the dynamic history of shocked rocks in terrestrial meteorite craters, because they could serve as indicators of extreme shock pressure. Additional constrains on shock pressures in meteorites are important to unravel the impact records of asteroids and planets in the early history of the solar system.
These cracks are similar to those formed around coesite in high pressure metamorphic rocks and indicate that the volume of the silica phase increased greatly with decompression. Expansion must have occurred when the maskelynite was solid because the cracks also cut through it (Fig. 1B). The lamellar texture appears as two sets of lamellae of different brightness in field-emission scanning electron microscopy (FESEM) images, recorded in back-scattered-electron (BSE) mode (Fig. 1B). Back-scattered-electron imaging with a field-emission SEM (FESEM) revealed that the original silica grains consist of a mosaic of many individual domains (10-50 mm in diameter) each with a distinct pattern of intersecting thin (< 300 nm) lamellae (Fig. 1C).
The scientists investigated the crystallinity and structure of SiO2 phases, using laser Raman microprobe spectroscopy, transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). The crystallinity and structure of SiO2 phases was investigated by using laser Raman microprobe spectroscopy, transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). The electron diffraction (SAED) data fit a post-stishovite structure with space group Pbcn that is similar to the a-PbO2 structure, a new polymorph of silica. Although the diffraction data are insufficient to exactly determine the space group, they fit an orthorhombic unit cell (a = 4.16 ± 0.03 Å b = 5.11 ± 0.04 Å, c = 4.55 ± 0.01 Å, V = 96.91 ± 0.63 Å3) and are consistent with the Pbcn space group of a-PbO2. Assuming four formula units per cell (Z = 4) as in Pbcn, the density is 4.12 gm/cm3.
The stability of (Mg,Fe)SiO3-perovskite in the deep lower mantle is dependent on the structures and free energies of the SiO2 phases. If (Mg,Fe)SiO3-perovskite decomposes to SiO2 plus magnesiowüstite in the lower mantle, the SiO2 would have a post-stishovite structures. The present results confirm the existence of such a structure in a natural sample.