New Natural Very Dense Polymorph Of Silica From The Martian Meteorite Shergotty: Implications For The Possible Heterogeneity Of Earth's Lower Mantle

Figure 1. BSE-mode SEM images of SiO2 and maskelynite. (A) SiO2 grain and maskelynite are surrounded by a fractured clinopyroxene (cpx) where the fractures radiate more than 500 µm from the SiO2. (B) FESEM image of a triangular SiO2 grain showing the tweed-like internal microstructure with some nearly orthogonal lamellae and fractures radiating outward into the surrounding maskelynite. (C) individual domains (10-50 mm in diameter) each with a distinct tweed pattern of intersecting thin (< 300 nm) lamellae. Shergotty meteorite is heavily shocked. It contains maskelynite that was long thought to be diaplectic plagioclase glass which formed by a solid-state transformation, that resulted from shock-wave propagation as a result of a hypervelocity large impact on the Martian surface. Maskelynite appears to have been quenched from a shock-induced melt. Shergotty also contains large silica grains (> 150 mm) that were previously interpreted to be birefringence shocked quartz with planar deformation features (PDFs). Many of the SiO2 grains are surrounded by radiating cracks (Fig. 1A). Full size image available through contact

A novel natural, very dense polymorph of silica was discovered in a Martian meteorite by Geoscientists from China, United States, and from the Max Planck Institute for Chemistry in Mainz/Germany (Science 28 May 1999). Based on the chondritic model, SiO₂ makes up 50 wt. % of Earth's bulk. This raises the question if free very dense silica polymorphs exists in Earth's deep interior. Although it is generally accepted that the SiO₂ component of the Earth's lower mantle occurs as (Mg,Fe)SiO₃-perovskite, recent experimental evidence for the dissociation of perovskite at about 80 GPa greatly increases the probability of free dense silica in the Earth's lower mantle.

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.

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