archive: SETI [ASTRO] Novel Natural, Very Dense Polymorph Of Silica

SETI [ASTRO] Novel Natural, Very Dense Polymorph Of Silica

Larry Klaes ( lklaes@bbn.com )
Fri, 28 May 1999 12:43:51 -0400

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>Date: Fri, 28 May 1999 15:55:49 GMT
>From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>To: astro@lists.mindspring.com
>Subject: [ASTRO] Novel Natural, Very Dense Polymorph Of Silica Discovered
In Martian Meteorite Shergotty
>Sender: owner-astro@brickbat12.mindspring.com
>Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>
>Max Planck Institute for Chemistry
>Mainz, Germany
>
>Contact: Ahmed El Goresy
>Phone: (+49 63 31) 305 - 2 92 Fax: (+49 63 31) 37 12 90
>
>May 25, 1999
>
>A new natural very dense polymorph of silica from the Martian meteorite
>Shergotty: Implications for the possible heterogeneity of the Earth's lower
>mantle
>
>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.
>
>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).
>
>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 3D 4.16 B1 0.03 C5 b 3D
>5.11 B1 0.04 C5, c 3D 4.55 B1 0.01 C5, V 3D 96.91 B1 0.63 C53) and are
>consistent with the Pbcn space group of a-PbO2. Assuming four formula units
>per cell (Z 3D 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 magnesiowFCstite 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.
>
>[Image caption: http://www.mpg.de/news99/news27_99.htm]
>
>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 B5m 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.
>
>
>