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Novel Natural, Very Dense Polymorph Of Silica Discovered In Martian Meteorite Shergotty

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.


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