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Formula: SiO2
Oxide, orthorhombic high pressure paramorph of
coesite (high pressure, monoclinic),
cristobalite (high temperature, tetragonal),
mogánite (monoclinic), quartz
(low pressure and temperature for alpha-quartz, trigonal), stishovite
(high pressure, tetragonal) and
tridymite (low pressure, high temperature, triclinic)
Crystal System: Orthorhombic
Specific gravity: 4.294 calculated
Environments
Seifertite is the densest and hardest polymorph of silica found
in nature so far (February 2024). Found first in the Shergotty meteorite, and since then in several other Martian
meteorites. Stable only above pressures of 780 kilobars, so it could theoretically also form in the Earth's mantle
at depths over 1,700 km, if any free silica is present. Suggested as a minor phase near the core-mantle boundary
(Mindat).
Localities
The type locality, the Shergotty Martian meteorite, Gaya District, Bihar, India, is a
basaltic
achondrite consisting of 70%
pyroxene and 24% glass with
labradorite composition, called "maskelynite". Minor constituents are
titanium-bearing magnetite,
ilmenite,
pyrrhotite and silica. Silica grains, up to 900 μm in size, are usually
enclosed in "maskelynite" and rarely border pyroxene. Rare shock melted
mesostasis (last-formed, finely crysralline) pockets with much smaller prismatic silica grains are also encountered.
All silica grains consist of a lamellar intergrowth of at least two dense
polymorphs and dense silica glass. The large seifertite-bearing
silica grains show the typical pre-shock morphology and habit of tridymite
or cristobalite but not quartz.
Other silica grains in Shergotty contain another monoclinic dense
polymorph, stishovite, plus
seifertite, as well as a secondary
cristobalite-stishovite
intergrowth and secondary
cristobalite
(EJM 20.523-528.
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