Stishovite

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Stishovite
Category Mineral
Chemical formula Silica (silicon dioxide, SiO2)
Identification
Color Clear (if no impurities); also see Varieties
Mohs Scale hardness 9.4
Refractive index nω = 1.81
Specific gravity 4.28 when pure
Melting point 1650 (±75) °C
Solubility 11.0 +/- 1.1 PPM @ 25 C
Diaphaneity Transparent to translucent

Stishovite (after S.M. Stishov, 20th-century Russian mineralogist) is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide. It was traditionally considered the hardest known oxide; however, boron suboxide was recently discovered to be much harder. At normal temperature and pressure, stishovite is metastable; it will eventually decay to quartz; however, this phase change is slow enough that it has never been observed. Stishovite was first discovered in nature and named by Edward C. T. Chao.

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[edit] Synthesis

The only known occurrences of stishovite in nature formed at the very high shock pressures (>100 kbar = 10 GPa) and temperatures (> 1200°C) present during hypervelocity meteorite impact into quartz-bearing rock. Stishovite may also be synthesized by duplicating these conditions in the laboratory, either isostatically or through shock (see shocked quartz).

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High-pressure silica polymorphs as hardest known oxides

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