Stishovite

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Stishovite
Specific gravity
4.35 (synthetic)
4.29 (calculated)
Optical propertiesUniaxial (+)
Refractive indexnω = 1.799–1.800
nε = 1.826–1.845
Birefringenceδ = 0.027
Melting point(decomposes)
References[3][4][5]

Stishovite is an extremely hard, dense

lower mantle.[6]

Stishovite was named after Sergey M. Stishov [ru], a Russian high-pressure physicist who first synthesized the mineral in 1961. It was discovered in Meteor Crater in 1962 by Edward C. T. Chao.[7]

Unlike other silica polymorphs, the crystal structure of stishovite resembles that of rutile (TiO2). The silicon in stishovite adopts an octahedral coordination geometry, being bound to six oxides. Similarly, the oxides are three-connected, unlike low-pressure forms of SiO2. In most silicates, silicon is tetrahedral, being bound to four oxides.[8] It was long considered the hardest known oxide (~30 GPa Vickers[2]); however, boron suboxide has been discovered[9] in 2002 to be much harder. At normal temperature and pressure, stishovite is metastable.

Stishovite can be separated from quartz by applying hydrogen fluoride (HF); unlike quartz, stishovite will not react.[7]

Appearance

Large natural crystals of stishovite are extremely rare and are usually found as clasts of 1 to 2 mm in length. When found, they can be difficult to distinguish from regular quartz without laboratory analysis. It has a vitreous luster, is transparent (or translucent), and is extremely hard. Stishovite usually sits as small rounded gravels in a matrix of other minerals.

Synthesis

Until recently, the only known occurrences of stishovite in nature formed at the very high shock pressures (>100 kbar, or 10 GPa) and temperatures (> 1200 °C) present during hypervelocity meteorite impact into quartz-bearing rock. Minute amounts of stishovite have been found within diamonds,[10] and post-stishovite phases were identified within ultra-high-pressure mantle rocks.[11] Stishovite may also be synthesized by duplicating these conditions in the laboratory, either isostatically or through shock (see shocked quartz).[12] At 4.287 g/cm3, it is the second densest polymorph of silica, after

tetragonal crystal symmetry, P42/mnm, No. 136, Pearson symbol tP6.[13]

See also

  • Coesite, a related mineral
  • Thaumasite, another rare mineral with hexacoordinated octahedral silica

References

  1. S2CID 235729616
    .
  2. ^
    doi:10.1016/j.physb.2007.06.011
    . and references therein
  3. ISBN 0962209716
    . Retrieved December 5, 2011.
  4. ^ Stishovite. Mindat.org.
  5. ^ Stishovite. Webmineral.com.
  6. doi:10.1073/pnas.0706113104
    .
  7. ^ a b Fleischer, Michael (1962). "New mineral names" (PDF). American Mineralogist. 47 (2). Mineralogical Society of America: 172–174.
  8. ^ Ross, Nancy L. (1990). "High pressure crystal chemistry of stishovite" (PDF). American Mineralogist. 75 (7). Mineralogical Society of America: 739–747.
  9. doi:10.1063/1.1494860
    .
  10. doi:10.1016/j.epsl.2007.04.041
    .
  11. doi:10.1016/j.epsl.2007.08.010. Archived from the original
    (PDF) on 2010-07-17.
  12. doi:10.1038/383401a0.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  13. S2CID 196903109
    .

External links
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