Stishovite
| Stishovite | |
|---|---|
| General | |
| Category | Mineral |
| Chemical formula | SiO2 |
| Strunz classification | 4.DA.40 |
| Crystal symmetry | Tetragonal ditetragonal dipyramidal H-M symbol: (4/m 2/m 2/m) Space group: P 4/mmm |
| Unit cell | a = 4.1772(7) Å, c = 2.6651(4) Å; Z=2 |
| Identification | |
| Color | Colorless (when pure) |
| Crystal system | Tetragonal |
| Mohs scale hardness | 7.5 - 8 |
| Luster | Vitreous |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 4.287 |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.799 - 1.800 nε = 1.826 - 1.845 |
| Birefringence | δ = 0.027 |
| Melting point | 1650 (±75) °C[citation needed] |
| References | [1][2][3] |
Stishovite (or shocked quartz) is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide. It was long considered the hardest known oxide; however, boron suboxide has recently been discovered to be much harder. At normal temperature and pressure, stishovite is metastable, as it will eventually decay to quartz; however, this phase change is slow enough that it has never been observed.
Stishovite was named for Sergey M. Stishov, 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.[4]
Unlike other silica polymorphs, the crystal structure of stishovite resembles that of rutile (TiO2). The silicon in stishovite, like in rutile, is in octahedral coordination with oxygen, with six oxygens surrounding every silicon. However, most silicates have silicon in tetrahedral, four-fold coordination with oxygen.[5]
Stishovite can be separated from quartz by applying hydrogen fluoride (HF); unlike quartz, stishovite will not react.[4]
Contents |
[edit] 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. Recently (2007), minute amounts of stishovite has been found within diamonds,[6] and post-stishovite phases were identified within ultra-high pressure mantle rocks.[7] Stishovite may also be synthesized by duplicating these conditions in the laboratory, either isostatically or through shock (see shocked quartz) [8] With a mass density of 4.287 g/cm3, stishovite is the second heaviest polymorph of silica, after seifertite. It has tetragonal crystal symmetry, P42/mnm, No.136, Pearson symbol tP6.[9]
[edit] See also
- Coesite, a related mineral
[edit] References
- ^ Handbook of Mineralogy
- ^ Stishovite on Mindat.org
- ^ Stishovite on Webmineral
- ^ a b Fleischer, Michael (1962). "New mineral names" (PDF). American Mineralogist (Mineralogical Society of America) 47 (2): 172–174. http://rruff.info/uploads/AM47_805.pdf.
- ^ Ross, Nancy L. (1990). "High pressure crystal chemistry of stishovite" (PDF). American Mineralogist (Mineralogical Society of America) 75 (7): 739–747. http://www.minsocam.org/ammin/AM75/AM75_739.pdf.
- ^ Wirth, R; Vollmer, C; Brenker, F; Matsyuk, S; Kaminsky, F (2007). "Inclusions of nanocrystalline hydrous aluminium silicate "Phase Egg" in superdeep diamonds from Juina (Mato Grosso State, Brazil)". Earth and Planetary Science Letters 259 (3–4): 384. Bibcode 2007E&PSL.259..384W. doi:10.1016/j.epsl.2007.04.041.
- ^ Liu, L; Zhang, J; Greenii, H; Jin, Z; Bozhilov, K (2007). "Evidence of former stishovite in metamorphosed sediments, implying subduction to >350 km". Earth and Planetary Science Letters. Bibcode 2007E&PSL.263..180L. doi:10.1016/j.epsl.2007.08.010. http://micron.ucr.edu/public/KNB-papers/Liu-et-al-2007.pdf.
- ^ J. M. Léger, J. Haines, M. Schmidt*, J. P. Petitet, A. S. Pereira & J. A. H. da Jornada (1996). "Discovery of hardest known oxide". Nature 383: 401. doi:10.1038/383401a0.
- ^ Smyth J.R., Swope R.J., Pawley A.R. (1995). "H in rutile-type compounds: II. Crystal chemistry of Al substitution in H-bearing stishovite". American Mineralogist 80: 454–456. http://rruff.geo.arizona.edu/doclib/am/vol80/AM80_454.pdf.
[edit] External links
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