| Stishovite | |
|---|---|
 Crystal structure of stishovite | |
| General | |
| Category | Tectosilicate,quartz group |
| Formula | SiO2 |
| IMA symbol | Sti[1] |
| Strunz classification | 4.DA.40 (Oxides) |
| Crystal system | Tetragonal |
| Crystal class | Ditetragonal dipyramidal (4/mmm) H–M symbol: (4/m 2/m 2>/m) |
| Space group | P42/mnm (No. 136) |
| Unit cell | a = 4.1772(7) Å, c = 2.6651(4) Å;Z = 2 |
| Identification | |
| Color | Colorless (when pure) |
| Mohs scale hardness | 9.5[2] |
| Luster | Vitreous |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 4.35 (synthetic) 4.29 (calculated) |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.799–1.800 nε = 1.826–1.845 |
| Birefringence | δ = 0.027 |
| Melting point | (decomposes) |
| References | [3][4][5] |
Stishovite is an extremely hard, densetetragonal form (polymorph) ofsilicon dioxide. It is very rare on the Earth's surface; however, it may be a predominant form of silicon dioxide in the Earth, especially in thelower mantle.[6]
Stishovite was named afterSergey M. Stishov [ru], a Russian high-pressure physicist who first synthesized the mineral in 1961. It was discovered inMeteor Crater in 1962 byEdward C. T. Chao.[7]
Unlike other silica polymorphs, the crystal structure of stishovite resembles that ofrutile (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 applyinghydrogen fluoride (HF); unlike quartz, stishovite will not react.[7]
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.
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 duringhypervelocitymeteoriteimpact intoquartz-bearingrock. 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 (seeshocked quartz).[12]At 4.287 g/cm3, it is the second densest polymorph of silica, afterseifertite. It hastetragonal crystal symmetry, P42/mnm, No. 136,Pearson symbol tP6.[13]
{{cite journal}}: CS1 maint: multiple names: authors list (link)| Crystalline | |||||||||
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