Stishovite
Stishovite is an extremely hard, dense tetragonal form (polymorph) of silicon 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 the lower mantle.
| Stishovite | |
|---|---|
Crystal structure of stishovite | |
| General | |
| Category | Tectosilicate, quartz group |
| Formula (repeating unit) | SiO2 |
| IMA symbol | Sti |
| 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 |
| 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 | |
Stishovite was named after 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.
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. It was long considered the hardest known oxide (~30 GPa Vickers); however, boron suboxide has been discovered 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.