Stishovite is an extremely hard, dense
tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...
form (
polymorph) of
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
. 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 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
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visib ...
(TiO
2). 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 SiO
2. 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
Boron suboxide (chemical formula B6O) is a solid compound with a structure built of eight icosahedra at the apexes of the rhombohedral unit cell. Each icosahedron is composed of twelve boron atoms. Two oxygen atoms are located in the interstices al ...
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.
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
Hypervelocity is very high velocity, approximately over 3,000 meters per second (6,700 mph, 11,000 km/h, 10,000 ft/s, or Mach 8.8). In particular, hypervelocity is velocity so high that the strength of materials upon impact is v ...
meteorite impact into quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
-bearing rock. Minute amounts of stishovite have been found within diamonds, and post-stishovite phases were identified within ultra-high-pressure mantle rocks. Stishovite may also be synthesized by duplicating these conditions in the laboratory, either isostatically or through shock (see shocked quartz).
At 4.287 g/cm3, it is the second densest polymorph of silica, after seifertite
Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of silica. It has only been found in Martian and lunar meteorites, where it is presumably formed from either tridymite or cristobalite – other polymo ...
. It has tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...
crystal symmetry, P42/mnm, No. 136, Pearson symbol tP6.
See also
* Coesite
Coesite is a form ( polymorph) of silicon dioxide Si O2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the ...
, a related mineral
* Thaumasite
Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedron, octahedral configuration, with chemical formula calcium, Ca3silicon, Si(Ohydrogen, H)6(carbon, Coxygen, O3)(sulfur, SO4)·12water, H2O, also sometimes more simply ...
, another rare mineral with hexacoordinated octahedral silica
References
External links
Properties of stishovite
{{Impact cratering on Earth
Superhard materials
Impact event minerals
Tetragonal minerals
Minerals in space group 136
Silica polymorphs
Soviet inventions