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 (geometry), cube becomes a rectangular Pri ...

form ( polymorph) of

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

. 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 , a Soviet high-pressure physicist who first synthesized the mineral in 1961. It was then discovered in Meteor Crater in 1962 by

Edward C. T. Chao Edward Ching-Te Chao (; November 30, 1919 – February 3, 2008) was one of the founders of the field of impact metamorphism, the study of the effects of meteorite impacts on the Earth's crust. Born in Suzhou, China, he was best known for di ...

. 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 vis ...

(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 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 ...

has been discovered in 2002 to be much harder. At normal temperature and pressure, stishovite is

metastable In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...

. Stishovite can be separated from quartz by applying

hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

(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

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

impact into

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

-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. It has

crystal symmetry, P4₂/mnm, No. 136, Pearson symbol tP6.

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

Appearance

Synthesis

See also

References

External links