Stishovite
Encyclopedia
Stishovite 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 synthesized by Stishov in 1961 and discovered in a meteor impact 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, 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.
Stishovite can be separated from quartz by applying hydrogen fluoride
(HF); unlike quartz, stishovite will not react.
impact
into quartz
-bearing rock
. Recently (2007), minute amounts of stishovite has 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
)
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.
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
) of silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
. It was long considered the hardest known oxide; however, boron suboxide
Boron suboxide
Boron suboxide is a solid compound of boron and oxygen.Its structure is 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 along the [111] rhombohedral direction...
has recently been discovered to be much harder. At normal temperature and pressure, stishovite is metastable, as it will eventually decay to quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
; however, this phase change is slow enough that it has never been observed. Stishovite was synthesized by Stishov in 1961 and discovered in a meteor impact crater in 1962 by Edward C. T. Chao
Edward C. T. Chao
Edward C. T. Chao was one of the founders of the field of impact metamorphism, the study of the effects of meteorite impacts on the Earth's crust....
.
Unlike other silica polymorphs, the crystal structure of stishovite resembles that of rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
(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.
Stishovite can be separated from quartz by applying hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
(HF); unlike quartz, stishovite will not react.
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 meteoriteMeteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
impact
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
into quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
-bearing rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
. Recently (2007), minute amounts of stishovite has 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
Shocked quartz
Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure , the crystalline structure of quartz will be deformed along planes inside the crystal...
)
With a mass density of 4.287 g/cm3, stishovite is the second heaviest polymorph of silica, after seifertite
Seifertite
Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of quartz. It has only been found in Martian and lunar meteorites where it is presumably formed from either tridymite or cristobalite – other polymorphs of quartz – as a result of heating during the...
. It has tetragonal crystal symmetry, P42/mnm, No.136, Pearson symbol
Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8...
tP6.