Stishovite

Stishovite is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide. It was long considered the hardest known oxide (~30 GPa Vickers); 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 named for 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, 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.

Production


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 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.