Stishovite

Stishovite is an extremely hard, dense <a href="/facts/Tetragonal/12Aop255">tetragonal</a> form (<a href="/facts/Polymorphism_(materials_science)/bu2tqsB9">polymorph</a>) of <a href="/facts/Silicon_dioxide/A3WILp6J">silicon dioxide</a>. 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 <a href="/facts/Lower_mantle_(Earth)/KQQBHWp8">lower mantle</a>.
Stishovite was named after Sergey M. Stishov [ru], a Russian high-pressure physicist who first synthesized the mineral in 1961. It was discovered in <a href="/facts/Meteor_Crater/J3KyXJ8w">Meteor Crater</a> in 1962 by <a href="/facts/Edward_C._T._Chao/Ll3nuhPM">Edward C. T. Chao</a>.
Unlike other silica polymorphs, the crystal structure of stishovite resembles that of <a href="/facts/Rutile/qwJH5ElN">rutile</a> (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, <a href="/facts/Boron_suboxide/91BkPrz4">boron suboxide</a> 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 <a href="/facts/Hydrogen_fluoride/XfQxlqR6">hydrogen fluoride</a> (HF); unlike quartz, stishovite will not react.

Stishovite open-in-new

Stishovite