Stishovite

Stishovite is an extremely hard, dense tetragonal form (polymorph) of silicon dioxide. 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.^[6]

Stishovite
Crystal structure of stishovite
General
Category	Tectosilicate, quartz group
Formula	SiO2
IMA symbol	Sti[1]
Strunz classification	4.DA.40 (Oxides)
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H–M symbol: (4/m 2/m 2>/m)
Space group	P42/mnm (No. 136)
Unit cell	a = 4.1772(7) Å, c = 2.6651(4) Å; Z = 2
Identification
Color	Colorless (when pure)
Mohs scale hardness	9.5[2]
Luster	Vitreous
Diaphaneity	Transparent to translucent
Specific gravity	4.35 (synthetic) 4.29 (calculated)
Optical properties	Uniaxial (+)
Refractive index	nω = 1.799–1.800 nε = 1.826–1.845
Birefringence	δ = 0.027
Melting point	(decomposes)
References	[3][4][5]

Stishovite was named after Sergey M. Stishov [ru], 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.^[7]

Unlike other silica polymorphs, the crystal structure of stishovite resembles that of rutile (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.^[8] It was long considered the hardest known oxide (~30 GPa Vickers^[2]); however, boron suboxide has been discovered^[9] 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.^[7]

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 impact into quartz-bearing rock. Minute amounts of stishovite have been found within diamonds,^[10] and post-stishovite phases were identified within ultra-high-pressure mantle rocks.^[11] Stishovite may also be synthesized by duplicating these conditions in the laboratory, either isostatically or through shock (see shocked quartz).^[12] At 4.287 g/cm³, it is the second densest polymorph of silica, after seifertite. It has tetragonal crystal symmetry, P4₂/mnm, No. 136, Pearson symbol tP6.^[13]

See also

• Coesite, a related mineral
• Thaumasite, another rare mineral with hexacoordinated octahedral silica