Another high pressure quartz polymorph features in today’s post, but this one is even more extreme than coesite. Stishovite has a similar history to coesite – it was first created in the laboratory and a natural occurrence was discovered soon after.
Stishovite (black) in the Muonionalusta Meteorite (image from mindat.com)
- Oxide mineral
- Formed in high pressure-temperature environments
- Type locality: Meteor Crater, Coconino County, Arizona, USA
- Formula: SiO2
- Crystal system: Tetragonal
- Hardness: 7.5-8
- Density: 4.35/cm3
Stishovite occurs as small crystals up to 2mm in size which are usually found in rocks from impact craters or as inclusions in diamonds. Stishovite formed by impact metamorphism experiences pressures of over 10GPa and temperatures exceeding 1200°C.
Even though the chemical formula is SiO2, its crystal structure is different to quartz and other silicates. Silicate minerals have tetrahedral coordination where each silicon atom is surrounded by four oxygen atoms, but in stishovite, the coordination is octahedral so silicon atoms are surrounded by six oxygen atoms. Because of this, stishovite is classed as an oxide mineral; it is isomorphous with rutile (has the same crystal structure). This crystal arrangement also makes stishovite denser than the other quartz polymorphs.
Ball and stick models of a silicon dioxide tetrahedron (left) and octahedron (right). Blue = silicon and red = oxygen (images by Ben Mills, left image from commons.wikipedia.org, right image from commons.wikipedia.org)
Stishovite is named after Sergei Mikhailovich Stishov who first synthesised the mineral. He was a geochemistry graduate student at Moscow State University in the early 1960s. A few years earlier, Francis Birch had predicted a high pressure phase of silica, so Stishov decided to create it with high pressure-temperature experiments. In 1961, he synthesised stishovite. Just one year later, Edward C. T. Chao discovered the mineral in Meteor Crater, Arizona, USA and the mineral was named in Stishov’s honour.
Stishov, S.M., 1995. Memoir on the discovery of high-density silica. International Journal of High Pressure Research, 13(5), pp.245-280