Stishovite
Stishovite is an extremely dense form of silica that forms at very high shock pressures and is a strong indicator of meteorite impact.
Stishovite is a polymorph of silica (SiO2), meaning the same chemical formula arranged in a different crystal structure. In ordinary quartz each silicon atom is bonded to four oxygen atoms; in stishovite it is bonded to six, in the rutile structure, giving a density of about 4.3 g/cm³ compared with 2.65 g/cm³ for quartz. It needs pressures above roughly 10 GPa to form. Physicist Sergey Stishov and colleagues synthesised it in 1961, and the next year it was found in nature at Barringer Crater (Meteor Crater), Arizona.
Because such pressures are not reached in Earth's crust, stishovite in surface rocks is a powerful piece of evidence for a hypervelocity impact. It is generally rarer than coesite at craters, because it forms only in the most intensely shocked material and tends to revert to glass when the rock remains hot after the shock wave passes. High-pressure silica phases have also been reported from some heavily shocked meteorites.
Recognising stishovite is strictly laboratory work. Grains are usually microscopic, so researchers use X-ray diffraction, Raman spectroscopy or transmission electron microscopy, sometimes after dissolving away the surrounding quartz to concentrate it.
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