Stishovite/

Introduction

Stishovite is a rare, high-pressure polymorph of silica that is tetragonal in crystal structure. It was first discovered in meteorite impact craters and named after its discoverer, Sergei M. Stishov, a Russian high-pressure physicist. Stishovite is one of the densest forms of silica, and its formation requires extremely high pressures and temperatures, conditions typically found in meteorite impacts or deep within the Earth's mantle.

Discovery and Naming

Stishovite was first discovered in 1961 in the Barringer Meteor Crater in Arizona by Sergei M. Stishov and L. F. Vereschchagin. The mineral was named in honor of Stishov for his contributions to high-pressure research. It was the first natural occurrence of a silica mineral in the tetragonal crystal system.

Physical Properties

Stishovite is a dense, hard mineral with a Mohs hardness of 7.5, comparable to that of quartz. However, unlike quartz, stishovite has a high density of 4.29 g/cm3 due to its compact crystal structure. It is optically uniaxial and positive with a refractive index of 1.799. Stishovite is colorless to white in appearance and has a vitreous luster.

Crystal Structure

Stishovite has a tetragonal crystal structure, which is unique among silica polymorphs. This structure is characterized by each silicon atom being surrounded by six oxygen atoms in an octahedral arrangement. This contrasts with the four oxygen atoms surrounding each silicon atom in a tetrahedral arrangement in other silica polymorphs, such as quartz.

Formation and Occurrence

Stishovite forms under extremely high pressures of at least 100,000 atmospheres and temperatures above 1200 °C. These conditions are typically found in meteorite impacts or deep within the Earth's mantle. Stishovite has been found in several meteorite impact craters around the world, including the Barringer Meteor Crater in Arizona, the Ries Crater in Germany, and the Popigai Crater in Russia.

Uses and Significance

Due to its rarity and the extreme conditions required for its formation, stishovite has few practical uses. However, it is of significant scientific interest due to its unique properties and formation conditions. The presence of stishovite in a geological sample can be used as an indicator of past high-pressure events, such as meteorite impacts.