Coesite

High-pressure polymorph of silicon dioxide (SiO₂). Has the same chemical composition as cristobaliteHigh temperature polymorph of silicon dioxide (SiO2). Has the same chemical composition as coesite, stishovite, seifertite and tridymite but possesses a different crystal structure. This silica group mineral occurs in terrestrial volcanic rocks, martian and lunar meteorites, chondrites and impact glasses like Libyan Desert Glass. Cristobalite has a very open, stishoviteDense, high-pressure phase of quartz; so far identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters. Stishovite was synthesized in 1961 before it was discovered at Meteor Crater, Arizona. Its structure consists of parallel chains of single SiO6 octahedra. The octahedra are on their sides, sharing opposing edges. Image, seifertite and tridymiteSilica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. but possesses a different crystal structureMutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal.. Coesite forms at intense pressures of above about 2.5 GPa (25 kbar) and temperature above about 700 °C, and was first found naturally on Earth in impact craters. More recently, coesite and other high pressure silicaSilicon dioxide, SiO2. polymorphs have been found in lunar specimens (sample return and meteorites) and eucrites.