Silica Polymorphs Framework structures of immense crystal-chemical importance
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Silica Polymorphs Description

Silica, SiO2, exists in a number of different crystalline forms. At low pressures and temperatures the most stable phase is the well-known mineral quartz, familiar as "beach sand" or as large "rock crystal" specimens. On increasing temperature, tridymite, and then cristobalite become more stable. These are low-density structures, in which each silicon atom is bonded to four oxygen atoms at the corners of a tetrahedron, and each tetrahedron is bonded to four other tetrahedra, so that the structure is a fully-polymerised three-dimensional framework. On increasing temperature, quartz, tridymite and cristobalite undergo series of displacive phase transitions involving distortions of their tetrahedral frameworks. At high pressures, denser silica structures are formed. The mineral coesite has a structure resembling that of feldspar. At higher pressures an even denser structure is formed - stishovite - in which six oxygen atoms pack tightly around each silicon atom. Crystals of stishovite have been found in meteorite impact craters, where desert sand (quartz) has been violently pressurised and transformed. The full version of CrystalMaker includes over 600 annotated structure files on disc, including nearly 300 distinct mineral phases - the major rock-forming minerals - plus organic and inorganic structures of technological importance. The Silica Polymorphs package provides framework structures of immense crystal-chemical importance.

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