Polyhedral models of
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.
The full version of CrystalMaker includes ca. 300 annotated mineral structures including all the major rock-forming minerals!
Silica Phase Diagram
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.
Download Silica polymorphs library (72 K zip archive)
Compatibility Note: These files require CrystalMaker 9 or later.