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Concept
Cristobalite
Summary
Cristobalite (krɪˈstoʊbəˌlaɪt) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group, which also include coesite, tridymite and stishovite. It is named after Cerro San Cristóbal in Pachuca Municipality, Hidalgo, Mexico.
It is used in dentistry as a component of alginate impression materials as well as for making models of teeth.
Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures. The persistence of cristobalite outside its thermodynamic stability range occurs because the transition from cristobalite to quartz or tridymite is "reconstructive", requiring the breaking up and reforming of the silica framework. These frameworks are composed of SiO4 tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula of silica is SiO2. The breaking of these bonds, required to convert cristobalite to tridymite and quartz, requires considerable activation energy and may not happen on a human time frame at room temperature. Framework silicates are also known as tectosilicates.
When devitrifying silica, cristobalite is usually the first phase to form, even when well outside its thermodynamic stability range. This is an example of Ostwald's step rule. The dynamically disordered nature of the β phase is partly responsible for the low enthalpy of fusion of silica.
There is more than one form of the cristobalite framework. At high temperatures, the structure is called β-cristobalite. It is in the cubic crystal system, space group Fdm (No. 227, Pearson symbol cF104). It has the diamond structure but with linked tetrahedra of silicon and oxygen where the carbon atoms are in diamond. A chiral tetragonal form called α-cristobalite (space group either P41212, No. 92, or P43212, No.
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