The room-temperature form of

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

, α-quartz, undergoes a reversible change in crystal structure at 573 °C to form β-quartz. This phenomenon is called an inversion, and for the α to β quartz inversion is accompanied by a linear expansion of 0.45%. This inversion can lead to cracking of

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...

ware if cooling occurs too quickly through the inversion temperature. This is called ''dunting'', and the resultant faults as ''dunts''. To avoid such thermal shock faults, cooling rates not exceeding 50 °C/hour have been recommended. At 870 °C quartz ceases to be stable but, in the absence of fluxes, does not alter until a much higher temperature is reached, when, depending on the temperature and nature of the fluxes present, it is converted into the polymorphs of

cristobalite Cristobalite 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 ...

and / or tridymite. These polymorphs also experience temperature-induced inversions. The inversion of cristobalite at 220 °C can be advantageous to achieve the ''cristobalite squeeze''. This puts the glazes into compression and so helps prevent

crazing Crazing is the phenomenon that produces a network of fine cracks on the surface of a material, for example in a glaze layer. Crazing frequently precedes fracture in some glassy thermoplastic polymers. As it only takes place under tensile stre ...

. The size of the silica particles influences inversions, conversions and other properties of the ceramic body."Influence Of The Fine Grinding Of Hard Materials On Porcelain And Vitreous Tableware." E.Signorini E. ''Industrial Ceramics.'' No.858, 1991, pg. 174–179. The presence of other ceramic raw materials can influence the thermal behaviour of quartz, including: * Talc promotes the conversion of quartz to

, and if sufficient alumina is available the formation of cordierite. * Nepheline syenite increases the dissolution of silica. * Petalite promotes the formation of cristobalite. * Alumina can react with silica to form mullite.

References

{{Reflist Ceramic materials

See also

References