Quartz inversion

The room-temperature form of quartz, 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 ware if cooling occurs too quickly through the inversion temperature. This is called dunting, and the resultant faults are known as dunts.^[1] To avoid such thermal shock faults, cooling rates not exceeding 50 °C/hour have been recommended.^[2]

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 and / or tridymite.^[3] 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.^[4][5][6]

The size of the silica particles influences inversions, conversions and other properties of the ceramic body.^[7][8][9] The presence of other ceramic raw materials can influence the thermal behaviour of quartz, including:

• Talc promotes the conversion of quartz to cristobalite, 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.

See also

• Veining (metallurgy), sand casting defect associated with the alpha to beta silica phase change