Quartz inversion
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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.
- Veining (metallurgy), sand casting defect associated with the alpha to beta silica phase change
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