Silica is a chemical compound, silicon dioxide, and a mineral of formula SiO2, current and very hard. It represents more than 60% of the mass of the earth's crust. Silica SiO2 is chemically stable and hardly attacked by hydrofluoric acid, a reaction used in glass etching.
Silica exists in several crystalline forms including:
- Quartz, the most widespread species in nature
- Tridymite (melting point 1670°C)
- Cristobalite (melting point 1713°C)
- In amorphous form: silica glass or vitreous silica
Each of the crystallized forms has several varieties. The transition from one species to another results in dimensional variations, some of which are important at the scale of ceramic materials. This often results in thermal shock fragility and other phenomena due to expansion tension.
The stability of the species will depend on the temperature range to which they are subjected, the fineness of the grains, the cooking atmosphere and the presence of other oxides acting as catalysts (mainly the alkalis: K2O, Na2O, Li2O and also alkaline-earths with CaO) also called "mineralisers".
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