Polymorphs of silica (SiO2 )

Polymorphs of SiO2

SiO2 exhibits a complex polymorphism characterised by the occurrence of a large number of reversible and irreversible phase transformations usually associated with important relative volume changes (ΔV/V). 

At low temperature and pressure beta-quartz (β-quartz) predominates, but above 573°C, it transforms reversibly into the high-temperature alpha-quartz (α-quartz) with a small volume change (0.8 to 1.3 vol.%):

β-quartz <—> α-quartz (573°C)

Quartz exhibits a very low coefficient of thermal expansion (0.5 μm/m.K) and an elevated Mohs hardness of seven. Large and pure single crystals of quartz of gem quality called lascas are used due to their high purity in the preparation of elemental silicon for semiconductors. At a temperature of 870°C, α-quartz transforms irreversibly into alpha-tridymite (α-tridymite, orthorhombic) with an important volume change of 14.4 vol.% as follows:

α-quartz <—> α-tridymite (870°C)

But in practice, the kinetic of the above reaction is too slow, and tridymite never forms below 1250°C, and hence at 1250°C or 1050°C in the presence of impurities, α-quartz transforms irreversibly into alpha-cristoballite (α-cristoballite, tetragonal) with an important volume change (17.4 vol.%) as follows:

α-quartz <—> α-cristoballite (1250°C)

However, if the temperature is raised to 1470°C, α-tridymite transforms also irreversibly into alpha-cristoballite (α-cristoballite) without any change in volume as follows:

α-tridymite <—> α-cristoballite (1470°C)

On cooling α-cristoballite transforms reversibly into beta-cristoballite (β-cristoballite, cubic) at 260°C with a volume change 0 2.0 to 2.8 vol.%:

α-cristoballite <—> β-cristoballite (260°C)

Finally, α-cristoballite melts at 1713°C while α-tridymite melts at 1670°C. Upon cooling silica melt yields amorphous fused silica. There also exist two high-pressure polymorphs of silica called coesite and stishovite that occur in strongly mechanically deformed metamorphic rocks (e.g., impactites), but these two phases are usually not encountered in ceramics, refractories, and glasses.

Industrially, silica products are classified into two main groups: natural silica products —quartzite, silica sand, and diatomite—and specialty silicas including fumed silica, silica gel, microsilica, precipitated silica, fused silica, and vitreous silica.

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