Hand-drawn diagrams of the polymorphs of silica
This diagram shows the basic tetrahedral SiO4 building block (Fig. 51) and α-quartz (Fig. 235), β-quartz (Fig. 236), β-cristobalite (Fig. 236) and β-tridymite (Fig. 237). Polymorphs have the same chemical formulae but different structures. Mellor describes the significance of these forms in the glazing process. 'Nature has set limits of temperature within which each form of silica is stable. If these limits be exceeded, the silica becomes unstable and undergoes a molecular cataclysm, whereby the structural units rearrange themselves of their own accord, slowly or quickly, according to a different architectural plan, so as to produce another kind of silica which is stable only in a new range of temperature' (Mellor, Crazing and Peeling of Glazes, 1935).
Joseph W. Mellor, “Hand-drawn diagrams of the polymorphs of silica,” ourheritage.ac.nz | OUR Heritage, accessed February 4, 2023, https://otago.ourheritage.ac.nz/items/show/9008.