Silica (the chemical compound SiO) has a number of distinct crystalline forms in addition to the quartz structure. Nearly all of the crystalline forms involve tetrahedral SiO units linked together by shared vertices in different arrangements (stishovite, composed of linked SiO octahedra, is the main exception). Si-O bond lengths vary between the different crystal forms. For example, in α-quartz the bond length is , whereas in α-tridymite it ranges from . The Si-O-Si bond angle also varies from 140° in α-tridymite to 144° in α-quartz to 180° in β-tridymite. Any deviations from these standard parameters constitute microstructural differences or variations that represent an approach to an amorphous, vitreous or glassy solid.
The transition temperature T in silicates is related to the energy required to break and re-form covalent bonds in an amorphous (or random network) lattice of covalent bonds. The T is clearly influenced by the chemistry of the glass. For example, addition of elements such as B, Na, K or Ca to a silica glass, which have a valency less than 4, helps in breaking up the network structure, thus reducing the T. Alternatively, P, which has a valency of 5, helps to reinforce an ordered lattice, and thus increases the T.
T is directly proportional to bond strength, e.g. it depends on quasi-equilibrium thermodynamic parameters of the bonds e.g. on the enthalpy H and entropy S of configurons – broken bonds: T = H / [S + R ln[(1 − f)/ f] where R is the gas constant and f is the percolation threshold. For strong melts such as SiO the percolation threshold in the above equation is the universal Scher–Zallen critical density in the 3-D space e.g. f = 0.15, however for fragile materials the percolation thresholds are material-dependent and f ≪ 1. The enthalpy H and the entropy S of configurons – broken bonds can be found from available experimental data on viscosity.