Pentacoordinated silicon (SiO5) has long been considered as a possible reactive intermediate in bond-swapping reactions in even ambient pressure viscous flow, diffusion, nucleation and crystallization. In this paper, new results are presented for potassium silicate glasses and for two lithium silicates. In the former, SiO5 is readily detectable by 29Si MAS NMR on 29Si-enriched glasses at concentrations as low as about 0.02 mol%; in the latter this species is below detection limits. SiO5 concentrations are higher at higher fictive temperatures, and first increase, then decrease, as K2O is added to SiO2. This pattern in compositional variation resembles those long-known in alkali borate and germanates, although high coordinate species (BO4 and GeO5/GeO6) are orders of magnitude more abundant than in ambient pressure silicates. A simple thermodynamic model, considering only the non-ideal mixing of bridging and non-bridging oxygens, at least qualitatively predicts the shape of the compositional variation of SiO5 in the potassium silicates, and is sensible with respect to known tendencies for clustering and unmixing; such a model also predicts curves of network cation coordination vs. compositon that resemble those known for borates and germanates, suggesting an underlying similarity in the energetics of mixing of anionic species. © 2019 The Author
