Replacing SiO₂ by alkali oxides sharply increases the glass-forming ability of high-silica melts

Replacing silica with alkali oxides significantly alters most glass properties, including two quantities that govern glass-forming ability (GFA): viscosity, η, and liquidus temperature, T_L. The conventional view is that higher silica content yields higher GFA. However, in binary SiO₂–alkali oxide systems, substituting SiO₂ by Li₂O, Na₂O or K₂O up to the first eutectic raises the viscosity at T_L, η(T_L), and thus reduces the critical cooling rate for crystal formation, R_c. Using literature data, thermodynamic phase equilibrium modeling, and viscosity modeling, we show here that this enhancement extends to multicomponent, naturally occurring high-silica compositions. We also demonstrate that the JeZiCA, GFA predictor, JeZiCA=η(T_L)/T_L², holds for the natural basalt–rhyolite series examined, with R² = 0.95 ± 0.02, log(R_c[K/s])=−0.747log(JeZiCA[Pa·s/K²])−3.01. This route simultaneously improves GFA and lowers processing costs, with direct relevance to industrial applications for silicate glasses.