Glass formation in silicates: Insights from composition

The composition dependence of glass formation is examined in a variety of silicate systems that include alkali and alkaline earth alumino-, titano-, ferro- and ferrisilicates. Empirically, there is a clear correlation between wide extent of glass formation, possible crystallization from the melt of numerous compounds, and moderate liquidus temperatures. Vitrification with usual cooling rates is in contrast impossible when binary and ternary compounds are scarce and liquidus temperatures are high. These correlations imply that vitrification is favored by moderately negative enthalpies of mixing in the melt but made difficult by high configurational heat capacities. The close connection between glass formation and viscosity is reviewed in the light of these melt properties. That bulk viscosity is in general not directly relevant to the kinetics of crystal nucleation in particular indicates that vitrification theories cannot be considered as by-products of crystallization theories.