The kinetic fragility of natural silicate melts

Newtonian viscosities of 19 multicomponent natural and synthetic silicate liquids, with variable contents of SiO₂ (41-79 wt%), Al₂O₃ (10-19 wt%), TiO₂ (0-3 wt%), FeO_tot (0-11 wt%); alkali oxides (5-17 wt%), alkaline-earth oxides (0-35 wt%), and minor oxides, obtained at ambient pressure using the high-temperature concentric cylinder, the low-temperature micropenetration, and the parallel plates techniques, have been analysed. For each silicate liquid, regression of the experimentally determined viscosities using the well known Vogel-Fulcher-Tammann (VFT) equation allowed the viscosity of all these silicates to be accurately described. The results of these fits, which provide the basis for the subsequent analysis here, permit qualitative and quantitative correlations to be made between the VFT adjustable parameters (A_VFT, B_VFT, and T₀). The values of B_VFT and T₀, calibrated via the VFT equation, are highly correlated. Kinetic fragility appears to be correlated with the number of nonbridging oxygens per tetrahedrally coordinated cation (NBO/T). This taken to infer that melt polymerization controls melt fragility in liquid silicates. Thus NBO/T might form an useful ingredient of a structure-based model of non-Arrhenian viscosity in multicomponent silicate melts.