The relationships between the chemical composition and the derivative rheological and thermodynamic values have been determined for two melt series in the anorthite–wollastonite–gehlenite (An–Wo–Geh) compatibility triangle. The melt series have 0.5 and 1 non-bridging oxygens per tetrahedrally coordinated cation (NBO/T), respectively. The influences of the ratio Si/(Si+AlCa1/2) and NBO/T on the fragility and the configurational entropy at Tg are evaluated. Linear dependencies of the viscosity, the glass transition temperature and the fragility on the ratio Si/(Si+AlCa1/2) are found for the two melt series. A crossover in the viscosity–temperature relationship is observed for both series, i.e. an inverse compositional dependence of viscosity in the high and low viscous range. The crossover presumably reflects different responses of the adjustment of melt structure to the substitution of Al3++1/2Ca2+ for Si4+ in the low versus the high viscous ranges. The crossover shifts to higher temperature with increasing NBO/T.