Viscosity of melts in the NaAlSiO4-KAlSiO4-SiO2 system: Configurational entropy modelling

We provide new viscosity data in the system SiO2-(Na,K)AlSiO4, for the nepheline-kalsilite and jadeite-leucite joins. We present a configurational entropy model for the viscosity of melts in the system as a function of Na/(Na+K) and Al/(Al+Si) ratios. Our modelling indicates that: i) Viscosity data are reproduced well by a non-ideal, symmetrical form of the parameters Sconf (Tg) and Be, ii) Na-K mixing is the main source of additional entropy in the system based on the limited dependence of Sconf (Tg) and Be parameters on Al/(Al+Si) ratio, iii) Ae likely varies as a function of Al/(Al+Si) ratio. Melt fragility in the system increases with increasing Al/(Al+Si) ratio and is greater for Na or K end-member melts than mixed melts. The viscosity of nominally fully-polymerized melts in the SiO2- (Na,K)AlSiO4 system can be modelled through chemical mixing, without explicit consideration of the important changes in structure related to changes in K/(Na+K) and Al/(Al+Si) ratios