Modeling the charge compensation effect in silica-rich Na2O-K2O-Al2O3-SiO2 melts

At high SiO2 contents, a model of Na2O-K2O-Al2O3-SiO2 melts must take into account the “charge compensation effect” whereby the replacement of a tetravalent Si4+ by a trivalent Al3+ in the silicate network is facilitated by the formation of (NaAl)4+ or (KAl)4+ issociates. This effect has been taken into account in the quasichemical model by treating the (NaAl)4+ and (KAl)4+ associates as separate species in the melt, which is then formally treated as consisting of the components NaO1/2, KO1/2, AlO3/2, SiO2, (NaAl)O2 and (KAl)O2. Optimizations of the Na2O-Al2O3-SiO2, K2O-Al2O3-SiO2 and NaAlSiO4-KAlSiO4-SiO2 systems at high SiO2 contents are reported.