The viscosity of melts in the Na2O-CaO-Al2O3-SiO2 system follows the same trend as a function of composition as that observed in the Na2O-Al2O3-SiO2 system; in that there is a local maximum in viscosity at the condition Na2O+CaO ~Al2O3. This is despite the difference in charge of Na+ and Ca2+, the similarity in their radius, and XANES data together with MD calculations from the literature that Ca2+ has an affinity to form NBOs with Si4+ tetrahedra, thus possibly leaving Na+ to act as a charge-balancer for Al3+ in tetrahedral co-ordination. The overall description of melt structure and the general flow mechanism appear to be the same for Na2O-Al2O3-SiO2 and CaO-Al2O3-SiO2 melts as there are no anomalous changes in viscosity (or glass density) which could be associated with a strong preference for Ca2+ to form NBOs; or geometrical difficulties accompanying the need for 2 Al3+ tetrahedra to be charge-balanced by the same Ca2+ in the Al3+-rich composition melts.

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