The viscosity of Na (sub 2) O-CaO-Al (sub 2) O (sub 3) -SiO (sub 2) melts

The viscosity of melts in the Na (sub 2) O-CaO-Al (sub 2) O (sub 3) -SiO (sub 2) system follows the same trend as a function of composition as that observed in the Na (sub 2) O-Al (sub 2) O (sub 3) -SiO (sub 2) system; in that there is a local maximum in viscosity at the condition Na (sub 2) O+CaO approximately Al (sub 2) O (sub 3) . This is despite the difference in charge of Na (super +) and Ca (super 2+) , the similarity in their radius, and XANES data together with MD calculations from the literature that Ca (super 2+) has an affinity to form NBOs with Si (super 4+) tetrahedra, thus possibly leaving Na (super +) to act as a charge-balancer for Al (super 3+) in tetrahedral co-ordination. The overall description of melt structure and the general flow mechanism appear to be the same for Na (sub 2) O-Al (sub 2) O (sub 3) -SiO (sub 2) and CaO-Al (sub 2) O (sub 3) -SiO (sub 2) melts as there are no anomalous changes in viscosity (or glass density) which could be associated with a strong preference for Ca (super 2+) to form NBOs; or geometrical difficulties accompanying the need for 2 Al (super 3+) tetrahedra to be charge-balanced by the same Ca (super 2+) in the Al (super 3+) -rich composition melts.