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The viscosity of Na₂O-CaO-Al₂O₃-SiO₂ melts

Mineralogy Department, Georg-August-University, Göttingen, Germany

^* Corresponding author, e-mail: swebb{at}gwdg.de

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.

Key-words: melt structure, CaO, Na₂O, viscosity, peraluminous.