Structure of Sodium Aluminosilicate Melts Containing at Least 50 mole % SiO2 at 1500°C

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Abstract
Viscosity and density data were obtained for several series of alkali aluminosilicate melts between 1200° and 1700°C with a counterbalanced sphere viscometer. Property comparisons are presented as functions of the SiO2 content, Na2O content, and constant Al/Na ratios at 1500° and 1700°C. The viscosity and expansion information support the formation of three‐dimensional cristobalitelike liquids on the Al/Na=1.0 line with some of the aluminum (III) in the form of AlO6 octahedra for Al/Na>1.0 melts. The molar volume models that coincide with experiment for Al/Na≤1.0 melts involve AlO4 tetrahedra and v̄ Na2O values that increase with increase of the Al2O3 content. The model that best agrees with experiment for melts with Al/Na>1.0 involves only some of the excess aluminum ions in octahedral coordination as well as a changing v̄ Na2O. Melt molar volume dependencies on composition can resemble those observed for similar glasses, which suggests structural similarities. The present liquid structure picture differs somewhat from previous concepts of alumina‐rich glass structures in this ternary system.