STRUCTURE OF MAGNESIUM ALUMINIOSILICATE LIQUIDS AT 1 700 °C

Abstract

An improved counterbalanced sphere viscometer–densitometer was used to obtain precise viscosity and density data for melts on (a) the silica •cordierite (2MgO•2Al₂O₃•5SiO₂) •spinel (MgO•Al₂O₃) join and (b) the 50 mole % SiO₂line at temperatures between 1 500 and 1 720 °C. The viscosity, activation energy for viscous flow, and expansion data are indicative of a general tightening of melt flow parameters and structure as either the silica or the alumina content is increased.The density-derived data have been interpreted in terms of a model that involves no significant change of the partial molar volume of SiO2 (Vsio₂). Crystalline coordination number information was used to develop a liquid molar volume model that agrees rather closely with the experimental information. This model involves AlO₄tetrahedra as the major aluminium (III) species for all melts studied with Al₂O₃/MgO ratios equal to or less than unity. The gradual formation of MgO₆octahedra from MgO₄tetrahedra appears to accompany the substitution of MgO for Al₂O₃in melts containing 50 mole % SiO₂.It has been demonstrated that a reasonable correlation can exist between the coordination number of a given cation in the crystalline state and in high-temperature liquids of similar composition.