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.
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