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3.6 d. Shear viscosity measurements close to the charge balanced join in the system MgO-Al2O3-SiO2 (M.J. Toplis and D.B. Dingwell)

It is generally thought that in aluminosilicate melts and glasses, aluminum occurs in tetrahedral coordination as long as sufficient metal cations, such as Na+ or Mg2+, are present to play a charge balancing role to satisfy the charge deficit of Al3+. In this case, the structural role of aluminum should change across the subaluminous join, when the ratio Mn+/Al = 1. This change in the structural role of aluminum should in turn result in a change of the behavior of shear viscosity as a function of composition across the subaluminous join. Results for the systems Na2O-Al2O3-SiO2 (NAS) and CaO-Al2O3-SiO2 (CAS) previously reported (see Annual Report 1996, 1997) show that although the variation of shear viscosity across the subaluminous join does indeed suggest a change in the structural role of one or more elements, this change does not in general occur exactly at the subaluminous join, implying an equilibrium between different aluminum bearing species in the melt. For example, it was suggested that aluminum may occur not only in tetrahedral coordination associated with metal cations, but also in tetrahedral coordination associated with a three-coordinated oxygen (in the case of both the NAS and CAS systems) and in a network modifying role, in the CAS system.

This detailed study of shear viscosities of ternary aluminosilicate melts close to the subaluminous join has been extended to the system MgO-Al2O3-SiO2 (MAS). Experiments were performed in the temperature range 1300 to 1650°C using the concentric cylinder technique, and studied compositions spanned the subaluminous join (molar Mg/(Mg+0.5Al) in the range 0.60 to 0.40) along silica isopleths at 50, 67 and 75 mol% SiO2.

The results obtained show that at 1600°C viscosities in the system MAS show very little variation for the compositions with the greatest excess of Mg over Al. However, with addition of aluminum a change to decreasing values of shear viscosity is observed at all three silica isopleths. Although at 50 mol% SiO2 the onset of this viscosity decrease occurs close to the subaluminous join, at 75 and 67 mol% SiO2 it occurs within the field of excess Mg. This is similar to the system CAS where a local viscosity maximum was observed to occur in the excess calcium field along the 75 and 67 mol% SiO2 isopleths, but not at 50 mol% SiO2, suggesting that the structural entity responsible for the viscosity maximum in the excess cation field is the same in the MAS and CAS systems. This feature may be attributed to a proportion of aluminum in the melt having a network modifying character (coordination number greater than four) at 1600°C, even in the excess cation field. In contrast to the NAS and CAS systems, no evidence for a viscosity maximum in the peraluminous field is observed in the system MAS, suggesting an important difference between the structure of melts in these different systems.

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