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To complement the detailed viscosity measurements in the system Na₂O-Al₂O₃-SiO₂ (NAS) previously presented (Annual Report, 1996) additional high precision viscosity measurements of 18 compositions in the system CaO-Al₂O₃-SiO₂ (CAS) were performed using the concentric cylinder technique. Compositions were close to the subaluminous join (Ca/(Ca+2Al) in the range 0.60 to 0.40) and covered four isopleths of silica content in the range 75 to 33 mol% SiO₂. In addition the viscosities of compositions along the 50 mol% silica isopleth were also determined in the glass transition range using a micropenetration technique.

In contrast to the results for compositions in the system NAS, results obtained in the system CAS show that at high temperature (1600°C) viscosity begins to flatten off within the peralkaline field. Viscosity is approximately constant over a wide range of composition close to the subaluminous join, but falls sharply within the peraluminous field. At 75 and 67 mol% SiO₂ the presence of two distinct viscosity maxima may be distinguished (one in the peralkaline and one in the peraluminous field). At 50 mol% SiO₂ only the peak in the peraluminous field may be discerned. With decreasing temperature, the peak in the peralkaline field decreases in magnitude compared to that in the peraluminous field. At the glass transition of melts close to anorthite in composition only a single well defined peak within the peraluminous field is observed. At 75 and 67 mol% SiO₂ the displacement of the peraluminous viscosity maximum at 1600°C is similar to that observed in the NAS system, but at 50 mol% SiO₂ the maximum does not return to the subaluminous join but rather occurs even deeper within the peraluminous field.

Whereas the presence of a viscosity maximum within the peraluminous field for the NAS system could be satisfactorily explained by a proportion of Al occuring in ´triclusters´ (three tetrahedra which share a common oxygen) containing one aluminate and two silicate tetrahedra, the situation for the CAS system is clearly more complex. We attribute the flattening off of viscosity within the peralkaline field at high temperature to the possible presence of a significant proportion of aluminium occuring with a coordination number higher than four: the network modifying character of such aluminium leading to a minimum number of non-bridging oxygens occuring in the peralkaline field. The disappearance of this feature with decreasing temperature is consistent with the results of MD simulations and direct spectroscopic study which infer that high-coordinated aluminium is favoured with increasing temperature. Although high coordinate aluminium may be present in CAS melts at high temperature, little or none is inferred to be present at the glass transition, consistent with spectroscopic studies of CAS glasses. The presence of a second viscosity maximum occurring within the peraluminous field, is interpretated to suggest that triclusters also occur in CAS glasses, as recently suggested by NMR spectroscopy. In contrast to the case of NAS melts, the deviation of the peraluminous viscosity maximum is not greatest at 67 mol% SiO₂ in the system CAS, implying that if triclusters are present, those containing only one aluminate tetrahedron do not dominate. However, it is not possible to determine whether the dominant triclusters are those consisting of 2 aluminate and 1 silicate, or 3 aluminate tetrahedra.

The dynamic formation and destruction of triclusters at high temperature may play an important role during processes of viscous and diffusive flow in highly polymerised melts, analogous to the role inferred for 5-fold coordinated silicon and/or aluminium in depolymerised compositions.