Abstract:
The temperature dependence of viscosity was investigated at 1 atm and at temperatures near the glass transition in the dry Qz-Ab-Or system [SiO2 (Qz)-NaAlSi3O8 (Ab)-KAlSi3O8 (Or)]. This system is particularly difficult for the interpretation of high-precision viscosity measurements because light compositional deviations of the agpaitic index (AI) [(Na2O+K2O)/Al2O3] from one and/or variations in trace water contents can have significant effects on the viscosity of the melts at comparable temperatures. The 16 investigated compositions contained joins with 82, 85 and 87 mol% SiO2, and various Na2O/(Na2O+K2O) ratios ranging from 0.9 to 0.1. The water contents of the nominally dry melts varied between 100 and 200 ppm.The results show that there is no pronounced mixed alkali effect on the viscosities of these melts. Rather, the viscosity increases with increasing potassium content along each silica isopleth (more than 1 log unit for 82 mol% SiO2), and increases slightly with increasing SiO2 content at fixed Na2O/(Na2O+K2O) ratios. Comparison of the observed data with the prediction of Hess and Dingwell [Hess, K.U., Dingwell, D.B., 1996. Viscosities of hydrous leucogranitic melts - a non-Arrhenian model. Am. Mineral. 81 (9-10), 1297-1300.] model for calculating the viscosities of calc-alkaline rhyolitic melts leads to the recommendation that the model can be used for viscosity estimation with Qz-Ab-Or proportions close to those of natural compositions.