Empirical method of calculating the viscosity of peraluminous silicate melts at high temperatures

Shawʹs method for the calculation of the viscosities of magmatic silicate liquids has been extended to multicomponent silicate melts in which Al2O3 exceeds the sum of alkali metal oxides and alkaline earth oxides. Al2O3 was converted to AlO2 and divided into the network-forming part, AlO20, and the excess part, AlO2*. AlO20 and AlO2* differed in their effect on viscosity. SAlO2*∘ value, which evaluates the effect of AlO2* on viscosity, was chosen to compensate for the difference in Arrhenius slope between the data obtained by viscosity measurement and the data calculated from the chemical compositions other than AlO2*. From the viscosity measurement data of synthesized multicomponent melts, SAlO2*∘ value was 5.6. This is smaller than the SAlO20∘ value of 6.7. Using the SAlO2*∘ value with other Si∘ values from Shaw (1972), the viscosities of peraluminous glassy rocks produced by the 1988–1989 eruptions of Tokachi-dake volcano (Hokkaido, Japan) were calculated. The calculated viscosities agreed well with the measured ones.