Abstract
Shaw's method for the calculation of the viscosities of magmatic silicate liquids has been extended to multicomponent silicate melts in which Al 2O 3 exceeds the sum of alkali metal oxides and alkaline earth oxides. Al 2O 3 was converted to AlO 2 and divided into the network-forming part, AlO 2 0, and the excess part, AlO 2 *. AlO 2 0 and AlO 2 * differed in their effect on viscosity. S AlO 2 * ∘ value, which evaluates the effect of AlO 2 * 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 AlO 2 *. From the viscosity measurement data of synthesized multicomponent melts, S AlO 2 * ∘ value was 5.6. This is smaller than the S AlO 2 0 ∘ value of 6.7. Using the S AlO 2 * ∘ value with other S i ∘ 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.
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