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The vast majority of viscosity data that has been obtained for silicic silicate melts has been concerned with simple synthetic compositions that serve as analogs for natural silicic melts. The feldspar compositions, albite and orthoclase, SiO₂, and several approximations of the 1-2 kbar P(H₂O) melt minimum compositions in the hydrous NaAlSi₃O₈-KAlSi₃O₈-SiO₂ system have been investigated. To understand how melt viscosity varies, in more detail, in the haplogranitic (NaAlSi₃O₈-KAlSi₃O₈-SiO₂) system, we have performed a high precision study
in the high viscosity range for melts HPG1-HPG16 (Fig. 3.6-3) spanning a considerable range
 

Fig. 3.6-3: Viscosities (log, Pa s) of melts in the system NaAlSi3O8 (Ab)-KAlSi3O8 (Or)-SiO2 (Qz) at a temperature of 950°C.

of the geo-relevant portion of the NaAlSi₃O₈-KAlSi₃O₈-SiO₂ system. Working in this dry system is especially difficult because small amounts of water and slight variation in the alkali/aluminum ratio both can lead to significant changes in melt viscosity. Thus the measured data for the HPG1-16 compositions have been corrected for nonstoichiometry both in water content and alkali/aluminum ratio. Following these corrections, the variation of viscosity with composition in the NaAlSi₃O₈-KAlSi₃O₈-SiO₂ system is small, smooth and linear from higher viscosities in melts near the KAlSi₃O₈-SiO₂ join to lower viscosities near the NaAlSi₃O₈-SiO₂ join. This trend is consistent with the relative viscosities of NaAlSi₃O₈ and KAlSi₃O₈ melts and lends further maintenance to how slight the variation is for geologically realistic metaluminous compositions. We anticipate, however, that even these subtle variations in viscosity will be removed by the addition of water to these melt compositions. Overall, this implies that the model of Hess and Dingwell (1996) to calculate viscosities of hydrous calcalkaline melts will also be acceptable for calculation of hydrous melt viscosities covering the compositional range illustrated by Fig. 3.6-3.