The viscosities of dry and hydrous XAlSi 3O 8 (X=Li, Na, K, Ca 0.5, Mg 0.5) melts

Abstract

The low-temperature viscosities of dry and hydrous X (X=Li, Na, K, Ca 0.5, Mg 0.5)AlSi 3O 8 melts have been investigated. The samples were hydrated via piston cylinder synthesis, and the water contents were subsequently determined by Karl-Fischer titration (KFT) and IR spectroscopy. Both the anhydrous and hydrous viscosities were measured using the micropenetration technique in the range of viscosities between 10 8.5 to 10 11.9 Pa s, at 1 atm pressure and in the temperature ranges of 745–990°C and 400–790°C for the dry and wet melts, respectively. The range of water content varied for all of the samples from 0.70 to 3.13 wt.% H 2O. The viscosities of dry melts vary, at fixed temperature, as a complex function of the identity of the cation in the order Li<Na<Ca≤Mg<K. This trend is interpreted as due to the combined effects of cation field strength and (Si, Al) distribution in these melts. With the introduction of water into these melts, the viscosity decreases for all of the compositions investigated. As water is further dissolved, the array of anhydrous viscosities converges into two distinct curves, for alkali-bearing and alkaline-earth-bearing aluminosilicate liquids, respectively. In contrast to the insensitivity of viscosity to alkali cation identity for hydrous melts, the alkali/aluminium ratio remains a sensitive control on viscosity. Thus, the viscosities of a slightly peralkaline albite glass (Na exc) are lower than all of the others, both for the dry and the hydrous systems. We suggest that, in the case of alkaline-earth-bearing melts, an aluminium pair must be closely related to a doubly charged cation, to maintain electrostatic neutrality. The increase in the size of smallest rearranging species, which participates in the viscous flow process, as well as clustering of silica-rich and alumina-rich domains on an “intermediate-range” scale, may be the factors resulting in the higher viscosities of Ca- and Mg-bearing compared to alkali-bearing liquids.