The viscosities of dry and hydrous XAlSi3O8 (X = Li, Na, K, Ca-0.5, Mg-0.5) melts

The low-temperature viscosities of dry and hydrous X (X = Li, Na, K, Ca-0.5 , Mg-0.5)AlSi3O8 melts have been investigated. The samples were hydrated vi a piston cylinder synthesis, and the water contents were subsequently deter mined by Karl-Fischer titration (KFT) and IR spectroscopy. Both the anhydro us and hydrous viscosities were measured using the micropenetration techniq ue in the range of viscosities between 10(8.5) to 10(11.9) Pa s, at 1 atm p ressure and in the temperature ranges of 745-990 degreesC and 400-790 degre esC for the dry and wet melts, respectively. The range of water content var ied for all of the samples from 0.70 to 3.13 wt.% H2O The viscosities of dr y melts vary, at fixed temperature, as a complex function of the identity o f the cation in the order Li < Na < Ca less than or equal to Mg < K. This t rend 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 fo r all of the compositions investigated. As water is further dissolved, the array of anhydrous viscosities converges into two distinct curves, for alka li-bearing and alkaline-earth-bearing aluminosilicate liquids, respectively . In contrast to the insensitivity of viscosity to alkali cation identity f or hydrous melts, the alkali/aluminium ratio remains a sensitive control on viscosity. Thus, the viscosities of a slightly peralkaline albite glass (N a-exc) are lower than all of the others, both for the dry and the hydrous s ystems. We suggest that, in the case of alkaline-earth-bearing melts, an al uminium pair must be closely related to a doubly charged cation, to maintai n electrostatic neutrality. The increase in the size of smallest rearrangin g species, which participates in the viscous now process, as well as cluste ring of silica-rich and alumina-rich domains on an "intermediate-range'' sc ale, may be the factors resulting in the higher viscosities of Ca- and Mg-b earing compared to alkali-bearing liquids. (C) 2001 Elsevier Science B.V, A ll rights reserved.