The influence of alkaline-earth oxides (BeO, MgO, CaO, SrO, BaO) on the viscosity of a haplogranitic melt: systematics of non-Arrhenian behaviour
Hess, Kai-Uwe; Dingwell, Donald B.; Webb, Sharon L.
European Journal of Mineralogy Volume 8 Number 2 (1996), p. 371 - 382
published: Apr 26, 1996
manuscript accepted: Oct 31, 1995
manuscript received: Jan 4, 1995
ArtNo. ESP147050802018, Price: 29.00 €
Abstract The effect of the addition of 5, 10 and 20 wt % of the alkaline-earth oxides on the viscosity of a haplogranitic melt composition has been investigated at 1 atm pressure and in the temperature range of 640 to 1675°C. The high-temperature viscosity data were obtained with concentric-cylinder viscometry and the lowtemperature viscosity data using micropenetration viscometry. The combined data sets for low and high temperature viscosities have been fit for each composition using the Tamann-Vogel-Fulcher (TVF) equation. The effect of alkaline-earth oxide on the viscosity of the present haplogranitic melt is extremely variable. The viscosity decreases with added alkaline-earth oxide content in a nonlinear fashion. The first few mole % of alkaline-earth oxide added decreases viscosity up to several orders of magnitude whereas subsequent addition of alkaline-earth oxide has a much smaller effect. The effects of each of the alkaline-earth oxides are quite distinct on a molar basis. The viscosity of the melts investigated here decreases with the size of the added alkaline-earth cation in the order Ba < Sr < Ca < Mg < Be. This trend of viscosity versus cation size is the opposite to that observed for the effects of alkali oxide additions on the melt viscosity. The alkaline-earth-bearing melts are significantly non- Arrhenian. The degree of "fragility" N or non-Arrhenian temperature-dependence of viscosity of these melts may be quantified using the ratio of the c parameter in the TVF fit to the value of temperature at which the viscosity equals 1012 Pa s, designated TN- The parameter N = C/TN varies smoothly as a single function of the number of moles of added cations for all melts of this study as well as all the alkali-oxide-rich peralkaline melts