The combined effects of chlorine and fluorine on the viscosity of aluminosilicate melts

Abstract

The effect of fluorine and fluorine + chlorine on melt viscosities in the system Na 2O–Fe 2O 3–Al 2O 3–SiO 2 has been investigated. Shear viscosities of melts ranging in composition from peraluminous [(Na 2O + FeO) < (Al 2O 3 + Fe 2O 3)] to peralkaline [(Na 2O + FeO) > (Al 2O 3 + Fe 2O 3)] were determined over a temperature range 560–890 °C at room pressure in a nitrogen atmosphere. Viscosities were determined using the micropenetration technique in the range of 10 8.8 to 10 12.0 Pa s. The compositions are based on addition of FeF 3 and FeCl 3 to aluminosilicate melts with a fixed amount of SiO 2 (67 mol%). Although there was a significant loss of F and Cl during glass syntheses, none occurred during the viscometry experiments. The presence of fluorine causes a decrease in the viscosity of all melts investigated. This is in agreement with the structural model that two fluorines replace one oxygen; resulting in a depolymerisation of the melt and thus a decrease in viscosity. The presence of both chlorine and fluorine results in a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. The variation in viscosity produced by the addition of both fluorine and chlorine is the opposite to that observed in the same composition melts, with the addition of chlorine alone (Zimova M. and Webb S.L. (2006) The effect of chlorine on the viscosity of Na 2O–Fe 2O 3–Al 2O 3–SiO 2 melts. Am. Mineral. 91, 344–352). This suggests that the structural interaction of chlorine and fluorine is not linear and the rheology of magmas containing both volatiles is more complex than previously assumed.