Melt viscosities in the system Na-Fe-Si-O-F-Cl; contrasting effects of F and Cl in alkaline melts

Abstract

The shear viscosities of melts in the system Na-Fe-Si-O-F-Cl were determined over a wide range of temperatures (400-1200 °C) at 1 atm pressure in air. The compositions are based on the addition of Fe 2 O 3 , FeCl 3 , and FeF 3 to a base melt composition corresponding to sodium disilicate (Na 2 S i 2O 5 ). Viscosities were determined using concentric cylinder and micropenetration methods and measurements span the range of 10 0.5 to 10 11 Pa·s. The chemical compositions of these melts were analyzed after the viscometry determinations. The iron is fully oxidized under the conditions of the viscometry. Although F and especially Cl are volatile elements in silicate melts, levels of Cl and F up to over 3 and 4 wt%, respectively, were stabilized in these melts, assisted presumably by the presence of Fe 3+ . Although some volatilization occurred during the original synthesis of these samples, none occurred during viscometry. The anionic substitutions Cl 2 O -1 and F 2 O -1 have very different influences on the viscosity. The F 2 O -1 substitution causes a drastic decrease in viscosity over the entire investigated range whereas the Cl 2 O -1 substitution causes a much smaller decrease in viscosity in the high viscosity range and a slight increase in viscosity in the low viscosity range. As a consequence, minor to major element abundance of Cl in strongly peralkaline undersaturated volcanic rocks are not likely to significantly influence melt viscosity.