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

The shear viscosities of melts in the system Na-Fe-Si-O-F-Cl were determined over a wide range of temperatures (400-1200 [degrees] C) at 1 atm pressure in air. The compositions are based on the addition of [Fe.sub.2][O.sub.3], Fe[Cl.sub.3], and Fe[F.sub.3] to a base melt composition corresponding to sodium disilicate ([Na.sub.2][Si.sub.2][O.sub.5]). Viscosities were determined using concentric cylinder and micropenetration methods and measurements span the range of [10.sup.0.5] to [10.sup.11] Pa[center dot]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.sup.3+]. Although some volatilization occurred during the original synthesis of these samples, none occurred during viscometry. The anionic substitutions [Cl.sub.2][O.sub.-1] and [F.sub.2][O.sub.-1] have very different influences on the viscosity. The [F.sub.2][O.sub.-1] substitution causes a drastic decrease in viscosity over the entire investigated range whereas the [Cl.sub.2][O.sub.-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.