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In nature, fluorine and chlorine can be significantly enriched in hypabbyssal and volcanic rocks. However, experimental studies of anionic substitution in silicate melts are lacking. While it is now well established that fluorine strongly decreases the viscosity of silicate melts (see previous yearbooks), almost nothing is known about the effects of the other halides on melt viscosities. Recently, (Annual Report 1997) we have demonstrated that chlorine contrasts sharply with fluorine in this aspect, leading to little or no viscosity decrease and perhaps even a viscosity increase in sodium iron silicate melts. The high volatility of the halides, particularly chlorine, was circumvented in that study by the stabilizing infuence of ferric iron in those melts. In order to attempt a generalisation of the results in terms of anionic substitutions in silicate melts we have expanded the study to include bromine and iodine. We have now successfully performed an investigation of the influence of bromine and iodine on melt viscosities where several wt% of each has been stably contained in high-temperature melts.

The experimental determination of viscosities were performed in the simple model system Na-Fe-Si-O-(F,Cl,Br,I) in the temperature range from 400 to 1200°C using concentric cylinder and micropenetration methods covering the viscosity range from 10⁰ to 10¹² Pa s. The compositions were based on a Na₂Si₂O₅-melt, to which individually variable amounts of Fe₂O₃, FeCl₃, FeF₃, FeBr₃ and FeI₃ were added. An exact chemical analysis was performed after the viscosity measurements. It was concluded from the analysis that during the initial fusion stage there has been volatilisation of some components but not during viscometry measurements. Concentrations of the halides up to 4 weight percent could be achieved. As noted above, the anionic substitutions of Cl₂O_-1 and F₂O_-1 influence the viscosity very differently. Substitution of F₂O_-1 decreases the viscosity dramatically over the total investigated temperature range, whereas the substitution of Cl₂O_-1 reduces the viscosity at low temperatures (glass-transition region) but increases the viscosity at high temperatures (superliquidus region). The effects of bromine and iodine on the viscosity of the sodium iron silicate melts are very slight and very similar to those of chlorine. Thus we infer that the distinct anionic properties of fluorine distinguish it from all other halides both in terms of its volatility and its influence on melt properties such as viscosity.