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Halogens are minor elements in a wide range of magmatic compositions found in both intrusive complexes and volcanic centres. Although both Cl and F are considered volatile, Cl possesses a far higher volatility in most naturally occurring melts than F. This difference in their volatilities could have significant implications for magmatic and volcanic systems where the halogen concentration of either can exceed 1 wt%. Fluorine concentrations are high in certain rhyolitic volcanics where evidence of dry, but relatively fluid effusive activity exists and the experimental evidence for a fluxing effect of fluorine is abundant. Chlorine concentrations are highest in more alkaline and less silicic systems such as pantellerites, yet there are extremely few reliable data for the influence of Cl on melt viscosity. Encouraged by evidence of pyromagmatic rocks very rich in Cl and Fe as well as the high Fe contents of pantellerites in comparison with other peralkaline natural melts, syntheses of melts in the Na-Si-O-Cl system have been attempted. Successful syntheses of melts with up to 2 wt% Cl have been achieved. Such melts have been subjected to viscometry using the concentric cylinder and the micropenetration methods. The addition of a few wt% Cl to melts in this system results in a slight decrease in viscosity (Fig. 3.6-3). This influence of chlorine on viscosity appears to contrast strongly with the influence of fluorine. Further investigations are underway to systematize the influence of Cl on the viscosity of simple melt compositions as well as to evaluate the effect in natural Fe-Cl rich compositions such as pantellerites.
 

Fig 3.6-3: Log viscosity as a function of reciprocal temperature. The addition of Cl reduces the viscosity slightly at low temperatures.