The effect of chlorine, fluorine and water on the viscosity of aluminosilicate melts

- The decreasing effects of Cl, F and H2O on density are independent and add linearly.
- F decreases viscosity more strongly in peraluminous than in peralkaline melts.
- The decrease in viscosity due to H2O is reduced by the presence of F.
- The decrease in viscosity due to H2O is not influenced by the presence of Cl.
- H2O and F influence each other's speciation & thus reduce their effects on viscosity.

The influence of 1.6 to 14.5 mol% water, 1.1 to 18.3 mol% fluorine (F) and 0.5 to 1.4 mol% chlorine (Cl) on the viscosity of peralkaline and peraluminous Na2O-CaO-Al2O3-SiO2 melts as a model system for phonolites was studied with the micropenetration method within the range of 109 to 1013 Pa s. Water and F alone and combined decrease viscosity non-linearly. With increasing F and water concentration the further addition of F and water becomes less effective in decreasing viscosity. Water decreases the viscosity of peraluminous melts more strongly than that of peralkaline melts. In the halogen-free peraluminous sample 5 mol% water decreases Tg12 (the temperature where the viscosity = 1012 Pa s) by 259 ± 5 K, while in the halogen-free peralkaline sample 5 mol% water decreases Tg12 by 189 ± 5 K. Fluorine (on a mol% basis) decreases the viscosity of Na2O-CaO-Al2O3-SiO2 melts less than water. In the peraluminous melts the addition of 5 mol% F decreases Tg12 by ~ 95 K. In the peralkaline melt 5 mol% F decreases Tg12 by ~ 30 K. However, a significant decrease in Tg12 was only observed up to a F concentration of 1.9 mol% F, another sample with 6.2 mol% F showed no further decrease of Tg12. In all hydrous peraluminous and peralkaline samples that contain F alone or combined with Cl, the decrease of viscosity due to the addition of water is smaller than in the halogen-free samples. In a peraluminous melt with 18.3 mol% F the addition of 5 mol% water decreases Tg12 by about 152 ± 5 K compared to the dry melt. In a peralkaline melt with 1.9 mol% F the addition of 5 mol% water decreases Tg12 by about 146 ± 5 K compared to the dry melt. In both cases this is significantly less than for the halogen-free samples. In peralkaline melts Cl has no influence on the decrease in viscosity upon the addition of water. The effects of water and Cl on viscosity are independent of each other and add together. The effects of water and F on viscosity are not independent, which could be, in the case of the peralkaline samples related to changes in F speciation due to the presence of water.