THE EFFECT OF FLUORINE ON MELT VISCOSITY IN JADEITE-LEUCITE MELTS

We synthesized glasses along the jadeite-leucite (NaAlSi₂O₆-KAlSi₂O₆) join with various amounts of dissolved fluorine (up to 4 wt.%). All fluorine contents quoted are nominal amounts. Na:K ratios synthesized include Jd₁₀₀, Jd₇₅Lct₂₅, Jd_62.5Lct_37.5, Jd₅₀Lct₅₀, Jd_37.5Lct_62.5, and Jd₂₅Lct₇₅, all of which have nominal ratios of non-bridging oxygen to tetrahedrally-coordinated cations (NBO/T) of 0. For Jd₅₀Lct₅₀, we synthesized glasses with 0, 1, 2, 3, and 4 wt.% F. For all other Na:K ratios, we only synthesized glasses with 0 and 2 wt.% F. We measured the viscosity of each melt by parallel-plate viscometry at temperatures between 655°C and 980°C. We use the temperature at which the viscosity is 10¹² Pa.s (T₁₂) to compare the effect of F on the different composition melts.

For F contents of 0 and 2 wt.%, at constant temperature, decreasing Na:K ratio increases viscosity. At 825°C, F-free Jd₁₀₀ has a viscosity ~1.75 log units lower than F-free Jd₂₅Lct₇₅. At the same temperature, but with 2 wt.% dissolved F, Jd₁₀₀ has a viscosity over ~3 log units lower than Jd₂₅Lct₇₅.

The addition of fluorine decreases the viscosity of each melt relative to its F-free equivalent. With the addition of 2 wt.% F, T₁₂ decreases between 45°C and 130°C for the samples studied. The smallest reduction in T₁₂ is for the Jd₂₅Lct₇₅ composition (45°C) while the largest reduction in T₁₂ is calculated for Jd₁₀₀ (130°C). The effect of F on viscosity therefore seems to be greater for Na-rich compositions. This is further confirmed in Jd₅₀Lct₅₀ melts for which we synthesized melts with additional nominal F contents. At 825°C, the addition of 1 wt.% F decreases viscosity by nearly 1 log unit. The addition of 2 and 3 wt.% F reduces viscosity by over 1.6 log units and 2 log units, respectively. The addition of 4 wt.% F reduces viscosity by almost 2.7 log units.

We conclude that increasing Na:K ratio and wt.% F both decrease the viscosity of the melts in this study and that dissolved F has a greater effect on the viscosity of Na-rich melts. Our results are consistent with the observed effect of Na:K ratio in more silica rich melts, with albite melt having a viscosity nearly 3 orders of magnitude lower than orthoclase melt at ~925°C. Our results also show a similar effect of F on the viscosity of jadeite and albite melts, with a reduction in T₁₂ for 2 wt.% F of ~130°C in both melts.