QUANTIFYING THE EFFECT OF FLUORINE ON THE VISCOSITY OF NEPHELINE-KALSILITE MELTS

We studied the effect of up to 4 wt.% nominal fluorine on the viscosity and density of (Na,K)AlSiO₄ melts with Na:K ratios of 100:0 (Ne₁₀₀), 75:25 (Ne₇₅Kls₂₅), 62.5:37.5 (Ne_62.3Kls_37.5), 50:50 (Ne₅₀Kls₅₀), 37.5:62.5 (Ne_37.5Kls_62.5), and 25:75 (Ne₂₅Kls₇₅). All fluorine contents reported are nominal. These melts are analogs for natural alkali-rich magmas and all have nominal volatile-free NBO/T values of 0. We measured viscosity by parallel-plate viscometry at temperatures between 680-945°C, representing a viscosity range of ~10^9-12 Pa s. We report T₁₂, the temperature at which a melt has a viscosity of 10¹² Pa s, to compare the effect of F on viscosity between the different melt compositions. The T₁₂ value was obtained by fitting the viscosity data for each melt to a TVF equation. We measured density using the Archimedean method with ethanol as the immersion fluid.

At constant temperature, the addition of fluorine reduces the viscosity of all melts studied. For 2 wt.% F, the reduction in T₁₂ is non-linear but generally increases with decreasing Na:K ratios. The addition of 2 wt.% dissolved F results in a T₁₂ reduction of ~55°C for Ne₁₀₀, ~125°C for Ne₇₅Kls₂₅, ~140°C for Ne₅₀Kls₅₀, and ~88°C for Ne₂₅Kls₇₅. At 2 wt.% F, the effect of Na:K ratio on T₁₂ for fluorine-bearing melts mirrors what is observed for F-free melts, with the greatest reduction in T₁₂ occurring at intermediate Na:K ratios and K-rich melts having a higher T₁₂ than Na-rich melts. We tested additional F contents of 1, 3, and 4 wt.% for the Ne₅₀Kls₅₀ melt only, and observed a greater reduction in T₁₂ at greater F content. F reduces the density in all glasses studied relative to F-free glasses, but the effect is small, on the order of 2-3 kg/m³ for 2 wt.% nominal dissolved F.

The effect of F on T₁₂ reduction in (Na,K)AlSiO₄ melts is smaller than that observed for melts with higher SiO₂ content in the same (Na,K)AlSiO₄-SiO₂ system, specifically along the jadeite-leucite and albite-orthoclase joins. Indeed, the addition of 2 wt.% F to Ne₁₀₀ melts (SiO₂ ~48%) results in a reduction in T₁₂ of only ~55°C, whereas it results in a reduction of ~130°C for both jadeite (SiO₂~67%) and albite (SiO₂ ~75%) melts.