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This study looks at the effect of up to 2.15 wt.% fluorine on the viscosity and density of (Na,K)AlSiO4 melts with varying Na:K ratios. These melts are analogs for natural alkali-rich magmas and are assumed to have nominal volatile-free NBO/T values of 0. Viscosity is measured via parallel-plate viscometry at temperatures between 680-945°C, representing a viscosity of of 9-12 log Pa◦s. T12, the temperature at which a melt has a viscosity of 12 log Pa◦s, is reported 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 Tamman-Volger-Fulcher 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. F reduces the density and fragility in almost all glasses studied relative to F-free glasses. The reduction in T12 increases with added F but is nonlinear in Ne50Kls50 melts, the only melt where multiple wt.% values were tested. The reduction in viscosity seen with added F does not appear to be a function of Al:Si. It is inferred that this is due to triclustered Al3+ structures observed in other subaluminous melts e.g. (Zimova and Webb, 2007) and (Baasner and Schmidt, 2013). It is unclear whether the reduction in viscosity was a function of Na:K. At intermediate Na:K ratios and ∼1.24 wt.%F, viscosity is very close to linear, indicating that the mixed alkali effect seen in F-free melts does not influence how much F reduces viscosity. However, two melts at Ne100 and Ne37.5Kls62.5 with ∼0.90 wt.% F added do show evidence of the mixed alkali effect as they deviate from linearity. It is inferred that F depolymerizes Ne-Kls melts, resulting in a decrease in viscosity, due to structural bonding with network-forming tetrahedra in place of oxygen, as is seen in other aluminosilicate melts, e.g. (Baasner and Schmidt, 2013) and (Romano et al., 2001).
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Restricted: Campus/Bates Community Only Access
Date of Graduation
Bachelor of Arts
Bruno, Madeline Stella and Robert, Genevieve, "The effect of fluorine on melt viscosity in the NaAlSiO4 - KAlSiO4 system" (2017). Standard Theses. 33.
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