Low temperature aqueous solubility of fluorite at temperatures of 5, 25, and 50°C and ionic strengths up to 0.72M

Abstract

Historical fluorite solubility research found that fluorite solubilities varied over three orders-of-magnitude and appear to be inconsistent with several recently determined fluorite solubilities. This research involved a comprehensive series of solubility experiments run over a period of three years using nine natural fluorites and one synthetic fluorite to determine their solubility in low temperature (5, 25, and 50oC) aqueous solutions and ionic strengths ranging from near zero to 0.72M. Equilibrium was approached from both undersaturated and supersaturated initial conditions. The fluorites were chemically and physically characterized using laboratory analyses, thin section petrography, UV fluorescence, and x-ray diffraction. The fluorite samples were generally found to have similar chemical and physical properties. The chemical composition was approximately stoichiometric, with a mean calcium concentration of 51.6 ± 0.6 wt % and mean fluorine concentration of 50.2 ± 0.7 wt %. Fluorite unit cell edge lengths, volumes, and densities averaged 5.4630 ± 0.0007 Å, 163.04 ± 0.06 Å3, and 3.181 ± 0.009 g/cm3, respectively. The fluorite solubility results showed that fluorite dissolved congruently and increased in solubility with both temperature and ionic strength. The fluorite solubilities determined were relatively consistent, with median log Ksp values that ranged from -10.81 ± 0.02 at 5oC, -10.63 ± 0.02 at 25oC, and -10.49 ± 0.03 at 50oC. An internally consistent set of fluorite thermodynamic data were determined from the log Ksp results which yielded a ∆G_f^0.of -1,177.7 ± 0.2 kJ/mol, ∆H_f^0 of -1,226.9 ± 1.1 kJ/mol, and S_f^0 of 76.0 ± 3.7 J/mol*K. However, the third law entropy value determined for this study was about 9% higher than the reported value Soof 68.9 ± 0.3 J/mol*K. This discrepancy in entropy values likely results from the range in log Ksp values determined for the fluorites during this study. This study’s results resolve the uncertainty in past work and provides an optimal solubility dataset for fluorite going forward.