The solubility of apatite in H2O, KCl-H2O, NaCl-H2O at 800 °C and 1.0 GPa: Implications for REE mobility in high-grade saline brines

Apatite is an important host for LREE, F, and Cl and thus can be used to monitor elemental mass transfer in high P-T settings. In this investigation, the solubilities of synthetic fluor- and chlorapatite and natural Durango fluorapatite were measured in H2O, KCl-H2O, and NaCl-H2O fluids at 800 °C and 1.0 GPa, using the piston-cylinder apparatus. The solubility of apatite is reported in parts/million by weight (ppm), and is calculated via the formula mAp = [(∆ mAp / H2O + KCl or NaCl + ∆ mAp) ∗ 106], where ∆ mAp = mAp-in − mAp-out. The experimental results indicate a strong increase in apatite solubility in aqueous fluids with moderate KCl and NaCl mole fractions (XKCl and XNaCl). Under all conditions, synthetic fluorapatite and synthetic chlorapatite dissolve congruently. The solubility of synthetic fluorapatite increases from 19 ppm in pure H2O to 2123 ppm at XKCl = 0.52. The solubility of synthetic chlorapatite rises from 37 ppm in pure H2O to 2590 ppm at XKCl = 0.50. At XKCl > 0.5 the solubility of synthetic fluorapatite decreases to 1691 (XKCl = 0.72) and the solubility of synthetic chlorapatite decreases to 2284 ppm (XKCl = 0.65). In contrast, natural Durango fluorapatite dissolves incongruently at XKCl < 0.2 to monazite + fluid and congruently at XKCl > 0.2. The solubility of natural Durango fluorapatite increases from 70 ppm in pure H2O to 2073 ppm at XKCl = 0.50 respectively. In contrast to the KCl-H2O system, apatite solubilities are considerably higher in the NaCl-H2O system. The solubility of synthetic fluorapatite increases to 4909 ppm at XNaCl = 0.51 and the solubility of synthetic chlorapatite rises to 5144 ppm at XNaCl = 0.50. Compared to fluorite, calcite and anhydrite, apatite solubility is significantly lower in H2O, NaCl-H2O, and KCl-H2O. After completion of each experiment, the pH was measured and a comprehensive step-by step procedure (limiting reactant concept) was used to model the observed quench pH in the system apatite-KCl-H2O.