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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.

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Title: 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.
Authors: Mair, Philipp1 philipp.mair@uibk.ac.at, Tropper, Peter1, Harlov, Daniel E.2,3, Manning, Craig E.4
Source: Chemical Geology. Oct2017, Vol. 470, p180-192. 13p.
Subjects: Apatite, Solubility, Potassium chloride, Salt, Mass transfer
Abstract: 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 H 2 O, KCl-H 2 O, and NaCl-H 2 O 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 m Ap = [(∆ m Ap / H 2 O + KCl or NaCl + ∆ m Ap ) ∗ 10 6 ], where ∆ m Ap = m Ap-in − m Ap-out . The experimental results indicate a strong increase in apatite solubility in aqueous fluids with moderate KCl and NaCl mole fractions (X KCl and X NaCl ). Under all conditions, synthetic fluorapatite and synthetic chlorapatite dissolve congruently. The solubility of synthetic fluorapatite increases from 19 ppm in pure H 2 O to 2123 ppm at X KCl = 0.52. The solubility of synthetic chlorapatite rises from 37 ppm in pure H 2 O to 2590 ppm at X KCl = 0.50. At X KCl > 0.5 the solubility of synthetic fluorapatite decreases to 1691 (X KCl = 0.72) and the solubility of synthetic chlorapatite decreases to 2284 ppm (X KCl = 0.65). In contrast, natural Durango fluorapatite dissolves incongruently at X KCl < 0.2 to monazite + fluid and congruently at X KCl > 0.2. The solubility of natural Durango fluorapatite increases from 70 ppm in pure H 2 O to 2073 ppm at X KCl = 0.50 respectively. In contrast to the KCl-H 2 O system, apatite solubilities are considerably higher in the NaCl-H 2 O system. The solubility of synthetic fluorapatite increases to 4909 ppm at X NaCl = 0.51 and the solubility of synthetic chlorapatite rises to 5144 ppm at X NaCl = 0.50. Compared to fluorite, calcite and anhydrite, apatite solubility is significantly lower in H 2 O, NaCl-H 2 O, and KCl-H 2 O. 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-H 2 O. [ABSTRACT FROM AUTHOR]
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Abstract: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 H 2 O, KCl-H 2 O, and NaCl-H 2 O 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 m Ap = [(∆ m Ap / H 2 O + KCl or NaCl + ∆ m Ap ) ∗ 10 6 ], where ∆ m Ap = m Ap-in − m Ap-out . The experimental results indicate a strong increase in apatite solubility in aqueous fluids with moderate KCl and NaCl mole fractions (X KCl and X NaCl ). Under all conditions, synthetic fluorapatite and synthetic chlorapatite dissolve congruently. The solubility of synthetic fluorapatite increases from 19 ppm in pure H 2 O to 2123 ppm at X KCl = 0.52. The solubility of synthetic chlorapatite rises from 37 ppm in pure H 2 O to 2590 ppm at X KCl = 0.50. At X KCl > 0.5 the solubility of synthetic fluorapatite decreases to 1691 (X KCl = 0.72) and the solubility of synthetic chlorapatite decreases to 2284 ppm (X KCl = 0.65). In contrast, natural Durango fluorapatite dissolves incongruently at X KCl < 0.2 to monazite + fluid and congruently at X KCl > 0.2. The solubility of natural Durango fluorapatite increases from 70 ppm in pure H 2 O to 2073 ppm at X KCl = 0.50 respectively. In contrast to the KCl-H 2 O system, apatite solubilities are considerably higher in the NaCl-H 2 O system. The solubility of synthetic fluorapatite increases to 4909 ppm at X NaCl = 0.51 and the solubility of synthetic chlorapatite rises to 5144 ppm at X NaCl = 0.50. Compared to fluorite, calcite and anhydrite, apatite solubility is significantly lower in H 2 O, NaCl-H 2 O, and KCl-H 2 O. 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-H 2 O. [ABSTRACT FROM AUTHOR]
ISSN:00092541
DOI:10.1016/j.chemgeo.2017.09.015