Solubility measurements of synthetic neodymium monazite as a function of temperature at 2 kbars, and aqueous neodymium speciation in equilibrium with monazite

The solubility of synthetic NdPO₄ monazite end-member was experimentally determined from 300 up to 800 °C, at 2000 bars in pure water, and in aqueous chloride or phosphate solutions. Both the classical weight-loss method and a new method based on isotope dilution coupled with thermal ionization mass spectrometer were used. In the range of temperature studied monazite showed a prograde solubility from 10^−5.4 m at 300 °C up to 10^−2.57 m at 800 °C. Experiments in H₂O-H₃PO₄-NaCl-HCl solutions suggested Nd(OH)₃⁰ was the major species that was formed at high temperature and pressure. The equilibrium constants (log K) for the reaction: