EXPERIMENTAL DETERMINATION OF SYNTHETIC NDPO4 MONAZITE END-MEMBER SOLUBILITY IN WATER FROM 21°C TO 300°C: IMPLICATIONS FOR RARE EARTH ELEMENT MOBILITY IN CRUSTAL FLUIDS

Abstract

The solubility of synthetic NdPO4 monazite end-member has been determined experimentally from 21 to 300°C in aqueous solutions at pH = 2, and at 21°C and pH = 2 for GdPO4. Measurements were performed in batch reactors, with regular solution sampling for pH measurement, rare earths and phosphorous analysis by inductively coupled plasma mass spectrometry (ICP-MS) coupled with a desolvation system. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to check that no reprecipitation of secondary phases occurred and that the mineral surfaces remained those of a monazite. Coupled with speciation calculations, measured solution compositions permitted the determination of NdPO4 and GdPO4 solubility products which are in general agreement with previous experimental determination on rhabdophane at 25°C, but showing that monazite is more than two orders of magnitude less soluble than inferred on the basis of previous thermodynamic estimates. The temperature evolution from 21 to 300°C of the equilibrium constant (K) of the NdPO4 monazite end-member dissolution reaction given by: