Solubilities of some hydrous REE phosphates with implications for diagenesis and sea water concentrations

Abstract

Solubility product determinations suggest that the hydrous phosphates of the rare earths, REPO 4 · xH 2O , are important in controlling the sea water REE concentrations. Two of these solids, rhabdophane, (P6 222) and “hydrous xenotime”, (14 1/amd), have been synthesized at 100°C via the acid hydrolysis of the respective REE pyrophosphate. The solubility products at infinite dilution were determined to be pK 0 = 24.5 , (La at 25°C); 26.0, (Pr at 100°C); 25.7, (Nd at 100°C): and 25.5, (Er at 100°C). On the basis of calculations involving the reaction of RE 3+ with apatite to form the hydrous phosphate, the lanthanum concentration in sea water is predicted to be about 140 pmol/L. Laboratory experiments support the hypothesis that apatite is a substrate for reactions with dissolved REE.