The solubility of xenotime-(Y) and other HREE phosphates (DyPO4, ErPO4 and YbPO4) in aqueous solutions from 100 to 250 °C and psat

- The solubility (Ks0) of HREE phosphates is retrograde from 100 to 250 °C.
- The Ks0 of xenotime-(Y) is much higher than predicted from previous calorimetric data.
- Available calorimetric entropy data were used to extrapolate our logKs0 values to 25 °C.
- The extrapolation allowed us to re-evaluate published enthalpy data.
- We recommend a new set of standard thermodynamic properties for HREE phosphates.

Xenotime (YPO4) is a heavy rare earth element (HREE) phosphate, the principal host to economic HREE mineralization and a common accessory mineral in igneous, metamorphic and sedimentary rocks. Many occurrences of xenotime-(Y) are of hydrothermal origin, and therefore, determination of its solubility in aqueous fluids may help to evaluate the mobility of HREE in crustal fluids. We have measured the solubility of the HREE phosphate end-members YPO4, ErPO4, DyPO4 and YbPO4 in aqueous HClO4-H3PO4 solutions at temperatures from 100 to 250 °C and saturated water vapor pressure (psat) corresponding to the reaction: HREEPO4 = REE3 + + PO43 − (Ks0). The logarithm of the solubility products (logKs0) determined for these end-members, can be evaluated as a function of temperature (in K) using the equation logKs0 = A + BT + C / T + Dlog(T). Reliable fits to our data yields the following coefficients: YPO4, A = − 876.93, B = − 0.25127, C = 2.327e+4, D = 342.82; DyPO4, A = − 1150.2, B = − 0.31778, C = 3.132e+4, D = 450.32; ErPO4, A = − 555.90, B = − 0.17933, C = 1.373e+4, D = 217.42; and YbPO4, A = − 419.76, B = − 0.15215, C = 8.941e+3, D = 165.27. A comparison of our solubility products to those calculated with the available enthalpy, entropy and heat capacity values from calorimetric determinations, and the corresponding properties of the aqueous species (Y3 +, Dy3 +, Er3 +, Yb3 +, PO43 −), shows that our solubility products for YPO4 and ErPO4 are orders of magnitude different from the calculated solubility products. By contrast, the calculated solubility products for YbPO4 are in good agreement with the measured values for the range of temperature considered in our experiments. There are no data available for the heat capacity of DyPO4, and consequently our experimental results provide the first data that permit the solubility of this HREE phosphate to be calculated for elevated temperature. We have developed a method that allows our data to be extrapolated to 25 °C and have reconciled the measured calorimetric and solubility data. The results are used to recommend standard thermodynamic properties for the HREE phosphates at 25 °C and 1 bar.