| Summary: | Rare Earth Elements (REE) phosphates (monazite, xenotime, and rhabdophane) are critical REE-bearing minerals typically formed in hydrothermal and magmatic ore deposits. The thermodynamic properties of those REE minerals are crucial to understanding the solubility, speciation, and transport of REE complexes. However, the standard-state Gibbs free energy of formation (∆<i>G</i>°<i><sub>f</sub></i>) values reported for these minerals in the literature vary by up to 25 kJ mol<sup>−1</sup>. Here, we present linear free energy relationships that allow the evaluation and estimation of the ∆<i>G</i>°<i><sub>f</sub></i> values at 25 °C and 1 bar for the three minerals from the ionic radius (<i>r</i><sub>REE</sub><sup>3+</sup>) and the non-solvation Gibbs free energy contribution to the REE<sup>3+</sup> aqua ion (∆<i>G</i>°<i><sub>n</sub></i><sub>, REE</sub><sup>3+</sup>): ∆<i>G</i>°<i><sub>f</sub></i><sub>,monazite</sub> − 399.71 <i>r</i><sub>REE</sub><sup>3+</sup> = 1.0059 ∆<i>G</i>°<i><sub>n</sub></i><sub>,REE</sub><sup>3+</sup> − 2522.51; ∆<i>G</i>°<i><sub>f</sub></i><sub>,xenotime</sub> − 344.08 <i>r</i><sub>REE</sub><sup>3+</sup> = 0.9909 ∆<i>G</i>°<i><sub>n</sub></i><sub>,REE</sub><sup>3+</sup> − 2451.53; and ∆<i>G</i>°<i><sub>f</sub></i><sub>,rhabdophane</sub> − 416.17 <i>r</i><sub>REE</sub><sup>3+</sup> = 1.0067 ∆<i>G</i>°<i><sub>n</sub></i><sub>, REE</sub><sup>3+</sup> − 2688.86. Moreover, based on the new dataset derived for REE end-members, we re-fitted the binary Margules parameter (<i>W</i>) from previous theoretical calculations into linear correlations: <i>W</i> + 0.00204 ∆<i>G</i>°<sup>’</sup><i><sub>n</sub></i><sub>,monazite</sub> = 39.3549 ∆<i>V</i> + 0.0641; <i>W</i> + 0.00255 ∆<i>G</i>°<sup>’</sup><i><sub>n</sub></i><sub>,xenotime</sub> = 25.4885 ∆<i>V</i> − 0.0062. The internally consistent thermodynamic properties of these REE phosphates are incorporated into the computer program Supcrtbl, which is available online at Zhu’s research website.
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