Mineral–Water Interface Structure of Xenotime (YPO4) {100}

Crystal truncation rod (CTR) measurements and density functional theory (DFT) calculations were performed to determine the atomic structure of the mineral–water interface of the {100} surface of xenotime (nominally YPO4). This mineral is important, because it incorporates a variety of rare earth elements (REEs) in its crystal structure. REEs are critical materials necessary for a variety of renewable and energy efficient technologies. Current beneficiation techniques are not highly selective for REE ore minerals, and large amounts go to waste; this is a first step toward designing more efficient beneficiation. Evidence is found for minor relaxation of the surface within the topmost monolayer with little or no relaxation in subsurface layers. Justification for ordered water at the interface is found, where water binds to surface cations and donates hydrogen bonds to surface phosphates. The average bond lengths between cations and oxygens on water are 228 pm in the best fit to the CTR data, versus 243 and 251 pm for the DFT. No agreement on water positions bound to surface phosphates is obtained. Overall, the findings suggest that ligands used in beneficiation with a single anionic headgroup, such as fatty acids, will have limited selectivity for xenotime relative to undesirable minerals.