Structural Form of Yttrium in Nanocrystalline Fluorapatite from Marine Sediments at 0.11 Å Resolution

Deep-sea mud is rich in rare earth elements and yttrium (REY), with yttrium (Y) exhibiting the highest concentration. REY are found in authigenic (a-CFA) and biogenic (b-CFA) carbonate fluorapatite (CFA, Ca₅(CO₃) _x (PO₄)_3-x F_1+x ). The presence of REY in both CFA types suggests different enrichment processes in abyssal environments, which may be traced through detailed structural analysis of REY's coordination chemistry. The bonding environment of Y in CFA was investigated in 2018 and 2023 using extended X-ray absorption fine structure (EXAFS) spectroscopy at a resolution of 0.15 Å. While these studies offered valuable insights into Y's short-range order, they also presented inconsistencies. Moreover, a resolution of 0.15 Å is insufficient to uncover the intricate local structure of Y in CFA. Here, we present EXAFS data at a resolution of 0.11 Å for Y in a-CFA and b-CFA collected several meters beneath the Pacific Ocean seafloor. Y is predominantly hydrated and bound to Ca and PO₄ in an amorphous phase surrounding the a-CFA and b-CFA nanocrystals and is secondarily incorporated into the crystal structure of a-CFA. There is no EXAFS evidence indicating the presence of polynuclear Y precipitate, which contrasts with a recent finding on cerium (Ce), nor supporting the formation of a Y-carbonate complex. The latter two findings are backed by density functional theory, which indicates that Y-Y pair formation is thermodynamically unfavorable and that the predicted Y-C distance is inconsistent with the EXAFS distances. This research highlights the geochemical enrichment of Y in abyssal sediments through the formation of a hydrated yttrium-calcium phosphate phase in a-CFA and b-CFA and Y for Ca substitution in authigenic a-CFA nanocrystals during the coprecipitation of calcium and phosphate.