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–xF1+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.