Oxidation state and coordination environment of Pb in U-bearing minerals

This study examines the oxidation and coordination of lead (Pb) in the uranium (U) minerals uraninite (UO₂), coffinite (USiO₄), and brannerite (UTi₂O₆), and minerals that have U as a minor constituent, zircon (ZrSiO₄) and titanite (CaTiSiO₅). The characterization of Pb was conducted through a combination of micro-synchrotron X-ray fluorescence mapping (µSXRF) and micro-X-ray absorption spectroscopy (µXAS), collected at the Pb L_II and L_III edge on characterized grains (electron microscopy and X-ray diffraction) from a range of geological settings and ages. For all minerals considered in this study, Pb L_II and L_III X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data indicate that Pb exists as Pb²⁺. In the case of uraninite, DFT structure optimization calculations suggest that the substitution of Pb²⁺ for U⁴⁺ is facilitated by the stereochemically active electron lone pair indicative of Pb²⁺, which effectively takes the place of an oxygen vacancy (coupled substitution Pb²⁺ + □ = U⁴⁺ + O²⁻). In detail, however, the EXAFS and XANES data, supported by first principle calculations of the XANES associated with Pb²⁺ replacing U⁴⁺ in the crystal lattice of uraninite, indicate that Pb²⁺ is present in a less distorted coordination than suggested by DFT, which may reflect a yet to be characterized link between interstitial oxygen defects and Pb²⁺ incorporation, rather than a simple oxygen vacancy. For coffinite, the XANES data demonstrate that Pb exists mainly as Pb²⁺ in the form of galena, indicating limited stability of the isomorphous substitution in coffinite. In contrast, Pb is homogeneously distributed in the metamict mineral brannerite, which displays a distinct XANES spectrum suggestive of Pb²⁺ bonded to oxygens in a highly distorted coordination environment. Pb L_II XANES indicate that radiogenic Pb exists predominantly as Pb²⁺ within the crystal lattice of zircon and as distinct mineral PbO nano-scale inclusions or domains. Additionally, Pb L_II XANES measurements also indicate that Pb²⁺ exists within titanite although the coordination environment is unclear. Overall, these data suggest that the previous indications of the presence of Pb⁴⁺ in radioactive minerals need to be reconsidered. The nature of radiogenic Pb²⁺ incorporation in U-bearing minerals controls the mobilization of Pb in hydrothermal fluids and the utility of a mineral as a geochronometer. The Pb valence and coordination data presented in this study provide fundamental constraints imperative for our understanding of the controls governing the mobilization of radiogenic Pb during water-rock interaction and metamorphism.