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

Drew D. Syverson, Barbara Etschmann, Weihua Liu, Rahul Ram, Yuan Mei, Tony Lanzirotti, Julien Mercadier, Joel Brugger

► To cite this version:

Drew D. Syverson, Barbara Etschmann, Weihua Liu, Rahul Ram, Yuan Mei, et al. Oxidation state and coordination environment of Pb in U-bearing minerals. Geochimica et Cosmochimica Acta, 2019, 265, pp.109-131. 10.1016/j.gca.2019.08.039. hal-02334123

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

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Drew D. Syverson¹*, Barbara Etschmann², Weihua Liu³,

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Rahul Ram², Yuan Mei²,³, Tony Lanzirotti⁴, Julien Mercadier⁵, Joël Brugger²

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¹Department of Geology and Geophysics, Yale University, New Haven, CT USA

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²School of Earth, Atmosphere and Environment, Monash University, VIC Australia

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³CSIRO Mineral Resources, Research Way, Clayton, Australia

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⁴Advanced Photon Source, Argonne National Laboratory, Lemont, IL USA

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⁵GeoRessources UMR 7359, Université de Lorraine, France

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*correspondence: drew.syverson@yale.edu
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Abstract

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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 L₁₁ and L₁₁ edge on characterized grains (electron microscopy and X-ray diffraction) from a range of geological settings and ages.

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For all minerals considered in this study, Pb L₁₁ and L₁₁ 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⁺ + O²⁻ = 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 L₁₁ 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 L₁₁ XANES measurements also indicate that Pb²⁺ exists within titanite although the coordination environment is unclear.

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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 Pb mobilization 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.
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# 1. Introduction

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## 1.1 Lead in Uranium Minerals

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Knowledge of the distribution, speciation, and retention of radiogenic daughter elements such as Pb, the ultimate product of the radioactive decay of U-bearing minerals, is imperative for interpreting the integrity of geochronological data and to effectively predict the mobility and degree of mass transfer of radionuclides upon mineral replacement and leaching reactions (Peterman et al., 2016; Schoene, 2014) (Fig. 1). To better constrain this effect, this study examines the coordination environment and valence/oxidation state of radiogenic Pb in U-bearing minerals through micro-Synchrotron X-Ray Fluorescence mapping (µSXRF) and micro-X-ray Absorption Spectroscopy (µXAS). Ultimately, this information assists in providing a framework to predict the mobility and degree of mass transfer of radiogenic elements upon aging, mineral alteration, and as a consequence of damage of the crystal lattice due to radioactive decay (i.e., metamictization).

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The valence and distribution of Pb in the minerals zircon and titanite, two of the most important U-Pb geochronometers, have been investigated by a number of studies. These minerals contain trace amounts (<<1 wt.%) of U, and micro-characterization techniques such as transmission electron microscopy (TEM; Kogawa et al., 2012; Kusiak et al., 2015; Kusiak et al., 2013, 2015; Utsunomiya et al., 2004), nanoSIMS (Whitehouse et al., 2014), and atom-probe tomography (Kirkland et al. 2018; Valley et al., 2014) show that Pb and U distributions are often decoupled at the µm- to nm-scale. Lead occurs in the form of nano-scale inclusions (e.g., metallic lead, Kusiak et al., 2015) or in substitution for Zr/Ti at concentrations < 1 wt% (Kogawa et al., 2012).

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The crystal chemical requirements that define a robust geochronometer are contradictory: the crystal lattice has to exclude Pb during the growth of the mineral (minimize ‘common’ lead), but the lattice has to be able to retain radiogenic Pb even when subjected to processes such as alteration and/or recrystallization. Kramers et al. (2009) proposed that the resilience of zircon as a geochronometer is due to the fact that radiogenic Pb exists in the tetravalent state: Pb²⁺ is excluded during zircon crystallization,
but radiogenic Pb⁴⁺ is compatible. In 8-fold coordination, Pb⁴⁺ has a smaller effective ionic radius (0.94 Å) than Pb²⁺ (1.29 Å), and therefore is compatible within the crystal lattice of a number of U⁴⁺-bearing minerals (the ionic radii of U⁴⁺ is 1.00 Å (VIII)) (Shannon, 1976; Shannon and Prewitt, 1970). However, the formation of Pb⁴⁺ is only thermodynamically favorable at a limited range of pressure, temperature, and oxidative conditions (White et al., 1961); in nature, the Pb⁴⁺-minerals plattnerite and scrutinyite (PbO₂ polymorphs) occur as oxidation of Pb²⁺-minerals in low-temperature, near-surface environments (Wang et al. 2010).

In general, µm- and nm-scale chemical imaging techniques do not provide direct evidence for the oxidation state of minor amounts of an element in a mineral (Utsunomiya et al., 2004), and spectroscopic methods are called upon to provide unambiguous information on oxidation state. X-ray Absorption Near-Edge Structure (XANES) spectroscopy is a well-established method that provides a number of advantages for the study of complex, inhomogeneous samples: it is suitable for elemental concentrations ranging from a few ppm to wt%; it is non-destructive and not surface-sensitive; and it can be used on standard petrographic thin sections at high spatial resolution (µm) (Cook et al. 2012; Etschmann et al., 2010; Sutton et al., 2002). To date, only two studies have applied XANES to constrain the nature of radiogenic Pb in minerals. Kramers et al. (2009) found that radiogenic Pb (96% radiogenic) in a 1 Gy titanite existed as Pb²⁺, but a Tertiary Alpine titanite containing mainly common Pb had a XANES spectrum suggestive of Pb⁴⁺. Tanaka et al. (2010) similarly suggested that zircons with lower radiation doses may contain some Pb⁴⁺ along with a majority of Pb²⁺. To date, there is no spectroscopic study of the chemical state of Pb in U-rich minerals. Recently, Macmillan et al. (2016b) showed that wt.% Pb exist in substitution for U in Proterozoic uraninite, and argued that this is best explained if most Pb is in the tetravalent state based on crystal-chemical considerations.

We combined synchrotron-based µSXRF with Pb L<sub>III</sub>/Pb L<sub>II</sub> µXANES to determine the distribution and oxidation state of Pb in a set of uraninite (UO<sub>2</sub>), brannerite (UTi<sub>2</sub>O<sub>6</sub>), coffinite (USiO<sub>4</sub>), zircon (ZrSiO<sub>4</sub>), and titanite (CaTiSiO<sub>5</sub>) samples from a number of geological localities at a range of ages
(Table 1). While our study focusses on minerals that contain U as an essential element (uraninite, coffinite, brannerite), new data on zircon and titanite were also collected to allow a direct comparison with published XAS data on radiogenic Pb (Kramers et al., 2009; Tanaka et al., 2010). Uraninite has the ability to host relatively large amounts of Pb in its crystal structure, making it an important model compound to understand the crystal chemistry of radiogenic Pb. The uraninite µXANES data were complemented by micro-extended X-ray absorption fine structure (µEXAFS) data, both collected on polished sections. Bulk U L₃₃ edge EXAFS were also collected on selected powdered uraninite samples. *First principle* XANES calculations were performed to support the interpretation of the uraninite XANES data, and Density Functional Theory (DFT) geometry optimizations were conducted to investigate the effect of the substitution of Pb²⁺ for U⁴⁺ at a range of Pb²⁺ concentrations, from Pb-free to 20 wt.% Pb in the uraninite crystal structure. Overall, coupling of the measured XAS data with the theoretical data provides a clearer understanding of how lattice-bound Pb exists in uraninite, and by extrapolation in other U-bearing minerals (Fig. 1).

## 1.2 Crystal Chemistry of Uraninite, Coffinite, and Brannerite

Uraninite has the fluorite crystal structure type, with space group *Fm-3m* (Clausen et al., 1983; Hoekstra and Katz, 1955). Each U⁴⁺ is coordinated by eight O atoms in a face centered cubic arrangement, and each O atom is bonded to four U⁴⁺ cations at the corner of a regular tetrahedron. Uraninite has a nominal formula of UO₂+x, where x=0-0.33 (Janeczek and Ewing, 1992b; Roth and Jonsson, 2008). Uraninite oxidation occurs by the uptake of O²⁻ at interstitial sites; oxidation beyond UO₂.₃₃ leads to major structural changes, accompanied by increased dissolution and formation of secondary precipitates (Roth and Jonsson, 2008). However, uraninite that contains U⁴⁺ exclusively is not known in nature, and typically exists with U⁶⁺ and cation impurities that substitute for U and effectively maintain charge balance (Fritsche and Dahlkamp, 1997). Geologically old uraninite contains up to ~20 wt.% of PbO due to radioactive breakdown of U (Berman, 1957; Janeczek and Ewing, 1991b, 1992a,b). Natural uraninites also contain up to 11 wt.% CaO and 12 wt.% REE₂O₃ (Burns and Finch, 1999; Finch and Murakami, 1999); further minor
impurities include Th, Fe, Si, P, Al, Mg, Mn, and K (Alexandre and Kyser, 2005; Janeczek et al., 1996; Ram et al. 2013). This demonstrates the flexibility of the uraninite structure, which also has the ability to sustain high levels of internal alpha-particle bombardment.

The impurity content of uraninite depends strongly upon the environment of deposition (Mercadier et al. 2011), as well as the conditions under which dissolved U has been transported and hence can provide insight into the genesis of uraninite and uraninite-fluid interactions. Impurities may also affect uraninite stability (Fritsche and Dahlkamp, 1997; Frondel, 1958). The decay of U to Pb also influences the average oxidation state of uraninite (Berman, 1957; Janeczek et al., 1996), since increased radiogenic Pb content leads to higher concentrations of U vacancies or vacancy pairs. To reflect these crystal chemical complexity, the following formula for the uraninite structure was proposed (Janeczek and Ewing, 1992b; Janeczek and Ewing, 1995):

$$ (U_{1-x-y-z}^{4+}U_x^{6+}REY_y^{3+}M_z^{2+} \Box \nu^{-})O_{2+x-0.5y-z-2\nu*} \quad (1) $$

There is a rough negative correlation of the UO$_{2+x}$ cell parameter as a function of x, though the unit cell size is also a function of element substitution for U$^{4+}$ (Fritsche and Dahlkamp 1997). For example, substitution of Th$^{4+}$ and REE$^{3+}$ increases the cell dimensions, whereas elements such as Si$^{4+}$, Ti$^{4+}$, and Fe$^{2+}$ cause a contraction of the unit cell (Fritsche and Dahlkamp 1997). Additionally, for the substitution of Ca$^{2+}$, the change in the size of the uraninite unit cell is a function of the degree of U oxidation, where substitution of Ca$^{2+}$ for U$^{4+}$ does not appreciably change the dimensions of the crystal lattice, whereas substitution of Ca$^{2+}$ for U$^{6+}$ results in an increase in the cell dimensions.

Brannerite is monoclinic with space group C2/m and ideal formula U$^{4+}$Ti$_2$O$_6$. The U and Ti atoms both exist in a distorted octahedral coordination (Ruh and Wadsley, 1966; Szymański and Scott, 1982). Stoichiometric brannerite contains 62.8 wt.% UO$_2$ and 37.2 wt.% TiO$_2$ (Frondel, 1958). Natural brannerite generally contains impurity elements including Pb, Ca, Th, Y and REE on the U-site; and Si, Al, Mn, Fe, Ni and Nb on the Ti-site (Lian et al., 2002; Charalambous et al. 2012); unaltered brannerite typically has Ti and U contents ranging from 1.8 to 2.1 and 0.4 to 0.9 atoms per formula unit, respectively, based on a
total of three metal cations (Lumpkin et al., 2012). Natural brannerite is metamict (X-ray amorphous) as a result of the α-decay damage from the constituent U and Th (Lian et al., 2002; Lumpkin et al., 2012). Additionally, uranium in brannerite is nearly always partly oxidized (Burns and Finch, 1999; Finch and Murakami, 1999). The presence of Pb is mainly due to the radioactive decay of U and Th (Zhang et al., 2006). Lumpkin et al. (2012) reported that deuteric alteration causes significant U loss, whereas Pb loss is observed in both unaltered and altered brannerite. Preferential retention of radiogenic Pb in the altered brannerite was also observed and attributed to Pb serving as network modifier within the Ti-Si-O framework of the glass-like structure of metamict brannerite.

Coffinite is tetragonal, with space group I4₁/amd, and with an ideal formula commonly given as USiO₄.nH₂O (Fuchs and Gebert, 1958). Ideal hydrated coffinite contains 73.75 wt% UO₂, 16.41 wt% SiO₂, and 9.84 wt% H₂O, whereas ideal anhydrous coffinite contains 81.80 wt% UO₂ and 18.20 wt% SiO₂ (Janeczek and Ewing, 1991a, 1996). Elements which are thought to replace U within coffinite include Zr, Th, Hf, Ca, Fe, and REE; whereas P(V), As(V), V(V), S(IV) are thought to replace Si; and F and OH can substitute for (O,H₂O) sites (Förster, 2006; Janeczek and Ewing, 1991a). Leroy and Holliger (1984) report the common occurrence of Ca- and Pb-poor, Si-enriched coffinite from various localities. They proposed that the coffinization of pitchblende/uraninite results in Pb release from the pitchblende lattice and subsequent galena/clausthallite precipitation in presence of H₂S/H₂Se. Macmillan et al. (2017) observed similar compositions at the Olympic Dam deposit, South Australia, and showed based on textural and compositional data that coffinite at Olympic Dam is a late-stage mineral. These occurrences of coffinite are consistent with the thermodynamic study of Xing et al. (2018), which indicates that coffinite is unlikely to form at T ≥300°C.

The most commonly proposed substitution mechanism for Pb²⁺ (and other divalent cations) in U-bearing minerals involves an oxygen vacancy to compensate for charge balance:

$$U^{4+} + O^{2-} = Pb^{2+} + (2)$$
In addition, the fact that U in uraninite, brannerite and coffinite is partially oxidized suggests that charge balance can be achieved following the coupled substitution:

$$2 U^{4+} = U^{6+} + Pb^{2+} (3)$$

Substitution (3) is consistent with the fact that some uraninite and coffinite contain significant amounts of $Pb^{2+}$, yet are not depleted in oxygen (Burns and Finch, 1999; Janeczek and Ewing, 1992b; Janeczek and Ewing, 1995).

## 1.3 Crystal Chemistry of Zircon and Titanite

Zircon ($ZrSiO_4$) and titanite ($CaTiSiO_5$) are routinely used as U-Pb geochronometers to decipher the timing of thermal and tectonic events throughout Earth history (Lee et al., 1997; Frost et al., 2001). Despite their low abundances, these accessory minerals exert significant influence on the trace-element and isotopic character of petrologic systems because they tend to concentrate radioactive species such as U (< 5,000 ppm) and Th, and other petrogenetically important trace elements (Chen and Zheng, 2015). This U and Th incorporation can lead to significant damage in these minerals as a result of radioactive decay, which influences trace element diffusivities.

Zircon and titanite differ greatly in their reactivity and ability to retain radiogenic Pb upon metamictization and metamorphic/hydrothermal alteration. As mentioned previously, Kramers et al. (2009) proposed that radiogenic Pb substitutes on the $Zr^{4+}$ site and exists in the tetravalent state: $Pb^{2+}$ is excluded during zircon crystallization, but radiogenic $Pb^{4+}$ is compatible. Further, it can be naturally assumed that radiogenic Pb atoms begin their existence in the (IV) valence state, given their U(IV) parentage. Conversely, Cherniak and Watson (2001) report that Pb likely exists in the divalent state in nature since Pb is only stable in the (IV) valence state at elevated temperatures at conditions of high $fO_2$. As Pb accumulates over time in a zircon hosted by a normal crustal rock (i.e., one containing no more than a few ppm Pb), the system diverges increasingly from partitioning equilibrium (Watson et al., 1997). Therefore, in order to counter the charge imbalance, it is proposed that charge compensation is accomplished by protons in hydrous environments, allowing stabilization of radiogenic Pb in zircon via the following mechanism:
$$U^{4+} = Pb^{2+} + 2 H^{+} (4)$$

Kogawa et al. (2012) was able to confirm this mechanism based on a study of synthetic zircon. Kogawa et al. (2012) also highlighted the importance of the following coupled substitution:

$$U^{4+} + 2 Si^{4+} = Pb^{2+} + 2 P^{5+} (5)$$

Lead diffusion in crystalline zircon is remarkably slow (Watson et al., 1997), but it is much faster in zircon that has been severely radiation-damaged, rendering Pb loss more likely in metamict zircons.

In the case of titanite, divalent Pb²⁺ can be hosted in the Ca²⁺ site via a simple cation exchange process. Cherniak (1993) reported that crystalline titanite is less retentive of Pb than zircon; further, metamict titanite is unlikely to retain its original Pb isotopic ratios following radiation damage unless significant repair of lattice damage occurs. Kirkland et al. (2018) observed the presence of Pb²⁺ in a metamorphic titanite, and suggested that the breakdown of precursor U-bearing phases produced a homogeneous Pb reservoir from which the metamorphic titanite grew. Similarly, Kramers et al. (2009) found no spectroscopic evidence of Pb(IV) in an old titanite grain; however, they suggested that common Pb in a Tertiary titanite was mainly in the Pb(IV) state.

## 2. Samples and Experimental Methods

### 2.1 Sample Selection and Characterization (SEM/μSXRF/μXAS)

Samples of uraninite, coffinite, and brannerite were chosen from a representative set of formation conditions and over a wide range of age and correlative radiogenic Pb contents (Table 1). This collection is complemented by two zircons commonly used as standards in geoanalysis, and zircon and titanite from the Mt Painter Inlier in South Australia (details in Table 1).

All samples are either in the form of polished thin sections or polished blocks and were analyzed by scanning electron microscopy (SEM). SEM images (Secondary and Backscatter electron modes; completed by chemical micro-analysis via Energy Dispersive Spectroscopy, EDS) were taken at the
Monash Centre for Electron Microscopy using a JEOL JSM-7001F FEG SEM and a FEI Quanta 3D FEG SEM; and at Adelaide Microscopy using a Philips XL30 FEG SEM.

These samples were further examined by µSXRF and Pb L<sub>II</sub>/L<sub>III</sub> edge µXAS at GSE-CARS (13-ID-E) at the Advanced Photon Source (APS) at Argonne National Laboratory in Chicago. The APS is a 7 GeV ring and was operated with a maximum current of 102 mA. 13-ID-E is an undulator beam line with a Si(111) monochromator with an energy resolution (ΔE/E) of 1.1 x 10<sup>-4</sup> and a flux of ~10<sup>10</sup> photons/second at ~10 keV. KB mirrors were used to focus the beam to ~2x2 µm<sup>2</sup>. A four element Vortex Si-drift detector was used for detecting fluorescence data. Pb L<sub>III</sub>-edge µXANES data were collected on a number of natural Pb-bearing minerals (Table SUPP 1), which were used to qualitatively constrain the interpretation of differences in Pb oxidation state, coordination, and bonding environments; EXAFS data were collected for uraninite samples. Pb L<sub>II</sub> XANES were collected for zircon (and titanite) samples, to suppress the interference of the Hf L<sub>γ</sub> fluorescence line.

In addition, bulk U L<sub>III</sub> XAS data were collected on some uraninites (Table SUPP 1) at the XAS beam line at the Australian Synchrotron (AS). The AS is a 3 GeV ring and was operated in top-up mode with a maximum current of 200 mA. The XAS beam line has a 1.9 T wiggler source and a Si(111) monochromator with an energy resolution (ΔE/E) of 1.5 x 10<sup>-4</sup> at 10 keV. The beam was focused vertically and then horizontal and vertical slits were used to reduce the beam size to ~300 µm<sup>2</sup>. Data were acquired in fluorescence mode using a Canberra 100-element Ge detector.

## 2.2 XAS + µSXRF Data Analysis

XAS data were analyzed using Athena and Artemis, which are part of the HORAE package (Newville, 2001; Ravel and Newville, 2005). The measured XAS data were processed by background fitting and calibration of the Pb L<sub>II</sub> and L<sub>III</sub> edge energy (Athena). EXAFS fitting was performed with the program Artemis, using FEFF 9.0 (Rehr et al., 2009). SXRF maps were interrogated using GSE-viewer, which is part of the Larch package (Newville, 2013).
243 **2.3 First Principle XANES Simulations**

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To provide an independent check on how the oxidation state and geometry of Pb influences the
observed Pb XANES of U-bearing minerals, theoretical calculations were performed via the FDMNES
program (Joly, 2001; Guda et al., 2015). FDMNES calculates element specific absorption cross sections
across edge ionization energies, such as Pb L<sub>III</sub>, to model the XANES of specific compounds. For this
study, the input to the theoretical calculations are the crystallographic information regarding the U-bearing
minerals of interest, and by perturbing the ideal theoretical system with the effect of replacing a U<sup>4+</sup> atom
in the crystal lattice with Pb<sup>2+</sup>. Ab initio XANES calculations were also used to explore the effect on order
length scale (i.e., first shell versus bulk crystal structure) on the XANES spectra (Borg et al., 2012; Brugger
et al., 2014). The calculations were performed using the Finite Difference Method (FDM) to solve the
Schrödinger equation, allowing for a free shape of the molecular potentials; density of states were obtained
from fully relativistic monoelectronic calculations (DFT-LSDA). The theoretical spectra were convoluted
using a core-hole value of 5.81 eV (6.48) and a Gaussian broadening 2.31 eV (2.69) at the Pb L<sub>III</sub> edge to
account for beamline resolutions at the Pb L<sub>III</sub> (Pb L<sub>II</sub>) edge.

257 **2.4 DFT Geometry Optimization: Coordination Environment of Pb in Uraninite**

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Structure optimizations of uraninite and uraninite substituted with Pb<sup>2+</sup> were carried out using
density functional theory (DFT) within the CPMD code (version 3.17.1; Car and Parrinello 1985). The
CPMD method implements DFT using plane-wave basis sets and pseudopotentials to simulate the presence
of the core electrons and the nucleus. The DFT-based simulations have been successfully used to simulate
the structure of uranyl complexes (UO<sub>2</sub><sup>2+</sup>) in aqueous solutions and solids (Bühl, 2009; Bühl et al., 2006;
2008). The calculation details are similar to those given in Liu et al. (2017) and Bühl (2009), and the key
parameters are repeated here. The BLYP (Becke, Lee, Yang and Parr) exchange correlation-functional was
employed (Becke, 1988; Lee et al., 1988) together with Martins–Troullier pseudo-potentials (Troullier and
Martins, 1991) with plane-wave cutoffs of 100 Ry. In each calculation, wavefunction optimization was
conducted initially and then followed by geometry optimization. Convergence criteria of 10<sup>-5</sup> and 10<sup>-3</sup> were
268 applied for wavefunction and geometry optimization, respectively. All calculations were conducted using
269 periodic boundary conditions.

270 Geometry optimization of uraninite was performed with 1 and 8 supercells each containing 12 atoms
271 (Inorganic Crystal Structure Database, ICSD cif # 77701); the cells were aligned in the order of 1-1-1, and
272 2-2-2 along a-b-c dimension; where 1-1-1 is defined to be one unit cell, and 2-2-2 consists of 2 unit cells
273 along a, b and c axis. The unit cell size was kept constant for all calculations.

274 The geometry optimization of Pb-substituted uraninite was performed by replacing one U-O pair
275 with one Pb, representing Pb concentrations of 2.4 to 20 w.t.% (see details in Tables 3 and SUPP 3). Four
276 calculations were performed with Pb²⁺-substituted uraninite, following the coupled substitution [U⁴⁺ + O²⁻
277 = Pb²⁺ + O]: (1) one Pb²⁺ substitution in a 1-1-1 uraninite unit cell (11 atoms, formula PbU₃O₇); (2) one
278 Pb²⁺ substitution of uraninite in 2-2-2 uraninite supercell (95 atoms, formula PbU₃₁O₆₃); (3-4) two Pb²⁺
279 substitution in uraninite 2-2-2 supercell (94 atoms, formula Pb₂U₃₀O₆₂) with two different initial Pb-Pb
280 distances of 4.31 Å and 7.92 Å, respectively.

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282 **3. Results**

283 **3.1 Pb L<sub>III</sub> XANES on Standards**

284 Comparison of the Pb L<sub>III</sub> edge XANES spectra of the Pb standards reveals significant differences
285 between the minerals considered in this study (Fig. 2a). With respect to the Pb-oxide minerals, the most
286 significant difference exists between the Pb<sup>4+</sup>-bearing oxide, plattnerite (PbO<sub>2</sub>), relative to the Pb<sup>2+</sup>-bearing
287 oxides (PbO polymorphs), massicot (orthorhombic) and litharge (tetragonal), as demonstrated by the shift
288 in the maximum of absorption intensity from approximately 13.057 keV to 13.051 keV (Fig. 2a). This
289 edge shift is a typical feature demonstrated by many redox-sensitive elements, where the white line of the
290 oxidized species is shifted to higher energies relative to the corresponding reduced species (Bunker, 2010;
291 Penner-Hahn, 2005). Furthermore, the magnitude and energy position of the pre-edge features of the Pb L<sub>III</sub>
292 XANES spectra differs significantly between plattnerite relative to massicot and litharge. The pre-edge
293 feature is influenced by the coordination environment of Pb with the surrounding bonding atoms (e.g.,
Wilke et al., 2001). Farges et al. (2006) noted that the intensity of the pre-edge transition also tends to be correlated with oxidation state, notably the higher the oxidation state the more intense the pre-edge transition; indeed, the XANES pre-edge feature of plattnerite is more pronounced than the pre-edge features of litharge and massicot. Pb⁴⁺ in plattnerite is coordinated to 4+2 oxygens (distorted octahedron) and litharge and massicot are coordinated to four oxygens, whereby all oxygens are below the plane, with the Pb²⁺ lone electron pair occupying the position above the plane (hemidirected coordination). Comparison of the Pb-oxide minerals with cerussite (PbCO₃; holodirected distorted 9-fold coordination with O), galena (PbS; regular octahedral coordination with 6 S), curienite (Pb(UO₂)₂V₂O₈·5H₂O; holodirected 8-fold coordination), and cleusonite (Pb²⁺,Sr)(U⁴⁺,⁶⁺)(Fe²⁺,Zn)₂(Ti,Fe²⁺,³⁺)₁₈(O,OH)₃₈; cleusonite is x-ray amorphous (Wülser et al., 2005), but assuming local structure is preserved a holodirected distorted 12-fold coordination with O is expected where Pb exists as Pb²⁺ in each mineral, demonstrating how different coordination environments and bonding ligands can influence the observed XANES spectra (Fig. 2a). The most distinguishing difference is that none of these exhibit pre-edge features, and the edge energy position of their respective maximum in absorption intensity is consistent with being shifted to lower energy relative to plattnerite, similar to the Pb²⁺-oxide species. Additionally, the edge energy for galena, despite being a Pb²⁺ mineral, is shifted to lower energy than any of the Pb²⁺-oxide minerals. This is typical for sulfide minerals and has been observed in many previous studies (e.g., Etschmann et al., 2017; Etschmann et al., 2019; Waldo et al., 1991).

## 3.2 Petrography

The µSXRF images demonstrate that the distribution and concentrations of Pb are spatially inhomogeneous in uraninite (Figs. 3, 4 and SUPP 1), even in the youngest sample used in this study, Bella Tolla (Table 1). The Lambert uraninite grains are euhedral in morphology and were the only uraninites to appear inclusion-free under the SEM (Fig. 3a). However, the µSXRF images demonstrate that the distribution and concentrations of Pb are actually spatially inhomogeneous, i.e., the µSXRF highlights localized regions where Pb is relatively enriched or depleted (Fig. 3b). Bella Tolla uraninite is highly
fractured and spatially inhomogeneous with respect to Pb distribution and concentration (Fig. 3c, d). The circular symbols on each µSXRF image (e.g., Fig. 3b, d) indicate the areas chosen for individual XAS measurements; in general, we selected zones from areas of relatively low concentrations of Pb and away from grain boundaries and inclusions/Pb hot spots, to avoid inclusions and reaction rims surrounding the minerals of interest.

The coffinite samples were all highly inhomogeneous in composition (Fig. SUPP 1i,j; Fig. 4), with numerous Pb-rich inclusions (mostly galena). Sample RX2753 from Olympic Dam, illustrated in Figure 4a, b, was described in detail by Macmillan et al. (2017). At Olympic Dam, coffinite is a late phase resulting from U-mobility post-deposition of the Cu-Fe-ores (Macmillan et al., 2017; Xing et al., 2018), and shows complex textures suggesting mineral replacement; it is associated with a uraninite showing numerous micro-inclusions of galena. In addition, a ‘cobweb’ uraninite described by Macmillan et al. (2016b) was analyzed (sample RX7269; Fig. 4d). This uraninite has been replaced by bornite along zones defined by primary oscillatory and sector zoning (Macmillan et al. 2016a).

The measured brannerite samples vary in age from Miocene to Proterozoic (Table 1). They appeared homogeneous under the SEM (Fig. SUPP 1k-q), but showed varying Pb distributions: the Iragna brannerite was homogenous with respect to Pb; La Gardette showed a Pb-rich rim, developed along fractures through the crystal; and Crockers’ Well brannerite showed a ‘spotty’ Pb distribution, as well as Pb enrichment along fractures.

The SEM backscatter images of the titanite and zircon samples show that some samples appear homogeneous (SUPP 2 a,b,d; Table 1). In contrast, zircons from Plešovice show clear evidence of chemical zoning during growth (oscillatory and sector zoning; SUPP 2 c). Most of the U-rich, Mesoproterozoic Yerila granite zircons are metamict (Kromkhun et al., 2013); the example in Fig. SUPP 2e shows a metamict core with numerous inclusions; Pb is distributed in a highly heterogeneous manner in such cores (Fig. SUPP 2f). The core of the crystal in Fig. SUPP 2e is surrounded by a homogeneous (under the SEM) rim that is highly fractured due to the volume expansion of the core upon metamictization. The SXRF maps
345 indicate mostly homogenous distribution of Pb in such rims, although a few Pb-rich inclusions (PbS or PbO) do occur even in these zones (Fig. SUPP 2f).

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348 **3.3 Pb-XANES Spectra of U-bearing Minerals**

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The Pb L<sub>III</sub> XANES spectra of the uraninite grains examined in this study are largely dissimilar
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relative to the Pb-standards (Figs. 2a, b), and exhibit significantly more spectral features relative to
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brannerite and coffinite (Figs. 2c, d). For example, the Pb L<sub>III</sub> XANES of uraninite does not exhibit any
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pre-edge features (Fig. 4c, d), however, the maxima of the uraninite spectra is shifted to lower energies
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relative to all other Pb<sup>2+</sup>-,<sup>4+</sup>-oxides (Fig. 2). Overall the measured Pb L<sub>III</sub> XANES data suggest that Pb exists
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as Pb<sup>2+</sup> in the crystal lattice of uraninite rather than Pb<sup>4+</sup>, as the latter would have an absorption intensity
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maximum at higher energy relative to Pb<sup>2+</sup>-bearing minerals.
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The Pb L<sub>III</sub> XANES spectra from the brannerite grains (Fig. 2c, d) are dampened and relatively
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featureless compared to the Pb-standards and Pb-bearing uraninite (Fig. 2a, b). Similar dampened spectra
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have been observed for Pb complexes in solution (Etschmann et al., 2018), Pb adsorbed onto mineral
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surfaces (Obrycki et al., 2016), or high-coordination number structures with disordered, holodirected
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coordination (e.g., cleusonite assuming preservation of local order; curienite; Fig. 2a); such spectra are
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usually indicative of large distortion/low symmetry in the first coordination sphere.
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The Pb L<sub>III</sub> XANES spectra of the Beverley “coffinite” (Fig. 2d) are in fact (i) galena (the spectrum
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for the Beverley hotspot closely resembles that of galena), (ii) uraninite (the other Beverley XANES
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spectrum is the same as Pb in uraninite, indicating a close intergrowth of coffinite and uraninite, typical of
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these deposits (e.g. Fig. 3 in Li et al., 2016; diffraction and composition data for this sample in Wülser et
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al., 2011). Similarly, the XANES spectra collected on ‘clean’ areas for the coffinite from Olympic Dam
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(Fig. 4b, c, points 2,5,6) are similar to galena, suggesting that most Pb is hosted as galena micro- to nano-
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inclusions. The coffinite from Nant de Drance (Fig. 2d) can be well modelled using a linear combination
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of galena (55%) and the relatively featureless spectrum of Iragna brannerite (45%) (Fig. 2c), suggesting
that a small amount of Pb may be incorporated into the crystal structure of this young hydrothermal
coffinite.

Most Pb L<sub>II</sub> XANES spectra measured on our zircon and titanite grains are similar to the spectrum
for PbO (Fig. 5). The exception is the U-rich, mostly metamict zircons from Yerila granite: their XANES
spectrum is shifted to lower energy and has a flatter shape than PbO; this could indicate a low coordination
number, highly distorted hemidirected coordination sphere (Fig. SUPP 2, 3).

## 3.4 Insights from First Principle XANES Simulations

The FDMNES code successfully modelled the XANES spectra of Pb<sup>2+</sup> and Pb<sup>4+</sup> oxide reference
compounds (Fig. SUPP 3A): the band positions are well reproduced, as well as the relative intensities of
the spectral features. The calculations for galena were less accurate: one band is missing, and the locations
of the two main bands are significantly shifted relative to the experimental data. FDMNES calculations
also reveal that much of the observed spectral features arise from the first coordination sphere (dotted lines
in Fig. SUPP 3A); a notable exception is galena, where the first shell spectrum is featureless compared to
the bulk spectrum.

We calculated the XANES spectrum of Pb-substituted uraninite by assuming that one Pb replaces
a U in the uraninite structure (Fig. 6a, b). In general, this simple model provides a good agreement with
the experimental spectra collected from uraninites across a range of geological and geochronological
settings: the lack of pre-edge features and the location of the three main bands are all correctly reproduced.
The main difference between the calculated and experimental XANES spectra is the magnitude of the first
absorbance band (white line), which is much more intense in the calculated spectrum (Fig. 6a). This
significant difference most likely reflects the fact that the calculated structure is highly ordered, whereas,
in reality, Pb<sup>2+</sup> incorporation will result in some local distortion around Pb in the crystal lattice. This is
discussed in detail in Section 3.6 on the basis of DFT calculations. The XANES spectra calculated from
the highly distorted local structures around Pb in uraninite calculated in Section 3.6 are shown in Fig. 6c.
These simulations reveal that (i) the intensity and shape of the white line in the XANES calculated from
the relaxed structures of single Pb replacing U in uraninite is well reproduced. In particular, a shoulder on the white line, which is absent from the simulations in Fig. 6b, c, is present; this shoulder was also observed in the experimental data (easily seen in the derivative plot, Fig. 6c); (ii) The XANES spectrum derived from the simulation of a Pb-Pb dimer (i.e., two Pb replacing 2 neighboring U) differs significantly from that obtained from single site substitution, suggesting that Pb does not tend to form clusters. Note that the simulations show a pre-edge at ~13.01 keV; this is due to the choice of the Fermi level, and does not reflect a discrepancy between experiment and observations.

The FDMNES spectrum for Pb substituting for U in the crystal structure of brannerite (Szymański and Scott, 1982) does not resemble the experimental spectra (Fig. SUPP 3B); in particular, a large pre-edge is predicted by the calculations, but absent from the experimental spectra. The closest approximation to the experimental spectra was the simulation with Pb surrounded by six oxygen atoms in a symmetric octahedron, which reproduced the shape and location of the white line well.

Similarly, the calculated XANES spectrum for Pb-doped into zircon (Fig. SUPP 3C) differs significantly from the experimental zircon spectra, as the peaks are more pronounced and shifted in energy (Fig. SUPP 3). A better approximation can be achieved by using a linear combination of the first-shell (i.e., lacking the pre-edge) FDMNES spectrum of litharge (~70%) and Pb-doped into zircon (~30%), which is consistent with some Pb substituting in the zircon crystal structure, while the remaining forms clusters or nanoparticles with a structure close to that of PbO.

## 3.5 Pb in Uraninite: Insights from Pb L<sub>III</sub> EXAFS

The Fourier transform of the EXAFS oscillations reflects the distribution of electronic density away from the central atom. Overlaying the EXAFS data of the U-L<sub>III</sub> and Pb-L<sub>III</sub> edges of measured uraninites (Fig. 7) shows a remarkably similar pattern, suggesting strong similarities among the average local geometries of U and Pb in these uraninite samples, as expected if Pb substituted for U in the uraninite structure.
The EXAFS data (k-space) and their Fourier transform (R-space) are shown with quantitative fits in Figure 8 at both the Pb L<sub>III</sub> (micro-EXAFS collected at APS, selected spectra away from Pb-rich inclusions) and U L<sub>III</sub> edges (bulk data collected on powdered samples at AS). The uraninite crystal structure (Clausen et al., 1983) was used as the starting point for all refinements, replacing the central U atom with Pb for the Pb EXAFS refinements. As shown in Table 2, Pb is coordinated to eight oxygen atoms in the first shell; the younger uraninites were best fitted with a single Pb-O bond length (2.49(1) Å) to 2.50(1) Å), while the older uraninites required 2 x Pb-O bond lengths (2.38(4) and 2.58(2) Å for Cardiff and 2.29(4) and 2.51(3) Å for OD) to fit the EXAFS data.

Initial attempts to refine the Pb coordination numbers resulted in large errors, however the results indicated a split in the first coordination shell for the older uraninites, with three oxygen ligands at a shorter distance and five at a longer distance. In the final fit presented here, the coordination numbers were fixed and the bond length and EXAFS Debye-Waller parameters were refined.

The U L<sub>III</sub> EXAFS data show that U is bonded to eight oxygen ligands in three configurations: 8 uniform U-O bond lengths (2.44(1) Å); 1 short U-O (1.92(2) Å) + 7 long U-O (2.458(8) Å) bond lengths; and finally, 2.6 short U-O (2.38(2) Å) + 5 long U-O (2.52(4) Å) bond lengths. There is no simple correlation of first coordination shell disorder with age or unit-cell (Table 4); however, there are more variables involved here, including the average oxidation state of U, and the incorporation of other trace elements that could affect the bonding of U.

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### 3.6 DFT Geometry Optimization: Bonding Environment of Pb-substituted Uraninite

There have been many DFT investigations on UO<sub>2</sub>(s) (e.g., Andersson et al., 2009; 2013; Dorado et al., 2009; 2010 and references therein). These studies noted that by only ascribing **1k** anti-ferromagnetic order to UO<sub>2</sub>, instead of the required **3k**, the point symmetry of U drops from O<sub>h</sub> to D<sub>4h</sub>; this has been described as a Jahn-Teller distortion. The consequence is that the 5f electrons are split into two two-fold degenerate and three non-degenerate levels; U(IV) has two 5f electrons and given this split in degeneracy
it now matters which orbitals the 5f electrons have populated, as DFT calculations of the UO₂(s) system can achieve different local minima depending on the initial electron configuration (Dorado et al., 2009). Subsequently, in order for the DFT simulations to converge to the ground state, structural symmetry constraints are relaxed, resulting in significant distortions from the ideal fluorite lattice (Andersson et al., 2013). Experimental studies have observed this Jahn-Teller distortion in uraninite below the Néel temperature (T_N ~ 30 K, e.g., Blackburn et al., 2005; Wilkins et al., 2006).

The DFT-based geometry optimization on uraninite in this study shows the typical DFT-induced Jahn-Teller distortions, with U-O distances ranging from 2.21 to 2.90 Å (2-2-2 cell, Table 3), with an average U-O bond length of 2.43 Å, identical to the U-O bond length in the crystal structure of UO₂(s) according to Clausen et al. (1983).

When one Pb²⁺ ion was inserted into the uraninite structure replacing one [U⁴⁺ + O²] unit, the simulations converged in both the 1-1-1 and 2-2-2 cells. The result show that Pb²⁺ fits comfortably into the structure, with the electron lone pair located within the oxygen vacancy (Fig. 9). U-O distances (Table 3) ranged from 2.21-2.47 Å (1-1-1) and 2.19-2.96 Å (2-2-2), similar to the original uraninite structure containing no Pb; the Pb-O distances showed severe distortion, with 4 Pb-O bond lengths ranging from 2.03-2.19 Å; the remaining oxygen atoms that were originally associated with this site were located at 3.13 – 3.45 Å from the Pb atom (2-2-2, similar results for the 1-1-1 cell, see Table 3). To check that the simulations converged to a significant minimum, another set of simulations were run, whereby the starting configuration was obtained by shifting the oxygen positions in the ideal uraninite structure slightly from their original positions, but the structure converged towards similar configurations as described above.

Further calculations were run, exploring the effect of adding two Pb²⁺ ions into the uraninite structure; with two different initial Pb-Pb distances: Pb-Pb = 4.31 Å (i.e., initial Pb cluster with Pb-Pb distance of 4.31 Å), and Pb-Pb = 7.94 Å (i.e. there are U-O units between the Pb atoms). The system with the two neighboring Pb²⁺ ions converged to four Pb-O bonds ranging from 1.99 – 2.05 Å (the distances were the same for both Pb²⁺ ions) and a further two Pb-O distances at 3.13 – 3.22 Å; with U-O distances
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varying from 2.18 to 3.02 Å (average distance = 2.39 Å). The system with the well separated two Pb²⁺ ions converged to four Pb-O bonds ranging from 2.03 – 2.09 Å (again, the distances were the same for both Pb²⁺ ions) and a further two Pb-O distances at 3.27 and 3.36 Å; with U-O distances varying from 2.16 to 3.16 Å (average distance = 2.31 Å). The results show no significant difference in Pb-O or U-O bond lengths when the two Pb atoms substituted into the uraninite structure were close together or further away; there is no evidence from these calculations to suggest that Pb clustering is favorable.

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Comparing the DFT-optimized uraninite geometries to EXAFS (section 3.5; Table 2), it can be seen that DFT calculations suggest significant distortion of the first coordination shell of Pb, with two different Pb-O bond lengths. This is at odds with the EXAFS results on the Phanerozoic uraninite, where Pb exists in a coordination environment similar to that of U (Table 2). Even for the Proterozoic samples, where EXAFS indicated two different O-distances, the DFT bond lengths differ substantially from those refined from fitting the EXAFS data: namely the short DFT Pb-O bond lengths are up to 0.3 Å shorter (EXAFS short bond lengths = 2.29 - 2.38 Å, DFT = 2.03 - 2.19), while the long Pb-O bond lengths are up to ~1.0 Å longer than the EXAFS bond lengths (EXAFS long bond lengths = 2.38 - 2.58 Å, DFT = 3.13-3.45 Å). MD/DFT bond lengths are frequently longer than those obtained from experimental measurements, and this is generally related to the level of theory used in the calculations (e.g. Etschmann et al., 2018).

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# 4. Discussion

488

## 4.1 Oxidation State and Coordination Chemistry of Pb in Uraninite

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Few data exist that can directly constrain the mechanisms of element substitution in the crystal lattice of U minerals, and as a result the valence state and coordination environment of Pb in the crystal lattice of uraninite has not yet been unequivocally determined (Macmillan et al., 2016b; Sunder et al., 1993). Uraninite can contain up to 20 wt% of radiogenic Pb. The crystal structure of uraninite has a truly remarkable flexibility: it can sustain extremely high radiation damage, with even the oldest samples remaining crystalline (Table 4), and it can accommodate a wide range of elemental impurities and oxidation

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(to UO₂.₃₃; see section 1.2). Groundbreaking early work (Willis 1963; 1964) shows that the uraninite structure accommodates hyper-oxidation via incorporation of interstitial oxygens that do not occupy the center of oxygen sub-lattice cubes but rather occupy two positions displaced approximately 1 Å from the octahedral site (Brincat et al., 2015); these so-called 'Willis defects' tend to cluster together.

The results from this study indicate that Pb exists as Pb²⁺, as evidenced by the Pb L<sub>III</sub> XANES spectra of uraninite grains relative to the Pb-oxide standards (Fig. 2a, b). The Pb L<sub>III</sub> XANES spectra of uraninite do not exhibit pre-edge features in contrast to those of Pb-oxides, suggesting that the Pb does not exist as a distinct Pb-oxide phase within the crystal lattice of uraninite, but rather replaces U⁴⁺ via a substitution mechanism that can compensate for the charge difference imposed by radiogenic Pb²⁺. Indeed, the XANES spectra obtained from uraninite away from Pb-rich inclusions are similar across the wide range of samples studied. The FDMNES *first principle* XANES calculations of Pb doped into the uraninite structure (Fig. 6a, b) matches the experimental XANES measurements with respect to the position of the peaks, even though the magnitude does not match well – a reflection of the overly simplistic substitution model used for the XANES simulation (no relaxation around the Pb²⁺ ion; see below).

The coordination environment of radiogenic Pb in uraninite, as determined by modeling the Pb L<sub>III</sub> EXAFS data, suggests that Pb may exist in at least two bonding environments (Fig. 7, 8 and Table 4). In both models, Pb is coordinated to eight oxygen atoms in the first shell (though given that EXAFS errors on coordination numbers at ≥10%, a 7-fold coordination cannot be excluded), however the younger uraninites can be fitted with a single Pb-O bond length, while the older uraninites require 2 x Pb-O bond lengths to fit the EXAFS data. The differences in coordination environment of Pb as a function of age could be due to increasing concentrations of radiogenic Pb and other elements substituting within the crystal lattice of uraninite, which is associated with increased radiation damage and possibly higher degree of U⁴⁺ oxidation to U⁶⁺, although the precise reason remains elusive.

We performed DFT geometry optimization on a simple model of coupled substitution by replacing the neighboring [U⁴⁺ + O²⁻] with Pb²⁺. The results need to be carefully interpreted, since the DFT predicts
structures that are much more distorted than the real structures for U⁴⁺ compounds (e.g., U⁴⁺ coordination in pure uraninite shows U-O bonds varying from 2.21 to 2.90 Å, instead of a regular 8-fold coordination). However, these simulations suggest that the Pb²⁺ ion can fit comfortably into the uraninite structure, with the non-bonding 6s² lone pair electron indicative of Pb²⁺ (Walsh and Watson, 2005) occupying the space left vacant by the O²⁻ vacancy (Fig. 9b). The DFT results also indicate that there is significant distortion of the local environment around the Pb²⁺ ion, resulting in a strongly distorted 4+4 oxygen coordination environment around Pb, at odd with the experimental EXAFS results. XANES simulations based on the DFT-optimized structures (Fig. 6c, corresponding to Fig. 9b, c) reproduce the shape and intensity of the white line well, but not the further bands. Altogether, results of XANES simulations in displayed in Fig. 6 are consistent with Pb²⁺ substitution on U⁴⁺ sites in uraninite: the positions of the most intense bands in the experimental XANES spectra are correctly reproduced by a simple model of U substitution in the ‘ideal’ uraninite lattice (Fig. 6a, b), while the white line is well reproduced by the distorted local Pb environment illustrated by the DFT simulations (Fig. 6c). In detail, however, further complexity exists, as highlighted by the differences in first-shell Pb coordination in old and young uraninites. In particular, we speculate that since all natural uraninites have significant O-excess (UO₂ₓ, with x ≥ 0.1; Fritsche and Dahlkamp 1997) associated with the formation of clusters of Willis defects (Brincat et al., 2015; Willis 1963; 1964), Pb²⁺ incorporation in uraninite may be linked to these defects.

Based on an extensive geochemical and micro- to nano-scale structural characterization of uraninite grains from the Olympic Dam deposit in South Australia, Macmillan et al. (2016b) demonstrated that for different grain textures, different degrees of alteration/metasomatism, and a large range in Pb and REE contents, uraninite retains its fluorite-type crystal structure, and can contain wt.% levels of Pb in solid solution rather than as nano-size or larger particles. Based on the fact that no distortions were observed in the HR-TEM images, Macmillan et al. (2016b) infer that Pb must be incorporated as Pb⁴⁺. However, the DFT and XAS data presented here show that incorporation of Pb²⁺ only results in small local distortion. While HR-TEM can achieve sub-angstrom resolution under ideal conditions (e.g., Spence et al., 2006),
such details cannot be observed on the scale (nm) of the images currently available for U-bearing minerals (e.g., Fig. 10 in Macmillan et al. 2016b for uraninite; Fig. 5 on zircon in Utsunomiya et al. 2004). The lack of distortion in diffraction (either electron or X-ray) is also not surprising – to date no experimental study has noted distortion in uraninite from the fluorite structure under ambient conditions, despite their highly variable compositions. Willis (1964) noted that at higher temperatures the oxygen atoms in uraninite relax from the fluorite structure (0.25 + δ, 0.25 + δ + 0.25 + δ, where δ = 0.016 at 1000 °C).

The inferences gained from the FDMNES calculations and measured XANES and EXAFS data, that suggest isomorphic substitution of U⁴⁺ by Pb²⁺, is counterintuitive since there is a large difference between the atomic radii of 8-fold coordinated U⁴⁺ and Pb²⁺; substitution of Pb²⁺ at first sight requires deformation of the crystal lattice in addition to a coupled substitution mechanism in keeping with charge balance constraints (Janeczek and Ewing, 1992b). One needs to remember, however, that the Goldschmidt rules for substitution are not absolute, due to the ability of some crystal structures to accommodate large amounts of local distortion. An extreme example is the substitution of S⁶⁺ (ionic radius 0.12 Å) for Mo⁶⁺ (ionic radius 0.41 Å; Shannon 1976) in the crystal structure of vergasovaite, accommodated by the flexibility of a Jahn-Teller affected Cu(II) ion (Berlepsh et al. 1999). So it may not be that surprising that uraninite can incorporate relatively large amounts of Pb²⁺. On the one hand, Pb²⁺ displays a highly diverse coordination chemistry, due in part to its stereochemically active electron lone pair (see Figure 4; Brugger et al. 2016; Etschmann et al. 2018; Makovicky and Balic-Zunic 1998). On the other hand, the crystal structure of uraninite displays exceptional flexibility. Janeczek and Ewing (1992b) showed conclusively that uraninite is not ideally UO₂, but rather composed of a limited fraction of U⁶⁺ relative to U⁴⁺, which is a record of the temperature and redox conditions of the hydrothermal fluid from which the uraninite formed, and composed of a range of trace elements, such as Ca and REEs, due to the alteration history of uraninite. The range in oxidation state and the associated change in atomic radii of U⁴⁺·⁶⁺(IV) in uraninite facilitates the compensation of charge balance and structural deformation effects due to radiogenic Pb incorporation in the crystal lattice in conjunction with impurities such as REEs, which are commonly associated with
570 deposition of U-bearing minerals (Mercadier et al. 2011), and with oxygen defects (Brincat et al., 2015; Willis 1963; 1964).
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## 4.2 Oxidation State of Pb in Brannerite, Coffinite, Zircon, and Titanite

572 The measured Pb L<sub>II</sub> and Pb L<sub>III</sub> XANES spectra of the minerals, brannerite, coffinite, zircon, and titanite in comparison with the Pb<sup>2+</sup> and Pb<sup>4+</sup> standards all indicate that these minerals contain Pb as Pb<sup>2+</sup>. However, in detail, there are significant differences among these minerals.

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In the case of coffinite, the XANES spectroscopy indicates that most Pb is located in micro- to nano-scale mineral inclusions, typically galena. The youngest coffinite studied (Nant de Drance) shows some indication for a fraction of lattice-bound Pb. In contrast to uraninites and coffinites, all studied brannerites were X-ray amorphous. They contain Pb in a form that is distinct from PbO(s) minerals.

The XANES spectra collected on different zircons in this study indicate that Pb is present as Pb<sup>2+</sup>. The spectra resemble those of PbO(s), except for the spectrum from a U-rich, metamict zircon from the Proterozoic Yerila granite. In the only other XANES study of Pb in zircon, Tanaka et al. (2010) formed the conclusion that the oldest zircon studied – exposed to the highest radiation dose; see Murakami et al. (1991) – contained Pb<sup>2+</sup>, while younger zircons (i.e., those exposed to lower doses) may contain some Pb<sup>4+</sup>. This conclusion was based on examination of the Pb L<sub>III</sub> XANES pre-edge, but upon further examination of their Fig. 1, the pre-edge for those samples aligns well with PbO, i.e., is indicative of Pb<sup>2+</sup>.

Spectroscopic evidence for the presence of Pb<sup>4+</sup> in titanite has been suggested by Kramers et al. (2009). Somewhat counter-intuitively, Kramers et al. (2009) noted that Pb in their old titanite sample (95% radiogenic Pb) exists as Pb<sup>2+</sup>, while the young titanite sample (containing predominantly common Pb) was Pb<sup>4+</sup> (see Fig. 3 in Kramers et al., 2009). Their argument in preferring radiogenic Pb to exist as Pb<sup>4+</sup> appears to be based on U<sup>4+</sup> decaying “straight” to Pb<sup>4+</sup> by the emission of a few alphas and betas; but this is a simplistic view of the U decay chain: Pb is at the end of the decay chain, where it is effectively de-coupled from U as most decay paths pass through Rn, which is a gas that can move/diffuse in/through the structure (Watson et al., 1997). The XANES spectrum shown by Kramers et al. (2009) for the mostly common Pb
in the Tertiary titanite is convincingly representative of Pb⁴⁺. However, we note that when we applied a
common baseline subtraction to all spectra, a few of the spectra from samples with low Pb concentrations
looked like Pb⁴⁺. Careful examination of these spectra revealed that this was an artefact of the
normalization. Hence, based on the fact that (i) none of the spectra collected in our study showed evidence
of Pb⁴⁺, (ii) the spectrum shown by Kramers et al. (2009) remains unique, and this spectrum was obtained
on a young sample with radiogenic Pb levels below the detection limits of XANES; and that (iii) there is
no good explanation for the origin of Pb⁴⁺ in such a sample, we conclude that to date, there is no evidence
for Pb⁴⁺ preservation in any U-bearing mineral.

### 4.3 U-minerals: Radiogenic Pb Mobilization

It has been shown frequently that the radiogenic Pb content of uraninite is a function of depositional
age and metamorphic/alteration history (Alexandre and Kyser, 2005; Janeczek and Ewing, 1995;
Macmillan et al., 2016b; Martz et al., 2019; Mercardier et al., 2013). With respect to metamorphism and
alteration history, radiogenic Pb in uraninite will be mobilized from the crystal lattice of uraninite upon
changes in the oxidation state of the metasomatic hydrothermal fluid, where U⁴⁺ in uraninite will mobilize
to soluble U⁶⁺ species upon oxidative dissolution. This is a consequence of the solubility of Pb²⁺ in
hydrothermal fluids being less sensitive to changes in redox conditions of the hydrothermal fluid relative
to U-bearing minerals, but more of a function of temperature and dissolved ligand concentration, such as
association with dissolved chloride and sulfide (Etschmann et al. 2018; Zhong et al. 2015). In contrast,
non-oxidative induced Pb²⁺ mobilization upon alteration of uraninite can be engendered from mineral
replacement reactions, for example, upon coffinization of uraninite to coffinite; during such replacement
reaction, extensive geochemical data demonstrate that product coffinite and partially altered zones of
uraninite grains become depleted in radiogenic Pb relative to the surrounding primary unaltered zones of
uraninite (Alexandre and Kyser, 2005; Janeczek and Ewing, 1995; Macmillan et al., 2016b). This
observation is consistent with the XANES data of coffinite examined in this study, where the data is
generally noisier than the uraninite data, as a consequence of the inherently low Pb content (Figs. 4a-c).

Radiogenic Pb can be readily incorporated within the crystal lattice of uraninite as Pb²⁺, however, depending on changes in hydrothermal fluid chemistry, Pb can be susceptible to mobilization as a consequence of the different mechanisms of U-bearing mineral alteration, such as dissolution-reprecipitation and/or mineral replacement, whereas mobilization of U is primarily susceptible to the oxidation state of the coexisting hydrothermal fluid.

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## 4.4 Zircon: Radiogenic Pb mobilization

Evidence for radiogenic Pb²⁺ incorporation into the crystal lattice of zircon is provided by the Pb L₁₁ XANES spectra and FDMNES XANES calculations reported in this study, consistent with inferences gained from past analytical studies on natural zircons with a wide range of ages (Utsunomiya et al., 2004; Tanaka et al., 2010). Access to, and mobilization of radiogenic Pb²⁺ derived from zircon upon alteration is dependent on the degree of metamictization and on the alteration mechanisms responsible, i.e. diffusion-reaction-recrystallization and/or coupled dissolution-reprecipitation. With either alteration mechanism, which may occur in tandem, radiogenic Pb²⁺ will be mobilized from zircon by fluids and melts, at a range of physiochemical conditions, from weathering to high temperature metamorphism, ultimately resulting in spurious age data derived from U-Pb geochronology due to preferential radiogenic Pb mobilization.

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For the diffusion-reaction-recrystallization mechanism, amorphous domains develop within the metamict zones of zircon, forming interconnected clusters with nanoscale porosity, allowing fluid species to readily diffuse and react at the recrystallization front within the reaction rims. This process promotes the exchange of primary and radiogenic trace elements within zircon, such as radiogenic Pb²⁺, with solvent cations, such as Ca²⁺, upon hydrothermal alteration and weathering of zircon. The coupled dissolution and reprecipitation of zircon is an additional mechanism that liberates radiogenic Pb²⁺ from the zircon crystal lattice in the presence of fluids and melts. This process is driven by the inherent structural strain of the zircon crystal lattice due to the incorporation of trace elements such as Pb, Hf, U, and Th. In particular,
this structural strain enhances the solubility of zircon, resulting in the dissolution of primary zircon and reprecipitation of secondary zircon depleted in “non-formula” trace and radiogenic elements. Ultimately, with either alteration mechanism, radiogenic Pb²⁺ will undergo mass transfer into solution; precipitation as distinct PbO/PbS/native-Pb inclusions, and/or migration within domains within recrystallizing zircon, depending on the physiochemical conditions imposed.

## 5. Conclusion

This experimental investigation examined the local oxidation state and coordination environment of radiogenic Pb in the crystal lattice of the U-minerals, brannerite, coffinite, and uraninite, through characterization by electron microscopy and synchrotron X-ray based analytical tools, µSXRF and XAS (XANES/EXAFS). In addition, we revisited the speciation of Pb in the U-bearing minerals zircon and titanite, in which previous XANES studies had claimed to detect evidence of Pb⁴⁺, despite the low stability of this oxidation state in geofluids. Coupling of the SEM images with µSXRF of the U-bearing mineral allowed determination of distinct zones to avoid for XAS analysis, that were composed of distinct Pb-minerals, such as galena, and not representative of Pb in the crystal lattice. The XANES data demonstrate that Pb exists as Pb²⁺, either as Pb-mineral inclusions (e.g., most Pb in coffinite) or part of the crystal lattice (most prominently in uraninite). In the case of uraninite, XANES and EXAFS data strongly suggest that Pb replaces U in the fluorite-type crystal structure; DFT geometry optimization results of Pb-substituted uraninite confirm that Pb²⁺ can be incorporated in uraninite, but in conjunction with the XAS data indicate that the mechanism is not simply a coupled substitution of Pb²⁺ replacing a U⁴⁺ and a neighboring oxygen vacancy.

This study emphasizes the need to combine spectroscopic studies, which are sensitive to low levels of element substitution in a crystal lattice, with micro-characterization techniques (e.g., SEM; HR-TEM), which have low sensitivity to minor element substitution but highlight the presence of nano- and micro-particles within a mineral matrix.
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Knowledge of the radiogenic Pb chemical form can be coupled with known solubility constraints for U- and Pb-bearing minerals and constituent dissolved species and can be used to predict differences in Pb and U mobility and the isotope compositions of which they impart. Importantly, Pb²⁺ solubility will be controlled by the availability of ligands such as chloride and bisulfide, whereas that of U will be strongly controlled by redox, given the low solubility of U⁴⁺ in low temperature hydrothermal fluids (Xing et al. 2018).

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## Acknowledgements

This research was funded by the Australian Research Council Australian Copper-Uranium Transformation Research Hub (project number IH130200033). We are grateful to BHP Olympic Dam and the South Australian Department of State Development for funding, and Kathy Ehrig and Edeltraud Macmillan for providing Olympic Dam samples and feedback. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy - Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Part of this research was also undertaken on the XAS beamline at the Australian Synchrotron, part of ANSTO. The DFT calculations in this work were supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. The authors acknowledge Prof. Michael Bühl for providing the pseudopotential of uranium for DFT calculations. Many people provided support for this research; we are particularly grateful to Susan Cumberland (help with data collection at AS), Matt Newville (help with data collection at APS), and Kirk Scheckel (EPA) for providing data for many Pb standards. Most notably, we appreciate the constructive comments addressed by two anonymous reviewers and Dr. Denis Fougerouse,
694 which were incredibly constructive. Finally, we thank the associate editor, Dr. Mario Villalobos, for

695 facilitating the completion of the manuscript.
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935 Ram, R., Charalambous, F.A., McMaster, S., Pownceby, M.I., Tardio, J. and Bhargava, S.K. (2013) 936 Chemical and micro-structural characterisation studies on natural uraninite and associated gangue 937 minerals. Minerals Engineering 45, 159-169.
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1024

Figure 1

1025

Schematic diagram of radiogenic Pb distribution as a consequence of radioactive decay of U from uraninite (UO₂), surrounding mineral radiation damage induced by α-decay (metamictization), and transport of intermediate radionuclides such as radon (²¹⁸,²¹⁹Rn). This experimental study focuses on the bonding environment of Pb in U-minerals, uraninite, coffinite, and brannerite, and in minerals which have U as minor constituent, zircon and titanite. The diagram exemplifies the different environments which radiogenic Pb may exist, either as sub-micron PbS/PbO/Pb inclusions and/or distributed within the crystal lattice of uraninite through substitution with U⁴⁺,⁶⁺.

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Figure 2

1034

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Pb L<sub>III</sub> X-ray absorption near edge structure (XANES) of the Pb-standards used for this study (a): plattnerite (PbO<sub>2</sub>), litharge and massicot (PbO), cerrusite (PbCO<sub>3</sub>), galena (PbS), curienite (Pb<sup>2+</sup>(U<sub>2</sub>O)<sub>2</sub>V<sub>2</sub>O<sub>8</sub>*(5H<sub>2</sub>O), and cleusonite (Pb<sup>2+</sup>,Sr)(U<sup>4+,6+</sup>)(Fe<sup>2+</sup>,Zn)<sub>2</sub>(Ti,Fe<sup>2+,3+</sup>)<sub>18</sub>(O,OH)<sub>38</sub>; natural uraninite (Fig. 2b); brannerite (c); and coffinite (d). The Pb-standard XANES and associated derivatives exhibit noticeable differences as a consequence of Pb oxidation states, for example, plattnerite (Pb<sup>4+</sup>) versus litharge/massicot (Pb<sup>2+</sup>), and bonding environments, PbS versus Pb-oxides. The dashed line highlights the maximum peak of litharge (Pb<sup>2+</sup>) and plattnerite (Pb<sup>4+</sup>), which occurs at higher energy compared to Pb<sup>2+</sup> minerals. In contrast, the XANES spectra of the brannerite and coffinite are relatively featureless.

1042

1043

Figure 3

1044

1045

1046

1047

Corresponding back-scattered electron SEM and SXRF images of the Lambert and Bella Tolla uraninite samples used for this study. The white circles on each SXRF image depict the area of XAS microprobe analysis. The color distribution, with respect to green, depicts the relative concentration of Pb in each sample; red = Fe, blue = Cu.

1048
1049

**Figure 4**

1050

Coffinite-uraninite from Olympic Dam. (a) micro-SXRF image (Ba in red, U in green, Si in blue). (b) BSE-SEM image of area highlighted in a). (c) XANES spectra collected from the points shows in (a) and (b).

1051

1052

1053

**Figure 5**

1054

XANES data on zircon and titanite, collected at the Pb L<sub>II</sub> edge, and compared to PbO(s) and PbO<sub>2</sub>(s) references.

1055

1056

1057

**Figure 6**

1058

Comparison of FDMNES theoretical Pb L<sub>III</sub> XANES calculations with measured XANES of Pb-bearing uraninite. (a, b) Simulations of one Pb atom replacing a U<sup>4+</sup> in the crystal structure of uraninite (Clausen et al., 1983). Model results are largely similar with respect to the shape of the spectra despite the large differences in magnitude of the theoretical calculation by FDMNES. The overall similarity between theoretical and measured XANES confirm that the observed experimental spectra is indicative of Pb bonding within the crystal lattice of uraninite. (c) Selected FDMNES XANES simulations of DFT-optimized structures described in the text and shown/described in Fig. 9.

1059

1060

1061

1062

1063

1064

1065

1066

**Figure 7**

1067

Overlapped EXAFS data of uraninite samples measured at the U L<sub>III</sub> edge (AS) and the Pb L<sub>III</sub> edge (APS). The similarity of the patterns suggests that U and Pb have similar coordination environments, providing additional evidence for the hypothesis that Pb is incorporated into the uraninite lattice.

1068

1069

1070
**Figure 8**

The modelled extended X-ray absorption fine structure (EXAFS, dashed lines) compared with the measured EXAFS (solid lines) of natural uraninite samples at the Pb L<sub>III</sub> edge a) and U L<sub>III</sub> edge c); the Fourier transform of the EXAFS at the Pb L<sub>III</sub> edge b) and U L<sub>III</sub> edge d).

1075

**Figure 9**

DFT-optimized geometry of (a) uraninite; (b) one Pb<sup>2+</sup> replacing one [U<sup>4+</sup> + O<sup>2</sup>] unit in uraninite to 3Å; (c) one Pb<sup>2+</sup> replacing one neighboring [U<sup>4+</sup> + O<sup>2</sup>] unit in uraninite, with dist<sub>Pb-Pb</sub> = 5 Å; and (d) two Pb<sup>2+</sup> replacing two non-neighboring [U<sup>4+</sup> + O<sup>2</sup>] units in uraninite, with dist<sub>Pb-Pb</sub> = 3Å in the 2-2-2 supercell. The yellow surfaces show the electron density at isosurface value of 0.85. Composition and geometrical details are shown in Table 3.

1077

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1079

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1081
<table><caption>**Table 1**</caption><caption>Table 1: Summary of geological settings for samples used in this study, arranged by mineral type and age.</caption><thead><tr><th>Mineral/deposit</th><th>Type of deposit</th><th>Age of mineralization</th><th>Regional geology</th></tr></thead><tbody><tr><td>Uraninite, Bella Tolla, Val d'Annivier, Switzerland</td><td>Disseminated in gneiss (flattened masses along schistosity), as well as hydrothermal mineral in quartz, albite and barite veins.</td><td>Alpine - i.e. Oligocene-Miocene in age.</td><td>Pre-Westphalien polymetamorphic (Variscan and Alpine) gneissic basement of the Siviez-Mischabel Nappe (Penninic paleogeographic domain; Thélin et al., 1993). The mineralization is located several meters away from the discordant contact with Triassic series (quartzites and arkoses).</td></tr><tr><td>†Uraninite, Ike-Nixon mine, Utah, USA</td><td>Irregular, amoeba-shaped masses that are concordant to bedding. Sandstone-hosted, roll-front type.</td><td>Oligocene-Pliocene(?) (Meek, 2014).</td><td>The Lisbon Valley uranium district (Huber, 1981) is located in the east-central part of the Colorado Plateau, approximately 50 km southeast of Moab, Utah. The majority of the uranium deposits occur in Triassic sandstones and conglomerates.</td></tr><tr><td>Uraninite, Chaméane, Central Massif, France</td><td>Vein in granite (Johan et al., 1982)</td><td>Carboniferous to Permian (multistage).</td><td>Granite-hosted polymetallic (U-As-Cu-Se), multistage quartz-barite-ankerite vein. Early uraninite coeval with granite (granite du Livradois; ~330 Ma); late generation related to ankerite-Se-rich stage, ~251 ± 10 Ma (Durand and Sanselme, 1963; Johan et al., 1982).</td></tr><tr><td>Uraninite, Château Lambert, Southern Vosges, France</td><td>Cu-Mo-U±Au, Ag in pegmatite overprinted by hydrothermal vein mineralization.</td><td>Early Carboniferous (Visean) 340–336 Ma (Tabaud et al., 2015).</td><td>Mineralized pegmatitic veins in Visean diorite intrusions. Pagel and Ruhlmann, 1979 identified three uraninite-bearing mineralization stages, grading from pegmatitic (400±30°C; studied samples) to low T hydrothermal (215±20°C).</td></tr><tr><td>†Uraninite, Kemp Uranium Prospect, Cardiff, Canada</td><td>Pegmatite - magmatic-hydrothermal (e.g. Desbarats et al., 2016; Lentz, 1996).</td><td>The most reliable ages of the uraninite-bearing skarns and veins are between 1010 and 1060 Ma (Easton, 1992). Note: Malczewski and Dziurowicz, 2015 cite an inconsistent age of 1250–1340 Ma.</td><td>Central Meta-sedimentary Belt (CMB) of the Proterozoic Grenville Province, a complex (ca. 1100 Ma) orogenic belt truncating older (Mid-Proterozoic accumulation) geological provinces (Easton, 1992). Uranium mineralization associated with the (1070-1040 Ma) Fenite-Carbonatite and (1050-1030 Ma) Granite Pegmatite post-tectonic plutonic events.</td></tr><tr><td>Uraninite, coffinite Olympic Dam IOCG deposit, Gawler Province, South Australia</td><td>Hydrothermal (Iron Oxide Copper Gold).</td><td>Early magmatic hydrothermal uraninite at ~1.59 Ga (Macmillan et al., 2016b). 1000 My of hydrothermal remobilization responding to major tectonic events mainly at 1300-1100, 825 and 500 Ma (Kirchenbaur et al., 2016). Brannerite and coffinite are texturally late (Macmillan et al., 2017).</td><td>Cu-U-Au-Ag-(REE, Mo, F) ores within hematite-rich breccia complex developed within a granite of the 1.59 Ga Gawler Silicic Province (South Australia). The main U minerals are coffinite, brannerite, and uraninite-pitchblende (Ehrig et al., 2012).</td></tr></tbody></table>
<table><thead><tr><td>Mineral/deposit</td><td>Type of deposit</td><td>Age of mineralization</td><td>Regional geology</td></tr></thead><tbody><tr><td>†Coffinite, Beverley sensu stricto orebody, South Australia</td><td>Sandstone-hosted, tabular-type U deposit.</td><td>Pliocene: 6.7–3.4 Ma (based on U-Pb dating of coffinite and carnotite; Wülser et al., 2011).</td><td>The Beverley U-deposit is located in terrestrial sediments in the Mesozoic-Quaternary Lake Frome basin, South Australia. Mineralization is hosted mainly in organic matter-poor Miocene lacustrine sands, and consists of coffinite and/or uraninite nodules.</td></tr><tr><td>Coffinite, Nant-de-Drance hydroelectric tunnel, Switzerland</td><td>Hydrothermal.</td><td>Alpine - Oligocene-Miocene in age (remobilization of pre-alpine concentration; Meisser et al., 2010).</td><td>Fresh sample recovered during construction of the Nant de Drace underground powerplant (2009-15). Mineralization is hosted in the Aiguilles Rouges massif, consisting of pre-Variscan gneissic basement and Carboniferous granite (Meisser et al., 2010).</td></tr><tr><td>†Brannerite, La Gardette Mine, France</td><td>Hydrothermal.</td><td>Alpine - Miocene in age, 5-11 Ma (Gasquet et al., 2010).</td><td>The La Gardette brannerite was recovered from the La Gardette Mine in France. The brannerite is associated with quartz-barite veins formed from hydrothermal activity initiated by Alpine metamorphism (Gasquet et al., 2010; Geffroy, 1963; Lumpkin et al., 2012).</td></tr><tr><td>†Brannerite, Iragna, Ticino, Switzerland</td><td>Pegmatite.</td><td>Post-peak Alpine metamorphism (&lt;35 Ma)</td><td>Leventina granite gneiss within the Leventina nappe of Lepontine (Bianconi and Simonetti, 1967). Brannerite occurs as idiomorphic crystals to 1 cm in size.</td></tr><tr><td>†Brannerite, Crocker's Well, Olary province, South Australia</td><td>Magmatic to magmatic hydrothermal.</td><td>Intrusion at 1579.2 ± 1.5 Ma; mineralization coeval or shortly thereafter.</td><td>Proterozoic granitoids and metamorphic rocks in the Olary province of the Willyama block of South Australia (an extension of the Gawler Silicic Province) host disseminated U-Th-Ti and U-Fe-Ti-Th minerals (Ludwig and Cooper, 1984).</td></tr><tr><td>†Titanite, diopside-titanite veins, near Arkaroola, Northern Flinders Ranges, South Australia</td><td>Magmatic hydrothermal.</td><td>443 ± 3 Ma—U-Pb dating on titanite (Elburg et al., 2003).</td><td>Pegmatitic Palaeozoic diopside-titanite veins in Mesoproterozoic metagranites and metasediments that constitute the basement (Mt Painter Inlier) of the Adelaide Fold Belt (South Australia; Bakker and Elburg, 2006; Brugger et al., 2011).</td></tr><tr><td>Zircon, Mt Painter Inlier, Northern Flinders Ranges, South Australia (Yerila granite & MtGee)</td><td>Magmatic (granite; Yerila granite) and detrital (MtGee).</td><td>Yerila Granite is dated at 1562 ± 26 Ma (Kromkhun et al., 2013). Most Yerila zircons are metamict and suffered extensive Pb loss. The MtGee zircons are Paleozoic in age (Brugger et al., 2011).</td><td>Yerila granite is a pale grey, porphyric granite with tabular feldspar phenocrysts (Kromkhun et al., 2013). Yerila granite is part of a suite of radiogenic, Mesoproterozoic acid magmatic rocks outcropping in the Mt Painter and Mt Babbage inliers. MtGee zircons were extracted from conglomerates associated with Paleozoic epithermal activity at Mt Gee; the population measured was Paleozoic in age (Brugger et al., 2011).</td></tr><tr><td>Zircon '91500', Kuehl Lake, Ontario, Canada</td><td>Magmatic (syenite pegmatite).</td><td><sup>238</sup>U-<sup>206</sup>Pb age of 1062.4 ± 0.4 Ma (Wiedenbeck et al., 1995).</td><td>The 91500 zircon is one of the most popular standards for dating and trace-element determinations (Orihashi et al., 2008). It is sourced from a syenitic pegmatite.</td></tr><tr><td>Zircon, Plešovice, Bohemian Massif, Czech Republic</td><td>Metamorphic (granulite facies).</td><td><sup>238</sup>U-<sup>206</sup>Pb age of 337.13 ± 0.37 Ma.</td><td>Plešovice zircon is a popular standard for dating and trace-element determinations, sourced from the Plešovice quarry, situated in the eastern part of the Blansky' les granulite body, ~5 km NNE of the town of Česky' Krumlov (Slama et al., 2008). The host rock is felsic, potassic granulite.</td></tr></tbody></table>

†Sample sourced from the collections of the South Australian Museum, Adelaide, Australia.
<table><caption>**Table 2:** Calculated coordination and bond lengths of Pb-O in measured uraninite samples. The symbols, nO1, nO2 and nU refer to the number of ligands coordinated to Pb, RO1, RO2 and RU refer to the bond length (Å), and σ² is EXAFS Debye-Waller factor (Å²). The 1-sigma fitting error is indicated in parentheses; values that were not refined during the fit are indicated by (fix).</caption><caption>Pb in uraninite: ΔE=0.7(7), χ<sup>2</sup><sub>red</sub> = 405, R-factor=0.034. Note: the Debye-Waller factors for O1 and O2 were constrained to be equal (i.e., σ<sup>2</sup>O1 = σ<sup>2</sup>O2).</caption><thead><tr><th>Sample</th><th>nO1</th><th>RO1 (Å)</th><th>nO2</th><th>RO2 (Å)</th><th>σ<sup>2</sup>O (Å<sup>2</sup>)</th><th>nU</th><th>RU (Å)</th><th>σ<sup>2</sup>U (Å<sup>2</sup>)</th><th>k range</th><th>R range</th></tr></thead><tbody><tr><td>Bella Tolla</td><td>8 (fix)</td><td>2.50(1)</td><td></td><td></td><td>0.012(1)</td><td>12 (fix)</td><td>3.85(1)</td><td>0.011(2)</td><td>3&lt;k&lt;9</td><td>1&lt;R&lt;4</td></tr><tr><td>Ike Nixon (grain1)</td><td>8 (fix)</td><td>2.50(1)</td><td></td><td></td><td>0.019(2)</td><td>12 (fix)</td><td>3.86(2)</td><td>0.019(3)</td><td>3&lt;k&lt;8</td><td>1.05&lt;R&lt;4</td></tr><tr><td>Ike Nixon (grain2)</td><td>8 (fix)</td><td>2.49(1)</td><td></td><td></td><td>0.020(1)</td><td>12 (fix)</td><td>3.86(2)</td><td>0.019(2)</td><td>3&lt;k&lt;8</td><td>1.05&lt;R&lt;4</td></tr><tr><td>Château Lambert</td><td>8 (fix)</td><td>2.51(1)</td><td></td><td></td><td>0.015(1)</td><td>12 (fix)</td><td>3.87(2)</td><td>0.014(1)</td><td>3&lt;k&lt;9</td><td>1&lt;R&lt;4</td></tr><tr><td>Cardiff</td><td>3 (fix)*</td><td>2.38(4)</td><td>5 (fix)*</td><td>2.58(2)</td><td>0.016(7)</td><td>12 (fix)</td><td>3.89(2)</td><td>0.024(3)</td><td>3&lt;k&lt;9</td><td>1.1&lt;R&lt;4</td></tr><tr><td>Olympic Dam</td><td>3 (fix)*</td><td>2.29(4)</td><td>5 (fix)*</td><td>2.51(3)</td><td>0.001(6)</td><td>12 (fix)</td><td>3.79(5)</td><td>0.019(8)</td><td>3&lt;k&lt;8</td><td>1.05&lt;R&lt;4</td></tr></tbody></table>

<table><caption>**U in uraninite:** ΔE = 0.1(4), χ²red = 214, R-factor=0.046</caption><thead><tr><th>Sample</th><th>nO1</th><th>RO1 (Å)</th><th>σO1 (Å<sup>2</sup>)</th><th>nO2</th><th>RO2 (Å)</th><th>σO2 (Å<sup>2</sup>)</th><th>nU</th><th>RU (Å)</th><th>σU (Å<sup>2</sup>)</th><th>k range</th><th>R range</th></tr></thead><tbody><tr><td>Nant de Drance</td><td>2.6(12)</td><td>2.38(2)</td><td>0.003 (fix)</td><td>5.0(12)</td><td>2.52(4)</td><td>0.01 (fix)</td><td>3.6(14)</td><td>3.90(1)</td><td>0.004(3)</td><td>2&lt;k&lt;11</td><td>1&lt;R&lt;4.5</td></tr><tr><td>Ike Nixon **</td><td>8 (fix)</td><td>2.44(1)</td><td>0.018(2)</td><td></td><td></td><td></td><td>2.1(12)</td><td>3.88(2)</td><td>0.004(4)</td><td>2&lt;k&lt;12</td><td>1&lt;R&lt;4.5</td></tr><tr><td>Chaméane</td><td>0.9(2)</td><td>1.92(2)</td><td>0.003 (fix)</td><td>7.1(9)</td><td>2.458(8)</td><td>0.009(2)</td><td>3.6(11)</td><td>3.91(1)</td><td>0.003(3)</td><td>2&lt;k&lt;10</td><td>1&lt;R&lt;4.5</td></tr></tbody></table>

*Refined to 3.3(11) & 4.7(11) - given the large errors, fixed values at 3 & 5.

**Fitting with two different O distances leads to similar results (i.e., RO1 ~ RO2).
<table><caption>**Table 3:** Geometry calculated from *Ab initio* molecular simulations</caption><thead><tr><th rowspan="3">Crystal</th><th rowspan="3">Super-cell<br>and formula</th><th rowspan="3">Pb<br>(wt. %)</th><th colspan="4">Bond length (Å) and coordination number</th><th rowspan="3">U-Pb**</th><th rowspan="3">U-U**</th><th rowspan="3">Pb-Pb**</th></tr><tr><th colspan="2">U-O</th><th colspan="2">Pb-O</th></tr><tr><th>R</th><th>CN</th><th>R</th><th>CN</th></tr></thead><tbody><tr><td>Uraninite</td><td>1-1-1, U<sub>4</sub>O<sub>8</sub></td><td>0</td><td>2.43</td><td>8</td><td>-</td><td>-</td><td>-</td><td>3.97***</td><td>-</td></tr><tr><td>Uraninite</td><td>2-2-2, U<sub>32</sub>O<sub>64</sub></td><td>0</td><td>2.21-2.90<br>(2.43)*</td><td>8</td><td>-</td><td>-</td><td>-</td><td>3.87-4.09</td><td>-</td></tr><tr><td>1Pb(II)-Uraninite</td><td>1-1-1, PbU<sub>3</sub>O<sub>7</sub></td><td>20</td><td>2.21-2.47</td><td>5.33</td><td>2.06-2.08</td><td>5</td><td>3.47</td><td>3.53</td><td>-</td></tr><tr><td>1Pb(II)-Uraninite</td><td>2-2-2, PbU<sub>31</sub>O<sub>63</sub></td><td>2.4</td><td>2.19-2.96</td><td>8</td><td>2.03-2.19</td><td>4</td><td>3.56</td><td>3.80-4.15</td><td>-</td></tr><tr><td>2Pb(II)-Uraninite<sup>a</sup></td><td>2-2-2, Pb<sub>2</sub>U<sub>30</sub>O<sub>62</sub></td><td>4.8</td><td>2.18-3.02<br>(2.39)*</td><td>8</td><td>1.99-2.05</td><td>4</td><td>3.37</td><td>3.75</td><td>4.31</td></tr><tr><td>2Pb(II)-Uraninite<sup>b</sup></td><td>2-2-2, Pb<sub>2</sub>U<sub>30</sub>O<sub>62</sub></td><td>4.8</td><td>2.16-3.16<br>(2.31)*</td><td>8</td><td>2.03-2.09</td><td>4</td><td>3.46</td><td>3.79</td><td>7.92</td></tr></tbody></table>

<footnote>* Peak of the Gaussian distribution; a-two Pb in neighboring sites; b-Pb in non-neighboring sites. ** Closest distance. When distance distribution is Gaussian, range is indicated, for random distance distribution with no clear cut off, shortest distance is given. *** All distances the same.</footnote>
1095 **Table 4.**

1096

<table><caption>**Table 4:** Lattice parameters determined by XRD analysis of uraninite and coffinite from various localities.</caption><thead><tr><th>Mineral</th><th>Locality</th><th>a</th><th>c</th></tr></thead><tbody><tr><td rowspan="5">Uraninite</td><td>Bella Tolla</td><td>5.471(3)</td><td>-</td></tr><tr><td>Cardiff</td><td>5.503(1)</td><td>-</td></tr><tr><td>Ike Nixon</td><td>5.3895(8)</td><td>-</td></tr><tr><td>Lambert</td><td>5.4640(2)</td><td>-</td></tr><tr><td>Nant de Drance*</td><td>5.4136(2)</td><td>-</td></tr><tr><td rowspan="2">Coffinite</td><td>Nant de Drance*</td><td>7.0901(9)</td><td>6.359(2)</td></tr><tr><td>Beverley†</td><td>6.971(2)</td><td>6.255(2)</td></tr></tbody></table>

1097

<footnote>*Nant de Drance was a mixture of coffinite (80%) and uraninite (20%). † Data from Wülser et al. (2011). While there is a rough correlation with lattice parameter with age, this is not exact and complicated by the presence of impurities.</footnote>
238U
...
218Rn
...
206Pb

235U
...
219Rn
...
207Pb

<figure><img src="image_2.png" /></figure>

U-bearing mineral/metamictic zone

Mobile radiogenic Rn

PbS (Galena)

Structurally incorporated/interstitial Pb?

20 µm
<figure><img src="image_3.png" /><figcaption>b)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>c)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>d)</figcaption></figure>

<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>
a) RX7253 (Olympic Dam)

<figure><img src="image_11.png" /></figure>

BaUSi

<figure><img src="image_12.png" /></figure>

c)

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
Normalized Absorbance (a.u.)

Pb L$_{II}$
Zircon and Titanite

†Zircon: Mt Gee, Australia

*Zircon: Kuel Lake, Canada

*Zircon: Yerila granite, Australia

†Titanite: Radium Camp, Australia

*Zircon: Plesovice, Czech Republic

PbO

PbO$_{2}$

† No smoothing

* Data smoothed (15 x boxcar)

Energy (keV)

15.18

15.20

15.22

15.24

15.26

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>
EXAFS oscillations for uraninite

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>
<figure><img src="image_24.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_27.png" /></figure>