Robert J. Finch

Rare earth elements in synthetic zircon: Part 1. Synthesis, and rare earth element and phosphorus doping

Abstract Zircon crystals were grown from a Li-Mo flux (7.5 mol% Li2MoO4; 86.5 mol% MoO3) to which equal molar proportions ZrO2 and Li2SiO3 were added (3 mol% each). The crystals were initially grown undoped, but later Dy was added to the flux without any other charge-compensating species. With Dy and P added, in equal molar proportions, the zircon crystals incorporated 1.37 mol% (6.99 wt%) Dy and 1.36 mol% (1.33 wt%) P, a factor of 5.3 increase in Dy over the crystals grown without P intentionally added to the flux. The other P+REE-doped zircon crystals displayed an approximately 1000-fold increase in REE and P from La through Lu, a result of decreasing ionic radii. The incorporation of P5+ allowed, in a general sense, the xenotime-type coupled substitution of (REE3+, Y3+) + P5+ = Zr4+ + Si4+. From La to Nd, however, P exceeds REE, from Sm to Gd, the REE are generally equal to P, and from Tb to Lu the REE exceed P. The Y- and P-doped zircon behaved more like middle-REE (MREE)-doped zircon than heavy-REE (HREE)-doped zircon crystals in their ability to incorporate Y (and P) and to maintain charge neutrality. To investigate the incorporation of Dy with no P added to the flux, the P to LREE excess, and the HREE to P excess in the doped zircon, secondary ion mass spectrometry (SIMS) analyses were completed on selected crystals. In the Dydoped crystals, the SIMS analyses revealed minor amounts of P, Li, and Mo in the crystals. These elements contributed to charge balance required by the excess Dy. In REE- and P-doped zircon, the SIMS analyses detected Li and Mo, and the Li and Mo may also provide charge balance for excess REE in the HREE+P-doped crystals.

Structure and Chemistry of Zircon and Zircon-Group Minerals

Zircon (ZrSiO4) is a common accessory mineral in nature, occurring in a wide variety of sedimentary, igneous, and metamorphic rocks. Known to incorporate an assortment of minor and trace elements, zircon has the ability to retain substantial chemical and isotopic information, leading to its use in a wide range of geochemical investigations, including studies on the evolution of Earth’s crust and mantle (e.g., Hanchar et al. 1994, Bowring 1995, Vervoort et al. 1996, Hoskin and Schaltegger, this volume; Valley, this volume) as well as age dating (e.g., Gibson and Ireland 1995, Bowring et al. 1998, Solar et al. 1998, Bowring and Schmitz, this volume; Ireland and Williams, this volume; Parrish et al., this volume). The physical and chemical durability of zircon is a major factor in it being the mineral by which many of Earth’s oldest known rocks have been dated (Bowring et al. 1989, Maas et al. 1992, Buick et al. 1995, Bowring and Williams 1999, Wilde et al. 2001) and is also an important factor in zircon being proposed as a candidate waste form for the geologic disposal of excess plutonium from dismantled nuclear weapons (Ewing and Lutze 1997, Ewing 1999, Burakov et al. 2002, Burakov et al. 2003, Ewing et al., this volume). The chemical and physical properties of zircon and its ability to incorporate and retain trace elements are largely determined by its crystal structure. The zircon structure is adopted by numerous minerals and synthetic compounds with the general formula AT O4, in which high field-strength T -site cations occupy isolated tetrahedra, and A -site cations occupy larger eight-coordinated structural sites. Zircon-type compounds share many physical properties, as well as displaying variable degrees of solid solution among end members. …

Rare-earth elements in synthetic zircon: Part 2. A single-crystal X-ray study of xenotime substitution

Abstract Zircon crystals synthesized in a Li-Mo oxide melt and doped with trivalent lanthanides and Y (REE), both with and without P, were examined by single-crystal X-ray diffraction (XRD). REE are incorporated into the Zr site in the zircon structure, and some Zr appears to be displaced to the Si site. Crystals doped with middle REE (MREE, Sm to Dy) and Y, plus P follow the xenotime substitution (REE3+ + P5+ = Zr4+ + Si4+) rather closely, whereas crystals doped with heavy REE (HREE, Er to Lu) deviate from the xenotime substitution, having REE:P atomic ratios significantly greater than one. Xenotime substitution requires that P5+ replace Si4+, but this substitution becomes limited by strain at the Si site in HREE-doped crystals. As Si sites become saturated with P5+, additional charge balance in synthetic zircon crystals may be provided by Mo6+ and Li+ from the flux entering interstitial sites, accounting for an additional 0.3 to 0.6 at% HREE beyond that balanced by P5+ ions. Heavy REE are more compatible in the zircon structure than are LREE and MREE, and HREE substitution is ultimately limited by the inability of the zircon structure to further accommodate charge-compensating elements. Thus the limit on REE concentrations in zircon is not a simple function of REE3+ ionic radii but depends in a complex way on structural strain at Zr and Si sites, which act together to limit REE and P incorporation. The mechanisms that limit the coupled xenotime substitution change from LREE to HREE. This change means that REE fractionation in zircon may vary according to the availability of charge-compensating elements. REE partition coefficients between zircon and melt must also depend in part on the availability of charge-compensating elements and their compatibility in the zircon structure.

Wyartite: Crystallographic evidence for the first pentavalent-uranium mineral

Abstract Determination of the structure of wyartite provides the first evidence for a pentavalent-U mineral. The structure of wyartite, CaU5+(UO2)2(CO3)O4(OH)(H2O)7, Z = 4, orthorhombic, a = 11.2706(8), b = 7.1055(5), c = 20.807(1) Å, V = 1666.3(3) Å3, space group P212121, was solved by direct methods and refined to an agreement index (R) of 4.9% for 2309 unique reflections collected using MoKα Xradiation and a CCD-based detector. The structure contains three unique U positions; two contain U6+ and involve uranyl ions with typical pentagonal-bipyramidal coordination. Seven anions coordinate the other U position, but there is no uranyl ion present. The polyhedral geometry, the bond-valence sum incident at this U site, and electroneutrality requirements, all indicate that this site contains U5+. The Uφ7 (f: O, OH, H2O) polyhedra share edges and corners to form a unique sheet in which a CO3 group shares an edge with the U5+φ7 polyhedron. The structure contains one Ca site coordinated by seven anions. The Ca atom and its associated H2O groups occupy interlayer sites, along with two H2O groups that are held in the structure by H bonds only. The Caφ7 polyhedron is linked to one adjacent sheet by sharing an edge with the CO3 group and an O atom with a U6+φ7 polyhedron. Structural units are linked together through hydrogen bonds only.

The crystal structure of ianthinite, [U24+(UO2)4O6(OH)4(H2O)4](H2O)5: a possible phase for Pu4+ incorporation during the oxidation of spent nuclear fuel

Abstract Ianthinite, [U 2 4+ (UO 2 ) 4 O 6 (OH) 4 (H 2 O) 4 ](H 2 O) 5 , is the only known uranyl oxide hydrate mineral that contains U 4+ , and it has been proposed that ianthinite may be an important Pu 4+ -bearing phase during the oxidative dissolution of spent nuclear fuel. The crystal structure of ianthinite, orthorhombic, a = 0.7178(2), b = 1.1473(2), c = 3.039(1) nm, V = 2.5027 nm 3 Z = 4, space group P 2 1 cn , has been solved by direct methods and refined by least-squares methods to an R index of 9.7% and a wR index of 12.6% using 888 unique observed [| F | ≥ 5 σ | F |] reflections. The structure contains both U 4+ . The U 6+ cations are present as roughly linear (U 6+ O 2 ) 2+ uranyl ion (Ur) that are in turn coordinated by five O 2− and OH − located at the equatorial positions of pentagonal bipyramids. The U 4+ cations are coordinated by O 2− , OH − and H 2 O in a distorted octahedral arrangement. The Ur φ 5 and U 4+ | 6 (φ: O 2− , OH − , H 2 O) polyhedra l sharing edges to for two symmetrically distinct sheets at z ≈ 0.0 and z ≈ 0.25 that are parallel to (001). The sheets have the β-U 3 O 8 sheet anion-topology. There are five symmetrically distinct H 2 O groips located at z ≈ 0.125 between the sheets of U φ n polyhedra, and the sheets of U φ n polyhedra are linked together only by hydrogen bonding to the intersheet H 2 O groups. The crystal-chemical requirements of U 4+ and Pu 4+ are very similar, suggesting that extensive Pu 4+ ↔ U 4+ substitution may occur within the sheets of U φ n polyhedra in trh structure of ianthinine.

Weathering of Natural Uranyl Oxide Hydrates: Schoepite Polytypes and Dehydration Effects

Partial dehydration of schoepite, U 0 3 · 2H 20, is reported to produce three discrete schoepite polytypes with characteristic unit cell parameters, but this has not been confirmed. The loss of structural water from the schoepite interlayer results in progressive modification to the structure; expansion parallel to schoepite cleavage planes, and extensive fracturing. Dehydration of schoepite commences at grain boundaries and progresses inward until the entire grain is converted to dehydrated schoepite, U 0 3 · 0.8H20. The volume decrease associated with dehydration results in expanded grain boundaries. These gaps can provide pathways for the access of groundwater, and uranyl silicates and uranyl carbonates have precipitated within these gaps, replacing both schoepite and dehydrated schoepite. Schoepite, however, is not observed to re-precipitate where in contact with dehydrated schoepite. Thus, while the formation of schoepite early during the corrosion of uraninite may be favored, schoepite is not a long-term solubility limiting phase for oxidized uranium in natural ground waters containing dissolved silica or carbonate.

Retention of Neptunium in Uranyl Alteration Phases Formed During Spent Fuel Corrosion

Uranyl oxide hydrate phases are known to form during contact of oxide spent nuclear fuel with water under oxidizing conditions; however, less is known about the fate of fission and neutron capture products during this alteration. We describe, the first time, evidence that neptunium can become incorporated into the uranyl secondary phase, dehydrated schoepite (UO{sub 3}{lg_bullet}0.8H{sub 2}O). Based on the long-term durability of natural schoepite, the retention of neptunium in this alteration phase may be significant during spent fuel corrosion in an unsaturated geologic repository.

Crystal chemistry of uranium (V) and plutonium (IV) in a titanate ceramic for disposition of surplus fissile material

Abstract We report X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra for the plutonium LIII and uranium LIII edges in titanate pyrochlore ceramic. The titanate ceramics studied are of the type proposed to serve as a matrix for the immobilization of surplus fissile materials. The samples studied contain approximately 10 wt% fissile plutonium and 20 wt% natural uranium, and are representative of material within the planned production envelope. Based upon natural analogue models, it had been previously assumed that both uranium and plutonium would occupy the calcium site in the pyrochlore crystal structure. While the XANES and EXAFS signals from the plutonium LIII are consistent with this substitution into the calcium site within pyrochlore, the uranium XANES is characteristic of pentavalent uranium. Furthermore, the EXAFS signal from the uranium has a distinct oxygen coordination shell at 2.07 A and a total oxygen coordination of about 6, which is inconsistent with the calcium site. These combined EXAFS and XANES results provide the first evidence of substantial pentavalent uranium in an octahedral site in pyrochlore. This may also explain the copious nucleation of rutile (TiO2) precipitates commonly observed in these materials as uranium displaces titanium from the octahedral sites.

Thermodynamic Stabilities of U(VI) Minerals: Estimated and Observed Relationships

Gibbs free energies of formation ({Delta}G{degree}{sub f}) for several structurally related U(VI) minerals are estimated by summing the Gibbs energy contributions from component oxides. The estimated {Delta}G{degree}{sub f} values are used to construct activity-activity (stability) diagrams, and the predicted stability fields are compared with observed mineral occurrences and reaction pathways. With some exceptions, natural occurrences agree well with the mineral stability fields estimated for the systems SiO{sub 2}-CaO-UO{sub 3}-H{sub 2}O and CO{sub 2}-CaO-UO{sub 3}H{sub 2}O, providing confidence in the estimated thermodynamic values. Activity-activity diagrams are sensitive to small differences in {Delta}G{degree}{sub f} values, and mineral compositions must be known accurately, including structurally bound H{sub 2}O. The estimated {Delta}G{degree}{sub f} values are not considered reliable for a few minerals for two major reasons: (1) the structures of the minerals in question are not closely similar to those used to estimate the {Delta}G{sub f}* values of the component oxides, and/or (2) the minerals in question are exceptionally fine grained, leading to large surface energies that increase the effective mineral solubilities. The thermodynamic stabilities of uranium(VI) minerals are of interest for understanding the role of these minerals in controlling uranium concentrations in oxidizing groundwaters associated with uranium ore bodies, uranium mining and mill tailings and geological repositories for nuclear waste.

Characterization and dissolution behavior of a becquerelite from Shinkolobwe, Zaire

Abstract The solubility of becquerelite (Ca(U0 2 ) 6 0 4 (OH) 6 ·8H 2 O) from Shinkolobwe (Zaire) has been determined at 25°C as a function of pH under a nitrogen atmosphere. Leached and unleached samples were characterized by a variety of analytical methods that include optical microscopy, x-ray powder diffraction (XRD), electron microprobe analysis (EMPA), and conventional scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopic analysis (EDS). A least-squares minimization of the measured total aqueous uranium concentration in equilibrium with natural becquerelite using the NEA uranium database (Grenthe et al., 1992) gives a solubility constant of log K s0 0 = 29 ± 1, significantly lower than values reported for synthetic becquerelite: log K s0 = 43.2 (Vochten and Van Haverbeke, 1990).