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The transport and enrichment of rare earth element (REE) ore bodies are dependent on the stability of aqueous metal ligand complexes and the solubility of REE bearing minerals. REE ores are commonly associated with igneous systems having aqueous fluids with high carbonate concentrations and REE solubilities have been shown to be dependent on temperature and associate anion aqueous ligands present in solution. Furthermore, this work presents solubility experiments of hydroxybastnäsite-(La) at elevated temperatures in aqueous solutions of varying carbonate concentrations. At lower temperatures, hydroxybastnäsite-(La) solubility is controlled by neutral mono-carbonate LaCO₃OH° but at higher temperatures and activities of carbonate species,more » charged di-carbonate La(CO₃)₂^- increases and predominates. This divergence, and the difference in solubility products of other hydroxybastnäsite-(REE) phases, provides a potential mechanism for REE fractionation in carbonate dominated aqueous solutions. To illustrate one such mechanism the solubility data of hydroxybastnäsite-(La) is compared with previously reported data of hydroxybastnäsite-(Nd) at elevated temperatures.« less

Bastnäsites (LnCO₃(F,OH)) are a group of common rare earth elements (REE)-bearing minerals and are one of the primary global sources of REE. Due to the chemical similarities among REE, bastnӓsites tend to occur as solid solutions instead of end members in REE containing ores. To better understand the processes and the mechanisms of formation of such deposits, it is essential to determine the thermodynamic properties of bastnӓsites, including hydroxylbastnӓsite (LnCO₃OH) solid solutions. In this work, we performed detailed structural and calorimetric investigations on synthetic hexagonal La–Nd hydroxylbastnӓsite (La_1-$$\chi$$NdCO₃OH, $$\chi$$ = 0, 0.25, 0.5, 0.75, 1) solid solutions. X-ray diffraction confirmsmore » the crystal structure of the solid solution series in the $$\ P$$6 space group, and pair distribution function (PDF) analysis reveals local bonding environments characterized by three different types of 9-coordinated metal-oxygen polyhedra. Unit cell parameters of La1–xNdxCO3OH exhibit a nearly linear relation with the Nd content x, suggesting a random distribution of La and Nd in the structure. Their standard enthalpies of formation (Δ$$\ H$$°_f) were determined by high temperature oxide melt drop solution calorimetry, from which the enthalpies of mixing (Δ$$\ H$$_mix) were derived. The Δ$$\ H$$_mix can be fitted by a regular solution model with an interaction parameter of 12.58 ± 0.16 kJ/mol, suggesting enthalpic metastability of La1–xNdxCO3OH relative to the two endmembers. Combining entropy and enthalpy, we further estimated the Gibbs free energies of mixing (Δ$$\ G$$_mix) at relevant temperatures, revealing favorable temperatures under which the intermediate La_1-$$\chi$$NdCO₃OH phases can be stabilized. Such entropy-driven stabilization, as is consistent with our geochemical modeling results, may explain the enhancement of thermal stability of the solid solutions in nature. Additionally, the temperature range constrained from this study may be used to estimate the thermal history of REE bastnӓsite deposit.« less

Heterogeneous distribution of trace elements (impurities) within individual calcite crystals is a phenomenon commonly observed in natural and laboratory systems. Changes in thermodynamic intensive parameters (mostly chemical potential and temperature) cannot always explain the inhomogeneous impurity patterns in calcite crystals and it has been suggested that growth rate and crystallographic orientation may exert strong effects on elemental incorporation into calcite. In addition, there are a number of experimental studies on micro-scale element (E) distribution between non-equivalent pairs of calcite vicinal faces (known as sectoral zoning); however, the variability of partition coefficients (e.g., K^E = (E/Ca)_calcite/(E/Ca)_fluid) within individual crystals remains undetermined.more » In this study, we have extended the work on elemental distribution between crystal sectors to evaluation of partition coefficients of trace and minor elements (Li, B, Mg, and Sr) in calcite crystal faces (10–14) and (01-12), whose growth rates were assessed. Growth entrapment model (GEM) and lattice strain theory were applied to explain K^E heterogeneity by varying near-surface diffusivity of Mg and Sr and by varying surface enrichment factor for Li, B, Mg, and Sr. Decoupling of sectoral and growth rate effects reveals that sectoral zoning plays a key role in elemental distribution. More specifically, K^Li and K^B vary by more than one order of magnitude and K^Mg varies by a factor of two within individual crystal faces. Strontium sectoral distribution is different from those of Li, B, and Mg and K^Sr varies by up to a factor of two. Finally, these behaviors likely reflect different mechanisms of incorporation of Li, B, Mg, and Sr into calcite.« less

Quantitative understanding of uranium transport by high temperature fluids is crucial for confident assessment of its migration in a number of natural and artificially induced contexts, such as hydrothermal uranium ore deposits and nuclear waste stored in geological repositories. An additional recent and atypical context would be the seawater inundated fuel of the Fukushima Daiichi Nuclear Power Plant. Given its wide applicability, understanding uranium transport will be useful regardless of whether nuclear power finds increased or decreased adoption in the future. The amount of uranium that can be carried by geofluids is enhanced by the formation of complexes with inorganicmore » ligands. Carbonate has long been touted as a critical transporting ligand for uranium in both ore deposit and waste repository contexts. However, this paradigm has only been supported by experiments conducted at ambient conditions. We have experimentally evaluated the ability of carbonate-bearing fluids to dissolve (and therefore transport) uranium at high temperature, and discovered that in fact, at temperatures above 100 °C, carbonate becomes almost completely irrelevant as a transporting ligand. This demands a re-evaluation of a number of hydrothermal uranium transport models, as carbonate can no longer be considered key to the formation of uranium ore deposits or as an enabler of uranium transport from nuclear waste repositories at elevated temperatures.« less

In this work, the solubilities of the rare earth chlorides REECl₃, where REE = (La, Nd, Er), were measured in HCl bearing water vapor from 350 – 425°C with water partial pressures ranging from 8 – 170 bar. Solubility data were fit to the Pitzer-Pabalan quasi-chemical model in order to extract thermodynamic parameters for the formation of the gaseous REE-chloride-water clusters REECl₃(H₂O)_n. The data show that the solubility of the REE chlorides are orders of magnitude higher than salts such as NaCl or CuCl at low water fugacities, despite their sublimation energies being substantially higher. This enhanced solubility is likelymore » due to the high enthalpy associated with binding a single water molecule to form the species REECl₃(H₂O), with derived enthalpies ranging from –378 to –465 kJ/mol. Addition of further water molecules to form higher order clusters (n > 1) involves enthalpy changes of ~ -20 kJ/mol, and are in effect thermodynamically suppressed over the temperature range 350 – 425°C. Despite the enhanced solubility of small REECl₃ water clusters, simulations of boiling processes demonstrate that the REE show highly conservative behavior, partitioning strongly into the dense aqueous phase. Not surprisingly, the presence of phosphates in the system makes this effect even more pronounced, completely immobilizing the REE. This would reduce transport in both the vapor and aqueous phase to negligible levels. However, we suggest that vapor phase transport of the REE may play a significant role in systems having a relatively low partial pressure of water (below the saturation point), where the relatively high stability of the first hydrated REE chloride clusters (REECl₃(H₂O) and REECl₃(H₂O)₂) will give a preference for gas transport of the REE relative to other elements. This can likely happen in systems involving a gas/melt exchange in fumarolic exhalations, where water vapor discharges at relatively low (close to atmospheric) pressures.« less

Following the Fukushima Daiichi accident, significant efforts from industry and the scientific community have been directed towards the development of alternative nuclear reactor fuels with enhanced accident tolerance. Among the proposed materials for such fuels is a uranium silicide compound (U₃Si₂), which has been selected for its enhanced thermal conductivity and high density of uranium compared to the reference commercial light water reactor (LWR) nuclear fuel, uranium oxide (UO₂). To be a viable candidate LWR fuel, however, U₃Si₂ must also demonstrate that, in the event of this fuel coming in contact with aqueous media, it will not degrade rapidly. Inmore » this contribution, we report the results of experiments investigating the stability of U₃Si₂ in pressurized water at elevated temperatures and identify the mechanisms that control the interaction of U₃Si₂ under these conditions. Our data indicate that the stability of this material is primarily controlled by the formation of a layer of USiO₄ (the mineral, coffinite) at the surface of U₃Si₂. The results also show that these layers are destabilized at T > 300 °C, leading to the complete decomposition of U₃Si₂ and its pulverization due to its full oxidation to UO₂.« less

Thorium mineralization is frequently hosted in carbonate-bearing rocks, and thorium commonly substitutes into the structures of carbonate-bearing minerals that have precipitated from or been modified by hydrothermal fluids. Given this common association, it is reasonable to consider the hypothesis that the presence of carbonate ligands in hydrothermal solutions promotes the transport of Th through the formation of stable aqueous complexes. Our ability to evaluate this hypothesis, however, is hindered by the lack of experimental data for Th-carbonate species at conditions beyond ambient. The low-temperature data indicate that carbonate is a strong complexing agent for Th. In this contribution, we investigatemore » the solubility of Th in carbonate-bearing fluids relevant to natural systems (0.05–0.5 m NaHCO₃/Na₂CO₃; pH_T ~ 7.8–9.8) at elevated temperature (175–250 °C). We demonstrate that, in contrast to the behavior of Th at low temperature, the stability of Th-carbonate complexes is not sufficient for them to predominate at these conditions. Instead, the solubility of Th is governed by hydrolysis reactions. Under the experimental conditions investigated, the predominant hydroxyl complexes are Th(OH)₄⁰ and Th(OH)₅-. Furthermore, thermodynamic formation constants were derived for these species at the temperatures considered in our experiments (log β₄ = 43.34 and 44.31 at 175 and 200 °C, respectively, and log β₅ = 46.15 and 47.9 at 225 and 250 °C, respectively) to permit forward modeling of Th mobility in natural systems. Our study indicates that carbonate ions are unlikely to play a role in transporting Th in hydrothermal fluids. Summarizing the results of this study and our previous studies of the solubility of Th in hydrothermal fluids, we conclude that SO₄^2- is the primary ligand responsible for the hydrothermal transport of Th.« less

This work evaluated the immobilization of uranium (U) through incorporation into calcite under reduced and oxidized conditions. We investigated how much U could be entrapped by calcite crystallizing in chloride solutions in autoclaves at temperatures from 162 to 300 °C. The oxidation state of U was set by controlling oxygen fugacity via redox buffers. Uranium was introduced into calcite growth media as a solid oxide compound or U aliquot. We found the uptake of tetravalent U by calcite is higher than that of hexavalent U by up to four orders of magnitude. Furthermore, we estimate that crystallization of a fewmore » mg of calcite immobilizes all dissolved U when 1 kg of solution is saturated with UO₂ under reduced hydrothermal conditions.« less

Thorium is the most abundant actinide in the Earth’s crust and has universally been considered one of the most immobile elements in natural aqueous systems. This view, however, is based almost exclusively on solubility data obtained at low temperature and their theoretical extrapolation to elevated temperature. The occurrence of hydrothermal deposits with high concentrations of Th challenges the Th immobility paradigm and strongly suggests that Th may be mobilized by some aqueous fluids. Here, we demonstrate experimentally that Th, indeed, is highly mobile at temperatures between 175 and 250°C in sulfate-bearing aqueous fluids due to the formation of the highlymore » stable Th(SO₄)₂ aqueous complex. The results of this study indicate that current models grossly underestimate the mobility of Th in hydrothermal fluids, and thus the behavior of Th in ore-forming systems and the nuclear fuel cycle needs to be re-evaluated.« less

The speciation of uranyl in sulfate-bearing solutions at temperatures up to 250 ºC has been investigated using in situ UV-Visible spectrophotometry. Formation constants for the reactions UO$$^{2+}_{2}$$ + SO$$^{2–}_{4}$$ $$\Longleftrightarrow$$ UO₂SO$$^{0}_{4}$$ (logβ₁ = 3.38, 4.40, 5.44, 6.33, 7.74) and UO$$^{2+}_{2}$$ + 2SO$$^{2–}_{4}$$ $$\Longleftrightarrow$$ UO₂(SO₄)$$^{2–}_{2}$$ (logβ₂ = 4.10, 5.26, 6.83, 8.00, 9.70) derived at 25, 100, 150, 200 and 250 ºC respectively. These formation constants were fitted to the modified Ryzhenko – Bryzgalin (MRB) model to derive pK(298), A(zz/a) and B(zz/a) values for both complexes: 3.262, 2.212, -197.96 (UO₂SO$$^{0}_{4}$$) and 4.189, 3.21, -473.14 UO₂(SO₄)$$^{2–}_{2}$$ respectively. Compared with values extrapolated using previouslymore » available data for 25, 70 and 75 ºC, our new data suggest higher stability of UO₂SO$$^{0}_{4}$$ at temperatures above 150 ºC and significantly lower stability of UO₂(SO₄)$$^{2–}_{2}$$ at all temperatures above 25Cº. Molar absorbances of both sulfate species were also derived. At 25 Cº we found our molar absorbance for UO₂SO$$^{0}_{4}$$ agreed well with those reported previously in the literature, however we report lower peak amplitudes for UO₂(SO₄)$$^{2–}_{2}$$ . We also noted significant temperature-dependent red shifts in the molar absorbance of UO₂SO$$^{0}_{4}$$. We imply that these shifts could be explained by changes in sulfate bonding behaviour around the uranyl O04 ion - namely shifts in the distribution of complexes with sulfate bound in either a monodentate or bidentate configuration. Simple models calculated with our new data suggest that sulfate complexes may readily predominate over chloride complexes even in solutions containing upwards of 20 wt% chloride and only 100 ppm sulfate. In addition, moderate concentrations of sulfate (~1 wt%) can increase the solubility of uranium by an order of magnitude. Therefore, we imply that sulfate could play a vital role in mobilising uranium in hydrothermal systems, and that the removal of sulfate via precipitation of sulfate minerals may act as a means of depositing uranium under oxidizing conditions. This in turn might explain the associations between sulfate minerals and uranium mineralisation occasionally seen in large uranium ore deposits such as Olympic Dam and MacArthur River.« less