Fang Xia

To meet growing demand for renewable technologies that will mitigate climate change, the world requires a sustainable supply of rare earth elements (REEs). Uranium (U) is a common impurity in REE ores that causes socio-environmental challenges during mining and processing. Sustainable REE extraction thus relies on targeting ores with little U, which necessitates an understanding of the conditions under which U accumulates in REE minerals. Here we show that rhabdophane and monazite – two common hydrothermal REE ore minerals – can incorporate U in its oxidised U(V) and U(VI) forms. Contrary to current understanding, this demonstrates that reducing conditions are not required for U to accumulate in hydrothermal REE minerals. Rather, the higher availabilities of U in oxidising and/or Ce-poor fluids can also drive significant accumulation. Four redox scenarios are consequently theorized for predicting the capacity of aqueous-fluid-affected REE ores for accumulating U. This framework may assist in evaluating the environmental sustainability of various REE deposits, thereby guiding decisions on exploration and extraction. Results also confirm the capacity of both minerals to immobilise fluid-soluble U(VI) during formation, with implications for their application as mineral barriers in U-contaminated systems.
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Protonic ceramic electrochemical cells (PCECs) have potential as long-duration energy storage systems. However, their operational stability is limited under industrially relevant conditions due to the intrinsic chemical instability of doped barium cerate-based electrolytes and oxygen electrodes against H2O, as well as the poor electrode–electrolyte interfacial contact. Here we present a conformally coated scaffold (CCS) design to comprehensively address these issues. A porous proton-conducting scaffold is constructed and conformally coated with Pr1.8Ba0.2NiO4.1 electrocatalyst, which has high chemical stability against H2O, triple conductivity and hydration capability, and protects vulnerable electrolytes from H2O. The CCS structure consolidates the electrode–electrolyte interfacial bonding to enable fast proton transfer in the percolated network. This design enables PCECs to reach electrolysis stability for 5,000 h at −1.5 A cm−2 and 600 °C in 40% H2O. This work provides a general strategy to stabilize PCECs and offers guidance for designing resilient and stable solid-state energy storage systems.

Windows are essential for advancing energy efficiency, yet fabricating window materials that simultaneously achieve high visible light transmission, exceptional and ultrabroadband near-infrared (NIR) shielding, neutral color appearance, excellent stability, and cost-effective, sustainable production remains a significant challenge. This study introduces ferrous ion-doped phosphosilicate glass (Fe2+-PSG), a material that meets all these criteria, establishing itself as a promising candidate for energy-efficient windows. Comprehensive characterization using Raman spectroscopy, synchrotron X-ray absorption near edge spectroscopy (XANES), UV–Vis-NIR transmission spectroscopy, and structural analysis reveals that the glass achieves superior visible light transmission performance (Tlum = 84.1 ± 0.3 %) and a neutral color temperature of 6753 ± 45 K. These remarkable properties are primarily attributed to the formation of colorless ferrous [FeO6] units, whose unique coordination environment is precisely controlled through phosphorus incorporation. Superior ultrabroadband NIR-shielding (750–2500 nm), with a high NIR-shielding figure of merit of 1.8 ± 0.1, is enabled by the incorporation of larger alkali ions such as K+ and Cs+. Additionally, Fe2+-PSG demonstrates exceptional stability, maintaining its performance after 12 months under ambient conditions and 24 h in hot water at 80 °C and 120 °C. These high-performance characteristics are attributed to structural modifications that alter the splitting energy of ferrous ions, enabling precise control over optical properties across the visible and NIR spectrum. Manufactured using the scalable and cost-effective melt-quench method, Fe2+-PSG offers a practical solution for sustainable window production, addressing limitations of current technologies and paving the way for real-world applications.

The accurate characterisation of regolith materials is crucial for mineral exploration, yet distinguishing visually indistinct clay-rich samples can be challenging and labour-intensive. This study conducts unsupervised k-means clustering and principal component analysis (PCA) on a geochemical dataset of over 3000 regolith samples from the Splinter Rock rare earth element (REE) prospect, Western Australia, to determine how unsupervised statistical methods may expedite the characterisation of regolith samples across large, buried and/or regolith-hosted ore deposits. K-means clustering identified five laterally consistent regolith horizons at Splinter Rock, which were manually interpreted into three REE-barren transported horizons and two mineralized saprolite-saprock horizons. The mineralogical and metallurgical features of all 3000 samples were then extrapolated from hyperspectral and metallurgical data of a select few reference samples within their clusters, to provide a preliminary understanding of the deposit’s overall structure and properties. Despite being a first-order approach, this method highlighted several consistent, statistically robust and previously unidentified patterns across the entire prospect: 1) the highest REE grades exist predominantly in the granitic saprolite and saprock; 2) relative to the light REEs (La–Sm), the heavy REEs (Eu–Lu) experience enrichment at the saprolite-saprock boundary and depletion with increasing depth in the saprock; 3) optimal metallurgical conditions occur near this saprolite-saprock interface; 4) relative accumulation of the economically- and environmentally-important ‘magnet’ REEs (MagREE, Pr, Nd, Tb, Dy) occurs mostly in the saprock; and 5) relative MagREE enrichment can be linked to the formation of negative Ce anomalies at lower stratigraphic positions. Lastly, PCA facilitated the development of tailored geochemical ratios to classify future samples into their appropriate horizons. This study highlights unsupervised statistical analysis of existing geochemical data as a robust, rapid and effective first-pass method for classifying and characterising extensive sets of regolith samples, as well as an efficient method of outlining deposit-scale trends and zones of consistent economic REE enrichment in large regolith-hosted deposits/prospects.
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The rare-earth elements (REEs, La–Lu, Y) are essential for the development of renewable technologies. Bastnäsite (REECO3F) is a common REE ore mineral that is often subject to hydrothermal alteration at all crustal levels. Mechanisms of hydrothermal bastnäsite alteration therefore govern the evolution of REE deposits, though these mechanisms remain poorly understood. This experimental work investigates the hydrothermal replacement of bastnäsite by rhabdophane (REEPO4∙xH2O, x = 0–1) and monazite (REEPO4) in phosphatic fluids. Two temperature-dependent alteration pathways were identified; both follow the coupled dissolution-reprecipitation (CDR) mechanism. At 90 °C, bastnäsite was replaced by highly-porous metastable rhabdophane which was then replaced by monazite, forming an inner layer of rhabdophane and an outer layer of monazite. At 220 °C, bastnäsite was replaced directly by monazite. Although replacement initiated more quickly at 220 °C, greater overall replacement occurred at 90 °C (~ 61 wt.% after 500 h, compared to ~ 13 wt.% at 220 °C) due to surface passivation by monazite at 220 °C. Geochemical analyses showed REE fractionation during bastnäsite alteration. At 90 °C, rhabdophane was enriched in heavy REEs (Eu–Lu, Y), likely due to the evolving fluid chemistry, while at 220 °C secondary monazite was enriched in Sm and Ho compared to bastnäsite. These results indicate that: 1) the hydrothermal alteration of bastnäsite by rhabdophane and monazite in ore deposits leads to REE immobilisation, with little net loss of REEs to solution; 2) rhabdophane is metastable relative to monazite at 90 °C, and; 3) variable temperatures can cause different mineral textures and REE fractionation trends during hydrothermal alteration and mineral replacement.

The stratiform and vein-hosted Kapunda Cu deposit in South Australia contains a saprolitized hydrothermal vein with 12.37 wt.% total rare earth oxide (TREO). The vein was analyzed by X-ray diffraction, scanning electron microscopy, synchrotron-based X-ray fluorescence microscopy and electron backscatter diffraction to understand the controls that govern high-grade REE accumulation during periods of intense weathering. Petrological assessments indicate the transformation of an apatite-calcite-aluminosilicate-bearing protolith to a supergene assemblage of Fe-oxides, kaolinite and REE-phosphate minerals that include rhabdophane-(Ce), monazite-(Ce) and florencite-(Ce). This transformation was facilitated by progressive acidification of the weathering fluid, which is indicated by: 1) the increasing crystallinity of authigenic Fe-oxides and kaolinite, which led to REE desorption; 2) the textural evolution and increase in grain size of authigenic REE-phosphates from nanoscopic crystallites, to acicular needles, to micro-scale hexagonal prisms; 3) the late dissolution of REE-phosphates; and 4) the replacement of goethite by jarosite, whose sulfate component originated from the oxidation and weathering of proximal sulfide minerals. Alongside the depletion of pH-buffering carbonate minerals that are indicated by the preservation of calcite menisci, this sulfide dissolution also facilitated acid generation. Results illustrate how highly acidic weathering fluids might facilitate either REE mobilization or REE accumulation in regolith. High-grade REE accumulation under acidic supergene conditions is prioritized when the host-rock contains a significant source of depositional ligands (i.e., phosphate in the form of apatite) that can be readily leached during intense weathering. Exploration companies should therefore assay routinely for REEs in any heavily weathered phosphatic rock, due to the observed efficiency of phosphate minerals as geochemical traps for REE accumulation.

Apatite, rhabdophane and monazite are actinide- and rare-earth element (REE)-bearing phosphate minerals with diverse geoscientific applications. However, their mineral-fluid reaction mechanisms below 200 °C are understudied. Insufficient understanding as to how these minerals behave during hydrothermal alteration impedes useful interpretations of REE-U-Th mobilisation in regolith profiles, diagenetic environments and low-temperature hydrothermal systems. This work experimentally investigates the replacement of apatite by rhabdophane in a Ce-doped acidic fluid at 30 °C, and of rhabdophane by monazite in REE-barren acidic fluids at 180 °C. Rhabdophane replaced apatite via the coupled dissolution-precipitation mechanism, forming nanoscale prisms clustered into spherulitic aggregates. Interstitial voids that formed between rhabdophane and apatite indicate how rhabdophane precipitation was the rate-limiting step. Void widths varied in response to apatite's anisotropic dissolution; rapid dissolution parallel to the c-axis formed the widest voids (50–125 μm) and thickest rhabdophane layers (20–50 μm), while slow dissolution perpendicular to the c-axis formed narrower voids (< 1 μm) and thinner layers (< 5 μm). At 180 °C, monazite also replaced rhabdophane via the dissolution-precipitation mechanism. Replacement rates increased with increasing phosphate concentrations, implying that monazite precipitation was the rate-limiting step. However, microscale rhabdophane aggregates were pseudomorphically preserved. This is attributed to the shared orientation of rhabdophane prisms (not pseudomorphically preserved) and their intergranular boundaries which, as microreactors, guided monazite precipitation. Both replacement reactions were accompanied by a loss of U to solution, which is attributed to the stabilisation of uranyl-phosphate complexes under acidic conditions. Results indicate that: 1) anisotropic dissolution can govern the extent of rate coupling between dissolution and precipitation during mineral replacement; 2) the pseudomorphic replacement of mineral aggregates can occur when precipitation is the rate-limiting step; 3) rhabdophane is a feasible (and perhaps necessary) metastable precursor to authigenic monazite precipitation in low temperature hydrothermal systems; 4) highly porous aggregates of both rhabdophane and monazite generate insufficient EPMA totals, such that they cannot be differentiated by EPMA alone, and; 5) phosphate complexes can mobilise U under certain hydrothermal conditions, with implications for geochronology, nuclear waste management and the formation of REE deposits.

Hydrogels demonstrate significant potential as materials for fabricating energy-saving smart windows. Nevertheless, their liquid nature poses challenges and renders them susceptible to leakage during operation. In this study, we developed an HPC-PAA-PAM semi-solid hydrogel through a two-step polymerization process at 80 °C, with acrylamide serving as the curing agent. The resulting hydrogel exhibited a precisely tunable lower critical solution temperature (LCST) spanning from 31.5 °C to 36.2 °C. Additionally, we constructed double-layered glass-encapsulated HPC-PAA-PAM semi-solid hydrogel smart windows, which demonstrated remarkable optical transmittance (Tlum = 88.3 %), excellent solar energy modulation (ΔTsol = 47.9 %), and remained stability through 100 heating and cooling cycles. Thermal regulation tests conducted in model houses revealed outstanding heat-shielding performance of these hydrogel smart windows, surpassing conventional double-glazed windows and double-glass encapsulated water windows. Notably, after 90 days of outdoor exposure, the HPC-PAA-PAM semi-solid hydrogel smart window experienced only a minimal reduction in solar modulation, underlining its exceptional weather durability in practical applications. Consequently, the HPC-PAA-PAM semi-solid hydrogel presents substantial promise for the production of smart windows, particularly suited for energy-efficient buildings in tropical and subtropical regions, where it can effectively prevent the intense summer sun's heat from penetrating indoors, fostering a more pleasant indoor temperature.
The HPC-PAA-PAM semi-solid hydrogel smart windows exhibit high optical transmittance (88.3 %), excellent solar energy modulation (47.9 %), and outstanding heat shielding performance. [Display omitted]

Acid pressure oxidation (POX) is a cost-efficient pre-treatment method for releasing refractory gold from arsenopyrite and pyrite prior to cyanidation. However, ongoing debate surrounds the mechanism of the dissolution, crystallization, and phase transformation during POX, in part due to the lack of thermodynamic data and in situ studies of the acid POX. In this study, we developed Eh-pH diagrams and used in situ powder X-ray diffraction (PXRD) to investigate mineralogical phase changes during acid POX of arsenopyrite. Based on the existing thermodynamic data, we established the Eh-pH diagrams for the Fe-As-S-H2O systems at 225 °C and 100 °C, which correctly predicted the predominant solid phases precipitating during POX of arsenopyrite at a pH range of 0 to 0.5, such as basic ferric sulphate (BFS: Fe(OH)SO4), szomolnokite (FeSO4·H2O), hydronium jarosite ((H3O)Fe3(SO4)2(OH)6) and angelellite (Fe4O3(AsO4)2). FeSO4·H2O solid is the stable phase during annealing stages as predicted by the Eh-pH diagram, which is in line with the observed solid phases using in situ PXRD. The influence of initial concentration of ferric ions and the presence of pyrite on mineral phase change was revealed. Our results confirmed the formation of FeSO4·H2O, BFS and hydronium jarosite during annealing stages, while BFS and/or hydronium jarosite dissolved during curing and cooling process. Especially, the formation of angelellite was observed in the presence of pyrite during cooling process, which is well-agreed with thermodynamic analysis.

This work presents the influence of glass matrix SiO2–B2O3–NaF on the formation of near-infrared (NIR)-shielding functional units in energy-saving glass windows. The structural analyses by XRD, XPS, and Raman confirmed that the NIR-shielding performance is insensitive to SiO2 or B2O3 glass network formers but is significantly impacted by the concentration of NaF. This is because the presence of a reasonable amount of NaF provides the much-needed sodium for the formation of NIR-shielding functional units Na5W14O44 but too much NaF (>22 mol.%) can inhibit the formation of Na5W14O44 by transforming the W–O–W chain in the interconnected [WO6] and [WO4] polyhedrons into W–O- bonds. By adjusting NaF to the optimal level (16 mol.%) and adding only a small amount of H2WO4 (1.9 mol.%) as the tungsten source in the raw materials, excellent optical properties and thermal insulation performance have been achieved in the produced bulk glass, with a high transmission of 65.8% at 550 nm in the visible light region and a low transmission of 6.6% at 1550 nm in the NIR region. This performance is much better than soda lime glass and ITO glass and is comparable to cesium tungsten bronze-based membrane coated glass. This study paves a way for glass composition design to achieve high NIR-shielding performance with the minimum amount of tungsten source, leading to cost-effective glass fabrication and enhanced mechanical strength of energy-saving glass windows.