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The Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin is regarded as an important reservoir with great potential for hydrocarbon exploration. It is previously indicated that the Xixiangchi carbonates have experienced extensive dolomitization, however, the origin of dolomitizing fluids and the dolomitization mechanism still remain uncertain. In this study, a set of petrographic and geochemical examinations, including rare earth elements (REE) and isotopic (C, O, and Sr) compositions were used to trace the origins of dolomitizing fluids and associated diagenetic processes. The petrographic examination revealed three types of matrix dolomites (D1, D2, D3) and one cement saddle dolomite (SD). These phases have crystal size ranges of less than 30 μm (very fine to fine crystals, D1), 30–100 μm (fine to medium crystals, D2), 100–300 μm (medium to coarsely crystalline dolomite, D3), and 0.3–4 mm (fracture filling cements, SD), respectively. D1 is characterized by non to very weak luminescence, weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02), strongly negative Eu anomalies (Eu/Eu* = 0.65 ± 0.03), and high 87Sr/86Sr ratios (0.71062 ± 0.00122). In combination with δ13C (−1.5‰ ± 0.2‰) and δ18O (−9.7‰ ± 0.5‰) compositions, D1 is interpreted to be formed by penecontemporaneous dolomitization in the near-surface environment with seawater as the dolomitizing fluid. In contrast, D2 is characterized by intercrystalline pores, dirty crystal surfaces, similar δ13C (−1.4‰ ± 0.4‰) compositions but higher δ18O (−8.9‰ ± 0.7‰) compositions, and lower 87Sr/86Sr ratios (0.70992 ± 0.00035), similar Ce anomalies (Ce/Ce* = 0.87 ± 0.04) and higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04). The coarser D2 is regarded to be formed by the post-penecontemporaneous seepage-reflux dolomitization or by recrystallization of D1 dolomite in a near-surface or shallow burial environment. D3 is distinguished by a cloudy core with clear rims, showing slightly higher Eu anomalies (Eu/Eu* = 0.88 ± 0.02) and similar Ce anomalies (Ce/Ce* = 0.88 ± 0.02) than those of D1 and D2. Combined with the δ18O compositions (−10.4‰ ± 0.4‰) and 87Sr/86Sr ratios (0.70989 ± 0.00048), D3 is thought to be formed by the overgrowth or recrystallization of D1 and D2 dolomites in a shallow to moderate burial environment. The fractures filling SD dolomite consists of nonplanar and much coarser crystals with undulatory extinctions and brighter red luminescence. The lower δ18O (−11.1‰ ± 0.3‰) compositions, lower negative Eu anomalies (Eu/Eu* = 0.70 ± 0.01) of SD than the matrix dolomites, and similar Ce anomalies (Ce/Ce* = 0.83 ± 0.01) are indicative of hydrothermal dolomitization, with possible fluids associated with the magma during the period of Emei taphrogenic movement. In addition, the 87Sr/86Sr ratios (0.70941 ± 0.00003) of SD suggest probable origin from the coeval seawater partially. Therefore, SD dolomite is interpreted to be formed by hydrothermal dolomitization with mixed dolomitizing fluid of seawater and hydrothermal fluids. In summary, all the matrix dolomites have almost the same ΣREE concentrations and exhibit similar near-flat REE partition patterns with weak LREE enrichments, weakly negative Ce anomalies, and negative Eu anomalies. Such characteristics of REE compositions are indicative of similar evolved dolomitizing fluid, such as seawater or seawater- derived fluids. By contrast, SD dolomites have a different REE partition pattern with left-leaning characteristics, LREE depletions, and negative Eu anomalies, thus suggesting a different dolomitizing fluid source from the matrix dolomites. In addition, the development of intercrystalline pores associated with D2 dolomite makes it more likely to be a potential reservoir, indicating that the dolomitizing history of carbonate has a strong influence on the quality of potential dolomite reservoirs. [ABSTRACT FROM AUTHOR]

KYb3F10:Ln3+ (Ln = Er, Ho, Tb, Gd, Eu, Sm, Ce and La) micro/nanocrystals with abundant morphologies were synthesized via a one-step hydrothermal route without employing any surfactants. Furthermore, the homogeneous and well-defined KYb3F10 crystals with truncated tetragonal bipyramidal, nanoparticle, nanospherical, microtubular and rugby ball shapes were achieved by manipulating the different environments of crystal growth, such as increasing the concentration of F−, adjusting the pH, using different fluorine sources and adding trisodium citrate. XRD, SEM, EDS and PL as well as lifetime analysis were used to characterize the products. The obtained KYb3F10:Ln3+ products showed bright emission colors red (Eu3+) and green (Tb3+), respectively. It has been found that larger crystal shapes/sizes can greatly enhance their luminescence properties and lifetimes due to their high crystallinity and few defects on the crystal surface. The as-synthesized KYb3F10:Eu3+/Tb3+ products might be potentially applied in multicolor lighting and displays. [ABSTRACT FROM AUTHOR]

• Experimental investigation of fluorite solubility and REE mobility in acidic, mildly acidic and alkaline solutions. • Reaction textures of fluorite are dissolution-controlled between 100 and 250 °C. • Addition of NaCl enhances fluorite dissolution and mobilizes REE in acidic and alkaline fluids. • Mass balance indicates REE-depleted fluorite and REE fluorides precipitation both control variations in fluid chemistry. • Fluorite solubility constants are derived between 25 and 250 °C. Fluorite is a common vein mineral in critical mineral deposits that are overprinted by hydrothermal processes. Its chemistry potentially reflects the mobilization conditions of rare earth elements (REE) in these hydrothermal fluids. However, the controls of temperature and fluid chemistry (i.e., pH and salinity) on fluorite solubility and coupled REE mobilization is still uncertain. In this study, batch-type experiments were conducted to investigate REE mobility in aqueous fluids controlled by the solubility of fluorite at temperatures between 100 and 250 °C. Natural fluorite crystals from Cooke's Peak (New Mexico) were equilibrated with aqueous solutions of variable starting pH (2–10), salinities (0 and 15 wt.% NaCl), and initial REE concentrations (0 and 0.5 ppm). Reacted fluorite crystals display dissolution-controlled reaction textures (i.e., etch pits and dissolution channels along fluorite cleavage planes), which are most pronounced in experiments conducted in acidic fluids and at elevated temperature. Addition of NaCl considerably enhances fluorite dissolution, in both acidic and alkaline solutions, which is also reflected by the compositions of the reacted solutions displaying an increase in Ca, F, and REE concentrations. The overall fluorite solubility is controlled by the association behavior of HF0 at acidic conditions, and by the formation of Ca chloride, Ca hydroxyl, and Na fluoride complexes, which become enhanced by addition of NaCl, including in mildly acidic and alkaline solutions. Mass balance calculations were used to compare the REE stoichiometry of unreacted fluorite crystals with measured REE concentrations in the reacted solutions, which reveal several possible controls on REE mobility: i) stoichiometric release of REE upon fluorite dissolution; ii) precipitation of secondary REE-depleted fluorite; and/or iii) precipitation of secondary light REE fluorides, which were observed to form on fluorite crystals surfaces in the REE-doped experiments. Fluorite solubility constants ( K sp ) were regressed with available low temperature solubility data using the equation log K sp = A + B T + C/ T + Dlog(T) (T , temperature in K) valid between 25–250 °C. Fitted coefficients yield: A = 171.180, B = 0.01477, C = −7794.10, D = −64.6634. The experimental results have important implications for geochemical modeling of natural systems, where fluorine controls the mobility of REE, and can be enhanced by addition of NaCl in both acidic and alkaline solutions. Therefore, the salinity of magmatic-hydrothermal fluids plays a crucial role in mobilizing REE during the formation of hydrothermal fluorite veins. [ABSTRACT FROM AUTHOR]

Deep-sea sediments highly enriched in rare-earth elements (REE), termed "REE-rich mud," have recently attracted attention as a potential mineral resource for industrially critical metals. Biogenic apatite or fish teeth and bone debris comprises the mineral phase which primarily hosts REE in deep-sea sediments; thus, biogenic apatite is a key material to decipher the formation process of REE-rich mud. We investigated the key process(es) controlling REE-enrichment in biogenic apatite through a novel data-driven approach. We analyzed the in situ chemical composition of biogenic apatite grains collected from REE-rich mud in the western North Pacific Ocean. Subsequently, the dataset of 685 analytical points × 35 elements was statistically analyzed via independent component analysis. Our new approach extracted four independent components (ICs) as geochemically meaningful and statistically independent signatures: substitution of elements into apatite lattice (IC1), the relatively rapid adsorption process that occurred at the sediment-water interface (IC2), elemental transfer from volcanic materials during early diagenesis (IC3), and metalliferous or detrital impurities physically adhering to or infiltrating apatite grains (IC4). Data distributions within the real and IC spaces provide insights into physicochemical processes involving elemental incorporation into biogenic apatite. We argue that after settlement on the seafloor, the fish bones and dentine in teeth adsorb REE from the overlying seawater and REE-enriched porewater at the sediment-water interface, resulting in the preferential enrichment of light rare-earth elements into apatite from these solutions. Further, that REE adsorbed on the apatite crystal surface was incorporated into the crystal lattice via substitution. The IC2 signal was intensified in the layers of "extremely REE-rich mud", suggesting that the REE adsorption process was facilitated during the formation of layers with remarkably high bulk-sediment REE contents. This may reflect an increased supply of REE-carrier substances such as organic matter, which degraded on the seafloor and released REE at the sediment-water interface, which is associated with changes in paleoceanographic conditions including ocean surface bioproductivity. • Dataset of 685 points × 35 elements in biogenic apatite was statistically analyzed. • Signatures of diagenetic processes were extracted as four independent components. • Elemental adsorption, substitution, and transfer were superimposed in the apatites. • Increased supply of rare-earth carriers was a key to form extremely REE-rich mud. [ABSTRACT FROM AUTHOR]

31P solid state NMR studies combined with DFT calculations were conducted over a chosen series of rare earth element phosphates (REEPO 4 s), selected on the basis of the size and magnetic properties of REEs (La, Sm, Lu and Yb). PXRD analysis revealed the presence of rhabdophane (La, Sm), monazite (La) and xenotime (Lu, Yb) phases of these phosphate compounds. The direct excitation and cross-polarization 31P NMR studies together with calculations confirmed the PXRD results for the abovementioned bulk structures, but also revealed presence of several local phosphorus environments on surfaces. NMR is sensitive to the atomic level local interactions, and we were able to show that the combination of experimental and theoretical NMR methods can provide information unavailable with other methods. Due to the distinct coordination of the water molecules to crystal surfaces with different Miller plane cleavages, we were able to identify from the NMR spectra the surface structures of the studied minerals. This adds to the knowledge of the bulk structures of REE phosphates and provides preliminary data for studies on coordination of various ligands on REE phosphate surfaces. This combination of experimental and computational methods can further be used for studies on surface chemistry, important for applications in catalysis and extraction of REEs from the minerals. Combination of phosphorus NMR and DFT calculations reveal information from bulk as well as from surfaces with distinct Miller planes in REEPO 4 materials. [Display omitted] • Water coordination on REEPO4 surfaces affects phosphorus NMR chemical shifts. • Information on surface hydration from combination of NMR and calculations. • CP-MAS and direct excitation solid-state NMR: spotting bulk and surface sites. • Information on local phosphate structures on surfaces. [ABSTRACT FROM AUTHOR]

High-resolution transmission electron microscopy observations of bastnäsite from the Maoniuping rare earth element (REE) deposit at Panxi, SW China, revealed the presence of nanoparticles in the surface of bastnäsite crystal. The nanoparticles are identified as the bastnäsite nanocrystals, with 5–30 nm in length, by energy-dispersive spectrometry and fast fourier transform patterns. This represents the first observation of bastnäsite nanoparticle in nature, confirming a new form of migration and precipitation of La and Ce in the hydrothermal fluids. Meanwhile, we herein reveal that random attachment of small bastnäsite nanoparticles during aggregation-based growth initially produces large bastnäsite nanoparticles, driven by the tectonic stress. Subsequent evolution of the random aggregation and orientation of large bastnäsite nanoparticles result in bastnäsite crystal formation, driven by the tectonic stress and inherent surface stress. This underlines the bastnäsite crystallization by random aggregation and the significant role of tectonic stress in forming the bastnäsite crystal in REE deposit. • Bastnäsite nanoparticles occurred in the Maoniuping REE deposit. • La and Ce can migrate and precipitate as the form of nanoparticle. • Tectonic stress has significant role in forming the bastnäsite crystal in REE deposit. [ABSTRACT FROM AUTHOR]

• CsPbBr 3 nanocrystals with EuBr 2 addition exhibit dual-band luminescence. • EuBr 2 induces phase transformation from CsPbBr 3 to Cs 4 PbBr 6. • The blue emission is originated from Eu3+-modified perovskite crystal surface. • EuBr 2 -solvent interaction influences perovskite crystallization process. Caesium lead bromide perovskite nanocrystals (PNCs) are synthesized using a room-temperature sonochemical method. By mixing equimolar of CsBr and PbBr 2 in Dimethylformamide (DMF), PNCs of CsPbBr 3 structure are formed. Under an identical reaction condition, when EuBr 2 is introduced with the CsBr/PbBr 2 precursor, the resulted product (Eu-PNCs) becomes zero-dimensional Cs 4 PbBr 6 nanocrystals. In addition to the typical green emission observed in both CsPbBr 3 and Cs 4 PbBr 6 , Eu-PNCs has a blue emission band. The overall photoluminescence quantum yield is much higher than the pristine PNCs. We perform detailed analysis of the crystal structure and luminescence properties of the nanocrystals with and without EuBr 2 modification. An X-ray absorption fine structure analysis reveals the local structure of Eu in the PNCs. We propose that the cation-solvent interaction strongly influences the crystallization process of ternary Cs x Pb y Br z , and the formation of Eu-solvent complexes further modifies the PNCs surface resulting in unique luminescence properties. [ABSTRACT FROM AUTHOR]