Deep-sea sediments hold large quantities of critical rare earth-elements and yttrium (REY) sequestered in nanoparticulate biogenic fluorapatite (Ca5(CO3) x (PO4)3-x F1+x ). Understanding their enrichment processes and improving recovery and mineral processing methods require atomic-scale information about their chemical form, but it is difficult to obtain. Here, we use novel high-energy-resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy to elucidate the local structure of gadolinium (Gd) in the highly enriched REY deposit from the Clarion-Clipperton fracture zone (CCFZ) in the Pacific Ocean. Our findings reveal that Gd is neither incorporated into the apatite structure nor precipitated alongside Ce in a Ce-PO4 precipitate. Instead, it is bound at short-range distances to Ca and PO4 in a defective apatite-type bonding environment within an amorphous matrix that encases fluorapatite nanocrystals. Density functional theory (DFT) suggests that Gd and Y, whose atomic fraction is ten times higher than that of Gd, are not dispersed throughout the amorphous matrix, but are likely segregated at medium-range distances. The entrapment of Ce, Gd, and Y within an amorphous matrix explains, at the microscopic level, why REY can be easily recovered through straightforward acid leaching. This is due to the intrinsic instability of disordered atomic structures compared to crystalline phases. This research highlights the complementarity of HERFD-EXAFS and DFT calculations for atomic-scale analysis of trace elements in complex natural matrices. It establishes a basis for their use across diverse terrestrial and marine materials.

This study investigates pressure-induced local structural changes of strontium (Sr2+) and yttrium (Y3+) in basaltic silicate glasses up to 130 GPa. To monitor local structural changes of these minor elements we conducted X-ray absorption spectroscopy (XAS) experiments combined with the diamond anvil cell technique at room temperature. Both elements exhibit a two-stage reorganization of the local structure: in the first stage (up to ∼20 GPa), the average cation‑oxygen distance and cation's coordination number increase (for Sr from 6 to 8, for Y from 6 to 7); in the second stage, above 20–30 GPa, a progressive shortening of cation‑oxygen distance is observed, associated to the compression of coordination polyhedra. The two-stage compression scheme differs from that of network-forming cations, such as Si and Al. Those change from a corner-shared tetrahedral network to the closer packing of edge-sharing octahedra starting at 20–30 GPa. Due to the similarity of structural changes of Sr and Y to those around Ca and Mg, we conclude that their behavior reflects a universal trend for large cations in silicate melts during compression to conditions of the lower mantle. We used bond valence theory to validate the pressure-evolution of cation‑oxygen distances and compared them with data of crystalline counterparts. We expanded the bond valence theory by adding an additional pressure correction to ensure applicability at extreme pressure. The presented data improve our knowledge on local structural changes in basaltic melts across Earth's mantle pressures. We qualitatively assess the consequences for element partitioning between minerals and melt under core-mantle boundary conditions.

Frustrated multiferroic manganites exhibit strong lattice-charge-magnetic coupling, enabling complex phase transitions and tunable magnetocaloric responses for cryogenic cooling. Here, we systematically investigate GdMn2O5 to elucidate its structure, magnetic ordering, magnetocaloric properties, and pressure response. Structural studies reveal an orthorhombic Pbam framework with no global phase transition from 4-300 K, but instead show anisotropic thermal expansion, including negative expansion along the b-axis, driven by magnetoelastic coupling. Microscopy and X-ray absorption spectroscopy confirm Mn3+/Mn4+ charge ordering between octahedral and pyramidal sites, with an effective coordination number of 5.5. The compliant Mn-Gd interface acts as the primary transducer of magnetoelasticity, exhibiting anomalous spin-phonon coupling. Magnetic measurements reveal successive transitions: TN ≈ 39 K (Mn3+/Mn4+ antiferromagnetic ordering), TSR ≈ 34 K (spin-reorientation), and TGd ≈ 3.5 K (Gd3+ ordering). Under 0-7 T, the material displays a conventional magnetocaloric effect with a maximum entropy change of 16.1 J/kg K at ∼8 K and a relative cooling power of 272 J/kg, surpassing prior reports. Hydrostatic pressure reveals opposing responses: TN decreases, consistent with weakened Mn-O-Mn exchange, while TGd increases, suggesting enhanced Mn-Gd coupling under compression. These results highlight GdMn2O5 as a promising cryogenic magnetocaloric material and reveal a sublattice-dependent pressure-tuning mechanism relevant to frustrated multiferroics.

The enrichment of platinum (Pt) in marine ferromanganese (FeMn) deposits has attracted persistent interest for over half a century; yet the chemical form of Pt remains unclear. Here, we collected Pt-enriched FeMn crusts and nodules from the world’s oceans and used high-energy-resolution X-ray absorption spectroscopy (XAS) to decipher how Pt is sequestered at the atomic level. Platinum occurs in its tetravalent form, resulting from the oxidation of divalent Pt in seawater upon contact with Mn oxides. Tetravalent platinum is precipitated as α-PtO 2 nanoparticles with longer Pt-Pt distances than well-crystallized α-PtO 2 . Density functional theory (DFT) shows that the local structure of Pt is well represented by α-PtO 2 layers topotactically stacked on vernadite phyllomanganates. Evidence of α-PtO 2 nanoparticles challenges previous hypotheses that Pt exists as discrete metallic particles or within phyllomanganate MnO 2 layers replacing Mn. Since α-PtO 2 is the most thermodynamically stable Pt oxide, this Pt form may represent the ultimate sedimentary sink of Pt in oceans.

The chemical conversion of phosphogypsum (PG) raises significant concerns regarding both economic viability and environmental impact, including the potential mobilization and accumulation of impurities (e.g., F, Cd, U) in effluents and synthesized products. In this study, the conversion of PG as the main source of Ca2+ into alpha-hemihydrate calcium sulfate whiskers (alpha-HH CSW) was investigated due to its dual benefits: it generates valuable by-products while enhancing our understanding of element behavior, with a focus on rare earth elements (REEs), to address the existing knowledge gap and assess the feasibility of their recovery during alpha-HH CSW production. The optimal conditions resulting in whiskers involve hydrothermal treatment in a 2.5 mol/L H2SO4 solution for 3 h, using 8% citric acid, a solid-to-liquid ratio of 1:24, and a temperature of 90 degrees C. The elements concentration monitoring during the process showed limited leachability of P (20%) and F (14%), mainly due to their insoluble form within the PG. Pb and Th showed comparable behavior, with leaching recoveries less than or similar to 15%, indicating predominant retention in the recrystallized product. Conversely, Cd (40%) and U (50%) displayed high solubility, highlighting the need for environmental monitoring of the resulting liquid. Regarding REE behavior, only about 6% were dissolved. Yttrium K-edge XAS analysis, used as a proxy for HREEs, confirmed their incorporation into the alpha-HH CSW structure during the transformation process. The conversion process offers a valuable pathway in promoting circular economic practices within the phosphate fertilizer industry. Nonetheless, the challenge of REE recrystallization requires further in-depth investigation to facilitate their extraction as valuable elements.

The reduction of tetravalent manganese (Mn-(IV)) to trivalent manganese (Mn-(III)) by HEPES Good's buffer is often used to modify the reactivity of δ-MnO2 and to distinguish between the Mn-(III) and Mn-(IV) oxidants in redox reactions. However, the structure of HEPES-reacted δ-MnO2 has remained elusive, hindering a detailed understanding of interfacial electron transfer between adsorbed species and structural Mn. Here, we characterized the structure of δ-MnO2 reacted with HEPES at pH 6 and 8 under low and high NaCl ionic strength, using chemical analysis, high-energy X-ray diffraction, pair distribution function (PDF), extended X-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (HRTEM) coupled with selected area electron diffraction (SAED). The average Mn oxidation state (AMOS) decreases from 3.92-3.87 to 3.71-3.59 after HEPES addition, depending on pH and ionic strength. HEPES-reacted δ-MnO2 has a distinctly different structure at low and high ionic strength. At low ionic strength, the δ-MnO2 HE crystallites are 3-6 nm across, and the MnO2 layers have approximately 23% vacant sites capped with mainly Mn-(III) and some Mn-(II). At high ionic strength and pH 8, δ-MnO2 HE contains large crystals, several hundred nanometers across, made up of crystallographically oriented nanodomains. Most SAED patterns show streaks along the [100]* direction, indicating a high degree of disorder in the close packing of the anionic sheets, in the Na position within the interlayer, and in the Mn-(IV)-Mn-(III) distribution within the layer. Some nanodiffraction patterns show distinct superstructure reflections along the streaks with A* = 3a*, as seen in well-crystallized triclinic birnessite, and A* = 6a*. High-ionic-strength δ-MnO2 HE has no interlayer Mn-(III), and the Na-(I) ions, along with the layer Mn-(III) and Mn-(IV) cations, are semiordered at the short- to medium-range scales and essentially disordered over longer distances. Identifying the two distinct structures of HEPES-reacted δ-MnO2 clarifies structural ambiguities reported in the literature and provides a solid foundation for exploring its redox reactivity and electrochemical performance.

The oxidation of cerium (Ce) from the soluble trivalent state (Ce(III)) to the insoluble tetravalent state (Ce(IV)) on manganese (Mn) oxides critically influences its environmental fate and geochemical cycle, and is also of interest in water treatment. However, a comprehensive mechanistic understanding of how Ce is immobilized upon interaction with Mn oxides in soils and marine sediments is still lacking. The bonding structure of Ce on δ-MnO2, the most abundant Mn oxide, was investigated by X-ray absorption spectroscopy at environmentally relevant pH and Ce concentration, and the oxidation reaction was modeled by atomistic calculation. Ce(III) is adsorbed as a six-coordinate complex at particle edges and a nine-coordinate complex at Mn(IV) vacancy sites of the MnO2 phyllomanganate layer. Ce(III) oxidation is nonspontaneous and requires hydrolysis of the sorption complexes to proceed. Gibbs free energy calculations of possible oxidation pathways show that electron transfers from Ce(III) to Mn(IV) at edge sites, and from Ce(III) to interlayer Mn(III) at vacancy sites, are thermodynamically favorable. Thus, the redox reactivity of δ-MnO2 depends on its crystallographic structure and the Mn valence. Our findings show that Mn(IV) and Mn(III) are kinetically more effective oxidants of Ce(III) than dissolved oxygen, and therefore, that cerium can be immobilized by Mn oxides even under suboxic conditions. The new mechanistic insights from this study improve understanding of the oxidative uptake of Ce by Mn oxides and its relevance to natural and engineered systems.

Understanding how critical metals such as platinum (Pt) are enriched in marine environments is essential for advancing knowledge of their geochemical cycles and the formation of oceanic mineral resources. Pt occurs in seawater at picomolar concentrations, yet deep-sea ferromanganese (FeMn) deposits contain up to eight orders of magnitude more Pt. The mechanism behind this enrichment remains unresolved. Although FeMn deposits exhibit high Pt partitioning, their low Pt levels and the co-occurrence of multiple elements pose challenges to investigating Pt oxidation state and uptake processes using traditional spectroscopic methods. In this study, high-energy-resolution X-ray absorption spectroscopy (XAS) was used to elucidate how Pt is sequestered at the atomic scale in FeMn crusts and nodules from the Pacific, Atlantic, and Indian oceans. Pt is present in the tetravalent form, resulting from the oxidation of dissolved divalent Pt in seawater upon interaction with Mn oxides. Tetravalent Pt is precipitated as α-PtO2 nanoparticles with longer Pt-Pt distances than in well-crystallized α-PtO2. Density functional theory calculations indicate that the local structure of Pt is best described by α-PtO2 layers topotactically stacked on vernadite phyllomanganates, similar to (Co,Ni)-asbolane. The Pt content depth profiles are constant during spot analyses by laser ablation-inductively coupled plasma-mass spectrometry, suggesting that the Pt nanoparticles are uniformly dispersed within the ablated volume, rather than occurring as discrete metallic nuggets. These findings reveal a new pathway for Pt immobilization in marine environments and suggest that thermodynamically stable α-PtO2 serves as a long-term sink for oceanic Pt. The advanced capabilities of high-energy-resolution XAS offer new opportunities to deepen understanding of how trace metals are sequestered in structurally disordered and chemically complex natural materials, with potential applications in georesource exploration.

The expansion of lithium mining in Europe is critical for securing strategic raw materials but poses growing challenges for the long-term management of mine tailings. Arsenic (As), a potentially harmful element present in trace amount in lithium ores, may report to some residues and tailings during lithium beneficiation, yet its mineral hosts and solid-phase speciation remain poorly constrained due to low bulk concentrations and diffuse distribution across multiple phases. This uncertainty limits reliable prediction of arsenic behaviour during processing, leaching, and storage, while current regulatory assessments largely rely on bulk concentrations and standardized leaching tests that may overlook key mineralogical controls. Here, we combine quantitative mineral deportment analysis with arsenic K-edge X-ray absorption near-edge structure (XANES) spectroscopy to characterise arsenic distribution and solid-phase speciation in ores and process residues from three major European lithium projects. Results show that arsenic is distributed among sulphide, silicate, oxide, and phosphate phases, many of which host arsenic at trace concentrations but occur in high modal abundance. Least-squares linear combination fitting reveals the predominance of As(-I) and As(V), with minor As(III) contributions in a subset of samples. XANES analyses of leaching residues demonstrate oxidation of arsenic toward As(V), showing that low arsenic release does not necessarily reflect stability, as sorption processes can mask underlying reactivity. This mineral-centric approach enables the estimation of the relative contributions of As(-I,III,V)-bearing minerals and their control on arsenic release across contrasting geochemical environments. The proposed framework supports evidence-based decisions for targeted tailings management solutions, thereby strengthening environmental strategies for cleaner lithium production.

Redox properties of Mn oxides play a central role in metal cycling and material reactivity because of their variable Mn(II)/Mn(III)/Mn(IV) contents. Typical chemical methods estimate the Mn(II)/Mn(III)/Mn(IV) proportions by measuring Mn(III) with pyrophosphate (PP) and calculating the average manganese oxidation state (AMOS) using redox-based techniques. In this study, we introduce a modified PP method (MPP) that allows measuring each Mn oxidation state individually without requiring a separate AMOS measurement. Eight poorly crystallized birnessite (delta-MnO2), with different AMOS levels, were synthesized under various chemical conditions, including changes in pH, ionic strength, and reductant treatment. These samples were analyzed with X-ray absorption near-edge structure (XANES) spectroscopy, the four most common redox-based methods, and the MPP method to evaluate their accuracy and precision. The redox methods, including oxalate back-titration, iodometry, potentiometry, and LBBI colorimetry, yielded inconsistent Mn(II)/Mn(III)/Mn(IV) proportions and AMOS values, with AMOS differences ranging from 0.15 to 0.78 across the four methods for the same sample. The AMOS values obtained with the MPP method agreed with those measured by XANES and potentiometry, and showed no agreement with those from the other three chemical methods. The MPP method provides higher precision for the Mn(II)/Mn(III)/Mn(IV) proportions of the eight delta-MnO2 samples (standard deviations, SD <= 0.01) compared to the four redox methods (SD up to 0.10). Since potentiometry and XANES are considered the most accurate methods for measuring AMOS, we conclude that the MPP method represents a significant advancement in quantifying Mn(II), Mn(III), and Mn(IV) in Mn oxides.

Deep-sea rare earth elements (REEs) have attracted global interest as terrestrial resources become less available and demand rises sharply. Seafloor REEs are hosted in authigenic carbonate fluorapatite (a-CFA), formed by the precipitation of calcium and phosphate, and in biogenic carbonate fluorapatite (b-CFA), derived from marine organisms’ skeletons. The ease with which REEs can be extracted from pelagic sediments via acid leaching challenges the common view that they are structurally incorporated into CFA, given fluorapatite’s low solubility. This apparent paradox was elucidated by investigating the nanostructure of samarium (Sm), used as a chemical REE probe. Using transmission electron microscopy and X-ray absorption spectroscopy, we found that the CFA crystals are nanosized and that Sm is not incorporated into the CFA lattice but instead primarily resides in an amorphous phase surrounding the a-CFA and b-CFA nanocrystals. At the atomic scale, Sm has a disordered apatitic bonding environment, and atomistic modeling suggests it clusters with other REEs. The facile extractability of REEs from pelagic sediments is attributed to the poor crystallinity of the host matrix at the atomic scale. This study demonstrates that understanding the location and atomic structure of critical elements enables quantitative prediction of their macroscopic properties. This knowledge can help improve recovery processes and inform the design of more efficient remediation strategies that protect the environment and public health. Rare earth elements accumulate at the fluorapatite-seawater interface in a poorly crystalline phase, making them both enriched in marine sediments and easy to extract by mild acid leaching, according to spectroscopy, geochemical and computational modeling analysis.

The expansion of lithium mining in Europe is essential for securing strategic raw materials, but it raises increasing challenges for long-term tailings management. Arsenic (As), present at trace levels in lithium ores, can partition into mine wastes during beneficiation, yet its mineral hosts and solid-phase speciation remain poorly constrained due to low concentrations and diffuse distribution. This limits predictions of arsenic behaviour during processing, leaching, and storage, while current assessments often rely on bulk data that overlook key mineralogical controls. Here, we combine quantitative mineral deportment with As K-edge XANES to characterise arsenic distribution and speciation in ores and residues from three major European lithium projects. Arsenic occurs across sulfide, sulfarsenide, silicate, oxide, and phosphate phases, typically at trace levels but within abundant minerals. Sulfarsenides are dominated by As(–I), whereas other phases show mixed speciation (As(–I), As(III), As(V)). Linear combination fitting indicates overall dominance of As(–I) and As(V), with As(III) minor and sample-specific. XANES of leaching residues reveal the oxidation of arsenic released during arsenopyrite dissolution to As(V), followed by its immobilisation under specific pH conditions, demonstrating that arsenic leaching behaviour at disposal sites is strongly controlled by weathering conditions. Coupled with traditional leaching approaches, this mineral-centric framework quantifies the contribution of As-bearing phases and their control on release across environments, supporting more targeted tailings management and improved environmental strategies for lithium production.

The oxidative enrichment and isotopic fractionation of cerium (Ce) in contact with vernadite (δ-MnO2) serve as a proxy for past redox conditions in both terrestrial and marine environments. However, the molecular processes that govern the scavenging of Ce from the dissolved 3+ to the insoluble 4+ oxidation states remain obscure. Adsorption experiments on synthetic δ-MnO2 suggest that aqueous Ce(III) precipitates as ceric hydroxide (Ce(OH)4), an unknown mineral. Here, the atomic-scale structure of Ce in natural vernadite from ferromanganese crusts collected across the Pacific, Atlantic, and Indian Oceans was examined using advanced high-energy-resolution extended X-ray absorption fine structure spectroscopy. The findings provide direct evidence for the uptake of Ce as mononuclear Ce(IV) complexes at the layer-edge sites (DES complex) and Mn(IV) vacancy sites of vernadite. Density functional theory-based Gibbs free-energy calculations indicate that hydrolysis of the DES complex promotes the oxidation of Ce(III) to Ce(IV). Quantum mechanical calculations predict that the equilibrium 136Ce/140Ce isotope fractionation factor between Ce(III) dissolved in seawater and the Ce(IV) complexes can reach 1.2-1.3 ‰ at 25 °C, indicating that the 136Ce/140Ce ratio has high potential as a new paleoredox proxy. Seawater cerium (III) forms mononuclear complexes in vernadite from global ferromanganese crusts and is oxidized to cerium (IV) during hydrolysis, as shown by atomic-scale analysis and quantum calculations. This leads to strong Ce isotope fractionation.

Simple binary oxide glasses can exhibit a compression behavior distinct from that of their crystalline counterparts. In this study, we employed high-pressure X-ray absorption spectroscopy coupled to the diamond anvil cell to investigate in detail local structural changes around Ge in glassy GeO_2 up to 158 GPa. We conducted four independent runs, both with and without pressure-transmitting media. Up to 30 GPa, we observed no significant influence of the pressure medium on the pressure dependence of the Ge-O bond length (). Between 10 and 30 GPa, the evolution of shows substantial variability across our experiments and previous works. The measured values lie close to those reported for crystalline polymorphs, including the rutile- and CaCl_2-type phase of GeO_2. This finding suggests that the amorphous structure possesses considerable flexibility to transition among different atomic configurations. From 30 GPa to 158 GPa, our results for both and the non-bonded cation-cation distance demonstrate that edge-sharing octahedra remain the main structural motives in glassy GeO_2. Up to 100 GPa, compaction proceeds primarily via distortions of octahedral O-Ge-O bond angles accompanied by octahedral bond shortening. Above 100 GPa, octahedral distortion becomes the prevailing mechanism. Compared to its crystalline analogues (α-PbO_2 and pyrite-like phase), glassy GeO_2 exhibits a slightly less efficient compaction mechanism, likely due to kinetic constraints that inhibit reconstructive lattice rearrangements.

Two-phase synthesis is a well-established approach for achieving precise control of the nanoparticle properties. However, studying and understanding chemical transformations in such a spatially heterogeneous system is challenging. In this work, we introduce a two-phase synthesis route for ZnS nanoparticles (ZnS NPs) at the water-toluene interface. By employing spatially resolved in situ high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) combined with density functional theory (DFT) calculations, we track the diffusion of Zn2+ species at the interface, identify key reaction intermediates, and monitor the nucleation and growth of ZnS NPs within the toluene phase. We propose the formation of a [Zn(H2O)6]2+ complex upon dissolving Zn(Ac)2 in water and the diffusion of Zn2+ ions from water to toluene driven by the formation of an octahedral [Zn(OA)6]2+ complex (OA = oleylamine). Furthermore, by complementing HERFD-XAS with total X-ray scattering analysis, we show the formation of an intermediate tetrahedral [Zn(SR)4]2+ complex at 60 °C and its successive transformation to noncrystalline ZnS nuclei at 80 °C and crystalline ZnS NPs starting at 100 °C. Thus, we demonstrate how in situ X-ray spectroscopy can elucidate the coordination and diffusion of Zn2+ ions, and, in combination with X-ray scattering studies, identify the emergence of atomic and electronic structures during the two-phase synthesis of ZnS nanoparticles.

The hydrous Ca-Al silicates lawsonite and epidote group minerals (EGMs) are key phases in subduction-zone H2O and element cycling. In high-pressure-low-temperature metamorphic rocks, Fe in both minerals is typically assumed to be entirely Fe3+, which substitutes for Al in octahedral sites as a major component in most EGMs and as a minor component in lawsonite and zoisite. New Fe micro-X-ray absorption near-edge spectroscopy (mu-XANES) analyses show substantial Fe2+ in lawsonite in blueschist from New Caledonia and zoisite from an unknown locality. Analysed Fe-rich EGMs (epidote, clinozoisite) contain primarily Fe3+. Lawsonite and some EGMs in subducted oceanic crust may contain more Fe2+ than is currently known, with possible implications for understanding subduction redox processes and conditions and why they vary in different subduction zones.

Deep-sea mud is rich in rare earth elements and yttrium (REY), with yttrium (Y) exhibiting the highest concentration. REY are found in authigenic (a-CFA) and biogenic (b-CFA) carbonate fluorapatite (CFA, Ca5(CO3) x (PO4)3-x F1+x ). The presence of REY in both CFA types suggests different enrichment processes in abyssal environments, which may be traced through detailed structural analysis of REY's coordination chemistry. The bonding environment of Y in CFA was investigated in 2018 and 2023 using extended X-ray absorption fine structure (EXAFS) spectroscopy at a resolution of 0.15 Å. While these studies offered valuable insights into Y's short-range order, they also presented inconsistencies. Moreover, a resolution of 0.15 Å is insufficient to uncover the intricate local structure of Y in CFA. Here, we present EXAFS data at a resolution of 0.11 Å for Y in a-CFA and b-CFA collected several meters beneath the Pacific Ocean seafloor. Y is predominantly hydrated and bound to Ca and PO4 in an amorphous phase surrounding the a-CFA and b-CFA nanocrystals and is secondarily incorporated into the crystal structure of a-CFA. There is no EXAFS evidence indicating the presence of polynuclear Y precipitate, which contrasts with a recent finding on cerium (Ce), nor supporting the formation of a Y-carbonate complex. The latter two findings are backed by density functional theory, which indicates that Y-Y pair formation is thermodynamically unfavorable and that the predicted Y-C distance is inconsistent with the EXAFS distances. This research highlights the geochemical enrichment of Y in abyssal sediments through the formation of a hydrated yttrium-calcium phosphate phase in a-CFA and b-CFA and Y for Ca substitution in authigenic a-CFA nanocrystals during the coprecipitation of calcium and phosphate.

Deep eutectic solvents (DES), a new class of solvents discovered twenty years ago, are increasingly proposed as a "green" alternative for extractive applications, such as the recovery of critical metals. However, little is known about the chemistry involved in DES, or what makes them special. This study focuses on reducing this knowledge gap by exploring the ability of DES to dissolve rare earth elements (REEs). It compares one of the most widely used systems in the literature, a choline chloride and lactic acid-based DES, with an aqueous solution of lactic acid (1 mol/L), pure H2O and other variations of the initial DES. Analysis of the dissolution yields and dissolution products in the case of concentrated REE phases (La, Nd, Gd, Dy, Y, Yb) revealed that the speciation and nature of REEs affect the dissolution ability of the solvents, but that both lactic acid-DES and aqueous solution show similar dissolution behaviors. X-ray absorption spectroscopy measurements showed that REEs tend to form lactate complexes in both solvents and that the hydrogen bond acceptor part of DES is not directly involved in the dissolution processes. The proportion and stability of these REE-lactate complexes in solution appear to be a crucial limitation for REE solubilization in high REE-concentrated materials, whereas this was not the case for REE-diluted materials. However, under the conditions of this study, all the DES tested showed a lower dissolution capacity than the lactic acid aqueous solution, raising the question of the value of using lactic/citric acid-based DES as dissolution solvents.

Sphalerite is a crucial host mineral for germanium (Ge) resources worldwide. However, the oxidation state (+2 or +4) of Ge and its substitution mechanism in sphalerite remain a subject of ongoing debate. The Huize and Maoping deposits are the largest and second largest Pb-Zn-Ge deposits in the Mississippi Valley Type (MVT) metallogenic province in southwest China, respectively. Four types of Ge-rich sphalerite have been identified within these two deposits: C1-Huize, C3-Huize, C1-Maoping, and C5-Maoping. This study employs synchrotron-based microscale X-ray absorption fine structure (mu-XAFS) methods, including X-ray absorption near-edge structure (mu-XANES) and extended X-ray absorption fine structure (mu-EXAFS) analyses, to investigate the Ge distribution, oxidation state, and neighboring atomic environment within the Huize and Maoping Ge-rich sphalerites. The results suggest that the incorporation of Ge4+ and Cu+ into sphalerite occurs at varying Cu/Ge molar ratios, depending on the availability/concentration of Cu in the sphalerite (i.e., Cu/Ge >= 2). On the other hand, Ge2+ and Ge4+ coexist when Ge and one vacancy (square) substitute for Zn in sphalerite. These different types of Ge substitutions influence the position of Zn atoms in the second neighboring atomic shell in the structure, while they have minor effects on the location of S atoms in the first neighboring atomic shell. The presence of vacancies strongly affects Zn atoms in the second neighboring atomic shell when coupled with Ge substitution in sphalerite, resulting in smaller interatomic distances and significant structural disorder (Debye-Waller factor). Additionally, two Cd2+ ions are required to co-replace two Zn2+ ions to fill the structural defects caused by vacancies. In contrast, the substitution of Cu+ and Ge4+ for two Zn ions results in a more ordered spatial structure, which is not distinctly controlled by the Fe content of sphalerite. Based on the characterization of the Ge oxidation state and local structure, we redefined the Ge substitution mechanisms inferred from element correlations: (1) Ge4+ +2Cd2++square -> 4Zn2+ and Ge2+-> Zn2+ in C1-Huize; (2) Ge4++2Cu+-> 3Zn2+ in C3-Huize and C1-Maoping; and (3) 3Cu++As3++Ge4+-> 5Zn2+ in C5-Maoping. This study not only elucidates the distribution of Ge in different valence states but also unveils its true spatial structure in sphalerite. These findings have significant implications for investigating Ge substitution and enrichment mechanisms in sphalerite.

Molybdenum (Mo) concentrations and isotopic signatures (S98Mo) are useful proxies for reconstructing paleoredox conditions and tracing material cycling in subduction systems. While the Mo isotope geochemistry of diverse geological settings has been extensively studied over the past two decades, the diagenetic effects on Mo behavior in pelagic oxic sediments remain poorly understood. Here, we investigate Mo cycling in Mn-rich pelagic sediments in an 847 cm long sediment core from the deep Pacific Ocean. We observe a systematic downcore decrease in Mo content (28.0 to 6.10 mu g/g) together with increasing Mo isotope ratios (S98Mo: -0.52 %o to +1.32 %o). Synchrotron X-ray absorption spectroscopy shows that the average oxidation state of Mn decreases with depth, which is consistent with the transformation of Mn minerals during sediment burial. In contrast to the canonical S98Mo (-0.7 %o) of Fe-Mn nodules and crusts, the depth-integrated S98Mo value of the Mn-rich sediments is unusually high, likely due to recycling of isotopically light Mo. If our findings are representative for larger areas of the deep ocean, they could potentially challenge the conventional global Mo budget. A revised isotopically heavier oxic Mo sink in the global ocean's Mo budget would imply a more extensive expansion of oceanic anoxia and/or reduction of the oxic sink during geological periods when seawater had lower S98Mo values.