Yuanming Pan

Quantifying sulfur (S) speciation in apatite‐group minerals (apatites) is critically important for understanding magmatic and magmatic‐hydrothermal processes but has generally been restricted to individual analytical techniques. In this study, we have quantified sulfur speciation in a suite of apatites (including fluorapatite, hydroxylapatite and chlorapatite) using three complementary analytical techniques: S K ‐edge X‐ray absorption near‐edge structure (XANES), S 2 p X‐ray photoelectron spectroscopy (XPS), and S K α wavelengths with electron probe micro analysis (EPMA). Our results show that all three techniques identify S ⁶⁺ as the dominant sulfur species, with minor contributions from S ⁴⁺ , in the apatite samples investigated in this study. Sulfur K ‐edge XANES offers the most precise quantification of S ⁶⁺ /ΣS values (where ΣS = S ⁶⁺ + S ⁴⁺ + S ^2‐ ) from 0.93 to 1.0, with the standard deviation from ±0.01 to ±0.06, although it requires synchrotron access and careful calibration. Sulfur 2 p XPS analyses yield a similar trend in S speciation but lower S ⁶⁺ /ΣS values from 0.69 to 0.78, attributable to partial radiation‐induced reduction of S ⁶⁺ to S ⁴⁺ related to prolonged X‐ray exposure. Similarly, the EPMA‐based (S K α wavelength shift) technique is capable of distinguishing between S ⁶⁺ and S ⁴⁺ , and yields S ⁶⁺ /ΣS values ranging from 0.56 to 0.88, again indicating significant radiation‐induced reduction. Therefore, the XPS and EPMA‐based techniques are also capable of quantifying sulfur speciation in apatites but require accurate corrections for radiation‐induced sulfur reduction. This study highlights the strengths and limitations of these three analytical techniques, with XANES offering the highest sensitivity. At the same time, XPS and EPMA, although less sensitive, are surface‐specific and widely accessible, respectively. It also guides their applications for quantifying sulfur speciation in apatites.

The sulfidation of Cd(II)-bearing ferrihydrite in anoxic environments is a key process controlling Cd(II) geochemical cycling. However, the transformation pathways of ferrihydrite and the resulting fate of Cd(II) under varying sulfide levels are not fully quantified. This study systematically investigated the Cd, S, and Fe speciation during ferrihydrite sulfidation transformation using Raman, XRD, HRTEM, XAS, and wet chemistry. Results show that both the initial concentration of Cd(II) and the ratio of S(-II)/Fe(III) govern ferrihydrite transformation pathways. At low S(-II)/Fe(III) (0.01), lepidocrocite and goethite dominated at ≤0.5 mM Cd(II), while hematite prevailed at 2 mM Cd(II). At higher S(-II)/Fe(III) (0.1–0.5), goethite formed under low Cd(II), whereas goethite and mackinawite dominated at high Cd(II). Elevated Cd(II) promoted hematite and mackinawite but inhibited lepidocrocite and goethite formation. Collectively, these sulfidation transformations reduced Cd(II) mobility by redirecting its speciation. Cd K-edge EXAFS and extraction results indicated approximately 23–33% Cd(II) incorporation into Fe oxides at low S(-II)/Fe(III), but >85% converted to greenockite at higher S(-II)/Fe(III). The results establish a foundation for subsequent research to assess the influences of both abiotic and biotic transformations of ferrihydrite, while also aiding in the prediction of Cd(II) geochemical cycling in wetland soils and sediments.

Chalcedony forms across a wide area in eastern Iran, particularly in the Sarbisheh district near Birjand city, with notable occurrences in the Qazdez-Bahamarz region. Here, cryptocrystalline quartz is found in hydrothermal veins and veinlets within volcanic rocks hosted by carbonate and intermediate igneous formations. Chalcedony samples of various colours—black, purple, green, blue, lavender, grey, lemon yellow and white—were analysed using diverse techniques. These chalcedony samples display fibrous and granular textures, comprising microcrystalline quartz, cryptocrystalline moganite, and opal-CT and opal-C interlayers. Elements that affect the colouration of chalcedony include iron (producing red and yellow tones), chromium (green), manganese (black, blue and grey patterns), nickel (purple) and copper (also purple). Altered carbonate host rocks are enriched in Fe, Cu, Zn and Cd, with Al and Mn depletion. Stable isotope analyses show δ ¹⁸ O values in agates range from +14.9‰ to +25.5‰, whereas δ ¹⁸ O and δ ¹³ C values in carbonate minerals and chalcedony range from +14.1‰ to +24.8‰ and –5.7‰ to +0.7‰, indicating agate formation from mantle-derived hydrothermal fluids mixed with meteoric waters. Raman spectroscopy detected moganite, α-quartz, goethite, aragonite and illite in agate interlayers. Analyses by field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) revealed minerals of Fe, Cr, Ni, Ti and Sn. X-ray diffraction confirmed chalcedony, moganite and opal-CT, whereas EPR spectroscopy showed strong magnetic backgrounds from goethite and silicon-vacancy centres formed by radioactive decay of U, Th and their byproducts.

Heavy rare earth elements (HREE), as critical materials for the global transition to a low-carbon economy, are mostly supplied by regolith-hosted ion adsorption REE deposits formed from the weathering of granites. Magmatic-hydrothermal titanite of pronounced HREE enrichment in granites has been suggested to play an important role in forming HREE-dominated regolith-hosted deposits. However, the crystal-chemical mechanism and geological process responsible for HREE enrichment in titanite remain unknown. In this study, we investigated two texturally distinct types of HREE-enriched titanite (titanite I and II) in granites from the Gucheng regolith-hosted HREE deposit in South China. Secondary ion mass spectrometry (SIMS) U-Pb dating of rutile associated with titanite I and II yielded similar ages of 101.0 ± 1.7 and 97.9 ± 6.2 Ma, respectively, supporting HREE remobilization during auto-metasomatism. Microbeam synchrotron X-ray absorption spectroscopic analyses show that titanite I and II have distinct Y K-edge X-ray absorption near-edge structure spectra. The best-fit results of Y K-edge extended X-ray absorption fine structure data suggest that titanite I and II have a Y-O first shell with coordination numbers and distances of 7.8 and 7.2 Å and 2.31–2.51 Å and 2.30–2.47 Å, respectively. Measured Y 3d and Dy 3d X-ray photoelectron spectroscopic data also support different Y,HREE-O first shells between titanite I and II. These results suggest that Y3+ and REE3+ occupy the 7-coordinated Ca site via three substitutions (Y,REE)3+ + (Al,Fe)3+ ↔ Ca2+ + Ti4+, 2(Y,REE)3+ + □Ca ↔ 3Ca2+, and 2(Y,REE)3+ + O2− ↔ 2Ca2+. First-principles calculations and the lattice strain model predict preferential uptakes of Y3+ and HREE3+ as the (Y,HREE)O8 polyhedra arising from the latter substitution mechanism at the Ca site in titanite. Moreover, we link the 2Y3+ + O2− ↔ 2Ca2+ substitution in titanite to highly oxidized and HREE-enriched melts/fluids originating from the subducted slab. These findings further support our previous suggestion that the crystallization of HREE-enriched titanite in granites plays an important role in forming HREE-dominated regolith-hosted deposits.

Niobium and tantalum are the so-called “geochemical twins,” sharing similar properties and having long been used to constrain magmatic sources and trace geological and planetary processes. Significant fractionations of Nb and Ta have recently been documented in diverse magmatic-hydrothermal and metamorphic systems, but their origin remains unclear. Nb and Ta are preferentially incorporated in Ti-rich minerals such as ilmenite, rutile, and titanite. In this study, representative samples of Nb-rich titanite and ilmenite exhibiting large Nb/Ta variations (5 to 65) from two anorogenic alkaline suites in the Fangcheng and Ziyang regions of China and associated hydrothermal alteration have been investigated by Raman spectroscopy, synchrotron X-ray adsorption spectroscopy (SXAS) and X-ray photoelectron spectroscopy (XPS). The Raman spectra of Nb-rich titanite and ilmenite show only minor degrees of metamictization. The Nb K-edge SXAS spectra of Nb-rich ilmenite with Nb/Ta > 30 are similar to those of Nb2O5, indicating a dominant presence of Nb5+ with minor Nb4+ and an Nb5+/ΣNb ratio range from 0.59 to 1. In contrast, the Nb K-edge SXAS spectra of titanite and ilmenite with Nb/Ta < 30 indicate mixed valences of Nb5+ and Nb4+, with Nb5+/ΣNb in the range of 0.22 to 0.55. Similarly, XPS measurements of titanite and ilmenite further support the presence of mixed valences of Nb, with Nb5+/ΣNb in the range of 0.49 to 0.85. In addition, Ta L3-edge SXAS spectra of titanite suggest the occupancy of Ta5+ at the octahedral site. Fitting results from Nb K-edge and Ta L3-edge extended X-ray adsorption fine structure (EXAFS) data further suggest that both Nb and Ta replace Ti via coupled substitutions of the type 2Ti4+ ↔ (Nb5+, Ta5+) + (Al, Fe)3+ and an isovalent substitution Ti4+ ↔ Nb4+. A positive correlation between the oxidation states of Nb and the Nb/Ta values has been observed in titanite and ilmenite, but not for Ta. These results suggest that the extreme enrichment and large variations in Nb contents, as well as significant fractionation of Nb from Ta in titanite and ilmenite, are controlled by the prevailing oxygen fugacity in magmatic-hydrothermal systems. Given these complexities involving variable valence states, circumspection is required to use Nb and Ta anomalies to interpret geotectonic settings and apply Nb/Ta systematics to constrain magmatic-hydrothermal sources.

Regolith-hosted rare earth element (REE) deposits are the world’s primary source of heavy REEs (HREEs) critical to the global clean-energy transition. Previous studies suggested that REEs in regolith-hosted deposits are largely inherited from their parent granites. However, several HREE-dominated deposits occur in the weathering crusts of light REE (LREE)-enriched granites, where the mechanisms of REE fractionation remain poorly understood. Also, the conventional mining method of regolith-hosted REE deposits has limited efficiencies in REE recovery while causing enormous environmental contamination. Herein, we have investigated the distribution and speciation of Y and REEs in three representative regolith-hosted REE deposits (i.e., Gucheng and Shangyou, HREE-dominated; Renju, LREE-dominated) as well as Y-sorbed birnessite from batch experiments. Our results show that birnessite in all three deposits is a minor constituent but contains anomalously high concentrations of REEs and contributes to 25.3, 23.4, and 26.5% of the HREE contents of mineralized saprolites. Measured Y K-edge X–ray absorption spectroscopic data suggest that Y3+ (representing HREE3+) is adsorbed on birnessite as YO8 complexes in all three deposits but via different linkages: i.e., the bidentate corner-sharing mode in the HREE-dominated deposits but a mixture of both bidentate corner-sharing and edge-sharing modes in the LREE-dominated deposit. These binding mechanisms are also observed in Y-sorbed birnessite prepared at different ionic strengths. Therefore, different binding mechanisms of Y and HREE sorption on birnessite together with its preferential adsorption of HREE not only are responsible for the formation of HREE-dominated deposits from LREE-enriched granites but have important implications for the sustainable development of regolith-hosted REE deposits.

Picromerite K₂Mg(SO₄)₂•6H₂O (also known as schoenite, schönite, or sulfate of potash) formed from alkaline lakes by evaporation is an increasingly important chlorine-free fertilizer and has been used to produce other organic fertilizers such as arcanite K₂SO₄ and boussingaultite (NH₄)₂Mg(SO₄)₂•6H₂O. Picromerite and boussingaultite, two Tutton’s salts, are also common secondary solid phases in diverse types of mine tailings and play important roles in controlling the mobility and bioavailability of various heavy metal(loid)s, including arsenic. In this study, picromerite crystals containing 64 ppm As have been synthesized at ambient conditions from aqueous solutions by slow evaporation. Arsenic K-edge X-ray absorption near-edge-structure and extended X-ray absorption fine structure data show that the dominant oxidation state is +5 and that As⁵⁺ occupies the S site in picromerite. Single-crystal and powder electron paramagnetic resonance spectra of gamma-ray-irradiated picromerite reveal three arsenic-associated oxyradicals: AsO₄ ²⁻, AsO₃ ²⁻, and AsO₂ ²⁻. The orientations of the principal ⁷⁵As hyperfine directions of the AsO₃ ²⁻ and AsO₂ ²⁻ radicals match the bonding directions of the SO₄ ²⁻ groups in the picromerite structure, further supporting substitutions of their respective diamagnetic precursors AsO₄ ³⁻ and AsO₃ ³⁻ for the sulfate group. These AsO₄ ²⁻, AsO₃ ²⁻, and AsO₂ ²⁻ radicals in picromerite are similar but not identical to their counterparts in boussingaultite and gypsum, suggesting that sulfate minerals are capable of sequestrating both arsenate and arsenite, with important implications for understanding the fate and bioavailability of arsenic associated with agricultural applications of organic fertilizers.

Prolonged and episodic magmatic-hydrothermal processes have been proposed to be important in forming large-tonnage Cu volcanogenic massive sulfide (VMS) deposits but are difficult to document and remain poorly understood in ancient VMS systems. In this study, we combine textural evidence and in situ analysis of sulfur isotopic composition with trace elements of pyrite from the well-preserved, large-tonnage Ashele VMS deposit (Central Asian Orogenic Belt, NW China) to elucidate the origin of the ore-forming materials and controlling factors of VMS deposits. The distribution of hydrothermal alteration and mineralization allows the Ashele deposit to be divided into five zones: massive sulfide, quartz-pyrite, chlorite-chalcopyrite-quartz-pyrite, quartz-chlorite-sericite-pyrite, and quartz-sericite-pyrite zones. Cu mineralization is mainly hosed in the massive sulfide and chlorite-chalcopyrite-quartz-pyrite zones. Three texturally and compositionally distinct types of pyrite from the Cu mineralization zones have been recognized. The first type of growth zones with high Cu, As, and volatile elements (e.g., Hg and Tl) concentrations and negative δ34S values (−7.83‰ to −0.35‰) are observed in pyrite grains from the massive sulfide zone most likely formed from the input of magmatic volatiles degassed from the underlying magmatic systems. The second type of Cu-As-rich growth zones in pyrite grains from the chlorite-chalcopyrite-pyrite zone is not enriched in volatile elements and has positive δ34S values (2.59–6.56‰), which are interpreted to have precipitated during fluid boiling. The third type of growth zone, without Cu-As enrichment but having positive δ34S values (0.30–9.76‰) in pyrite grains from both mineralization zones, was probably formed without boiling or magmatic volatile input. Our results suggest that multi-stage magmatic degassing or fluid boiling accompanied by Cu precipitation recorded in individual pyrite grains can reveal episodic magmatic-hydrothermal activity, which is an essential factor in forming large-tonnage VMS deposits.

High-entropy LnBaCo₂O_5+δ perovskites are explored as rSOC air electrodes, though high configuration entropy (S_config) alone poorly correlates with performance due to multifactorial interactions. We systematically engineer LnBaCo₂O_5+δ perovskites (Ln = lanthanides) with tunable S_config and 20 consistent parameters, employing Bayesian-optimized symbolic regression to decode activity descriptors. The model identifies synergistic contributions from S_config, ionic radius, and electronegativity, enabling screening of 177,100 compositions. Three validated oxides exhibit superior activity/durability, particularly (Pr_0.05La_0.4Nd_0.2Sm_0.1Y_0.25)BaCo₂O_5+δ, showing enhanced oxygen vacancy concentration and disordered transport pathways. First-principles studies reveal optimized charge transfer kinetics via cobalt-oxygen bond modulation. Further, the interplay between first ionization energy, atomic mass, and ionic Lewis acidity dictates stability. This data-driven approach establishes a quantitative framework bridging entropy engineering and catalytic functionality in complex oxides.