Low-cost transition metal oxide catalysts are promising for catalytic mitigation of low concentrations of methane emissions by complete oxidation but suffer from high temperature and water-induced deactivation. Here, we design ZIF-67-derived nanocage catalysts: single-shelled Co3O4 (SCO), double-shelled Co3O4/Co3O4 (DCO), and Ni-doped double-shelled NixCo3-xO4/Co3O4 (NCO). While DCO shows superior initial activity under dry gas stream conditions (T-90 = 368 degrees C) due to abundant surface-adsorbed oxygen, it rapidly deactivates at 500 degrees C. In contrast, NCO maintains > 93 % methane conversion over 190 h under both dry and humidified gas stream conditions. Comprehensive characterization (O-2-TPD, XANES, in situ DRIFTS) reveals that high temperature and water-induced restructuring during the reaction reduces surface oxygen species and the activity of lattice oxygen. NCO's exceptional stability stems from its highly active lattice oxygen stabilized by Ni3 + sites, which resist hydrothermal sintering. This work provides mechanistic insights into the development of long-term stable catalysts for methane emissions abatement.

Fluorine is a key component in many hydrothermal ore-forming fluids, commonly occurring as fluorite and in minerals such as apatite, fluorite, and mica. The availability of fluoride ions (F-) for forming metal fluoride complexes is governed by the equilibrium thermodynamics of hydrofluoric acid [HF(aq); Migdisov et al. Chem. Geol. 2016, 439, 13-42 and Xing et al. Geofluids 2018, 2018 (1), 6835346]. However, obtaining reliable thermodynamic properties of HF(aq) at high pressure-temperature (P-T) conditions, typical of the Earth's crust and upper mantle, has proven challenging due to limitations in experimental techniques. This study employs an automated machine-learning molecular dynamics (MLMD) approach that is trained on ab initio data [Wang et al. J. Chem. Phys. 2022, 157 (2), 24103 and Wang et al. J. Am. Chem. Soc. 2024, 146 (21), 14566-14575] to calculate the pK a of HF(aq) across a wide range of temperatures (150-600 degrees C) and pressures (500 bar to 20 kbar). The calculated pK a values were then fitted into the Ryzhenko-Bryzgalin thermodynamic model with a density dependence to derive properties that can predict the role of HF(aq) in controlling the element mobility and pH in deep-Earth fluids. These results suggest that HF(aq) is a stronger acid than previously estimated, particularly under supercritical conditions, leading to enhanced dissociation and increased availability of fluoride ions (F-), which may improve the solubility and transport of metals such as rare-earth elements and uranium.

Understanding the incorporation of rare earth elements (REE) into their principal host minerals is fundamental to tracing geological processes, improving ore deposit formation models, assessing the resource potential of a deposit, and designingeffective and environmentally sustainable extraction processes. This study investigates the speciation and fractionation of europium (Eu) and cerium (Ce) in hydrothermally synthesized sulfate (anhydrite) and sulfide (sphalerite) minerals. Electron probe microanalysis reveals that Ce and Eu preferentially partition into anhydrite rather than sphalerite, consistent with the Goldschmidt Rules. Furthermore, we show that Eu concentration and oxidation state are decoupled within single anhydrite crystals: synchrotron X-ray absorption near-edge structure (XANES) and microprobe cathodoluminescence (CL) data both show that Eu3+ predominates in the core and Eu2+ increases from the core to the rim, whereas Ce3+ is the only oxidation state of Ce in anhydrite under the same conditions. In contrast, Eu and/or Ce concentrations oscillate from the core to the rim of anhydrite crystals, with the lowest concentration around the rim. Correlations among elemental concentrations in anhydrite show that Eu3+ and Ce3+ replace Ca2+ via a coupled substitution with Na+, and Eu2+ replaces Ca2+ via an isomorphous substitution. Complementary ab initio molecular simulations indicate that the incorporation of Eu and Ce caused only small local distortions of the anhydrite crystal structure, with more substantial distortion commonly observed around vacancies and Na sites; the extent of distortion is proportional to the differences between the effective ionic radii of Ca2+ and substituted Eu, Ce, and Na ions. This study demonstrates that hydrothermal anhydrite can incorporate up to several weight percent of REE, with a complex incorporation process controlled by the evolution of fluid composition and local fluid-mineral interactions. Spatially resolved oxidation state and concentration analyses provide deeper insights into fluid evolution than bulk REE measurements.

Understanding elements uptake and release from minerals in source rocks is crucial for comprehending critical metals accumulation, yet the mechanisms and kinetics of element mobilization at the atomic scale remain mostly unknown. Here, we analyzed the distribution of cobalt (Co) in natural pyrite from a Cu-Co ore deposit and found that metals distribution is best described by steady-state diffusion with constant flux and concentration-dependent diffusivities, rather than transient-state diffusion with time-evolving concentrations. First-principles calculations and diffusion modelling further demonstrate that this diffusion is accelerated by vacancy pathways and is far more efficient than traditional vacancy-mediated lattice diffusion, with element transfer rates higher by almost two orders of magnitude. We conclude that steady-state lattice diffusion induced by vacancies in the presence of fluid can be an efficient mechanism promoting the preferential release of metals into ore fluids and the accumulation of metals during ore formation.

Phase separation is a ubiquitous process in marine hydrothermal systems, significantly influencing the chemical and isotopic composition of vent fluids. Understanding its effects on elemental partitioning and isotopic fractionation is essential for using vent fluid chemistry as an accurate predictor of the chemical and physical conditions at depth in the oceanic crust. Here, we report the first laboratory experimental study investigating stable potassium isotope (K-41/K-39) fractionation during phase separation of alkali (Na, K) chloride fluids under temperature and pressure conditions relevant to natural hydrothermal systems. Two distinct fluid compositions were tested in our experiments at 400 degrees C, including a mixed NaCl-KCl solution with a starting Na/K molar ratio of 10, and a pure KCl solution. This compositional difference allows for evaluating the role of Na on K isotope fractionation between coexisting vapor and liquid phases. For the NaCl-KCl-H2O system, all experiments showed preferential enrichment of light K isotopes in the vapor phase during phase separation, yielding an average K isotope fractionation factor of -0.12 parts per thousand (+/- 0.04 parts per thousand, 1SD). In sharp contrast with the NaCl-KCl-H2O experiments, the Na-free KCl-H2O experiments revealed no measurable K isotope fractionation during phase separation. These results suggest that Na plays a critical role in modifying K bonding environments, likely through the formation of multi-cation polynuclear species in the concentrated liquid phase. The coexistence of the multi-cation polynuclear species in the liquid and compositionally simpler K free ions or KCl degrees(aq) ion pairs in the vapor may account for the observed K isotope fractionation in the NaCl-KCl-H2O system. The absence of Na in the KCl-H2O system prevents significant differences in K bonding environments between vapor and liquid phases, thereby precluding measurable K isotope fractionation. Using the K isotope fractionation factor quantified here, a Rayleigh fractionation model can successfully explain the previously unexplained low delta K-41 values (similar to-0.8 parts per thousand) reported for vapor-dominated fluids at Main Endeavour Field (NE Pacific Ocean). These results highlight the importance of considering phase separation alongside seawater interaction with the oceanic crust to fully understand K isotope variations in hydrothermal systems.

The high field strength elements (HFSE) Ti, Nb, and rare earth elements (REE) are commonly regarded as immobile during hydrothermal activity and metamorphism, making them important tracers of geological processes. Here, we report a Ti-REE-Nb-As mineralization recently discovered in quartz, feldspar, muscovite +/- biotite, fluorapatite, hematite, epidote, and dravite-schorl veins hosted in quartz +/- feldspar +/- muscovite +/- biotite gneisses from the Monte Leone nappe (Switzerland/Italy). The veins formed during prograde metamorphism and were boudinaged and/or folded during peak metamorphism under lower amphibolite facies. The mineralization consists of megacrysts (>> 2 cm) of allanite-(Ce) and Nb-REE-rich titanite-(I). Titanite-(I) displays prominent primary oscillatory- and sector-zonings in Y+REE and Nb. Allanite-(Ce) and titanite are also present as metamorphic minerals disseminated in the host-rock. The vein-hosted megacrysts and their host rocks have identical Nd isotope systematics, indicating that the HFSE mineralization results from small-scale remobilization of host-rock components.Localized, fluid-assisted dissolution of vein-hosted allanite-(Ce), epidote, and dravite-schorl during retrograde alpine deuteric alteration resulted in cavities lined with chlorite, muscovite, hematite, and diverse REE minerals. The same fluids caused titanite-(I) to break down into a porous assemblage of acicular niobian rutile with lamellae of crichtonite-group minerals and/or hematite and a suite of REE-Nb-Ti micro-minerals. A few titanite (titanite-II) crystals preserve an intermediate stage of the dissolution-reprecipitation process. Unlike titanite-(I), they display a patchwork-like micro-texture (100 mu m sized subgrains with inhomogeneous Nb concentrations); they host lamellae of crichtonite-group minerals within cleavage planes of the parent titanite, as well as secondary Y+Nb+REE oxides and calcite along subgrain boundaries. The occurrence of calcite indicates that CO2-enriched fluids promoted the destabilization of titanite-(I). Highly localized fluid flow accounts for the common occurrence of fresh and altered allanite-(Ce) and titanite in close proximity.The HFSE-enriched veins reveal a complex history of mobility of minor elements (Ti, Nb, REE, As +/- B, Be) together with major components (Si, Al, K, Na, Fe) from the host rock, resulting in their early (prograde) concentration within the veins, and their remobilization upon the action of oxidized CO2-bearing fluids during retrograde metamorphism. In general, crystallization of enriched phases during prograde metamorphism may be an important step in determining the fertility of a source rock for hydrothermal HFSE deposits.

The Beiya Au-base metal deposit in southwest China is characterised by a huge amount of iron associated with gold mineralization. The formation of the Beiya deposit is generally thought to be related to Tertiary potassic intrusion, as porphyry- and skarn-type mineralization are the two most important types of mineralization in the region. However, the lack of direct evidence to constrain the source of iron and gold as well as other metals makes it difficult to accurately conceptualise the ore genesis model. In this study, we report high-precision (0.03 %o; 2sd) Fe isotope data on Fe-bearing minerals including magnetite, pyrite and chalcopyrite, and major igneous rocks exposed in the area, including monzogranite porphyry (MGP), mafic microgranular enclave (MME), lamprophyre and basalt. Magnetites have a wide range of delta Fe from 0.15 to 0.64 %o, reflecting substantial isotopic fractionation at different mineralization stages. MMEs have heaviest Fe isotopic composition (0.35 %o to 0.88 %o) compared to MGPs (0.19 %o to 0.48 %o), lamprophyre (-0.2 %o) and basalt (-0.0 %o). A negative correlation between delta 56Fe and TFe2O3 is found for both MPGs and MMEs, which indicates the leaching of iron by hydrothermal fluids mobilizes light Fe and leaves behind isotopically heavy Fe in the remaining source rock. Fe isotopic compositions of MPG and MME overlap with those of magnetite, both of which are proposed to be the source of Fe in magnetite. The basalt has delta 56Fe value of 0 +/- 0.03 %o, similar to those of global basalts, suggesting a negligible contribution to the formation of iron ores. Therefore, we propose that magmas parental to the MGP and MME may be the main source of iron to form the Beiya deposit; post-magmatism hydrothermal activities play a key role in leaching the metals out and transporting them to the mineralization area.

In a series of four extended industrial column leach experiments spanning 542 days, aimed at examining Curecovery from low-grade sulfide ores, all instances yielded suboptimal Cu recovery rates (<27%). Through a nuanced analysis integrating time-resolved chemical data on leachate with detailed mineralogical investigations, we highlight that the dissolution of aluminosilicate gangue minerals is a pivotal factor constraining Cu recovery. The source materials, extracted from a low-grade segment of a Cu-Au porphyry deposit (-0.32-0.36 wt% Cu), underwent diverse leaching conditions employing various lixiviants, curing phases, redox conditions, and particle size distributions. Notably, poor Cu recovery is closely tied to gangue mineralogy. The ammonia leach column experienced limited Cu leaching due to an inert gangue shielding chalcopyrite from the lixiviant. In acid columns, swift precipitation of silica-rich phases impeded effective Cu leaching at multiple scales, from largescale-column to molecular levels. Identifying gangue mineralogy and considering their impact during acid leaching are crucial for formulating effective lixiviant strategies for Cu recovery from low-grade primary sulfide ores. The curing phase, requiring high acid concentrations (-50 g/L H2SO4), emerges as a critical aspect for promoting effective gangue dissolution while preventing refractory silica-rich phase precipitation.

The regolith-hosted rare earth element (REE) deposits in the Nanling Mountain Range, South China, are the most important source of heavy REE (HREE) in the world. A perplexing thing is that only a few of these deposits are relatively enriched in HREE and most of them are actually light REE (LREE)-enriched, although they are all originated from the granitic bedrocks with similar HREE concentrations. This is supposed to be related to the redistribution of REE in the weathered bedrocks due to groundwater-rock interaction, however, it remains enigmatic how REEs are mobilized and fractionated in the interaction. Titanite is an important REE contributor to the regolith-hosted REE deposit. Here, we investigate textural and compositional variations of the titanite from the granitic bedrocks to weathered profiles in the Zuokeng regolith-hosted REE deposit, the largest one that is newly discovered in this region. Together with thermodynamic modelling, we deduce the nature of the fluids, temperature range for titanite alteration, and the mechanism of REE fractionation. Primary titanite is of magmatic in origin and shows relatively high REE contents and negative Eu anomaly on the chondrite-normalized REE pattern. Secondary titanite is commonly associated with chloritized biotite and is characterized by extremely low REE, high Al 2 O 3 , F and significant positive Eu anomaly, typical of authigenic origin due to chloritization of biotite under oxidized conditions in supergene environment. From the bedrocks upward to the surface, primary titanite is gradually replaced by calcite, synchysite-(Ce) and TiO 2 in the weathered bedrocks. This is interpreted as the result of infiltration of the F, carbonate-rich, alkaline fluids derived from local groundwater through the bedrocks. The irregular dark patches in primary titanite display typical alteration texture and may have formed through fluid leaching of primary titanite. The patches in primary titanite have lower REE concentrations and higher LREE/HREE ratios than magmatic domains, indicating that the groundwater preferentially scavenges HREE from primary titanite, leaving behind LREE in weatherable secondary minerals, such as synchysite-(Ce) and allanite. Mass balance calculation results also confirm that more HREE are lost from weathered bedrocks than LREE. Therefore, the interaction of groundwater and granitic bedrocks may be favorable to enrich LREE rather than HREE in the regolith. This may explain why most regolith-hosted REE deposits like Zuokeng in South China are of predominant in LREE. This study highlights the role of groundwater in mobilizing, fractionating and enriching REE from the granitic bedrocks to resultant regolith-hosted REE deposits.

The solubility and speciation of zinc (Zn) in chloride-bearing aqueous fluids at high temperature and pressure are important for understanding Zn transport in natural hydrothermal systems and associated mineralizing processes. Here, we measured sphalerite solubility in NaCl-HCl-H2O fluids using a fixed-volume titanium alloy hydrothermal reactor equipped with a newly designed gas-tight titanium piston sampler. This novel reactor-sampling system is capable of acquiring internally filtered fluids at high temperature and pressure. The experiments were conducted at 300-450 degrees C, 500 bar, in fluid with 0.5 m and 1 m NaCl, respectively. The measured sphalerite solubilities are consistent with predicted values using previous thermodynamic data at 300-400 degrees C, but diverge significantly above 400 degrees C. To resolve this discrepancy, we adjusted the solubility product of Zn minerals by modifying the heat capacity and Born coefficients that describe the Gibbs Free Energy of formation from the elements of the Zn2+ aqua ion based on the new solubility data. The refined Helgeson-Kirkham-Flowers (HKF) equation of state (EoS) of Zn2+ empirically reproduces the solubility data of Zn minerals from previous experimental studies well over the covered T-P range (25-600 degrees C, Psat to 2 kbar), but extends accurate predictions to conditions typical of deep sea hydrothermal systems, down to fluid densities of 0.35 g/cm(3). Thermodynamic modelling using the revised EoS of Zn2+ shows that higher temperatures, chlorinity and lower pH increase Zn solubility, and that Zn chloride complexes are the predominant species. The influence from salinity on Zn solubility is less significant in fluids with low pH. Applied to seafloor hydrothermal systems, our results suggest that in addition to temperature, pH and total dissolved chloride, fluid/rock ratio may be an important factor contributing to Zn concentrations in vent fluids at Mid Ocean Ridges. (c) 2022 Elsevier Ltd. All rights reserved.

Densely assembled graphene-based membranes have attracted substantial interest for their widespread applications, such as compact capacitive energy storage, ion/molecular separation, gas barrier films, and flexible electronics. However, the multiscale structure of densely packed graphene membranes remains ambiguously understood. This article combines X-ray and light scattering techniques as well as dynamic electrosorption analysis to uncover the stacking structure of the densely stacked reduced graphene oxide (rGO) membranes. The membranes are produced by reducing graphene oxide (GO) membranes with hydrazine, during which the colloidal interactions between GO sheets are modulated by the electrolyte solution. In contrast to the common notion that direct reduction of densely assembled GO sheets in parallel tends to result in significant "graphitization", this article unexpectedly discovers that the resultant densely packed rGO membrane can still retain the interconnected network nanochannels and show good capacitive performances. This inspires the development of a hierarchical structural model to describe the densely packed rGO membranes. This article further shows that the nanochannel network can be fine-tuned at the sub-nanometer level by tailoring the salt concentration and the reduction temperature to render exceptional volumetric capacitance and good rate performance for rGO membranes even with increased packing density.

In seafloor hydrothermal systems, vent fluids usually contain elevated dissolved iron (Fe) that is significantly enriched relative to deep ocean seawater. It is commonly thought that Fe is preferentially transported in dense Cl-rich fluids due to the formation of aqueous Fe-Cl complexes. However, Fe enrichment in vapor-rich low-density fluids with low Cl concentrations (<550 mmol/kg) underscores the efficacy of the low-Cl vapor-rich phase to transport Fe in both subaerial and submarine hydrothermal systems. Currently, transport of Fe in low-density vapor-rich fluids is poorly understood due to the lack of high temperature-pressure (T-P) solubility experiments and requisite thermodynamic data. Here, we report new data of Fe solubility from experiments conducted at 400-500 degrees C, 215-510 bar, targeting fluids with low-density (similar to 0.1-0.35 g/cm(3)). The experiments were performed in the KCl-H2O system with hematite-magnetite and K-feldspar-muscovite-quartz as mineral buffering assemblages. Our results show that Fe solubility positively correlates with density and fluid chlorinity, which are affected by temperature and pressure. The equilibrium constants (log K-hm) for Fe-buffering reaction Fe3O4(s) + 2HCl(aq) = Fe2O3(s) + FeCl2(aq) + H2O were determined. The new data and the data calculated using Helgeson-Kirkham-Flowers (HKF) equation of state were fit into a density model to extrapolate log Khm for hematite-magnetite Fe buffering reaction over a wide T-P range. The density models for magnetite dissolution reaction and pyrite-pyrrhotite equilibrium were also fit based on HKF to allow redox constraints. We show that calculated Fe solubility are in good agreement with measured values in vapor-rich fluids formed via phase separation in mineral buffered and basalt alteration experiments at elevated T-P. The density model was further applied to model Fe transport in fluids at the Brandon hydrothermal field at East Pacific Rise (EPR) 21 degrees S, with T-P constrained by Si-Cl geothermobarometer. The calculations suggest that the reported Fe concentrations of vent fluids at Brandon reflect phase separation occurring at depth in the seafloor, with T-P up to 450 degrees C, 400 bar, and redox conditions buffered by pyrite-pyrrhotite-magnetite equilibrium. (C) 2021 Elsevier Ltd. All rights reserved.

The dynamic evolutions of fluid-mineral systems driving large-scale geochemical transformations in the Earth's crust remain poorly understood. We observed experimentally that successive sodic and potassic alterations of feldspar can occur via a single self-evolved, originally Na-only, hydrothermal fluid. At 600 °C, 2 kbar, sanidine ((K,Na)AlSi3O8) reacted rapidly with a NaCl fluid to form albite (NaAlSi3O8); over time, some of this albite was replaced by K-feldspar (KAlSi3O8), in contrast to predictions from equilibrium reaction modelling. Fluorine accelerated the process, resulting in near-complete back-replacement of albite within 1 day. These findings reveal that potassic alteration can be triggered by Na-rich fluids, indicating that pervasive sequential sodic and potassic alterations associated with mineralization in some of the world's largest ore deposits may not necessarily reflect externally-driven changes in fluid alkali contents. Here, we show that these reactions are promoted at the micro-scale by a self-evolving, kinetically-driven process; such positive feedbacks between equilibrium and kinetic factors may be essential in driving pervasive mineral transformations.

Reaction-induced porosity is a key factor enabling protracted fluid-rock interactions in the Earth’s crust, promoting large-scale mineralogical changes during diagenesis, metamorphism, and ore formation. Here, we show experimentally that the presence of trace amounts of dissolved cerium increases the porosity of hematite (Fe 2 O 3 ) formed via fluid-induced, redox-independent replacement of magnetite (Fe 3 O 4 ), thereby increasing the efficiency of coupled magnetite replacement, fluid flow, and element mass transfer. Cerium acts as a catalyst affecting the nucleation and growth of hematite by modifying the Fe 2+ (aq)/Fe 3+ (aq) ratio at the reaction interface. Our results demonstrate that trace elements can enhance fluid-mediated mineral replacement reactions, ultimately controlling the kinetics, texture, and composition of fluid-mineral systems. Applied to some of the world’s most valuable orebodies, these results provide new insights into how early formation of extensive magnetite alteration may have preconditioned these ore systems for later enhanced metal accumulation, contributing to their sizes and metal endowment.