Mineral Interface Research Articles

Coupling and decoupling of Au and As in pyrite from Carlin-type Au deposits, southwest China

The low-temperature geochemistry of gold (Au) and arsenic (As) in pyrite from Carlin-type Au deposits is significant for understanding the enrichment mechanisms of Au in pyrite, but remains poorly understood due to the complex processes that lead to the precipitation of micron-scale pyrite grains. This paper concerns the geochemical behaviors of Au and As during the precipitation of pyrite from six Carlin-type Au deposits (the Nibao, Shuiyindong, Banqi, Jinya, Yata, and Zimudang deposits), southwest China. These deposits are located in different locations and ore-bearing horizons of the Youjiang Basin. Arsenian pyrite and/or arsenopyrite are the dominant Au-bearing minerals. Gold-bearing pyrite grains are typically zoned (core, intermediate, and rim), with multiple, oscillatory banded sub-zones. In situ analysis demonstrates that invisible Au exists mainly as cationic Au+ in the growth zones of pyrite. Gold and As are positively correlated (i.e., coupled) in the Nibao and Shuiyindong deposits, negatively correlated in the Banqi, Jinya, and Yata deposits, and poorly correlated in the Zimudang deposit. Arsenic and S substitution in pyrite and the presence of a reduced fluid promoted the incorporation of Au, resulting in the coupling of Au and As. The Au precipitation was affected by changes in the physicochemical conditions of the hydrothermal systems, independent of the As concentration, resulting in decoupling of Au and As. As such, invisible Au precipitation is controlled by the extrinsic physicochemical conditions of the ore-forming fluids and microscopic reactions at the fluid–mineral interface, and also the episodic incursion of Au-rich hydrothermal fluids. High As contents in pyrite favor Au incorporation, but are not solely responsible for Au enrichment. Gold and As contents in pyrite in Carlin-type Au deposits may be coupled or decoupled.

Alteration of trioctahedral micas in the presence of inorganic and organic acids

The alteration of two trioctahedral micas, biotite and phlogopite, was investigated at the meso, micro, and nanoscale using three complementary microscopy techniques to better understand mica surface reactivity. In situ and ex situ experiments were performed to monitor the mineral interface during dissolution in acidic solutions (nitric and oxalic acid, pH ∼ 1–2), over a temperature range of 25–100°C. The inorganic acid was used as a benchmark condition to elucidate the effect of the organic acid on the dissolution behavior. The observed topographical changes that arose during mineral alteration revealed the simultaneous occurrence of different processes that heterogeneously shaped the mica surface: 1) the retreat of pre-existing and newly formed steps (edge surface reactivity). In the case of biotite, layer curling and peeling-off occurred in the presence of nitric acid whereas dendritic-shaped step edges resulted from the effect of oxalic acid; 2) the nucleation of etch pits and the formation of dissolution channels on the (001) surface. Oxalic acid promoted the growth of the pits to such an extent that they were discernible at each scale and resolution investigated; and 3) precipitation of secondary phases. Overall, a multi-scale approach offers new insights into the dissolution behavior of biotite versus phlogopite and provides and enhances understanding of the effect that oxalic acid has on the surface reactivity of mica.

Molecular Origin of Wetting Characteristics on Mineral Surfaces

Accurate determination of the wetting characteristics on mineral surfaces is critical for many natural processes and industrial applications where multiphase flow in porous media is involved. The wetting behaviors on mineral surfaces are controlled by water-mineral interactions, giving rise to various wetting characteristics, including contact line advancement, formation of precursor films, etc. However, a fundamental understanding of wetting characteristics on different mineral surfaces is still lacking at the molecular level. Here, utilizing a comprehensive set of molecular dynamics simulations, we investigate the wetting characteristics of water on various mineral surfaces and obtain the corresponding water-mineral interaction properties (including the areal density of water-mineral interaction energy and the work of adhesion of the water-mineral interface), mineral wettability, and structural and diffusion properties of water molecules near the surface. We show that the diffusion properties of water molecules on mineral surfaces play an important role in wetting characteristics. We find that the contact line tends to advance forward in the jumping mode or the rolling mode during the wetting process, which depends on the diffusion capacity of the water molecules on mineral surfaces. The corresponding evolution of the solid-liquid friction coefficient during dynamic spreading is also analyzed. We further demonstrate the strong impact of isomorphic substitution and charge-balancing counterions on wetting characteristics on the surfaces of clay minerals. It is shown that the introduction of charge-balancing counterions can shift the mineral surface from strongly hydrophilic to strongly hydrophobic and lead to completely different wetting characteristics. Our results provide a clearer picture of the molecular underpinnings in mineral wetting phenomena and deepen the understanding of the control of water-mineral interactions on the wetting properties.

The influence mechanism and the contribution of capillary force and gravity to recovery in spontaneous imbibition in low permeability reservoirs

Spontaneous imbibition is one of the important means to enhance oil recovery in low permeability reservoirs, and it has good field application effects. Through the interfacial tension (IFT), contact angle (CA), and adsorption experiments, the effects of SDBS surfactant on IFT and reservoir wettability were studied, and the adsorption rule of SDBS at the mineral surface was obtained. On this basis, the imbibition recovery and mechanism of cores with different permeability are studied. The study showed that in low permeability reservoir, gravity and capillary force are the main controlling factors of imbibition. In the early stage, gravity determines the recovery, because the imbibition rate is 2.75 times that dominated by capillary force. The capillary force can continue to produce oil in small pores, and the imbibition time is twice that of gravity, which determines the recovery in the late imbibition stage. Therefore, the synergistic effect of gravity and capillary force can optimize the imbibition recovery, in low permeability reservoirs. When the concentration of SDBS solution is CMC (0.1%), due to the mineral interface adsorbs, the capillary force decreases by 10.76%, and the adhesion work increases by 3.42 times, resulting in a decrease in the imbibition recovery. The imbibition driving force is different for different reservoir properties, which can be adjusted by adding a surfactant to change IFT and CA.

An Insight into the Mechanical and Fracture Characterization of Minerals and Mineral Interfaces in Granite Using Nanoindentation and Micro X-Ray Computed Tomography

An Insight into the Mechanical and Fracture Characterization of Minerals and Mineral Interfaces in Granite Using Nanoindentation and Micro X-Ray Computed Tomography

Describing the adsorption of PAM on coal/kaolinite surface in aquatic by combining experiments and MD simulation

Hydration is an important factor influencing the adsorption of reagent molecules at the mineral interface. Understanding the micro-scale characteristics of hydration and the molecular adsorption at the interface is fundamental to improving the effectiveness of solid–liquid separation. In this paper, the flocculation processes of coal sludge in water and in kaolinite solutions were investigated, and the surface properties of the settling materials were analysed using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that as the concentration of polyacrylamide (PAM) increased, the flocculation effect showed a trend of first increasing and then decreasing. The kaolinite particles covered the coal surface, reducing the exposed area of the coal and the amount of PAM molecules adsorbed. Under the same conditions, the settling time of particles in kaolinite solutions is 60% longer than that in water. Further, molecular dynamics (MD) simulation was used to study the distribution of water molecules on the surface of coal and kaolinite, and the micro-scale configurations of PAM at the coal-water and kaolinite-water interfaces were analysed. The results showed that the coal surface was weakly hydrated, and PAM molecules can replace some of the water molecules near the coal surface. Kaolinite surface was strongly hydrated and the maximum density of the hydration layer is close to that of the kaolinite. PAM cannot cross the hydration layer with three layers of water molecules at the aluminium-oxygen octahedra (001) surface of kaolinite. However, there was only one layer of water molecules between PAM and silicon-oxygen tetrahedra (00–1) surface. When the kaolinite covered the coal surface, it was difficult for the PAM to replace the water at the surface, the 00–1 surface was the key dissociation surface to promote flocculation. This study illustrated microscopic insights into the effect of mineral hydration on the adsorption of flocculant molecules and may enrich the theory of mineral flocculation and sedimentation.

Drilling fluid for stabilizing the wellbore in compound-salt formation based on multiple-synergism-method

The complex features of the compound-salt formation result in wellbore instability during drilling operations. The mechanism of wellbore instability in compound-salt formation is studied by the analysis of mineral compositions, microstructures, physical-chemical properties, and interface mechanical properties of compound salts from different areas. The results show that the existing micro-scale pores and fractures, creep and dissolution of salt, hydration of gypsum-mudstones are the key causes of wellbore instability in compound-salt formation. In order to stabilize the wellbore in compound-salt, we propose the drilling strategies based on the synergism of multiple methods of strengthening the wellbore by the sealing and consolidating effect of drilling fluid, inhibiting the dissolution of salt and the hydration of gypsum-mudstones, balancing the water activity between rocks and drilling fluid, providing effective stress to support the wellbore. Based on the effect of water activity on the shale hydration swelling and dispersion properties, the design basis of the safe window of water activity for drilling fluids is determined. According to the wellbore instability problems when drilling in compound-salt formations in M Oil Field in the Middle East, H-S Oil Field of Uzbekistan, and Xinjiang Uygur Autonomous Region of China, the drilling fluids are designed respectively based on the proposed wellbore stabilizing technology. The field applications show that the drilling fluid technology greatly improves the wellbore stability in compound salt formations and reduces downhole problems, thereby improving drilling efficiency.

A mechanism of ion exchange by interface-coupled dissolution-precipitation in the presence of an aqueous fluid

The equilibration of minerals in the presence of an aqueous fluid phase, with which it is out of equilibrium, has been described in terms of a number of potential mechanisms, specifically mainly either by solid state exchange of elements within the solid phase and the aqueous solution, or by interface-coupled dissolution-precipitation where equilibration is approached by the incremental dissolution of the parent solid and the coupled precipitation of a new solid. The conditions determining the equilibration mechanism can be defined by the specific chemical potential differences at the mineral interface, the kinetics of potential reactions, the solubility of the solid phase in the specific fluid and physical properties such as the mineral: fluid ratio as well as the surface area: fluid ratio, temperature and pressure. This review focusses on the mechanism of ion exchange in a range of solids by interface-coupled dissolution-precipitation in the presence of an aqueous solution.

Coronitic and Symplectitic Textures in Meta-Troctolite Reveal the Transition from Magmatism to Granulite-Facies Metamorphism in the Early Paleozoic Tongbai Orogen, Central China

AbstractNewly discovered meta-troctolite enclaves in Early Paleozoic Huanggang diorite within dominantly meta-volcaniclastic greenschist to amphibolite facies Erlangping unit in central China extends the known extent of granulite-facies metamorphism in the Tongbai Orogen. Evidence for this granulite facies event is preserved in coronitic and symplectitic textures developed in the meta-troctolites enclaves. The primary assemblage comprises olivine and plagioclase with minor orthopyroxene, clinopyroxene, spinel and amphibole. The late magmatic stage comprising an orthopyroxene (Opx1) corona around olivine and clinopyroxene corona around spinel formed by reaction of cumulus and intercumulus phases with trapped melt at around 1040–1060°C near the solidus. During troctolite cooling, metamorphic solid-state replacement of primary olivine and plagioclase produced orthopyroxene (Opx1)/clinopyroxene (Cpx1) + vermicular spinel (Sp1) symplectite, followed by an inclusion-free amphibole (Amp1) layer surrounded by amphibole + spinel symplectite (Amp2 + Sp2). Microstructural, pseudosection and mineral isopleth analysis coupled with prominent compositional zoning in the Opx1 corona and chrome-rich spinel (Cr-Sp) suggests that the symplectites formed by diffusion-controlled solid-state replacement at the interface of reactant minerals, probably during cooling to granulite-facies conditions of ~820–880°C and 6.7–7.0 kbar. U–Pb dates of rare metamorphic zircon, as indicated by growth zonation and identification of metamorphic minerals enclosed in grains by Raman probe, are ca. 450 Ma. The reaction textures record the transition from late magmatic to granulite-facies metamorphism with the heat supplied by magma upwelling at the base of Erlangping arc triggered by extension of Erlangping back-arc.

Effects of Microbial-Mineral Interactions on Organic Carbon Stabilization in a Ponderosa Pine Root Zone: A Micro-Scale Approach

Soil microbial communities affect the formation of micro-scale mineral-associated organic matter (MAOM) where complex processes, including adhesion, aggregate formation, microbial mineral weathering and soil organic matter stabilization occur in a narrow zone of large biogeochemical gradients. Here we designed a field study to examine carbon stabilization mechanisms by using in-growth mesh bags containing biotite that were placed in a ponderosa pine root zone for 6 months and compared to the surrounding bulk soil. We sought to determine the composition of the microbial community in the mesh bags compared to the surrounding soils, analyze the direct interactions between microbes and biotite, and finally identify the nature of the newly formed MAOM within the mesh-bags. Our results revealed that minerals in the mesh bags were colonized by a microbial community that produced organic matter in situ. The 16S rRNA gene sequencing and ITS2 region characterization showed phylogenetic similarity between the mesh bag and bulk soil archaea/bacteria and fungi microbiomes, with significant differences in alpha- and beta-diversity and species abundances. Organic matter pools in the mesh bags, analyzed by Fourier transform ion cyclotron resonance mass spectrometry, contained protein- (peptides) and lipid-like compounds while the bulk soil OM was comprised of lignin-like and carboxyl-rich alicyclic molecules. These results support that the newly formed biotite associated organic compounds have a microbial signature in the mesh bags. High-resolution electron microscopy documented strongly adhered organic compounds to biotite surfaces, formation of microaggregates, elemental uptake at the microbe (organic matter)-mineral interface, and distortion of biotite layers. Overall, this study shows the direct and indirect involvement of soil microbial communities from the root zone of ponderosa pine in the formation of MAOM, soil organic carbon stabilization, microaggregation, and mineral weathering at micro- and nano-scales.

Adsorption, Structure, and Dynamics of Short- and Long-Chain PFAS Molecules in Kaolinite: Molecular-Level Insights.

The ubiquitous presence of poly- and perfluoroalkyl substances (PFAS) in different natural settings poses a serious threat to environmental and human health. Soils and sediments represent one of the important exposure pathways of PFAS for humans and animals. With increasing bioaccumulation and mobility, it is extremely important to understand the interactions of PFAS molecules with the dominant constituents of soils such as clay minerals. This study reports for the first time the fundamental molecular-level insights into the adsorption, interfacial structure, and dynamics of short- and long-chain PFAS molecules at the water-saturated mesopores of kaolinite clay using classical molecular dynamics (MD) simulations. At environmental conditions, all the PFAS molecules are exclusively adsorbed near the hydroxyl surface of the kaolinite, irrespective of the terminal functional groups and metal cations. The interfacial adsorption structures and coordination environments of PFAS are strongly dependent on the nature of the functional groups and their hydrophobic chain length. The formation of large, aggregated clusters of long-chain PFAS at the hydroxyl surface of kaolinite is responsible for their restricted dynamics in comparison to short-chain PFAS molecules. Such comprehensive knowledge of PFAS at the clay mineral interface is critical to developing novel site-specific degradation and mitigation strategies.

Energy dissipation of osteopontin at a HAp mineral interface: Implications for bone biomechanics

Osteopontin (OPN) is a one of the most abundant non-collagenous proteins in the bone's organic matrix. OPN is responsible for mediating bonding at mineral interfaces in the extrafibrillar space and recent evidence shows that it is a major contributor to bone's fracture resistance. While several experimental studies have identified an important role for calcium ions in mediating energy dissipation in OPN protein networks, the underlying molecular mechanisms remain largely unknown. In the current study, the role of calcium ions on energy dissipation at OPN interface with hydroxyapatite (HAp) as the main bone mineral was investigated. For the first time, the three-dimensional structure of OPN proteins were predicted, and it was found that calcium ions greatly influenced the final protein configuration and energy dissipation performance. Under small deformation, the compact cOPN structure, resulting from calcium ions presence, facilitated greater energy dissipation through sacrificial bond breaking and mechanisms mediated by the surface-bound calcium. At larger deformation, the compact structure also enabled cOPN to dissipate higher energy. Moreover, it was found that phosphorylation of OPN played an important role in energy dissipation. While previous studies have shown that OPN dissipated energy by forming aggregate networks, this study also showed that network formation is not necessary and that individual OPN proteins can dissipate large amounts of energy at HAp interfaces.

The stoic tooth root: how the mineral and extracellular matrix counterbalance to keep aged dentin stable

Aging is a physiological process with profound impact on the biology and function of biosystems, including the human dentition. While resilient, human teeth undergo wear and disease, affecting overall physical, psychological, and social human health. However, the underlying mechanisms of tooth aging remain largely unknown. Root dentin is integral to tooth function in that it anchors and dissipates mechanical load stresses of the tooth-bone system. Here, we assess the viscoelastic behavior, composition, and ultrastructure of young and old root dentin using nano-dynamic mechanical analysis, micro-Raman spectroscopy, small angle X-ray scattering, atomic force and transmission electron microscopies. We find that the root dentin overall stiffness increases with age. Unlike other mineralized tissues and even coronal dentin, however, the ability of root dentin to dissipate energy during deformation does not decay with age. Using a deconstruction method to dissect the contribution of mineral and organic matrix, we find that the damping factor of the organic matrix does deteriorate. Compositional and ultrastructural analyses revealed higher mineral-to-matrix ratio, altered enzymatic and non-enzymatic collagen cross-linking, increased collagen d-spacing and fibril diameter, and decreased abundance of proteoglycans and sulfation pattern of glycosaminoglycans . Therefore, even in the absence of remodeling, the extracellular matrix of root dentin shares traits of aging with other tissues. To explain this discrepancy, we propose that altered matrix-mineral interactions, possibly mediated by carbonate ions sequestered at the mineral interface and/or altered glycosaminoglycans counteract the deleterious effects of aging on the structural components of the extracellular matrix. Statement of significanceGlobally, a quarter of the population will be over 65 years old by 2050. Because many will retain their dentition, it will become increasingly important to understand and manage how aging affects teeth. Dentin is integral to the protective, biomechanical, and regenerative features of teeth. Here, we demonstrate that older root dentin not only has altered mechanical properties, but shows characteristic shifts in mineralization, composition, and post-translational modifications of the matrix. This strongly suggests that there is a mechanistic link between mineral and matrix components to the biomechanical performance of aging dentin with implications for efforts to slow or even reverse the aging process.

Surface corrosion by microbial flora enhances the application potential of phosphate rock for cadmium remediation

Heavy metal pollution restricts the intensification of land and endangers human health through the food chain. Surface adsorption of heavy metals by soil natural minerals affects their mobility and biotoxicity, yet little is known about the fate of heavy metals after microorganisms affect the surface structure of minerals. Our study shows that ureolytic microbes corrode the surface of phosphate minerals through carbonate modification without changing their crystal structure but significantly enhancing their specific surface area. Compared with natural phosphate rock (PR), the concave surface of the flora-phosphate rock (FPR) exposes more Ca2+ exchange sites, more active functional groups, and more straightforward to release phosphates that co-precipitate with Cd2+, resulting in an approximately tenfold increase in Cd2+ adsorption. The adsorption of Cd2+ by FPR is a spontaneous endothermic single-layer adsorption, which can quickly remove Cd2+ in wastewater. FPR can also obviously change the pH of the soil by releasing phosphate to increase the stabilization effect of Cd2+. These results provide a sustainable approach to developing novel, cost-effective minerals for environmental remediation and contribute to understanding the function and role of microorganisms in immobilizing heavy metals at the mineral interface.

Mechanical Instability of Methane Hydrate-Mineral Interface Systems.

Massive methane hydrates occur on sediment matrices in nature. Therefore, sediment-based methane hydrate systems play an essential role in the society and hydrate community, including energy resources, global climate changes, and geohazards. However, a fundamental understanding of mechanical properties of methane hydrate-mineral interface systems is largely limited due to insufficient experimental techniques. Herein, by using large-scale molecular simulations, we show that the mechanical properties of methane hydrate-mineral (silica, kaolinite, and Wyoming-type montmorillonite) interface systems are strongly dictated by the chemical components of sedimentary minerals that determine interfacial microstructures between methane hydrates and minerals. The tensile strengths of hydrate-mineral systems are found to decrease following the order of Wyoming-type montmorillonite- > silica- > kaolinite-based methane hydrate systems, all of which show a brittle failure at the interface between methane hydrates and minerals under tension. In contrast, upon compression, methane hydrates decompose into water and methane molecules, resulting from a large strain-induced mechanical instability. In particular, the failure of Wyoming-type montmorillonite-based methane hydrate systems under compression is characterized by a sudden decrease in the compressive stress at a strain of around 0.23, distinguishing it from those of silica- and kaolinite-based methane hydrate systems under compression. Our findings thus provide a molecular insight into the potential mechanisms of mechanical instability of gas hydrate-bearing sediment systems on Earth.

Competitive kinetics of Ni(II)/Co(II) and Cr(VI)/P(V) adsorption and desorption on goethite: A unified thermodynamically based model

Predicting the dynamic behavior of coexisting ions on mineral interface is essential to understanding their lability in soil matrix, but a mechanical kinetic model for predicting competitive adsorption is lacking. In this study, the thermodynamic and kinetic adsorption behaviors of Ni(II), Co(II), Cr(VI), and P(V) on goethite under various condition were investigated by batch and stirred-flow experiments, respectively. The equilibrium model CD-MUSIC was developed to describe their equilibrium behavior, followed by the development of a multi-rate kinetic model constrained by the equilibrium model to describe their kinetic behavior. Ni(II) and Co(II) exhibit similar adsorption affinities, while the adsorption of P(V) was stronger and faster than that of Cr(VI). The two surface species of Cr(VI) and P(V) differed in dynamic features, a finding confirmed by in-situ ATR-FTIR spectroscopy. The kinetic model was successfully used to predict the binary competitive adsorption of Ni(II)-Co(II) and Cr(VI)-P(V), and especially the overshooting of Cr(VI) induced by P(V). Our results showed that an integrated thermodynamic-kinetic model obtained from a single-ion experiment can be extended to describe complex multi-ion interactions, indicating the robustness and scalability of the model’s parameters. This approach can be used to construct more comprehensive equilibrium and dynamic models of the actual soil environment.

Intrinsically Disordered Osteopontin Fragment Orders During Interfacial Calcium Oxalate Mineralization.

Calcium oxalate (CaC2O4) is the major component of kidney stone. The acidic osteopontin (OPN) protein in human urine can effectively inhibit the growth of CaC2O4 crystals, thereby acting as a potent stone preventer. Previous studies in bulk solution all attest to the importance of binding and recognition of OPN at the CaC2O4 mineral surface, yet molecular level insights into the active interface during CaC2O4 mineralization are still lacking. Here, we probe the structure of the central OPN fragment and its interaction with Ca2+ and CaC2O4 at the water–air interface using surface‐specific non‐linear vibrational spectroscopy. While OPN peptides remain largely disordered in solution, our results reveal that the bidentate binding of Ca2+ ions refold the interfacial peptides into well‐ordered and assembled β‐turn motifs. One critical intermediate directs mineralization by releasing structural freedom of backbone and binding side chains. These insights into the mineral interface are crucial for understanding the pathological development of kidney stones and possibly relevant for calcium oxalate biomineralization in general.

Pyrophyllite dissolution at elevated pressure conditions: An ab initio study

The atomistic mechanism of dissolution reactions at mineral fluid interface is essential for understanding the reaction kinetics and the adsorption and thermodynamic equilibria at mineral surfaces. In this study we investigate the initial state of the dissolution process on (0 1 0) surface of pyrophyllite at elevated pressure by applying large-scale ab initio Meta-Dynamics simulations. The system setup provides a realistic representation of the clay mineral edge surface structure and takes into account explicit dynamics of solvent molecules. The model parameters follow the procedure recently tested for the simulation of the (1 1 0) pyrophyllite edge. The simulation reveals that dissolution of a single tetrahedral or an octahedral unit from the clay mineral edge is a complex multi-step process with several reaction intermediates. Typically, each reaction step changes the denticity of the reacting site in a step-by-step manner and leads, eventually, to the leaching of the ions forming the octahedral and tetrahedral sheets of the phyllosilicate. The solvent rearrangement and the proton transfer reactions in the first and second coordination shell of the dissolving unit play a critical role in the stabilization of reaction intermediates and the net progress of the dissolution reactions. In contrast to ambient conditions, the transition complexes formed during the dissolution at elevated conditions were found to have an increased coordination number. The simulation reveals new insight into the coordination environment of the dissolving complexes at the mineral fluid interface.

Interfacial Molecular Fractionation Induces Preferential Protection of Biorefractory Organic Matter by Ferrihydrite

Minerals are known to protect dissolved organic matter (DOM) against biodegradation and thus play an important role in the long-term persistence of organic carbon (OC) in soils. However, as a super-mixture of organic compounds, it is expected that not all components in DOM can be protected by minerals due to selective adsorption. The present study examined how molecular fractionation of plant-derived DOM at ferrihydrite–water interface affects their biodegradation. The results of Fourier transform ion cyclotron resonance mass spectrometry analysis showed that highly unsaturated and oxygenated moieties in DOM displayed high affinities, while highly saturated compounds rich in aliphatic groups exhibited low affinities to ferrihydrite, resulting in the molecular fractionation of DOM at the water–mineral interface. Comparison of the biodegradation of DOM extracted from different phases after adsorption on ferrihydrite indicated that compounds with lower affinities to ferrihydrite had higher degradation degrees, while compounds with higher affinities to ferrihydrite had lower degradation degrees. Considering the protective role of minerals on adsorbed OC, molecular fractionation at the ferrihydrite–water interface leads to inherently refractory OC being more stable, while labile OC is more easily degraded in soils.

Experimental investigation of the reactions between pyrite and aqueous Cu(I) chloride solution at 100–250 °C

Replacement of pyrite by copper-bearing sulfides is widespread in mineral deposits, with zonation of chalcocite, bornite, and chalcopyrite being a common result of this process. Copper occurs predominantly as Cu(I) in hydrothermal fluids, but the mechanistic details of reactions between pyrite and aqueous Cu(I) and their relevance to the zonation of Cu sulfides in hydrothermal deposits remain poorly understood, because of a lack of relevant experimental constraints. In this study, we investigated the reactions between pyrite and aqueous Cu(I) chloride solutions over a wide range of physicochemical conditions (100–250 °C; pH = 1–8; saturated vapor pressure). The experimental results show that Cu-bearing sulfides form at temperatures of >150 °C, and that pH exerts an important control on the mineralogy of the reaction products. In strongly acidic conditions (pH = 1.1), pyrite dissolution occurs; in acidic-to-neutral conditions (pH = 3.0–6.0), chalcopyrite and bornite are the main products; in weakly alkaline conditions (pH = 8.2), the replacement products are chalcocite and bornite. Chalcopyrite, bornite, and chalcocite generally occur in a zoned sequence outward from the pyrite reaction front to the bulk solution, and the replacement of pyrite by chalcopyrite is heterogeneous. In situ Raman observation and S isotope analyses suggest that pyrite replacement by chalcopyrite is a redox-coupled dissolution–precipitation reaction, with the oxidative dissolution of pyrite to form sulfate, generating the electrons that maintain the reductive replacement of pyrite by chalcopyrite. The sites of oxidative dissolution and reductive replacement can be spatially separated, but coupled by electron transfer. By contrast, the further replacement of chalcopyrite by bornite and chalcocite does not involve the oxidative dissolution of sulfur, but release of iron and H+, which could be promoted at higher pH. Notably, when the zonation of sulfides is established in a hydrothermal experiment of pyrite replacement by Cu(I) solution, gradients of H2S (or HS–), Cu(I), and Fe(II) are developed between the pyrite reaction front and bulk solution via diffusion through interstitial pore space. As such, the inner zones of chalcopyrite and bornite are not in equilibrium with the bulk solution as confirmed by thermodynamic calculations. The results imply that the zonation of Cu-bearing sulfides in mineral deposits records heterogeneous chemical environments, from the mineral interface- to the ore deposit-scale during dynamic mineralization processes.