Bulk Pyrite Research Articles

Platinum-group elements (PGE) in the New Afton alkalic Cu-Au porphyry deposit, Canadian Cordillera, I: relationships between PGE, accessory metals and sulfur isotopes in pyrite

The second part of this article can be found here: 10.3389/feart.2023.819109 (DOI). At the late Triassic New Afton alkalic porphyry Cu-Au deposit (British Columbia, Canada), pyrite is a widely distributed minor sulfide phase within hypogene ore where it predates Cu mineralization and hosts significant concentrations of Pd and Pt. Here we characterize pyrite major, minor and trace element composition by EPMA and LA-ICP-MS, and S isotopes (bulk pyrite and in situ SIMS in individual growth zones) to elucidate compositional variations at different stages of pyrite growth with respect to PGE deposition. At least two cycles of zoned Co-Ni-Pd-Pt-Se-As co-enrichment are recorded over two stages of pyrite growth at the New Afton deposit. Concentrations of Co (up to ∼5.5 wt%; highest observed in any reported ore-forming system) and Ni (up to 1 wt%) overlap with pyrite from mafic-ultramafic platinum-group element (PGE) deposits, iron oxide±apatite and iron oxide-copper-gold deposits (IOA-IOCG), and mantle peridotite-associated base metal exhalative deposits. In early hypogene (type I) pyrite, high Pt (up to ∼24 ppm) occurs in crystal cores that have high Co/Ni ratio (>∼7), high Co (>∼ 1 wt%) and are poor in Ni, Se, and As. With progressive growth, early hypogene pyrite rims and late hypogene (type II) cores record an initial Ni-Pd-As-Se (±Co) co-enrichment stage, followed by oscillations in composition (from “barren” to variably Co-Ni-Pd-Pt-As-Se-enriched). Pd in pyrite (up to ∼70 ppm) is inversely correlated to Co/Ni ratio, being enriched when Co/Ni < ∼7 and Ni > ∼1000 ppm. The highest levels of Pd enrichment occur in the most Ni- and Se-enriched growth zones. The transition from early, high Co/Ni (Pt-enriched) to later, low Co/Ni (Pd-enriched) growth zones is accompanied by a decrease in pyrite δ34SVCDT of up to ∼7‰ (4‰ range in single grains) with a total range in composition measured between −5.5‰ and +1.4‰. Subsequent to the shift to lower values, overgrowths of high Co/Ni pyrite formed with values of δ34SVCDT similar to the earliest Co-Pt-rich growth zones. Some combination of fluctuations in temperature and oxygen fugacity related to episodic cooling and hydrothermal recharge involving new pulses of metal-rich magmatic fluids is required to explain the observed metal enrichment patterns and variations in S isotope values. Co-Ni-rich pyrite may be a valuable exploration vector to PGE enrichment in porphyry deposits.

Stability of NaO2 and LiO2 discharge products in metal-air batteries: density functional theory study

Rechargeable metal-air batteries are considered the next generation energy storage devices with the highest energy density of ~200 Wh/kg (which is much higher than the Li-ion batteries with ~150 Wh/kg). However, their practical applications are widely affected by the formation of unstable discharge products (like Na/LiO2, Na/Li2O, and Na/LiO) which react with electrolyte or the porous electrode material leading to either fire or explosion. Here, we explore the stability of the bulk pyrite and marcasite lithium/sodium superoxide (Li/NaO2) discharge products produced in Li and Na-air batteries using density functional theory calculations. The calculated lattice and heats of formations are consistent with available reported data. The electronic properties of show the Pnmm LiO2 as the most stable structure with the least density of states at the Fermi. The vibrational properties also show no imaginary vibrations in all directions for Pnmm LiO2. This finding can help direct and give an insight into the stability of major discharge products and give research direction towards controlling the formation of desired M-O discharge products in the batteries.

Testing the global significance of the sulfur isotope record of the ca. 2.0 Ga Zaonega Formation: A micro-scale S isotope investigation

The Paleoproterozoic metasedimentary rocks of the Zaonega Formation (Onega Basin, NW Russia) are important archives of inferred global environmental change following the initial oxygenation of the Earth’s atmosphere and oceans. However, the geochemical signals preserved in these exceptionally organic-and pyrite-rich metasedimentary rocks and their environmental meaning remain contested. In particular, the Zaonega Formation’s unusually high pyrite sulfur isotope ratios (δ34S) have been explained by either global or local forcings acting on sulfur cycling processes. We tested former interpretations of the Zaonega Formation’s sedimentary pyrite record by integrating bulk and micro-scale δ34S analysis to discriminate the isotopic signatures of different generations of pyrite and determine the underlying mechanisms contributing to δ34S variability. We show that the prolonged genesis of pyrite occurred via multiple stages and included precipitation from early diagenetic fluids, organic matter pyritization, and late-stage alteration fluids. Our results demonstrate that early-stage pyrite typically carries more variable and lower δ34S values than late-stage pyrite. Although the early pyrite captures pore water S isotope signatures least evolved from the seawater, their contribution to the bulk δ34S results can be dwarfed by the greater volume of late-stage coarse pyrite. Consequently, determining the sequence of pyrite precipitation and δ34S characteristics of individual generations in any given sample are fundamental to interpreting bulk δ34S records. Our micro-scale results suggest that previous estimates based on bulk pyrite data (ca. 6 to 18‰) should not be related to the original seawater sulfate’s isotopic composition. These results demonstrate that a thorough understanding of the geological context and mechanisms associated with S-cycling, and pyrite formation is necessary to interpret bulk δ34S records accurately.

Multiple sulfur isotopes discriminate organoclastic and methane-based sulfate reduction by sub-seafloor pyrite formation

The marine sulfate inventory represents the largest standing pool of electron acceptors, which, via microbial sulfate reduction, is responsible for roughly half of the organic matter mineralization globally in marine sediments. In addition to the oxidation of buried organic matter, sulfate reduction can be coupled to the oxidation of methane migrating upward. Multiple sulfur isotope ratios were measured for porewater sulfate, sulfide, elemental sulfur and pyrite from core samples collected from continental shelf (19–96 m water depth) in the Baltic Sea and slope (1098 m water depth) in the Andaman Sea, in order to test if their isotope systematics can be used to trace the two different sulfate reduction processes. For shallow shelf sediments (<43 cm below seafloor), both δ34S and Δ33S values of porewater sulfate increased with increasing depth as a result of organoclastic sulfate reduction (OSR). However, the Δ33S values of both porewater sulfate and sulfide decreased as the δ34S values increased downcore at 43–73 cm depth where sulfate reduction was coupled to anaerobic oxidation of methane (AOM). Pyrite in shelf sediments yielded positive Δ33S values, reflecting Δ33S of porewater sulfide produced by OSR. In contrast, pyrite in slope sediments yielded negative Δ33S values as low as −0.15‰, as a result of mixing of sulfides derived from OSR and AOM. This is consistent with the slowly migrating sulfate–methane transition (SMT) in slope sediments inferred from sulfur and iron speciation, carbon isotope ratios of carbonates, and magnetic susceptibility data. A stable SMT, focusing AOM at roughly the same depth over the past few millennia, further provides favorable conditions for prolonged pyrite formation with the development of cubic crystals, while the sulfur isotope difference between bulk pyrite and macroscopic pyrite reflects the significance of OSR throughout the sediment column. We conclude that minor sulfur isotopes provide a unique proxy that can be applied to differentiate OSR and AOM and the origin of pyrite in diverse sediments and sedimentary rocks. This contribution highlights strong local diagenetic controls on pyrite formation and its multiple sulfur isotopic composition in various marine settings.

Rapid preparation and characterization of pyrite materials under HPHT: A new method

This paper reports the synthesis of bulk pyrite (FeS2) polycrystals at high pressure and high temperature (HPHT) for the first time. The HPHT method markedly improves the speed of synthesis, reducing the time for synthesizing bulk pyrite from several hundred hours to several hours. A unique honeycomb-type structure was introduced into the system by the primary reaction to refine the size of the reactants. On this basis, the sulfur-deficient contamination phase Fe7S8 was effectively eliminated by secondary synthesis. Using Raman spectroscopy and X-ray diffraction (XRD), the crystalline phase of the product was confirmed to be the cubic pyrite phase. The crystal structure and initial crystal parameters of pyrite crystals synthesized under HPHT conditions were obtained for the first time by Rietveld refinement with GASA software. In combination with a scanning electron microscope (SEM) assay, the reaction process was traced, and the significance of secondary synthesis for HPHT was analyzed. Seebeck data showed that as the temperature increased, the sample changed from p-type to an n-type semiconductor and the conductivity increased monotonically, revealing the typical semiconductor characteristics of pyrite. UV reflectance spectra and VSM tests revealed the bandgap width and magnetic properties of pyrite.

Preparation and densification of bulk pyrite, FeS2

A favourable electronic structure, together with a high abundance of pyrite (FeS2) in the Earth's crust, make this sulfide a promising material for thermoelectric applications. However, the experimental work published to date underlines a challenging scenario, with FeS2 presenting systematically small power factors. Knowing that sulfur vacancies are a key aspect for controlling the electronic transport properties of FeS2, this work focuses on the preparation of pyrite by hot-pressing and entails a careful characterization of its microstructure, chemical composition, thermal stability, and electrical transport properties. Dense pellets (>90% of the theoretical density) were obtained, consisting mostly of pyrite with trace amounts of Fe7S8. Different hot-pressing conditions led to similar chemical compositions, with no significant lattice distortions. Pyrite proved to be thermally stable up to 400 °C under an inert atmosphere. Analysis of the electronic band structure and density of states by density functional theory implies a p-type semiconductivity. However, measurements of Seebeck coefficient indicated n-type semiconductivity for pyrite produced by this experimental procedure. The highest power factor measured in this work, 0.23 μW K−2 m−1, is still far from what can be considered a promising value for thermoelectric materials, but different ways of improving it are discussed.

Mechanism of pyrite oxidation in copper(II)-ethylenediamine-thiosulphate gold leaching system

Although the Cu2+–ethylenediamine–S2O32− system is a promising alternative to cyanide-based gold leaching systems because of its low cost, nontoxicity, and environmental friendliness, its performance is strongly affected by the presence of pyrite. Therefore, a deep understanding of the mechanism of pyrite oxidation in this system is of high practical importance. Herein, this mechanism was probed to demonstrate the occurrence of rapid reactions on the pyrite surface at high S2O32− concentrations and reveal that the dissolution of bulk pyrite was most favoured at a certain optimal S2O32− concentration, i.e., S2O32− ions strongly affected the surface properties of pyrite. Anodic processes occurring in active, passive, and transpassive dissolution regions were studied by linear sweep voltammetry, with additional insights into the properties of pyrite electrodes and the related electrochemical reactions obtained by electrochemical impedance spectroscopy and equivalent circuit fitting. With increasing potential at range of 0.20 to 0.80 V, the reactions on the pyrite surface accelerated, and its roughness significantly increased. The products of pyrite surface oxidation were identified by Raman spectroscopy and X-ray photoelectron spectroscopy, and a corresponding mechanism was proposed. Thus, this work provides a theoretical basis for the regulation and control of gold leaching reagents in the Cu2+–ethylenediamine–S2O32− system.

Hydrothermal Monodisperse Microspherulite Pyrite: Novel Synthesis Process and Electrochemical Study of Its Oxidation.

A simple one-step hydrothermal method was developed to synthesize pyrite (FeS2) sheet- and bulklike pyrite mineral. The Fe/S molar ratio determines the phase of FeS2, including pyrite and marcasite. The reaction temperature and time are key factors to regulate the structure, morphology, and size of pyrite. Scanning electron and transmission electron microscopy showed the formation of monodisperse microspherulite within 1 h reaction time, and the particles aggregated to large irregular polyhedron particles with increasing reaction time up to 4 h. Electrochemical oxidation tests demonstrated that their electrochemical activity significantly decreased with increasing synthesis time. At an elevated temperature of 200 °C, bulk pyrite was obtained after a 24 h reaction time, which could have promising applications in hydrothermal pyrite ore oxidation research.

A simple method for the precise determination of multi-elements in pyrite and magnetite by ICP-MS and ICP-OES with matrix removal

A simple, economical, and rapid chemical method was developed for the precise measurement of major and trace elements in bulk pyrite and magnetite. This method, which required only 10–20 mg of sample, used inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES) with a polyurethane foam (PUF)-based matrix removal step. Interference derived from the iron-rich mineral matrix was reduced by the removal of iron from the sample solution using 0.18 g PUF in 6 M HCl. The extraction conditions (e.g., temperature, shaking time, and initial iron concentration) were intensively investigated to achieve high iron separation efficiency. An iron removal rate of 92.7 ± 0.7% (n = 3, SD) and recoveries of 97–103% for 17 trace elements were obtained under the optimized separation conditions. To evaluate the feasibility of our method, reference materials for pyrite (GBW07267) and iron formation (FER-1 and FER-2) were analyzed. Our results were consistent with the certified values or literature values, and the precision was comparable to the certified uncertainty or that of previous methods in which 100 mg of sample powder was digested. These results confirmed that this novel method is an efficient technique for the precise measurement of multi-elements in pyrite and magnetite. Relative to other methods, our protocol has great potential for analyzing a wide variety of trace elements in small amounts (<10 mg) of iron-rich samples with applicability in geochemistry, cosmochemistry, and environmental studies.

A novel antiferromagnetic semiconductor hidden in pyrite

In this paper, a new two-dimensional antiferromagnetic material, 2D pyrite, was reported, which exhibited a completely planar pentagonal structure. Pentagonal FeS2 were revealed by the first-principles calculation with GGA + U (Ueff = 2 eV) method and proved excellent mechanically stable, dynamically stable and thermally stable. Intriguingly, this single-layer structure was perfectly in line with the geometric characteristic of “Cairo pentagon”. Completely different from the bulk pyrite, the two-dimensional FeS2 was proved an amazing magnetic semiconductor and the ground state configuration of the magnetic coupling is antiferromagnetic with a 0.31 eV band gap. Considering the spin–orbit coupling effect, the magnetocrystalline anisotropy energy (MAE) in different orientation was calculated and the easy axis was proved along the c axis. The highest MAE in our GGA + U calculation could reach 454.77 μeV per Fe atom. Furthermore, biaxial strain was applied to regulate the electronic structure of 2D FeS2 and the results suggested that 2D pyrite would be converted into ferromagnetic configuration when the strain ranging from 2.2% to 5.0%. Therefore, our research predicted a new two-dimensional spintronic materials and proved a new thought for the application of pyrite.

In situ observations of thermally induced phase transformations in iron sulfide nanoparticles

Iron pyrite (FeS2) exhibits dynamic thermal stability and represents an ideal model system to probe complex phase transformations in materials. At elevated temperatures, the sublimation of S atoms from the FeS2 lattice results in a phase transformation to one of the many polymorphic forms of pyrrhotite, i.e., from Fe1-XS (where 0 ≤ X < 0.2) to stoichiometric FeS. The complex nature of this phase transformation remains relatively unexplored at the nanoscale. Here, we use in situ transmission electron microscopy (TEM) and in situ X-ray diffraction (XRD) to observe the phase transformation of ~150 nm pyrite nanoparticles to pyrrhotite in vacuum. Although the overall shape of the nanoparticles remains cubic, the crystal structure of the nanoparticles changes from cubic pyrite to hexagonal pyrrhotite at ~400–450oC, which is ~100–150oC lower than reported values of bulk pyrite. Interestingly, our in situ observations do not evidence a core-shell transformation model which has been reported for the phase transformation of bulk pyrite to pyrrhotite, indicating the role of shorter length scales for diffusion in nanoparticles, as well as the role of S vacancies that facilitate faster atomic diffusion and rearrangements. In addition, the heating ramp rate was found to influence the phase transition temperature with lower temperatures of transformation seen for higher heating rates indicating the role of kinetic effects on phase transformation. We expect our detailed insights will help advance the use of FeS2 nanoparticles in high-temperature applications including catalysis and high-temperature batteries.

Hydrogenation of Functionalized Nitroarenes Catalyzed by Single-Phase Pyrite FeS2 Nanoparticles on N,S-Codoped Porous Carbon.

Catalytic hydrogenation of nitroarenes is an industrially very important and environmentally friendly process for the production of anilines; however, highly chemoselective reduction of nitroarenes decorated with one or more reducible groups in a nitroarene molecule remains a challenge. Herein, a novel hybrid non-noble iron-based nanocatalyst (named as FeS2 /NSC) was developed, which was prepared from biomass as C and N source together with inexpensive Fe(NO3 )3 as Fe source through high-temperature pyrolysis in a straightforward and cost-effective procedure. Comprehensive characterization revealed that single-phase pyrite FeS2 nanoparticles with precisely defined composition and uniform size were homogeneously dispersed on N,S-codoped porous carbon with large specific surface area, hierarchical porous channels, and high pore volume. The resultant catalyst FeS2 /NSC demonstrated good catalytic activity for hydrogenation of functionalized nitroarenes with good tolerance of various functional groups in water as a sustainable and green solvent. Compared with bulk pyrite FeS2 and other non-noble metal-based heterogeneous catalysts reported in the literature, a remarkably enhanced activity was observed under mild reaction conditions. More importantly, FeS2 /NSC displayed exclusive chemoselectivity for the reduction of nitro groups for nitroarenes bearing varying readily reducible groups.

Transport Evidence for Sulfur Vacancies as the Origin of Unintentional n-Type Doping in Pyrite FeS2.

Pyrite FeS2 has long been considered a potential earth-abundant low-cost photovoltaic material for thin-film solar cells but has been plagued by low power conversion efficiencies and open-circuit voltages. Recent efforts have identified a lack of understanding and control of doping, as well as uncontrolled surface conduction, as key roadblocks to the development of pyrite photovoltaics. In particular, while n-type bulk behavior in unintentionally doped single crystals and thin films is speculated to arise from sulfur vacancies (VS), proof remains elusive. Here, we provide strong evidence, from extensive electronic transport measurements on high-quality crystals, that VS are deep donors in bulk pyrite. Otherwise identical crystals grown via chemical vapor transport under varied S vapor pressures are thoroughly characterized structurally and chemically, and shown to exhibit systematically different electronic transport. Decreased S vapor pressure during growth leads to reduced bulk resistivity, increased bulk Hall electron density, reduced transport activation energy, onset of positive temperature coefficient of resistivity, and approach to an insulator-metal transition, all as would be expected from increased VS donor density. Impurity analyses show that these trends are uncorrelated with metal impurity concentration and that extracted donor densities significantly exceedtotal impurity concentrations, directly evidencing a native defect. Well-controlled, wide-range n-doping of pyrite is thus achieved via the control of VS concentration, with substantial implications for photovoltaic and other applications. The location of the VS state within the gap, the influence of specific impurities,unusual aspects to the insulator-metal transition, and the influence of doping on surface conduction are also discussed.

Vanadium: prices reach the highest level since Sep 2008

The Jiaodong gold district of eastern China, the largest gold producing district in China, is located on the eastern margin of the North China Craton. It consists of three mineralisation belts: the western Zhao-Ye belt, the middle Qixia belt, and the eastern Muping–Rushan (Muru) belt. Over 85% of mineralisation is hosted in the Zhao-Ye belt, which is bordered by the mantle-tapping Tan Lu fault zone. Pyrite crystals from three deposits in the Zhao-Ye belt and three deposits in the Muru belt were studied using a combination of optical petrography, bulk pyrite geochemistry, and in-situ laser ablation ICP-MS. Results show that although mineralisation is broadly similar between the two belts, there are significant differences in ore and gangue mineral textures, pyrite geochemistry, and style of gold mineralisation.Texturally, pyrite grains from the Zhao-Ye belt are generally cubic and do not exhibit zoning. In contrast, Muru pyrite grains are more often pyritohedral, commonly exhibit well-defined concentric zoning, and display textures in ore and gangue minerals indicative of open space growth. Bulk pyrite geochemistry suggests a distinct enrichment in Pb, Bi, Au, Ag and Te in the Zhao-Ye belt, whereas the Muru belt pyrite is significantly enriched in As, Cu and Co. In situ pyrite geochemistry indicates that Au and As are variably correlated in the Zhao-Ye belt, typically only exhibiting correlation at low Au concentrations. Most gold occurs as visible electrum along pyrite fractures and grain boundaries, with a minor generation of invisible gold formed through As-facilitated uptake into pyrite. In the Muru belt, Au and As have a strong correlation and there is limited occurrence of gold particles, indicating that most gold in the Muru belt is invisible gold contained in the crystal structure of As-rich pyrite.The differences in style of gold mineralisation between the belts indicates an inherent difference in timing of gold introduction: in the Zhao-Ye belt, the visible electrum accounting for most of the gold endowment is formed post-pyrite, whereas the invisible gold in the Zhao-Ye and Muru belts is formed syn-pyrite. The heterogeneity in gold distribution in the Jiaodong district is attributed to melting of metallogenically fertile Archean crust at the base of the well-endowed Zhao-Ye belt, and the lack of a similarly fertile source region beneath the Muru belt.

Chemical Forms of Mercury in Pyrite: Implications for Predicting Mercury Releases in Acid Mine Drainage Settings.

Pyrite (cubic FeS2) is the most abundant metal sulfide in nature and also the main host mineral of toxic mercury (Hg). Release of mercury in acid mine drainage resulting from the oxidative dissolution of pyrite in coal and ore and rock resulting from mining, processing, waste management, reclamation, and large construction activities is an ongoing environmental challenge. The fate of mercury depends on its chemical forms at the point source, which in turn depends on how it occurs in pyrite. Here, we show that pyrite in coal, sedimentary rocks, and hydrothermal ore deposits can host varying structural forms of Hg which can be identified with high energy-resolution XANES (HR-XANES) spectroscopy. Nominally divalent Hg is incorporated at the Fe site in pyrite from coal and at a marcasite-type Fe site in pyrite from sedimentary rocks. Distinction of the two Hg bonding environments offers a mean to detect microscopic marcasite inclusions (orthorhombic FeS2) in bulk pyrite. In epigenetic pyrite from Carlin-type Au deposit, up to 55 ± 6 at. % of the total Hg occurs as metacinnabar nanoparticles (β-HgSNP), with the remainder being substitutional at the Fe site. Pyritic mercury from Idrija-type Hg deposit (α-HgS ore) is partly divalent and substitutional and partly reduced into elemental form (liquid). Divalent mercury ions, mercury sulfide nanoparticles, and elemental mercury released by the oxidation of pyrite in acid mine drainage settings would have different environmental pathways. Our results could find important applications for designing control strategies of mercury released to land and water in mine-impacted watersheds.

Inflexible stoichiometry in bulk pyrite FeS2 as viewed by in situ and high-resolution X-ray diffraction.

Non-stoichiometry is considered to be one of the main problems limiting iron pyrite, FeS2, as a photovoltaic absorber material. Although some historical diffraction experiments have implied a large solubility range of FeS2-δ with δ up to 0.25, the current consensus based on calculated formation energies of intrinsic defects has lent support to line-compound behavior. Here it is shown that pyrite stoichiometry is relatively inflexible in both reductive conditions and in autogenous sulfur partial pressure, which produces samples with precise stoichiometry of FeS2 even at different Fe/S ratios. By properly standardizing in situ gas-flow X-ray diffraction measurements, no significant changes in the lattice parameter of FeS2 can be resolved, which portrays iron pyrite as prone to forming sulfur-deficient compounds, but not intrinsic defects in the manner of NiS2-δ.

The effect of temperature on the kinetics of the ferric-ferrous redox couple on pyrite

This article presents two methods for the study of ferric-ferrous reaction kinetics on the surface of an electro-active electrode. The working electrode in this study was a high quality bulk pyrite mineral, and the experimental results have been collected in a 0.5 M sulfuric acid solution within a temperature range of 22 to 65 °C. Two electrochemical methods, based on mixed potential theory, are introduced to isolate the Fe(II)/Fe(III) reaction from the other interfering reactions of pyrite oxidation and oxygen reduction, and to study its kinetics. The two methods use chronoamperometry or cyclic voltammetry for the kinetics analysis. By subtracting the current density of pyrite oxidation in an Fe-free acidic solution from that in an Fe-containing solution the Fe(II)/Fe(III) kinetics were isolated and studied.Tafel behavior was established by both chronoamperometry and cyclic voltammetry to analyze the kinetics of the Fe(II)/Fe(III) redox reactions. The results showed that the exchange current densities increased slightly with temperature, however they remained in the order of 10−5 A cm−2. The activation energy for Fe(II) oxidation and Fe(III) reduction on the pyrite surfaces was calculated to be 102.0 and 32.7 kJ mol−1, respectively. Anodic transfer coefficients also increased by increasing temperature from 0.24 to 0.39. Electrochemical impedance spectroscopy measurements showed a typical two-step pyrite oxidation process. The EIS measurement under different temperature showed that the predominant reaction in an Fe-free solution is pyrite oxidation, which has a large Rct value, starting from 4200 Ω at 22 °C and decreasing to 1490 Ω at 65 °C. Meanwhile the Rct values in an Fe-containing solution reduces from 1760 to 511 Ω by increasing the temperature from 22 °C to 65 °C, as the Fe(II)/Fe(III) redox reaction is the predominant reaction. The diffusion coefficients of Fe(II) during pyrite polarization was related to the Warburg impedance and calculated to be 1.9×10−6 cm2 S−1 at 22 °C and increases to 7.1×10−6 cm2 S−1 at 65 °C.

Green synthesis of pyrite nanoparticles for energy conversion and storage: a spectroscopic investigation

Pyrite, FeS2, nanoparticles were obtained through a one-pot solvothermal synthesis, without surfactants, carried out at room pressure and mild temperature (≈180°C). A thorough characterization of the products was thus performed, including scanning-electron-microscope micromorphology, X-ray diffraction (XRD), diffuse reflectance spectroscopy, and X-ray absorption spectroscopy (both in the XANES and EXAFS regions). Monophasic pyrite products are obtained as aggregates having an approximate dimension of few hundreds of nm. The Scherrer analysis of the Bragg reflections suggests that these aggregates are clusters of smaller units, having a mean size of ~25 nm. The XRD measurements point to a small but significant increase of the Fe–S bond distance (+1.7 %) with respect to reference data. The optical behavior of the pyrite nanoparticles is indistinguishable from that of the bulk pyrite. These results point to the one-pot synthesis as an efficient and “green” way of obtaining pyrite nanoparticles exhibiting the same technological properties as bulk pyrite. Indeed these nanoparticles can be considered as a product viable for numerous technological applications in the solar-energy conversion and storage fields.

Iron isotope investigation of hydrothermal and sedimentary pyrite and their aqueous dissolution products

Oxidative dissolution experiments were carried out on pyrite from multiple petrogenetic environments (hydrothermal, sedimentary, and coal-related nodules) to investigate possible variations in the iron isotopic composition of pyrite and the products of pyrite dissolution. The experimental materials were leached under carefully controlled abiotic conditions, and a subset of leachates and starting materials from these experiments was analyzed for 56Fe/54Fe by multicollector ICP-MS. Bulk pyrite δ56Fe values (relative to IRMM-014) ranged from −0.1 to +1.3‰, with hydrothermal bulk pyrite values <+0.5‰ and those of coal and sedimentary nodular pyrite ≥+0.5‰, higher than most previously measured values for Phanerozoic sedimentary pyrite. We suggest that this reflects precipitation of coal pyrite from a high-δ56Fe continental source, such as Fe derived from dissolution of Fe(III) oxides. This interpretation is consistent with pyrite rare earth element (REE) patterns. This could allow differentiation of Fe contributed from coal- and shale-related pyrite at abandoned mine drainage sites. Leachates from oxidative dissolution of the pyrite at pH=3 yielded, with few exceptions, δ56Fe values equal to or lower than those of the coexisting bulk pyrite, by up to about 1‰. These shifts are consistent in direction (but not magnitude) with equilibrium isotope fractionation predictions from theory, with possible second order effects from isotopic heterogeneity within individual natural pyrite samples.

Characteristics of authigenic pyrite and its sulfur isotopes influenced by methane seep at Core A, Site 79 of the middle Okinawa Trough

The anaerobic oxidation of methane (AOM) has strongly developed at Core A, Site 79 of the middle Okinawa Trough, East China Sea, and a large amount of authigenic pyrite is preserved in the surface sediment. In this study, we analyze the characteristics of the authigenic pyrite and its sulfur isotopic values. The authigenic pyrite is stripy and tubular, and there were foraminifera compartments filled with pyrite. The pyrite is extracted using chromium reduction, and the values of δ 34S are found to lie between -41.20‰ and 8.92‰ V-CDT. The bulk pyrite tends to be more enriched in 34S with increasing depth. Particularly, the δ 34S value of the pyrite lies between -32.73‰ and -41.20‰ V-CDT above 278 cmbsf, but it quickly increases below this depth (-21.49‰–8.92‰ V-CDT). At the same time, the total sulfur content of the pyrite shows an abrupt increase above 100 cmbsf but is otherwise stable between 1.04% and 0.55% below 100 cmbsf. The stable and negative values of δ 34S and the decreasing values of total sulfur above 278 cmbsf indicate reduced AOM activities in 17.18–5.3 ka. In addition, the increasing δ 34S and pyrite content indicate strong AOM development and methane seep below 278 cmbsf in 18.8–17.18 ka. In particular, the highest positive value of δ 34S occurring in 18.78 ka indicates the most intense AOM activity. The shallow sulfate-methane interface (SMI) and high methane flux below marine sediments also strongly support this activity.