Natural Arsenopyrite Research Articles

Crystallography and Surface Oxidation of Stoichiometric Arsenopyrite from Šumadija-Kopaonik Pb-Zn/Polymetallic Ore District (Serbia)

The behaviour and stability of arsenopyrite largely depend on its crystal structure, but studies of crystallography and structural defects are scarce. For clarification, we investigated a series of natural arsenopyrite crystals from the polymetallic Pb-Zn-(±Cu,Ag,Au,Sb,Bi,W) Šumadija-Kopaonik ore district (Serbia) by powder X-ray diffraction (XRPD). Among them, samples from the Drenjak gold mineralisation showed near-stoichiometric FeAsS composition and were further investigated by conventional and high-resolution transmission electron microscopy methods. TEM revealed abundant planar crystallographic defects and epitaxial surface oxide layer formation upon air exposure. Our findings present the fundamental work on the prototype arsenopyrite structure and have further implications for studies on ion incorporation into FeAsS structure and its environmental stability and oxidation mechanisms as a source of anthropogenic As release into the environment.

Dual-anion etching induced in situ interfacial engineering for high-efficiency oxygen evolution

Designing novel catalysts for oxygen evolution reaction (OER) with high cost-effectiveness plays a central role in sustainably driving renewable energy conversion and storage. Here we demonstrate the in situ interfacial engineering for constructing efficient OER catalysts based on the electrochemical dual-anion etching of natural arsenopyrite. The OER catalyst (FeAsS) prepared from natural arsenopyrite via an environment-friendly ball milling approach achieves a current density of 10 mA cm−2 at an overpotential of 200 mV, outperforming many state-of-the-art catalysts. The in-depth study indicates that the co-etching of lattice As and S under the OER conditions triggers the in situ surface self-reconstruction, and a self-optimized catalytic active and stable FeAsS/α-FeOOH interface has been developed. Computational studies further confirm that the strong electronic coupling effect between α-FeOOH and FeAsS significantly tunes the binding energy between reaction intermediates and active sites, finally leading to an enhanced OER activity. The dual-anion etching of precatalysts induced in situ interfacial engineering demonstrated here expands the way of exploring other multiple nonmetallic elements involved nanomaterials as efficient OER precatalysts. This study also stimulates further study on the eco-design of electroactive materials for advanced energy conversion/storage applications from earth-abundance natural resources.

Composition and Ligand Microstructure of Arsenopyrite from Gold Ore Deposits of the Yenisei Ridge (Eastern Siberia, Russia)

The Mössbauer spectroscopy method was used to study the ligand microstructure of natural arsenopyrite (31 specimens) from the ores of the major gold deposits of the Yenisei Ridge (Eastern Siberia, Russia). Arsenopyrite and native gold are paragenetic minerals in the ore; meanwhile, arsenopyrite is frequently a gold carrier. We detected iron positions with variable distribution of sulfur and arsenic anions at the vertexes of the coordination octahedron {6S}, {5S1As}, {4S2As}, {3S3As}, {2S4As}, {1S5As}, {6As} in the mineral structure. Iron atoms with reduced local symmetry in tetrahedral cavities, as well as iron in the high-spin condition with a high local symmetry of the first coordination sphere, were identified. The configuration {3S3As} typical for the stoichiometric arsenopyrite is the most occupied. The occupation degree of other configurations is not subordinated to the statistic distribution and varies within a wide range. The presence of configurations {6S}, {3S3As}, {6As} and their variable occupation degree indicate that natural arsenopyrites are solid pyrite {6S}, arsenopyrite {3S3As}, and loellingite {6As} solutions, with the thermodynamic preference to the formation of configurations in the arsenopyrite–pyrite–loellingite order. It is assumed that in the variations as part of the coordination octahedron, the iron output to the tetrahedral positions and the presence of high-spin Fe cations depend on the physical and chemical conditions of the mineral formation. It was identified that the increased gold concentrations are typical for arsenopyrites with an elevated content of sulfur or arsenic and correlate with the increase of the occupation degree of configurations {5S1As}, {4S2As}, {1S5As}, reduction of the share of {3S3As}, and the amount of iron in tetrahedral cavities.

“Invisible” gold in synthetic and natural arsenopyrite crystals (Vorontsovka deposit, Northern Urals)

In many types of hydrothermal ore deposits Au occurs in invisible state in most common minerals of the Fe-As-S system. It is supposed that the state of theinvisible Au may beeither non-structural (nano-sized inclusions of metal and its compounds) or chemically bound (isomorphous solid solution). Here we report results of investigation of the state and the concentration range ofinvisible Au in synthetic and natural arsenopyrites FeAsS (Vorontsovka deposit, North Urals, type Carlin). Conditions that favor the formation of Au-bearing arsenopyrite were identified. The synthesis experiments were carried out in Au-saturated system by means of salt flux method with a stationary temperature gradient. The temperature at the cold end of the ampole was 400500С. The chemical composition of arsenopyrite was determined by electron probe microanalysis. The composition of the synthesized arsenopyrite varied within [at.%]: Fe from 32.6 to34.4, As from 30.0 to 36.5, S from 29.4 to36.0. The Au content in arsenopyrite varied from the detection limit ( 45ppm) to 3wt.%. A strong negative correlation between the concentrations of Au and Fe was observed in the synthesized arsenopyrite grains. The slope of the correlation lines corresponds to the formation of the Au-bearing solid solution where Au isomorphically substitutes for Fe at the parameters of the synthesis experiments. In addition, there is a weaker positive correlation between Au and As: higher Au concentrations are characteristic of arsenic-rich compositions (As/S [at.%] 1) and those close to stoichiometric arsenopyrite, whereas in sulfur-rich arsenopyrite the concentration of Au is lower and does not exceed 0.25wt.%. The positive Au-As correlation appears not only on a local level within a single crystal of synthetic and natural arsenopyrite, but is valid on the Vorontsovka deposit scale: As-rich arsenopyrite formed at lower temperature and sulfur fugacity (t= 250370C, logfS2= 1217) contains more Au than the As-poor early arsenopyrite (t= 270400C, logfS2= 79). Comparison of these results with the literature data shows that the positive correlation between the concentrations of Au and As in arsenopyrite and the negative correlation between the concentrations of Au and Fe are the common features of ores of the Carlin-type deposits. We suggest that, in contrast to negative correlation Au-Fe, the positive correlation Au-As cannot be explained in terms of crystal chemistry, but can result from the effect of external factors among which are the difference in composition of ore-forming hydrothermal fluids and the sulfur fugacity.

“Invisible” Gold in Synthetic and Natural Arsenopyrite Crystals, Vorontsovka Deposit, Northern Urals

In many types of hydrothermal ore deposits Au occurs in “invisible” state in most common minerals of the Fe-As-S system. It is supposed that the state of the “invisible” Au may be either non-structural (nano-sized inclusions of metal and its compounds) or chemically bound (isomorphous solid solution). Here we report results of investigation of the state and the concentration range of “invisible” Au in synthetic and natural arsenopyrites FeAsS (Vorontsovka deposit, North Urals, type Carlin). Conditions that favor the formation of Au-bearing arsenopyrite were identified. The synthesis experiments were carried out in Au-saturated system by means of salt flux method with a stationary temperature gradient. The temperature at the cold end of the ampole was 400–500 °С. The chemical composition of arsenopyrite was determined by electron probe microanalysis. The composition of the synthesized arsenopyrite varied within [at.%]: Fe from 32.6 to 34.4, As from 30.0 to 36.5, S from 29.4 to 36.0. The Au content in arsenopyrite varied from the detection limit ( 1) and those close to stoichiometric arsenopyrite, whereas in sulfur-rich arsenopyrite the concentration of Au is lower and does not exceed 0.25 wt.%. The positive Au-As correlation appears not only on a local level within a single crystal of synthetic and natural arsenopyrite, but is valid on the Vorontsovka deposit scale: As-rich arsenopyrite formed at lower temperature and sulfur fugacity ( t = 250–370 °C, log f S 2 = –12 ÷ –17) contains more Au than the As-poor early arsenopyrite ( t = 270–400 °C, log f S 2 = –7 ÷ –9). Comparison of these results with the literature data shows that the positive correlation between the concentrations of Au and As in arsenopyrite and the negative correlation between the concentrations of Au and Fe are the common features of ores of the Carlin-type deposits. We suggest that, in contrast to negative correlation Au-Fe, the positive correlation Au-As cannot be explained in terms of crystal chemistry, but can result from the effect of external factors among which are the difference in composition of ore-forming hydrothermal fluids and the sulfur fugacity.

X-ray diffraction study of arsenopyrite at high pressure

The high-pressure X-ray diffraction study of a natural arsenopyrite was investigated up to 28.2 GPa using in situ angle-dispersive X-ray diffraction and a diamond anvil cell at National Synchrotron Light Source, Brookhaven National Laboratory. The 16:3:1 methanol–ethanol–water mixture was used as a pressure-transmitting medium. Pressures were measured using the ruby-fluorescence method. No phase change has been observed up to 28.2 GPa. The isothermal equation of state (EOS) was determined. The values of K0, and K′0 refined with a third-order Birch–Murnaghan EOS are K0 = 123(9) GPa, and K′0 = 5.2(8). Furthermore, we confirm that the linear compressibilities (β) along a, b and c directions of arsenopyrite is elastically isotropic (βa = 6.82 × 10−4, βb = 6.17 × 10−4 and βc = 6.57 × 10−4 GPa−1).

Arsenic Mining Waste in the Catchment Area of the Madrid Detrital Aquifer (Spain)

In recent years, elevated arsenic concentrations in groundwater used for drinking water supplies have been recognised in the Madrid Tertiary detrital aquifer. Although only natural causes have been suggested as the source of arsenic, this study aims to highlight that the anthropogenic contribution cannot be disregarded. During the sub-catchment’s areas sampling, we found many geographical sites where natural arsenopyrite [FeAsS] originally encapsulated in pegmatite bodies and quartz veins, was artificially outcropped and dumped out, since mining wastes were scattered and exposed to weathering. Several mineral and ground specimens were collected to analyse its mineralogical and chemical composition by X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF) spectrometry and by Environmental Scanning Electron Microscope (ESEM). Both, the abundant existence of secondary phases, such as scorodite [FeAsO4⋅2H2O] and jarosite [KFe3(SO4)2(OH)6], much more soluble than arsenopyrite, and the lixiviation experiments of arsenopyrite in acidic media to simulate acid mine drainage (AMD) conditions, usually found in old mining districts, point to a potential risk of arsenic contamination of surface water bodies, which operate as recharged waters of the aquifer in the studied area. The elemental determination of heavy metals present in ground samples by XRF analyses, reaching up to 1,173 mg kg–1 of copper, 347 mg kg–1 of lead and 113,702 mg kg–1 of arsenic; and the physicochemical and arsenic fractionation studies of soil samples, led us to classify the soil as Spolic Technosol (Toxic). The contamination of the area due to old mining activities could release arsenic to Madrid water supplies; accordingly, additional decontamination studies should be performed.

Oxidation of arsenopyrite and deposition of gold on the oxidized surfaces: A scanning probe microscopy, tunneling spectroscopy and XPS study

We have used ex situ atomic force microscopy (AFM), scanning tunneling microscopy and spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) to study the surfaces of natural arsenopyrite samples that were electrochemically polarized in 1 M HCl, or leached in acidic solutions containing ferric iron salts, and then reacted with aqueous gold (III) chloride at ambient temperatures. For arsenopyrite oxidized on a positive-going potential sweep, progressively increasing amounts of surface Fe(III)–O and As–O species, and of S/Fe and S/As ratios in a non-stoichiometric sulfidic layer were found. The products formed in the sweep to a potential of 0.6 V (Ag/AgCl) of the passivity region are shaped in about 100 nm protrusions of two sorts, which are arranged in micrometer-size separate areas, while they are largely mixed at higher, “transpassive” potentials. The quantities of surface alteration substances notably decrease after leaching in ferric chloride and ferric sulfate acidic solutions. Passivation of arsenopyrite was suggested to associate with the disordered, metal-deficient surface layer having moderate excess of sulfur rather than with the products of arsenopyrite oxidation. Exposure of arsenopyrite to 10 −5–10 −3 M AuCl 4 - (pH 2) solutions results in the deposition of 8–50 nm gold particles; only a small fraction of the gold is present as Au(I)–S species. The electrochemical oxidation at 0.6 V or ageing of arsenopyrite in air promotes the subsequent gold deposition; in contrast, the amount of Au deposited on arsenopyrite that was treated by leaching in ferric chloride and sulfate solutions was about 10 times smaller than with polished arsenopyrite samples. It has been concluded that reducing agents formed as intermediates of arsenopyrite decomposition facilitate the Au 0 cementation although other factors related to the surface state of the arsenopyrite play a role as well. A decrease in the tunneling current magnitudes with decreasing the Au 0 particle size has been revealed using STS. This effect along with the increase by 0.2–0.5 eV in the XPS Au 4f binding energies were tentatively ascribed to retarding the electron transitions by emerging electrostatic charge on gold nanoparticles (Coulomb blockade). Possible mechanisms for the effects, and their potential role in the deposition and hydrometallurgy of “invisible” gold are discussed.

Stability and solubility of arsenopyrite, FeAsS, in crustal fluids

The stability and solubility of natural arsenopyrite (FeAsS) in pure water and moderately acid to slightly basic aqueous solutions buffered or not with H 2 and/or H 2S were studied at temperatures from 300 to 450°C and pressures from 100 to 1000 bar. The solubilities of FeAsS in pure water and dilute HCl/NaOH solutions without buffering are consistent with the formation of the As(OH) 3 0(aq) species and precipitation of magnetite. At more acid pH (pH ≤2), arsenopyrite dissolves either stoichiometrically or with formation of the As-FeAsS assemblage. In H 2S-rich and H 2-rich aqueous solutions, arsenopyrite dissolution results in the formation of pyrrhotite (±pyrite) and iron arsenide(s), respectively, which form stable assemblages with arsenopyrite. Arsenic concentrations measured in equilibrium with FeAsS in slightly acid to neutral aqueous solutions with H 2 and H 2S fugacities buffered by the pyrite-pyrrhotite-magnetite assemblage are 0.0006 ± 0.0002, 0.0055 ± 0.0010, 0.07 ± 0.01, and 0.32 ± 0.03 mol/kg H 2O at 300°C/400 bar, 350°C/500 bar, 400°C/500 bar, and 450°C/500 bar, respectively. These values were combined with the available thermodynamic data on As(OH) 3 0(aq) (Pokrovski et al., 1996) to derive the Gibbs free energy of FeAsS at each corresponding temperature and pressure. Extrapolation of these values to 25°C and 1 bar, using the available heat capacity and entropy data for FeAsS (Pashinkin et al., 1989), yields a value of −141.6 ± 6.0 kJ/mol for the standard Gibbs free energy of formation of arsenopyrite. This value implies a higher stability of FeAsS in hydrothermal environments than was widely assumed. Calculations carried out using the new thermodynamic properties of FeAsS demonstrate that this mineral controls As transport and deposition by high-temperature (>∼300°C) crustal fluids during the formation of magmatic-hydrothermal Sn-W-Cu-(Au) deposits. The equilibrium between As-bearing pyrite and the fluid is likely to account for the As concentrations measured in modern high- and moderate-temperature (150 ≤ T ≤ 350°C) hydrothermal systems. Calculations indicate that the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid, which is likely to be an effective mechanism for precipitating gold from hydrothermal solutions. This could be a possible explanation for the gold-arsenopyrite association commonly observed in many hydrothermal gold deposits.

Mobilisation and attenuation of arsenic around gold mines, east Otago, New Zealand

Arsenic is released into the environment by decomposition of natural arsenopyrite in gold‐bearing veins in east Otago. Natural release is slow, but is accelerated by mining activity. Waters from old mine adits have pH of 6–7, compared to normal groundwater pH of 7–8, and adit waters have arsenic concentrations up to 4 ppm compared to background levels of c. 0.01 ppm. Tailings disposed about 80–90 years ago into what is now a wetland at Barewood have arsenic concentrations of up to 33 000 ppm, principally in the form of the secondary arsenic mineral scorodite. The maximum dissolved arsenic concentration in the Barewood tailings is 0.75 ppm at pH of c. 5.5. Dissolved arsenic release at that pH may be restricted to levels near 1 ppm by the formation of scorodite with solubility and/or kinetic controls on scorodite redissolution. Modern mining activity in the Macraes area has resulted in localised dissolved arsenic concentrations in excess of 200 ppm at pH near 10. Evaporation of this water produces scorodite precipitates. Dissolved arsenic released from arsenopyrite decomposition is attenuated by fine‐grained minerals, predominantly phyllosilicates, in soils around weathered veins, and in wetlands downstream from mine workings. Lithic soils can hold at least 50–100 ppm As on the fine fraction (c. 1 μ,m), and wetland soils can hold hundreds to thousands of parts per million on their fine fractions. Both soil adsorption and scorodite mediation slow the release of arsenic into the environment, allowing more effective dilution by downstream waters to safe arsenic levels.

New Semiconducting Arsenopyrite-type Compounds

IN a recent paper1 we reported on semiconductivity in CoSb2-type compounds. The ternary analogues of these polycompounds are formed by the arsenopyrite group. The monoclinic minerals arsenopyrite, FeAsS, and gud-mundite, FeSbS, closely correspond to the mineral safflorite, CoAs2. They have the same number of valence electrons and we therefore expect their cations to have the same d-electron configuration. In CoAs2 we have found a magneton number much smaller than that corresponding to one unpaired spin1. A measurement of the magnetic susceptibility of a natural arsenopyrite crystal (from Altenberg, Silesia, the same on which resistivity measurements were performed earlier2) of unknown purity showed a very similar behaviour (see Table 1).