Iron Sulfide Grains Research Articles

Submicroscopic magnetite may be ubiquitous in the lunar regolith of the high-Ti region

Magnetite is rare on the Moon. The ubiquitous presence of magnetite in lunar soil has been hypothesized in previous Apollo Mössbauer spectroscopy and electron spin resonance studies, but there is currently no mineralogical evidence to prove it. Here, we report a large number of submicroscopic magnetite particles embedded within iron-sulfide on the surface of Chang’e-5 glass, with a close positive correlation between magnetite content and the TiO 2 content of the surrounding glass. The morphology and mineralogy of the iron-sulfide grains suggest that these magnetite particles formed via an impact process between iron-sulfide droplets and silicate glass melt, and ilmenite is necessary for magnetite formation. Magnetite in lunar glass is a potential candidate for the “magnetite-like” phase detected in the Apollo era and suggests that impact-induced submicroscopic magnetite may be ubiquitous in high-Ti regions of the Moon. Moreover, these impact-induced magnetite particles may be crucial for understanding the lunar magnetic anomalies and mineral components of the deep Moon.

Carbonate dissolution enhances particulate iron sulfide mobilization during water-shale interaction

The interaction between shale and water in the environment can lead to the mobilization of micrometer-sized iron sulfide (pyrite) grains. Because these grains often contain high concentrations of toxic elements, including arsenic, this process can pose a significant hazard. While previous studies have suggested that calcite dissolution plays a major role in iron sulfide mobilization, the relationship between rock composition and the release of particulate matter is unclear. Here, we performed laboratory experiments that simulated water–rock interaction in the subsurface on 5 different shale formations: Eagle Ford, Marcellus, Mancos, and Barnett from the USA, and Ein Zeitim from Israel. We used high resolution imaging to evaluate the impact of a reactive fluid on the shale surface, and used image analysis software to determine the rate of iron sulfide grain mobilization. Comparison of the shale surfaces before and after the experiments showed that the dissolution of calcite cement had a major impact on sulfide mobilization: grains that were primarily surrounded by calcite cement were up to 85 times more likely to be mobilized than grains embedded in the shale matrix, which compromises a complex mixture of submicrometer-scale phases, including phyllosilicates, organics, and carbonates. By contrast, iron sulfide embedded in organic matter typically remained cemented in place. This suggests that while calcite dissolution is a crucial phase in facilitating iron sulfide detachment in shales, organic matter might act as an adhesive that suppresses particulate mobilization. However, the adhesive effect of organic matter is only likely to be of secondary importance. Overall, our results indicate that during hydraulic fracturing operations, which involve the injection of fluid into shales in the subsurface, formations with high levels of carbonate cement are more likely to release particulate iron sulfide, thereby reducing the quality of flowback water and potentially increasing treatment costs.

Mid-infrared evidence for iron-rich dust in the multi-ringed inner disk of HD 144432

Context. Rocky planets form by the concentration of solid particles in the inner few au regions of planet-forming disks. Their chemical composition reflects the materials in the disk available in the solid phase at the time the planets were forming. Studying the dust before it gets incorporated in planets provides a valuable diagnostic for the material composition. Aims. We aim to constrain the structure and dust composition of the inner disk of the young Herbig Ae star HD 144432, using an extensive set of infrared interferometric data taken by the Very Large Telescope Interferometer (VLTI), combining PIONIER, GRAVITY, and MATISSE observations. Methods. We introduced a new physical disk model, TGMdust, to image the interferometric data, and to fit the disk structure and dust composition. We also performed equilibrium condensation calculations with GGchem to assess the hidden diversity of minerals occurring in a planet-forming disk such as HD 144432. Results. Our best-fit model has three disk zones with ring-like structures at 0.15, 1.3, and 4.1 au. Assuming that the dark regions in the disk at ~0.9 au and at ~3 au are gaps opened by planets, we estimate the masses of the putative gap-opening planets to be around a Jupiter mass. We find evidence for an optically thin emission (τ < 0.4) from the inner two disk zones (r < 4 au) at λ > 3 µm. Our silicate compositional fits confirm radial mineralogy gradients, as for the mass fraction of crystalline silicates we get around 61% in the innermost zone (r < 1.3 au), mostly from enstatite, while only ~20% in the outer two zones. To identify the dust component responsible for the infrared continuum emission, we explore two cases for the dust composition, one with a silicate+iron mixture and the other with a silicate+carbon one. We find that the iron-rich model provides a better fit to the spectral energy distribution. Our GGchem calculations also support an iron-rich and carbon-poor dust composition in the warm disk regions (r < 5 au, T > 300 K). Conclusions. We propose that in the warm inner regions (r < 5 au) of typical planet-forming disks, most if not all carbon is in the gas phase, while iron and iron sulfide grains are major constituents of the solid mixture along with forsterite and enstatite. Our analysis demonstrates the need for detailed studies of the dust in inner disks with new mid-infrared instruments such as MATISSE and JWST/MIRI.

Organic-rich bimineralic ooids record biological processes in Shark Bay, Western Australia.

Marine ooids have formed in microbially colonized environments for billions of years, but the microbial contributions to mineral formation in ooids continue to be debated. Here we provide evidence of these contributions in ooids from Carbla Beach, Shark Bay, Western Australia. Dark 100-240 μm diameter ooids from Carbla Beach contain two different carbonate minerals. These ooids have 50-100 μm-diameter dark nuclei that contain aragonite, amorphous iron sulfide, detrital aluminosilicate grains and organic matter, and 10-20 μm-thick layers of high-Mg calcite that separate nuclei from aragonitic outer cortices. Raman spectroscopy indicates organic enrichments in the nuclei and high-Mg calcite layers. Synchrotron-based microfocused X-ray fluorescence mapping reveals high-Mg calcite layers and the presence of iron sulfides and detrital grains in the peloidal nuclei. Iron sulfide grains within the nuclei indicate past sulfate reduction in the presence of iron. The preservation of organic signals in and around high-Mg calcite layers and the absence of iron sulfide suggest that organics stabilized high-Mg calcite under less sulfidic conditions. Aragonitic cortices that surround the nuclei and Mg-calcite layers do not preserve microporosity, iron sulfide minerals nor organic enrichments, indicating growth under more oxidizing conditions. These morphological, compositional, and mineralogical signals of microbial processes in dark ooids from Shark Bay, Western Australia, record the formation of ooid nuclei and the accretion of magnesium-rich cortical layers in benthic, reducing, microbially colonized areas.

Sub-microscopic magnetite and metallic iron particles formed by eutectic reaction in Chang’E-5 lunar soil

Ferric iron as well as magnetite are rarely found in lunar samples, and their distribution and formation mechanisms on the Moon have not been well studied. Here, we discover sub-microscopic magnetite particles in Chang’E-5 lunar soil. Magnetite and pure metallic iron particles are embedded in oxygen-dissolved iron-sulfide grains from the Chang’E-5 samples. This mineral assemblage indicates a FeO eutectoid reaction (4FeO = Fe3O4 + Fe) for formation of magnetite. The iron-sulfide grains’ morphology features and the oxygen’s distribution suggest that a gas–melt phase reaction occurred during large-impact events. This could provide an effective method to form ubiquitous sub-microscopic magnetite in fine lunar soils and be a contributor to the presentation of ferric iron on the surface of the Moon. Additionally, the formation of sub-microscopic magnetite and metallic iron by eutectoid reaction may provide an alternative way for the formation of magnetic anomalies observed on the Moon.

Morphological Evolution of Ordinary Chondrite Microstructure during Heating: Implications for Atmospheric Entry

Abstract Meteoroid entry physics remain poorly understood, owing to the extreme conditions experienced by the meteoroid during hypervelocity atmospheric passage, which are not reproducible in ground-test facilities. Therefore, much of our knowledge of fundamental processes is based on bolide lightcurve inference and a posteriori analysis of recovered meteorites. Here, novel in situ microtomography experiments at entry-relevant temperatures were performed on samples of two ordinary chondrites: Tamdakht (H5) and Tenham (L6). The two meteorites were imaged while undergoing a temperature ramp from room temperature to 1200°C. A machine-learning mediated analysis of the microstructural evolution reveals incongruent melting of the meteorite, initiated by the meteoritic iron and iron sulfide grains, and subsequent flow through microcracks that leads to the evolution of large voids. This behavior is correlated to a broad, high-temperature endotherm, noted from differential scanning calorimetry analysis, indicative of the heat of fusion of the melting grains. Correspondingly, a surface elemental analysis indicates that the sulfur species in iron sulfide are highly mobile, which can result in the formation of nonstoichiometric iron–sulfur compounds with melting points that span the temperature range of the observed endotherm. The implications for entry phenomena, in particular meteoroid ablation, are discussed.

A chemical dynamics study on the gas phase formation of thioformaldehyde (H2CS) and its thiohydroxycarbene isomer (HCSH)

Complex organosulfur molecules are ubiquitous in interstellar molecular clouds, but their fundamental formation mechanisms have remained largely elusive. These processes are of critical importance in initiating a series of elementary chemical reactions, leading eventually to organosulfur molecules-among them potential precursors to iron-sulfide grains and to astrobiologically important molecules, such as the amino acid cysteine. Here, we reveal through laboratory experiments, electronic-structure theory, quasi-classical trajectory studies, and astrochemical modeling that the organosulfur chemistry can be initiated in star-forming regions via the elementary gas-phase reaction of methylidyne radicals with hydrogen sulfide, leading to thioformaldehyde (H2CS) and its thiohydroxycarbene isomer (HCSH). The facile route to two of the simplest organosulfur molecules via a single-collision event affords persuasive evidence for a likely source of organosulfur molecules in star-forming regions. These fundamental reaction mechanisms are valuable to facilitate an understanding of the origin and evolution of the molecular universe and, in particular, of sulfur in our Galaxy.

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo-environmental information from modern and ancient sediments.

Sulfur isotope ratio measurements of bulk sulfide from marine sediments have often been used to reconstruct environmental conditions associated with their formation. In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10 μm) grains or for detecting intra-grain variability. Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f-GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of 20-130 μm, using an electron multiplier (EM) detector to collect counts of 32 S- and 34 S- for each pixel (128 × 128 pixel grids) for between 20 and 960 cycles. The extraction procedure did not discernibly alter pyrite grain-size distributions. The apparent inter-grain variability in 34 S/32 S in 1-4 μm-sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was ~ ±2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤ ±2‰, 1σ). In contrast, grain-specific 34 S/32 S ratios in modern and ancient sedimentary pyrites and marcasites can have inter- and intra-grain variability >60‰. The distributions of intra-sample isotopic variability are consistent with bulk 34 S/32 S values. SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34 S/32 S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S-isotopic composition of pore water sulfide in their S-isotopic compositions. These data allow past local environmental conditions to be inferred.

Melting efficiency of troilite-iron assemblages in shock-darkening: Insight from numerical modeling

We studied shock-darkening in ordinary chondrites by observing the propagation of shock waves and melting through mixtures of silicates, metals and iron sulfides. We used the shock physics code iSALE at the mesoscale to simulate shock compression of modeled ordinary chondrites (using olivine, iron and troilite). We introduced FeS-FeNi eutectic properties and partial melting in a series of chosen configurations of iron and troilite grains mixtures in a sample plate. We observed, at a nominal pressure of 45 GPa, partial melting of troilite in all models. Only few of the models showed partial melting of iron (a phase difficult to melt in shock heating) due to the eutectic properties of the mixtures. Iron melting only occurred in models presenting either strong shock wave concentration effects or effects of heating by pore crushing, for which we provided more details. Further effects are discussed such as the frictional heating between iron and troilite and the heat diffusion in scenarios with strongly heated troilite. We also characterized troilite melting in the 32–60 GPa nominal pressure range. We concluded that specific dispositions of iron and troilite grains in mixtures allow for melting of iron and explain why it is possible to find a wide textural variety of melted and unmelted metal and iron sulfide grains in shock-darkened ordinary chondrites. We finally observe shock-melting of albite within a few iron and troilite grain models, and investigate the effects of higher porosity within the olivine matrix in the single iron and troilite grain models.

Factors Controlling In Situ Biogeochemical Transformation of Trichloroethene: Field Survey

AbstractWe have previously defined in situ biogeochemical transformation as the biogenic formation of reactive minerals that are capable of abiotically degrading chlorinated solvents such as trichloroethene without accumulation of degradation products such as vinyl chloride (AFCEE et al. ). This process has been implemented in biowalls used to intercept contaminated groundwater. Abiotic patterns of contaminant degradation were observed at Altus Air Force Base (AFB) and in an associated column study, but not at other sites including Dover AFB. These abiotic patterns were associated with biogenic formation of reactive iron sulfide minerals. Iron sulfides in the form of small individual grains, coatings on magnetite, and sulfur‐deficient pyrite framboids were observed in samples collected from the Altus AFB biowalls and one of the EPA columns. Larger iron sulfide grains coated with oxide layers were observed in samples collected from Dover AFB. Altus AFB and the EPA column differed from Dover AFB in that groundwater flow at Dover AFB was relatively slow and potentially reversing. High volumetric sulfate consumption rates, an abiotic pattern of trichloroethene (TCE) degradation, and the formation of small, high surface area iron sulfide particles were associated with relatively high rates of TCE removal via an abiotic pattern. Geochemical modeling demonstrated that iron monosulfides such as mackinawite were near saturation, and iron disulfides such as pyrite were supersaturated at all sites. This environmental condition can be supportive of nucleation of small particles rather than crystal growth leading to larger particles. When nucleation is dominant, small, high surface area, and reactive particles result. When crystal growth dominates the crystals are larger and have lower specific surface area and reactivity. These results taken together suggest that creation of a dynamic environment can promote biogeochemical transformation based on generation of reactive iron sulfides.

Accumulation and Preservation of Reworked Marine Pyrite Beneath an Oxygen-Rich Devonian Atmosphere: Constraints from Sulfur Isotopes and Framboid Textures

Abstract The Devonian Appalachian Basin provides a unique window to the unusual geochemical conditions required to form and preserve detrital metal sulfide deposits during the oxygen-rich Phanerozoic Eon. The Devonian Leicester Pyrite Member, a marine transgressive lag, occurs in the northern Appalachian Basin of western New York and is a reworked and redeposited acid-resistant residuum dominated by a wide range of pyritic clasts in the absence of carbonate grains. These iron sulfide grains include 10−2 m nodules, framboids, and pyritized fossil material (e.g., cephalopods, burrows, and steinkerns). A previous model for the formation of the Leicester Pyrite Member requires establishment of a strong and persistent density and chemical stratification in the Appalachian Basin. Although this model permits erosion under the low-oxygen conditions required for pyrite preservation, little geochemical evidence has been provided in corroboration, and the chemical conditions responsible for the formation, deposition, and preservation of the pyrite lag have remained speculative. Using a combined geochemical and petrographic approach focusing on the pyritic fraction and emphasizing sulfur isotopes and pyrite framboid size distributions, the specific redox conditions responsible for the formation of the Leicester Pyrite Member are addressed. The wide ranges of textural and isotopic data observed for the Leicester Pyrite Member, with δ34S values that range from −22 to +54% at a single location, are consistent with the general idea of lag concentration of pyritic debris formed under diverse diagenetic and possibly water-column (syngenetic) conditions. Energy propagation, possibly storm related, along the redox transition of the pycnocline and chemocline would have favored pyrite preservation, while still permitting enough oxidation and concomitant acid production to dissolve associated hydrodynamically equivalent carbonate debris. Intermittence in the stability of water-column stratification would have enhanced this oxidation, although an association with the overall oxygen deficiency that characterized the deposition of the overlying Geneseo Shale Member is demonstrated by the occurrence of microlaminated, Geneseo Shale Member-like clasts within the Leicester Pyrite Member. Our geochemical results confirm that during times of atmospheric oxygenation, pyrite reworking and lag formation require high-energy but low-oxygen conditions at pycnocline depths in a subaqueous marine setting. While not unique to the Leicester Pyrite Member, the specific sets of environmental conditions required for pyrite preservation differ from those of the fluvial settings in which Archean detrital pyrite and uraninite deposits formed. We suggest, therefore, that the presence of Phanerozoic pyrite lags such as the Leicester Pyrite Member does not compromise the utility of Archean detrital pyrite as a paleoredox proxy delineating early deficiency of atmospheric oxygen.

Pyrite and Marcasite Coated Grains in the Ordovician Winnipeg Formation, Canada: An Intertwined Record of Surface Conditions, Stratigraphic Condensation, Geochemical "Reworking," and Microbial Activity

Sulfide-coated grains are enigmatic sedimentary particles that occur in sediments ranging in age from Archean to Jurassic. Although their oolitic and pisoidal texture suggests energetic and agitated conditions, their mineralogy seems to require a reducing and stagnant environment. Because of this conflict, they have long been considered diagenetic replacements of precursor iron oxide, chamosite, or carbonate ooids. Coated iron sulfide (CIS) grains from the Ordovician Winnipeg Formation of Saskatchewan, Canada, formed as primary diagenetic precipitates under shallow burial conditions, involving intermittent episodes of tractional reworking, grain abrasion, reburial, and precipitation of further concentric laminae. These iron sulfide grains are found in shallow marine sandstones with indications of episodic wave reworking, and indicate sufficient organic-matter burial to promote reducing pore waters and iron sulfide formation. Horizons with strong enrichment of iron sulfide grains (up to 40%) indicate negative net sedimentation to facilitate geochemical reworking of iron. CIS grains formed during intervals of maximum sediment starvation, when organic matter was enriched in surface sediments, and rare storm waves intermittently reworked the surface sediment. Alternating laminae of pyrite and marcasite within these grains point to substantial fluctuations in pore-water pH, driven by intermittent oxidation of previously formed iron sulfides. Filamentous and coccoid structures within cortical laminae suggest microbial involvement in lamina formation through providing a growth medium for sulfate-reducing bacteria and potentially a template for mineral growth. Coated iron sulfide grains of the Winnipeg Formation preserve a combined record of condensation, seafloor reworking, fluctuating chemical conditions in the surface sediment, and high surface productivity. This information-rich record is made even more valuable by the fact that the host sediment consists largely of heavily bioturbated monotonous sandstones that could not provide these insights on their own merit.

Chromium-Removal Processes during Groundwater Remediation by a Zerovalent Iron Permeable Reactive Barrier

Solid-phase associations of chromium were examined in core materials collected from a full-scale, zerovalent iron permeable reactive barrier (PRB) at the U.S. Coast Guard Support Center located near Elizabeth City, NC. The PRB was installed in 1996 to treat groundwater contaminated with hexavalent chromium. After eight years of operation, the PRB remains effective at reducing concentrations of Cr from average values >1500 microg L(-1) in groundwater hydraulically upgradient of the PRB to values <1 microg L(-1) in groundwater within and hydraulically downgradient of the PRB. Chromium removal from groundwater occurs at the leading edge of the PRB and also within the aquifer immediately upgradient of the PRB. These regions also witness the greatest amount of secondary mineral formation due to steep geochemical gradients that result from the corrosion of zerovalent iron. X-ray absorption near-edge structure (XANES) spectroscopy indicated that chromium is predominantly in the trivalent oxidation state, confirming that reductive processes are responsible for Cr sequestration. XANES spectra and microscopy results suggest that Cr is, in part, associated with iron sulfide grains formed as a consequence of microbially mediated sulfate reduction in and around the PRB. Results of this study provide evidence that secondary iron-bearing mineral products may enhance the capacity of zerovalent iron systems to remediate Cr in groundwater, either through redox reactions at the mineral-water interface or by the release of Fe(II) to solution via mineral dissolution and/or metal corrosion.

Identification and origins of iron sulfides in Czech Loess

Multiple rock‐magnetic and non‐magnetic techniques were employed to identify iron sulfides in late Pleistocene Czech loess deposits. The results indicate that iron sulfides in the loess at Znojmo section are mainly pyrrhotite and pyrite, with stoichiometry ranging from Fe10S11 to FeS2. Although early pedogenic origin of fine sulfide fraction can not be ruled out, scanning electron microscopy (SEM) of individual grains and microscopic observations of magnetic extracts show that iron sulfide grains are probably of detrital origin. Potential sources of these sulfides may have been eluvium loams above hypothetical ore deposits in the vicinity of the section or glacifluvial sediments from nearby glacial margins. Framboidal pyrite forms, now completely replaced by iron oxides, suggest that some particles of biogenic sulfides may have also been transported into loess from presumably fluvial sediments.

An Evaporation Model for Formation of Carbonates in the ALH84001 Martian Meteorite

Small, discoid globules and networks of magnesium-iron-calcium carbonates occur within impact-produced fracture zones in the ALH84001 Martian meteorite. Because these carbonates contain or are associated with the hydrocarbons, single-domain magnetite and iron-sulfide grains, and purported microfossils that collectively have been cited as evidence for ancient Martian life, it is critically important to understand their formation. Previous hypotheses for the origin of the carbonates involve either alteration of the rock by hydrothermal fluids at relatively low temperatures, or formation from a CO2-rich vapor at high temperatures. This paper explores an alternative mechanism–direct precipitation from a ponded evaporating brine infiltrating into fractures in the floor of an impact crater. Such a model can be reconciled with the observed carbonate compositional zoning and extreme stable-isotopic fractionations. If the carbonates formed in this manner, this removes a possible obstacle to the proposed existence of microbial remains in ALH84001; however, the cited evidence for life can be better explained by inorganic processes expected from brines in an evaporating alkaline lake, with an overprint of shock metamorphism and subsequent contamination by organic matter after falling to Earth.

Iron Sulfide Grains Produced by Coalescence of Fe and S Smoke Grains.

A new experimental method for preparing ultrafine iron sulfide grains is proposed. Troilite, marcasite and pyrrhotite grains predominantly, were produced by the coalescence between solid iron grains and liquidus sulfur grains at temperatures above 100°C. Troilite and marcasite grains were grown as single crystals of a complicated polyhedral shape. Pyrrhotite grains exhibited a typical DDSS (diffusion-dependence shell structure) shape produced by the diffusion of iron atoms to the surface layer. Iron grains covered with a thin sulfur layer were observed among the collected grains produced at the temperature of 50°C.

Trace-element siting in iron sulfides in Ohio coals determined by secondary ion mass spectrometry (SIMS)

Ion images of site-specific concentrations of various minor and trace elements at the surface of intact samples of iron sulfides in coals have been obtained using secondary ion mass spectrometry (SIMS). Problems related to intefeering images of complex secondary molecular ions have been largely overcome by combining SIMS imaging with ion probe step scanning. Although SIMS, when used in this way, is qualitative, it is sensitive in the parts-per-million (ppm) range and has the capability of micron-scale resolution for ion mapping. Iron sulfides in samples of Ohio coal bneds contain Mn, Co, Cu, As and Pb. The elements Si, Al, K, V, Cr, Ni and Co occur outside the iron-sulfide grains and appear to be associated with clay minerals in the coal. Where Si, Al and K occur within sulfide grains they are associated with clay minerals contained in porous pyrite. Arsenic also may be associated with clays present in porous pyrite. Of the elements found within the iron sulfides, non appear to generally favor pyrite or marcasite, but in some individual samples there can be elemental preference for one or the other sulfide phase. SIMS images clearly indicate that the occurrences of some trace elements are related to the growth of the iron-sulfide grains, and as a result, their concentrations are considered to have been controlled, at least in part, by the microenvironments that existed during the formation of the iron sulfides. Site-specific determination of the occurrences of trace elements in intact coal-sulfide samples is important with regard to development of methods of environmental management of the wastes derived from the uses of coal, and also should be useful in determining the relative organic affinity versus inorganic affinity of minor elements in coals.