Including Iron Oxide Research Articles

Microstructure and Deformability of Cast Zr-Nb-Fe-O Alloy with High Iron and Oxygen Content

In order to investigate the possible effect of the scrap inclusions in recycling, microstructure and deformability of Zr-Nb based alloys cast with the addition of iron and iron oxide were studied. No visible iron oxide inclusions were observed in the arc-melted cake, suggesting that Zr and Fe2O3 reacted completely to dissociate Fe2O3 into Fe and O in the Zr. No oxide peaks were observed by XRD analyses. In Zr-Nb alloy melted with iron oxide, the oxygen content reached up to 3150 ppm. Zr-1.2 Nb alloys with high oxygen contents exhibited needle-shaped α phase. and the the thickness of needles decreased. In Zr-1.2Nb alloys with over 2000 ppm they are brittle and cracked during rolling process. In some region of the fracture surface brittle fracture feature such as vein-like pattern was observed. In the fractograph, no brittle second phase particles were observed. EDS spectra. It is apparent that Zr-Nb alloys with the addition of iron oxide during the handling of scrap materials exhibit the extremely brittle behavior.

Reflectance spectral characterization of acid sulphate soil in South Yunderup, Western Australia

Acid sulphate soils (ASS) are widely spread worldwide and are detrimental to the environment. South Yunderup is one of the coastal areas of Western Australia heavily affected by ASS. Conventional investigation is costly and time-consuming, and thus there is an urgent need to rapidly characterize and identify this type of soil. This paper aims to characterize these soils using reflectance spectra, which may be one of the most significant steps in effectively identifying them and mapping their extent by remote sensing. The ASS from the study area were divided into several groups and subtypes according to both pH measurements and mineral composition as identified by X-ray diffraction analysis. Each group and subtype was then characterized by its spectral reflectance features. We found that the spectral characteristics of ASS are governed by the spectral features of its compositional minerals. In particular, some secondary iron-bearing minerals produced by the formation of ASS, together with surrounding minerals such as carbonate, play vitally important roles in influencing the spectral characterization of ASS. These iron-bearing minerals, including iron oxides, hydroxides/oxyhydroxides (e.g. haematite, goethite, and ferrihydrite), and iron sulphates (e.g. jarosite and copiapite), have diagnostic spectral features and are therefore detectable in the reflectance range. Moreover, these secondary iron-bearing minerals could be indicators suggesting the pH conditions in which they formed. The results of this study include the overall mineral distribution of the study area, the spectral characterization of different groups and subtypes of ASS, and the linkages between spectral features and pH ranges.

A multifunctional amphiphilic polymer as a platform for surface-functionalizing metallic and other inorganic nanostructures.

We designed a new set of polymer ligands that combine multiple metal-coordinating groups and short polyethylene glycol (PEG) moieties in the same structure. The ligand design relies on the controlled grafting of a large number of amine-terminated histamines and PEG short chains onto a poly(isobutylene-alt-maleic anhydride) backbone, via a one-step nucleophilic addition reaction. This addition reaction is highly efficient, can be carried out in organic media and does not require additional reagents. We show that when imidazole groups are used the resulting polymer ligand can strongly ligate onto metal nanostructures such as nanoparticles (NPs) and nanorods (NRs) made of gold cores. The resulting polymer-coated NPs and NRs exhibit good colloidal stability to pH changes and added electrolytes. This constitutes a departure from the use of thiol-based ligands to coordinate on Au surfaces. The present chemical approach also opens up additional opportunities for designing hydrophilic and reactive platforms where the polymer coating can be adjusted to various metal and metal oxide surfaces by simply modifying or combining the addition reaction with other metal coordinating groups. These could include iron oxide NPs and semiconductor QDs. These polymer-capped NPs and NRs can be used to develop biologically-active platforms with potential use for drug delivery and sensing.

Mathematical simulation of direct reduction process in zinc-bearing pellets

A one-dimensional unsteady mathematical model was established to describe direct reduction in a composite pellet made of metallurgical dust. The model considered heat transfer, mass transfer, and chemical reactions including iron oxide reductions, zinc oxide reduction and carbon gasification, and it was numerically solved by the tridiagonal matrix algorithm (TDMA). In order to verify the model, an experiment was performed, in which the profiles of temperature and zinc removal rate were measured during the reduction process. Results calculated by the mathematical model were in fairly good agreement with experimental data. Finally, the effects of furnace temperature, pellet size, and carbon content were investigated by model calculations. It is found that the pellet temperature curve can be divided into four parts according to heating rate. Also, the zinc removal rate increases with the increase of furnace temperature and the decrease of pellet size, and carbon content in the pellet has little influence on the zinc removal rate.

The effect of friction modifiers on wheel/rail isolation at low axle loads

Signalling block occupancy is triggered by the wheelset of a rail vehicle ‘shunting’ the track circuit. The main cause of loss of shunting is the many materials that can be present on the rail head. These include iron oxides, leaves, ballast dust, oil, as well as products deliberately applied to the rail/wheel such as grease, friction modifiers, lubricants and sand. These may lead to an indication that a signal block is unoccupied when a train is actually present. Friction modifiers (FMs) are being increasingly used for reduction of noise, lateral forces, wear, rolling contact fatigue, etc. The Kelsan® high positive friction (HPF) solid stick FM is applied directly to the wheel tread creating a thin film. Although no effects have been observed during field operation, the intention in this work was to build on previous studies and evaluate conductance across a wider range of contact conditions as studies have shown that loss of shunt is more common with light axle loads where films could have a greater effect. The testing was performed at a contact pressure of 470 MPa (equating to a 4.9 tonne axle load). An HPF stick was spring loaded against the rotating wheel disc to generate a FM film at the contact. Tests were run to measure the impedance across the discs using a modified TI21 track circuit. Static testing was also performed using discs with a pre-generated HPF film. Analysis of the results showed that the application of HPF friction modifier had no significant effect on the measured level of impedance. The highest impedance levels were recorded under pure rolling (0% slip) where FMs were not being applied. In cases where the application of FM increased impedance, the recorded impedance values were still lower than the highest values measured in the absence of a FM. This is of key relevance as several light rail operators coast their trains.

Micromagnetics and magnetomineralogy of ultrafine magnetite inclusions in the Modipe Gabbro

Iron oxide inclusions in pyroxene crystals from the Modipe Gabbro, Botswana, have been studied to determine their recording fidelity. Hysteresis parameters, first‐order reversal curves, isothermal remanent magnetization acquisition curves, and thermomagnetic data all indicate that the iron oxide occurs as stoichiometric magnetite in the form of single‐domain and/or small pseudo single‐domain particles. Using distributions for the grain shape and nearest‐neighbor spacings determined from optical micrographs as input, a first‐order reversal curve (FORC) diagram was simulated using a numerical micromagnetic model. The simulated FORC diagram was found to closely match the measured FORC distributions. Analysis of the interaction fields in the model found that the standard deviation of the interaction field distribution was 2.7 mT compared to a bulk coercivity of ~20 mT, suggesting that the majority of particles are unlikely to be affected by magnetostatic interactions. Thellier analysis of the samples induced with an initial laboratory thermoremanence yielded near‐perfect behavior. It is suggested that the Modipe Gabbro dated at 2784.0 ± 1.5 (2σ) Ma is potentially an ideal recorder of the geomagnetic field.

Magnetic Targeted Delivery of Dexamethasone Acetate Across the Round Window Membrane in Guinea Pigs

Magnetically susceptible PLGA nanoparticles will effectively target the round window membrane (RWM) for delivery of dexamethasone-acetate (Dex-Ac) to the scala tympani. Targeted delivery of therapeutics to specific tissues can be accomplished using different targeting mechanisms. One technology includes iron oxide nanoparticles, susceptible to external magnetic fields. If a nanocomposite composed of biocompatible polymer (PLGA), magnetite, and Dex-Ac can be pulled into and across the mammalian RWM, drug delivery can be enhanced. In vitro targeting and release kinetics of PLGA-magnetite-Dex-Ac nanoparticles first were measured using a RWM model. Next, these optimized nanocomposites were targeted to the RWM by filling the niche in anesthetized guinea pigs. A permanent magnet was placed opposite the RWM for 1 hour. Cochlear soft tissues, perilymph, and RWM were harvested after euthanasia and steroid levels were measured using HPLC. Membrane transport, in vitro, proved optimal targeting using a lower particle magnetite concentration (1 versus 5 or 10 mg/ml). In vivo targeted PLGA-magnetite-Dex-Ac particles had an average size of 482.8 ± 158 nm (DLS) and an average zeta potential -19.9 ± 3.3 mV. In 1 hour, there was significantly increased cochlear targeted delivery of Dex or Dex-Ac, compared with diffusion alone. Superparamagnetic PLGA-magnetite-Dex-Ac nanoparticles under an external magnetic field (0.26 mT) for 1 hour significantly increased Dex-Ac delivery to the inner ear. The RWM was not completely permeated and also became loaded with nanocomposites, indicating that delivery to the cochlea would continue for weeks by PLGA degradation and passive diffusion.

Individual inorganic nanoparticles: preparation, functionalization and in vitro biomedical diagnostic applications.

Inorganic nanoparticles have become the focus of modern materials science due to their potential technological importance, particularly in bionanotechnology, which stems from their unique physical properties including size-dependent optical, magnetic, electronic, and catalytic properties. The present article provides an overview on the currently used individual inorganic nanoparticles for in vitro biomedical domains. These inorganic nanoparticles include iron oxides, gold, silver, silica, quantum dots (QDs) and second harmonic generation (SHG) particles. For each of these interesting nanoparticles, the main issues starting from preparation up to bio-related applications are presented.

Improved dynamic response assessment for intra‐articular injected iron oxide nanoparticles

The emerging importance of nanoparticle technology, including iron oxide nanoparticles for monitoring development, progression, and treatment of inflammatory diseases such as arthritis, drives development of imaging techniques. Studies require an imaging protocol that is sensitive and quantifiable for the detection of iron oxide over a wide range of concentrations. Conventional signal loss measurements of iron oxide nanoparticle containing tissues saturate at medium concentrations and show a nonlinear/nonproportional intensity to concentration profile due to the competing effects of T₁ and T₂ relaxation. A concentration calibration phantom and an in vivo study of intra-articular injection in a rat knee of known concentrations of iron oxide were assessed using the difference-ultrashort echo time sequence giving a positive, quantifiable, unambiguous iron signal and monotonic, increasing concentration response over a wide concentration range in the phantom with limited susceptibility artifacts and high contrast in vivo to all other tissues. This improved dynamic response to concentration opens possibilities for quantification due to its linear nature at physiologically relevant concentrations.

Nanostructured metal oxide-based materials as advanced anodes for lithium-ion batteries

The search for new electrode materials for lithium-ion batteries (LIBs) has been an important way to satisfy the ever-growing demands for better performance with higher energy/power densities, improved safety and longer cycle life. Nanostructured metal oxides exhibit good electrochemical properties, and they are regarded as promising anode materials for high-performance LIBs. In this feature article, we will focus on three different categories of metal oxides with distinct lithium storage mechanisms: tin dioxide (SnO(2)), which utilizes alloying/dealloying processes to reversibly store/release lithium ions during charge/discharge; titanium dioxide (TiO(2)), where lithium ions are inserted/deinserted into/out of the TiO(2) crystal framework; and transition metal oxides including iron oxide and cobalt oxide, which react with lithium ions via an unusual conversion reaction. For all three systems, we will emphasize that creating nanomaterials with unique structures could effectively improve the lithium storage properties of these metal oxides. We will also highlight that the lithium storage capability can be further enhanced through designing advanced nanocomposite materials containing metal oxides and other carbonaceous supports. By providing such a rather systematic survey, we aim to stress the importance of proper nanostructuring and advanced compositing that would result in improved physicochemical properties of metal oxides, thus making them promising negative electrodes for next-generation LIBs.

Iron-containing coatings obtained by microplasma method on aluminum with usage of homogeneous electrolytes

The microplasma synthesis method of multicomponent oxide coatings including iron oxide on an aluminum alloy surface has been studied. The homogeneous electrolytes containing iron complexes with Edta4− under pH 9–10 have been used. The dynamics reducing the pH of the electrolyte during formation process have been studied. The content of iron in the coatings has been measured spectrophotometricaly. Morphlogy and phase composition of the coatings have been studied by the scanning electron microscopy and X-ray powder diffraction.

Morphology and microstructure of magnetite and ilmenite inclusions in plagioclase from Adirondack anorthositic gneiss

It is well known that oriented iron and titanium oxide inclusions occur in pyroxenes and plagioclase of anorthosites and granulites, and they are attributed to exsolution at subsolidus conditions. The oxides occur as needles or platelets. In this study, we determine the morphology of oxide needles as well as their orientation in plagioclase (An 30–35) in anorthosite gneiss from the Adirondack mountains (New York). The investigation was done with electron backscatter diffraction (EBSD) in a scanning electron microscope, as well as Laue diffraction with a microfocus synchrotron X-ray beam at the Advanced Light Source in Berkeley. It was observed that the needle direction is [110] in magnetite and [1010] in ilmenite. The needle direction is consistently parallel to [001] of plagioclase. Furthermore, (111) of magnetite and (0001) of ilmenite are sub-parallel to (120) and (120) of plagioclase. We note that for directions [110] in the magnetite structure and [1010] in ilmenite, O atoms are close-packed, and (111) and (0001) are close-packed planes, correspondingly. In plagioclase, [001] is a direction with open channels as well as approximate alignment of Si tetrahedral edges, thus providing nucleation sites with a coincidence lattice relationship. (120) and (120) in this triclinic mineral are planes with approximate tetrahedral sides so that the relationship is structurally plausible. From Laue diffraction, we can determine that the magnetite needle axis is subject to an extensional stress, most likely attained during cooling of the inclusions within the plagioclase host.

Self-Assembly of Phospholipid–PEG Coating on Nanoparticles through Dual Solvent Exchange

We coated nanoparticles including iron oxide nanoparticles and quantum dots with phospholipid-PEG using the newly developed dual solvent exchange method and demonstrated that, compared with the conventional film hydration method, the coating efficiency and quality of coated nanoparticles can be significantly improved. A better control of surface coating density and the amount of reactive groups on nanoparticle surface is achieved, allowing conjugation of different moieties with desirable surface concentrations, thus facilitating biomedical applications of nanoparticles.

Multidisciplinary approach of the hyperarid desert of Pampas de La Joya in southern Peru as a new Mars-like soil analog

The distribution of living organisms, organic matter, and chemical properties in Mars-like environments on Earth can be used as a model to guide the investigation of possible habitable environments on Mars. This work aims to demonstrate that the place known informally as the “Mar de Cuarzo” (Sea of Quartz) in the Pampas de La Joya desert southern Peru (between 16°S and 17°S latitude), contains soils with characteristics similar to those found on the Martian surface. Using a multidisciplinary approach, we studied the environmental data, geology, organic matter content, oxidant activity, and microbiology of this area. Our data show that (1) Mar de Cuarzo is a hyper arid area with a lower concentration of organic matter than those found in the Mars-like soils from Yungay area (Atacama Desert in Chile), while at the same time having, comparable extreme environmental conditions, and very low levels of microorganisms. (2) The detrital components of the soils come essentially from the Andean volcanic chain and local outcrops of Precambrian gneisses and Cretaceous granitic batholiths. (3) The presence of microclimates, geomorphological features, and the high influence of the “El Niño Southern Oscillation (ENSO)” allowed the formation of exotic and heterogeneous chemical deposits in these soils, including iron oxides, sulfates, and other evaporites. (4) Thermal volatilization in these soils (using methods similar to the Viking and Phoenix instruments) shows high oxidant activity. (5) Labeled release experiment (similar to the Viking instrument) shows high degradation of nutrients added in these soils. Altogether, the Mar de Cuarzo area in the Pampas de La Joya is an interesting place for astrobiological studies as a new analog to Mars, and for comparative analyses with other hyperarid analogs as Yungay.

Rate of Iron Carburization by Carbon in Slags through Carbon/Slag and Slag/Metal Reactions at 1723 K

The carbide capacity of CaO–SiO2–Al2O3 slags and the rate of iron carburization through successive phases of carbon, slag, and metal are measured at 1723 K. The carbide capacity of high-basicity slag is higher than that of simulated blast-furnace (BF) slag. The iron carburization rate through CaO–SiO2–Al2O3 slags is much smaller than the rate of direct carburization or that through slags including iron oxide. It is found that the rate of carburization of iron through slags increases with increasing slag basicity. The apparent rate constant of the carburization reaction is derived to be k = 2.71 × 10–5 mol/(m2 s) for the simulated BF slag and k = 8.00 × 10–5 mol/(m2 s) for the high-basicity slag. The rate increases with increasing slag basicity, increasing carbide capacity, and decreasing oxygen partial pressure.

Characterization of a calcium phospho-silicated apatite with iron oxide inclusions

An iron oxide containing calcium phosphate–silicate hydroxyapatite was synthesized by calcination at 900 °C of a sample obtained by precipitation in basic aqueous solution of Ca, P, Si, Fe and Mg containing acidic solution made from dissolution of natural minerals. XRD and FTIR were used for crystallographic characterization of the main apatitic phase. Its composition was determined using ICP-AES. EDX coupled with SEM and TEM evidenced the heterogeneity of this compound and the existence of iron–magnesium oxide. Magnetic analyses highlighted that this phase was non-stoichiometric magnesioferrite (Mg 1.2Fe 1.8O 3.9) spherical nanoparticles. Those analyses also put into evidence the role of calcination in synthesis. Carbonates detected by FTIR and estimated by SEM-EDX in non-calcinated sample were removed from apatitic structure, and crystallization of apatite was enhanced during heating. Moreover, there was phase segregation that led to magnesioferrite formation.

Pigments from the Middle Palaeolithic levels of Es-Skhul (Mount Carmel, Israel)

Discovery of pigments at Middle Palaeolithic sites is of interest in the context of the ongoing debate about the tempo and mode of the emergence of modern human behaviour. Here we analyse four previously undescribed fragments of pigmental material from Es-Skhul shelter, layer B, Israel, McCown excavations, identified at the Department of Palaeontology, Natural History Museum, London. One of them is still partially embedded in the hard breccia characteristic of this layer. Inspection of breccia fragments from layer B has led to the identification of small pieces of red and orange pigmental material still enclosed in large clasts, further corroborating the attribution of the larger pieces analysed in this study to layer B. The four objects are studied using optical microscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX), X-ray micro-diffraction (μ-XRD), Particle-Induced X-Ray Emission Spectrometry (PIXE), and Proton-Induced Gamma-ray Emission (PIGE). The specimens display various hues of yellow, orange, red, and one of them presents a gradual variation of shade from yellow to dark orange. SEM/EDX analysis identifies two specimens (Skhul 1 and 2) as being composed of iron-rich calcium phosphate, the third (Skhul 3) of potassium-rich clay with titanium-rich iron oxide inclusions, and the fourth (Skhul 4) of pure iron oxide crystals. TEM/EDX and μ-XRD analysis demonstrate that three pieces (Skhul 1, 2 and 4) were heated to at least 300 °C, a process that has partially or completely dehydrated goethite into haematite and changed their pristine yellow colour into orange or red. Skhul 3 shows no sign of heating, suggesting that its haematite content has a geological origin. The different mineral composition of the pieces suggests that they must come from a variety of sources. This implies that the associated collection strategies included the selection of materials that differed not only with respect to colour but also with respect to other physical and chemical properties. Although no formal proof exists that these lumps of pigmental material were deliberately heated, results obtained are consistent with this explanation.

Synthesis and stability of iron nanoparticles for lunar environment studies

Abstract– Simulants of lunar dust are needed when researching the lunar environment. However, unlike the true lunar dust, today’s simulants do not contain nanophase iron. Two different processes have been developed to fabricate nanophase iron to be used as part of a lunar dust simulant. (1) The first is to sequentially treat a mixture of ferric chloride, fluorinated carbon, and soda lime glass beads at about 300 °C in nitrogen, at room temperature in air, and then at 1050 °C in nitrogen. The product includes glass beads that are gray in color, can be attracted by a magnet, and contains α-iron nanoparticles (which seem to slowly lose their lattice structure in ambient air during a period of 12 months). This product may have some similarity to the lunar glassy agglutinate, which contains FeO. (2) The second is to heat a mixture of carbon black and a lunar simulant (a mixed metal oxide that includes iron oxide) at 1050 °C in nitrogen. This process simulates lunar dust reactions with the carbon in a micrometeorite at the time of impact. The product contains a chemically modified simulant that can be attracted by a magnet and has a surface layer whose iron concentration increased during the reaction. The iron was found to be α-iron and Fe3O4 nanoparticles, which appear to grow after the fabrication process. This growth became undetectable after 6 months of ambient air storage, but may last for several years or longer.

Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits

The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxide-associated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, high-grade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supercontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supercontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate world-class IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated.

Fe, C, and O isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in ~2.5 Ga marine environments

Combined Fe, C, and O isotope measurements of ~ 2.5 Ga banded iron formation (BIF) carbonates from the Kuruman Iron Formation and underlying BIF and platform Ca–Mg carbonates of the Gamohaan Formation, South Africa, constrain the biologic and abiologic formation pathways in these extensive BIF deposits. Vertical intervals of up to 100 m were sampled in three cores that cover a lateral extent of ~ 250 km. BIF Fe carbonates have significant Fe isotope variability ( δ 56Fe = + 1 to − 1‰) and relatively low δ 13C (down to − 12‰) and δ 18O values ( δ 18O ~ + 21‰). In contrast, Gamohaan and stratigraphically-equivalent Campbellrand Ca–Mg carbonates have near-zero δ 13C values and higher δ 18O values. These findings argue against siderite precipitation from seawater as the origin of BIF Fe-rich carbonates. Instead, the C, O, and Fe isotope compositions of BIF Fe carbonates reflect authigenic pathways of formation in the sedimentary pile prior to lithification, where microbial dissimilatory iron reduction (DIR) was the major process that controlled the C, O, and Fe isotope compositions of siderite. Isotope mass-balance reactions indicate that the low- δ 13C and low- δ 18O values of BIF siderite, relative to those expected for precipitation from seawater, reflect inheritance of C and O isotope compositions of precursor organic carbon and ferric hydroxide that were generated in the photic zone and deposited on the seafloor. Carbon–Fe isotope relations suggest that BIF Fe carbonates formed through two end-member pathways: low- δ 13C, low- δ 56Fe Fe carbonates formed from remobilized, low- δ 56Fe aqueous Fe 2+ produced by partial DIR of iron oxide, whereas low- δ 13C, high- δ 56Fe Fe carbonates formed by near-complete DIR of high- δ 56Fe iron oxides that were residual from prior partial DIR. An important observation is the common occurrence of iron oxide inclusions in the high- δ 56Fe siderite, supporting a model where such compositions reflect DIR “in place” in the soft sediment. In contrast, the isotopic composition of low-Fe carbonates in limestone/dolomite may constitute a record of seawater environments, although our petrographic studies indicate that the presence of pyrite in most low-Fe carbonates may influence the Fe isotope compositions. The combined Fe, C, and O isotope data from Kuruman BIF carbonates indicate that BIF siderites that have negative, near-zero, or positive δ 56Fe values may all record biological Fe cycling, where the range in δ 56Fe values records differential Fe mobilization via DIR in the sediment prior to lithification. Our results demonstrate that the inventory of low- δ 56Fe marine sedimentary rocks of Neoarchean to Paleoproterozoic age, although impressive in volume, may represent only a minimum of the total inventory of Fe that was cycled by bacteria.