Showing papers on "Hematite published in 1995"

The Growth Mechanism of α-Fe2O3 Ellipsoidal Particles in Solution

Abstract: The mechanism of formation of ellipsoidal hematite (α-Fe 2 O 3 ) particles, obtained by homogeneous precipitation from ferric salt solutions under different experimental conditions, is reported. Monocrystalline ellipsoidal particles having an axial ratio of 6, precipitated by aging FeCl 3 solutions in the presence of small amounts of phosphate anions, are formed by an ordered aggregation of smaller ellipsoids. The nature of the forces involved in this process and the role played by phosphate anions are discussed. Ellipsoidal hematite particles (axial ratio 1.65) obtained by aging Fe(NO 3 ) 3 acidic (HNO 3 ) solutions were found to contain a certain amount (≈8%) of goethite (α-FeOOH) in the particle core. A formation mechanism involving the heterogeneous nucleation of hematite on initially precipitated goethite cores is suggested.

Canted Antiferromagnetism: Hematite

Abstract: Crystal growth and characterization of hematite theory magnetization and susceptibility phase transitions heat capacity and other thermodynamic quantities electrical transport and optical properties elastic and magneto-elastic interactions electron magnetic resonance hyperfine interactions small particles and thin films surface and rock magnetism domains magnetization processes doped hematite.

Dehydroxylation of Aluminous Goethite: Unit Cell Dimensions, Crystal Size and Surface Area

Abstract: This work investigates unit cell dimensions, crystal size and specific surface area of aluminous goethite that was progressively dehydroxylated to form hematite. Goethite synthesized from the ferrous system altered to hematite with DTGA maximum increasing from 236° to 273°C for 0 to 30.1 mole % Al-substitution. Unit cell dimensions of goethite and hematite decreased as Al-substitution increased and increased as excess OH increased. The crystallographically equivalent a axis of goethite and c axis of hematite were more sensitive than other axes to the presence of excess structural OH associated with Al-substitution. Specific surface area increased from 147 to 288 m2/g for goethite and from 171 to 230 m2/g for hematite as Al-substitution increased. An increase in specific surface area on heating goethite at temperatures between 200° and 240°C is related to a decrease in the size of coherently diffracting domains of goethite crystals and to the development of pore and structural defects associated with the formation of hematite. The decrease in specific surface area for heating temperatures above 240°C is attributed to the growth of hematite crystals by diffusion.

Influence of Nonstoichiometry and the Presence of Maghemite on the Mössbauer Spectrum of Magnetite

Abstract: Several samples of large- and small-particle magnetite (Fe3O4), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mossbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly- or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400°C, transforms to a mixture of well-crystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300°C in a controlled O2 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mossbauer spectra of the decomposition products at T < 200°C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.

Evolution of Hematite Surface Microtopography Upon Dissolution by Simple Organic Acids

Abstract: The surface microtopography of hematite over the course of dissolution in oxalic and citric acids was examined by in-situ and ex-situ atomic-force microscopy, ln-situ imaging of the basal-plane surface of a centimeter-scale natural hematite sample immersed in 2 mM citric acid demonstrated that the basal-plane surface was relatively unreactive; rather, dissolution occurred along step edges and via etch-pit formation. Ex-situ imaging of synthetic hematite particles following batch dissolution in 1 mM oxalic acid showed similar dissolution features on basal-plane surfaces; in addition, etching along particle edges was apparent. The presence of etch features is consistent with a surface-controlled dissolution reaction. The results are in agreement with previous investigations suggesting that the basal-plane surface is relatively unreactive with respect to ligand exchange. Both in-situ and ex-situ imaging of particle surfaces can provide valuable information on the roles of surface structures and microtopographic features in mineral dissolution.

Hematite, pyroxene, and phyllosilicates on Mars: Implications from oxidized impact melt rocks from Manicouagan Crater, Quebec, Canada

Abstract: Visible and near-IR refectivity, Moessbauer, and X ray diffraction data were obtained on powders of impact melt rock from the Manicouagan Impact Crater located in Quebec, Canada. The iron mineralogy is dominated by pyroxene for the least oxidized samples and by hematite for the most oxidized samples. Phyllosilicate (smectite) contents up to approximately 15 wt % were found in some heavily oxidized samples. Nanophase hematite and/or paramagnetic ferric iron is observed in all samples. No hydrous ferric oxides (e.g., goethite, lepidocrocite, and ferrihydrite) were detected, which implies the alteration occurred above 250 C. Oxidative alteration is thought to have occurred predominantly during late-stage crystallization and subsolidus cooling of the impact melt by invasion of oxidizing vapors and/or solutions while the impact melt rocks were still hot. The near-IR band minimum correlated with the extent of aleration Fe(3+)/Fe(sub tot) and ranged from approximately 1000 nm (high-Ca pyroxene) to approximately 850 nm (bulk, well-crystalline hematite) for least and most oxidized samples, respectively. Intermediate band positions (900-920 nm) are attributed to low-Ca pyroxene and/or a composite band from hematite-pyroxene assemblages. Manicouagan data are consistent with previous assignments of hematite and pyroxene to the approximately 850 and approximately 1000nm bands observed in Martian reflectivity spectra. Manicouagan data also show that possible assignments for intermediate band positions (900-920 nm) in Martian spectra are pyroxene and/or hematite-pyroxene assemblages. By analogy with impact melt sheets and in agreement with observables for Mars, oxidative alteration of Martian impact melt sheets above 250 C and subsequent erosion could produce rocks and soils with variable proportions of hematite (both bulk and nanophase), pyroxene, and phyllosilicates as iron-bearing mineralogies. If this process is dominant, these phases on Mars were formed rapidly at relativly high temperatures on a sporadic basis throughout the history of the planet. The Manicouagan samples also show that this mineralogical diversity can be accomplished at constant chemical composition, which is also indicated for Mars from the analyses of soil at the two Viking landing sites.

Mossbauer spectroscopic evidence for citrate-bicarbonate-dithionite extraction of maghemite from soils

Abstract: In a previous paper, we used powder X-ray diffraction and changes in magnetic susceptibility to argue the importance of pedogenic maghemite to soils and the efficacy of the chemical extractant citrate-bicarbonate-dithionite (CBD) to preferentially remove pedogenic maghemite from soil samples while not removing coarse-grained magnetite. Although X-ray diffraction provides strong support for this contention, M6ssbauer spectroscopy is the method of choice for determining the oxidation state of iron in minerals and for inferring mineralogy of the iron oxide phases. Our objective in this work was to seek confirming evidence of the importance of maghemite as a pedogenic mineral and the usefulness of the CBD procedure in separating pedogenic maghemite from lithogenic magnetite. We present M6ssbauer data on magnetic fractions from pre- and post-CBD treated soil samples. Six of the l0 samples had only maghemite as the sextet component and after CBD treatment, four lost between 96 and 100% of the magnetic susceptibility. Two samples were interpreted as highly oxidized magnetite or a mixture of magnetite and maghemite. We cannot distinguish between these with M6ssbauer spectroscopy. In the remaining two samples, iron existed as hematite, ilmenite, magnetite and minor (< 10%) amounts of maghemite. Our results provide additional support for pedogenic maghemite in soils and for the pref- erential removal of maghemite by the CBD procedure.

Complex cementation textures and authigenic mineral assemblages in recent concretions from the Lincolnshire Wash (east coast, UK) driven by Fe(0) to Fe(II) oxidation

Abstract: Concretions are common in some of the modern intertidal sediments on the Lincolnshire coast of the Wash. The mineralogy and geochemistry of numerous examples of these concretions have been studied in detail. The majority have metallic nuclei and those that do not, exhibit textures which suggest that they originally did so. Petrographic observations indicate that the cements within these concretions precipitated in a distinct sequence that is spatially developed around the metallic nucleus and that are arranged from core to periphery: (a) a ferrous hydroxy chloride mineral (similar in bulk composition to akaganeite) together with iron monosulphide (amorphous FeS and mackinawite), pyrite and possibly elemental sulphur; (b) ferroan carbonate cements (including siderite, ankerite and calcite); (c) mixed ferrous and ferric minerals (“green rust” together with magnetite and possibly greigite); (d) fully oxidized minerals (including akaganeite, goethite, hematite, gypsum and a complex basic sulphate of Fe, Ca, Mg, Si and Al). This latter material has not been possible to characterize fully but is probably an amorphous mixture. The initiating reaction for the precipitation of these cements is anaerobic corrosion of iron at zero valence state with sulphate as an oxidant. Metal, hydroxide and monosulphide are found in close spatial association, indicating the reaction: 4 Fe 0 + S O 4 2 − + 4 H 2 O → 3 Fe ( OH ) 2 + 2 OH − + FeS Further away from the metallic nucleus, carbonates cement the host clastic sediment. These are the products of reaction between the first-formed hydroxides and pore water solutes (principally HCO −3 , Mg 2+ and Ca 2+ ). When oxygen gains access to the growing concretion, Fe(II) minerals are replaced very rapidly by akaganeite and either goethite or hematite. Gypsum and other sulphate minerals also precipitate as new cements around the periphery of the concretions. The cementation process in these concretions is driven by the extreme instability of metallic iron (here, relic military armaments and shrapnel fragments) in contact with saline, anaerobic water and is indicative more of cathodic corrosion than the growth of ancient carbonate–sulphide concretions.

Viscosity and Structural State of Iron in Coal Ash Slags under Gasification Conditions

Abstract: The effect of Fe 3+ /Fe 2+ redox equilibrium in ferrosilicate melts and ash slags on their viscosities is discussed. Viscosity experiments were carried out on two ash slags with low (Rochelle) and high (Illinois No. 6) iron content in CO 2 /CO (40/60) and air atmospheres as a function of temperature and as a function of time at a constant temperature. The lower values of viscosities measured in both ash slags under reducing conditions are assigned to the transformation of network-forming ferric iron to the network-modifying ferrous iron. Anomalies in viscosity are apparent in the reduced form of Illinois No. 6 ash slag during its oxidation in air at 1370°C (slightly above the temperature of critical viscosity of the oxidized form of slag). First the viscosity significantly increases and then decreases to that of the oxidized form. The Fe 3+ /ΣFe ratio increases from 0.12 to 0.84. The conclusion is that changes of local configuration in the melt (coordination number and charge of iron ions and stability and size of crystalline phases such as hematite and quartz) determine the viscosity in Illinois No. 6 ash slag. Redox ratios of iron and structural positions of Fe 3+ and Fe 2+ ions in quenched slags were determined with Mossbauer spectroscopy

Effect of Mixed-grinding on Reduction Process of Carbonaceous Material and Iron Oxide Composite

Abstract: Mixing state and structure of composite significantly influence reduction kinetics of carbonaceous materials/iron oxides mixture. The present study reports the effect of mixed-grinding of the mixture on the reduction of the composite by indirect heating. Mixtures prepared by different combinations of hematite and graphite reagents, iron ores and metallurgical coke were mixed-ground by two types of ball mills. TG-DTA measurements for the composites were carried out under different heating patterns and total pressures. Reaction temperature remarkably decreases by the mixed-grinding operation in any combinations of materials and total pressures. A catalytic effect of iron produced during reaction is observed when using graphite reagents but seems to be inhibited when using metallurgical coke. A simple kinetic analysis suggests that gasification of carbon is promoted through enhancement of the catalytic effect and/or change of rate limiting step from the gasification of carbon to the direct reactions between iron oxides and carbon.

Acid dissolution of synthetic aluminous goethite before and after transformation to hematite by heating

Abstract: Measurements of the kinetics of acid dissolution of synthetic aluminous goethites and corresponding hematites produced by heating of parent Al-goethites at various temperatures were carried out in 1 M HCI at 30, 40 and 50~ Dissolution-time curves show sigmoidal shapes for the goethites (110~ whereas deceleratory shapes were obtained for most of the partly and fully dehydroxylated samples. The dissolution rate for all materials decreased with increasing AI substitution and increased with increasing dissolution temperature, specific surface area and heating temperature. On a unit surface area basis, hematite dissolved ~ 2-8 times faster than goethite. Dissolution kinetics of most heated goethite samples (200-260~ were quite well described (R 2 > 0.96) by the modified first-order Kabai equation. The activation energy and frequency factor for dissolution increased with increasing A1 substitution.

Mössbauer spectroscopy analysis of iron‐containing minerals in the Chinese loess

Abstract: Chinese loess has been analyzed in bulk, using Mossbauer spectroscopy, in order to investigate the effects of climate on the overall iron-containing mineralogy. Strongly weathered loess is found to differ from weakly weathered loess in two significant respects. It contains a much larger content of fine-grained (superparamagnetic) hematite and the ratio of paramagnetic Fe 3+ /Fe 2+ is substantially increased. The suggestion is that weathering results in the liberation of Fe from silicate minerals and the formation of fine-grained hematite. Measurements of total iron content by atomic absorption spectrometry highlight that the strongly weathered loess has a higher content of iron than the weakly weathered loess, and that this is too large an effect to be explained by the leaching of carbonate. The indication is that the mineralogy of the dust input to the loess plateau varies as a function of climate. One possibility is that the grain size (and hence mineralogy) of the dust fluctuates in response to vegetational cover in proximal source areas. During interglacial times the vegetational cover would expand to stabilize these proximal sources, resulting in a bias to finer dust from more distant sources. Thus the mechanisms linking climate and mineralogy in the Chinese loess may be more complex than previously thought.

The Use of Small Angle Light Scattering to Study Structure of Flocs

Abstract: In this paper, a new approach using small angle light scattering to study the structure of large micron sized flocs is described. The flocs were made up from uniformly sized, approximately spherical hematite particles, induced to aggregate using 250 mM KCl to ensure that growth was governed by diffusion limited aggregation alone. The fractal dimensions of the hematite aggregates changed from 1.73 to 2.23 ± 0.05 as the aggregation proceeded, indicating that restructuring occurred during aggregation. The fractal dimensions obtained from the small angle light scattering technique are compared with those obtained from the combination of volume fraction and floc size measurement. The steady state fractal dimensions (2.23 ± 0.05) obtained by small angle light scattering are comparable to the fractal dimensions obtained from determining the slope of the relationship between floc volume fraction and floc size (2.09 and 2.14 ± 0.05). The discrepancy between the fractal dimensions obtained by these techniques during the initial stages of aggregation is due to the fact that the latter method measures the average fractal dimensions, whilst the former method measures the fractal dimensions at any particular instant.

Mid-infrared transmission spectra of crystalline and nanophase iron oxides/oxyhydroxides and implications for remote sensing of Mars

Abstract: Ferric-iron-bearing materials play an important role in the interpretation of visible to near-IR Mars spectra, and they may play a similarly important role in the analysis of new mid-IR spacecraft spectral observations to be obtained over the next decade. We review exisiting data on mid-IR transmission spectra of ferric oxides/oxyhydroxides and present new transmission spectra for ferric-bearing materials spanning a wide range of mineralogy and crystallinity. These materials include 11 samples of well-crystallized ferric oxides (hematite, maghemite, and magnetite) and ferric oxyhydroxides (goethite, lepidocrocite). We also report the first transmission spectra for purely nanophase ferric oxide samples that have been shown to exhibit spectral similarities to Mars in the visible to near-IR and we compare these data to previous and new transmission spectra of terrestial palagonites. Most of these samples show numerous, diagnostic absorption features in the mid-IR due to Fe(3+) - 0(2-) vibrational transitions, structural and/or bound OH, and/or silicates. These data indicate that high spatial resolution, moderate spectral resolution mid-IR ground-based and spacecraft observations of Mars may be able to detect and uniquely discriminate among different ferric-iron-bearing phases on the Martian surface or in the airborne dust.

Particle adhesion on gelatin-coated glass surfaces

Abstract: The adhesion phenomena of monodispersed spherical hematite particles on gelatin-coated glass beads were evaluated using the packed column technique and compared with the same system in the absence of the protein. Uptake is observed when the collector beads, with or without gelatin, and the hematite particles have opposite signs of charge, although coating lowers the particle deposition rate and enhances the multilayer attachment. Furthermore, hematite particles are bound irreversibly to the gelatin-covered collector, whereas they can be fully removed from the clean glass. In either case the surface charge of interacting surfaces must be altered by rinsing the column with solutions of appropriate pH in order to cause repulsion. This behavior suggests that the interaction between hematite and glass is affected by physical forces only, whereas in the presence of gelatin, chemical bonds are formed between the particles and the substrate.

Analysis of iron oxidation at high temperatures

Abstract: A new theory for the high-temperature oxidation of iron is proposed, in which the rate-limiting step is ternary diffusion of ferric, ferrous, and oxygen ions in the iron oxides that are formed. The predictions of this theory are compared with previously published experimental data. The only thermodynamic information required is a phase diagram.