Showing papers on "Magnetite published in 1995"
TL;DR: In this article, the effect of citrate ions on the growth of magnetite particles is investigated and the characterisation of the maghemite particles by various techniques (X-ray diffraction, transmission electron microscopy, magnetic measurements) is described.
462 citations
TL;DR: In this paper, the effect of activation and reaction treatments on the resulting phase transformations in a commercial, precipitated, and spray-dried Fe2O3-CuO-K2O Fischer-Tropsch catalyst has been studied.
336 citations
TL;DR: This study shows that marine and freshwater strains of the bacterium Shewanella putrefaciens are capable of the rapid dissolution and reduction of magnetite, converting millimolar amounts to soluble Fe(II)in a few days at room temperature, and suggests that magnetite reduction is coupled to carbon metabolism in S. putreficiens.
Abstract: Magnetite (Fe3O4) is an iron oxide of mixed oxidation state [Fe(II), Fe(III)] that contributes largely to geomagnetism and plays a significant role in diagenesis in marine and freshwater sediments. Magnetic data are the primary evidence for ocean floor spreading and accurate interpretation of the sedimentary magnetic record depends on an understanding of the conditions under which magnetite is stable. Though chemical reduction of magnetite by dissolved sulfide is well known, biological reduction has not been considered likely based upon thermodynamic considerations. This study shows that marine and freshwater strains of the bacterium Shewanella putrefaciens are capable of the rapid dissolution and reduction of magnetite, converting millimolar amounts to soluble Fe(II)in a few days at room temperature. Conditions under which magnetite reduction is optimal (pH 5-6, 22-37 degrees C) are consistent with an enzymatic process and not with simple chemical reduction. Magnetite reduction requires viable cells and cell contact, and it appears to be coupled to electron transport and growth. In a minimal medium with formate or lactate as the electron donor, more than 10 times the amount of magnetite was reduced over no carbon controls. These data suggest that magnetite reduction is coupled to carbon metabolism in S. putrefaciens. Bacterial reduction rates of magnetite are of the same order of magnitude as those estimated for reduction by sulfide. If such remobilization of magnetite occurs in nature, it could have a major impact on sediment magnetism and diagenesis.
275 citations
TL;DR: The results show that in some magnetotactic bacteria, external environmental conditions such as redox and/or oxygen or hydrogen sulfide concentrations may affect the composition of the nonmetal part of the magnetosome mineral phase.
Abstract: A slowly moving, rod-shaped magnetotactic bacterium was found in relatively large numbers at and below the oxic-anoxic transition zone of a semianaerobic estuarine basin. Unlike all magnetotactic bacteria described to date, cells of this organism produce single-magnetic-domain particles of an iron oxide, magnetite (Fe(inf3)O(inf4)), and an iron sulfide, greigite (Fe(inf3)S(inf4)), within their magnetosomes. The crystals had different morphologies, being arrowhead or tooth shaped for the magnetite particles and roughly rectangular for the greigite particles, and were coorganized within the same chain(s) in the same cell with their long axes along the chain direction. Because the two crystal types have different crystallochemical characteristics, the findings presented here suggest that the formation of the crystal types is controlled by separate biomineralization processes and that the assembly of the magnetosome chain is controlled by a third ultrastructural process. In addition, our results show that in some magnetotactic bacteria, external environmental conditions such as redox and/or oxygen or hydrogen sulfide concentrations may affect the composition of the nonmetal part of the magnetosome mineral phase.
251 citations
TL;DR: In this paper, a commercial promoted precipitated iron catalyst (100 Fe/5 Cu/4.2 K/25 SiO2 by weight) was characterized after different pretreatment conditions and after Fischer-Tropsch (FT) synthesis in a fixed bed reactor.
206 citations
TL;DR: In this article, the relationship between the magnetic and magnetite shape fabrics has been examined using shape fabric analysis on selected and oriented thin sections of the geology of the pluton of Gameleiras in Brazil.
165 citations
TL;DR: In this paper, the progress in spinel ferrite research is discussed and a large number of oxides with a metal-oxygen ratio of 0.75 as composition is known to crystallize into the spinel structure.
Abstract: Publisher Summary This chapter discusses the progress in spinel ferrite research. A large number of oxides with a metal–oxygen ratio of 0.75 as composition is known to crystallize into the spinel structure. Among these oxides, magnetite Fe 3 O 4 is an important compound, from which the spinel ferrites can be derived by partial substitution of the iron ions by other cations. Between most of the spinel oxides solid solutions can be formed, which means that a great variety of spinel oxides is possible. The simple spinel ferrites are those ferrites in which x = 0 and the bivalent metal ion is Mg or Cd or one of the bivalent transition metal elements Mn, Fe, Co, Ni, Cu, and Zn. Partial solid solutions of the simple ferrites are also possible with compounds that either do not exist or that do not have the spinel structure. A great variety in the chemical composition of spinel ferrites is possible. The diversity in composition results into a large range of physical properties that permits the tuning of the properties for specific applications and makes spinel ferrites of particular interest.
121 citations
TL;DR: The magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains as mentioned in this paper.
Abstract: The magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains. Measurement of the low-field magnetic anisotropy of single multi-domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio. Interactions between two elongated grains were studied experimentally using two types of grain arrangement: an “aligned” configuration and a “side-by-side” configuration. For a distance between the grain centers equal to approximately twice the average grain size, the magnetic susceptibility and its anisotropy are enhanced in both configurations, and the direction of kmax, the easiest magnetization axis, is stable in the “aligned” configuration, whereas it rotates toward an orthogonal direction in the “side-by-side” configuration. Depending on the distribution of the interacting magnetite grains, magnetic interactions may therefore either increase the whole-rock anisotropy magnitude, or reduce it as in the given example of the granitic rocks from Madagascar.
121 citations
96 citations
TL;DR: H hippocampal material from deceased normal and epileptic subjects, and from the surgically removed epileptogenic zone of a living patient, had magnetic characteristics similar to those reported for other parts of the brain, which indicate that the magnetic material is present in a wide range of grain sizes.
95 citations
TL;DR: In this paper, it is shown that it is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.
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.
TL;DR: In this article, the superparamagnetic grains, ultra-fine (d < 0.05µm) particles unable to retain a remanence at room temperature, have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks.
Abstract: Superparamagnetic grains, ultra-fine (d < 0.05µm) particles unable to retain a remanence at room temperature, have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks. In marine sediments, fine-grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction. On the basis of new magnetic hysteresis, low-temperature, and geochemical studies of pelagic sediments from the western equatorial Pacific Ocean, it is proposed that reduction processes increase rather than deplete the ultra-fine magnetic grain population. At the modern Fe-redox boundary enhanced superparamagnetism coincides with a coarsening of remanence-carrying grains. Superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically-driven fluctuations in organic carbon (Corg) supply. Together with hysteresis characteristics, changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady-state magnetic mineral reduction.
TL;DR: The opaque minerals of 19 CK chondrites from Antarctica and the Nullarbor Region, Australia were studied mineralogically by optical and analytical methods as discussed by the authors, and the most striking feature of these meteorites is the presence of magnetite as the most abundant opaque phase.
TL;DR: In this paper, composite microspheres of core-shell type were prepared by a seeded polymerization using monodispersed polystyrene seed latex (Ps) combined with an in situ dispersion of magnetite (Fe3O4) fine particles.
Abstract: Composite microspheres of core-shell type were prepared by a seeded polymerization using monodispersed polystyrene seed latex (Ps) combined with an in situ dispersion of magnetite (Fe3O4) fine particles. The heterogeneous polymerization was carried out in aqueous dispersions of the Fe3O4 particles modified with sodium oleate. All the synthetic processes were carried out in a wet state to avoid serious agglomeration. The morphology of the composite particle and the size distribution were examined to discuss the effects on the polymerization parameters, such as monomer concentration, type and concentration of an initiator, magnetite particle concentration and the method of surface modification of Fe3O4.
TL;DR: Li et al. as mentioned in this paper conducted a rock magnetic study of selected Chinese loess samples before and after citrate-bicarbonate-dithionite (CBD) treatment, revealing new details of the magnetic carriers which are related to paleoclimate change.
TL;DR: In this article, it was shown that the fayalite formed through reaction of SiO(g), released by decomposition of enstatite, with magnetite and sulfide.
TL;DR: In this article, the authors 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 magnetite from soil samples.
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.
TL;DR: In this article, the supercritical fluid phase exsolved during cooling after the consolidation of the plutons (800-900°C), results in a H+, Cl− and sodic enrichment, and in the sequential leaching of Fe (at less than 700°C) then Ca and Mg (between 600 and 500°C).
Abstract: North of El Algarrobo (one of the four main deposits of the Chilean Iron Belt), the iron-mineralization (magnetite-etrmolite/actinolite-apatite) is related to clinoand orthopyroxene diorite intrusions which have crystallized at shallow depth (4km) under increasing oxygen fugacities. The supercritical fluid phase exsolved during cooling after the consolidation of the plutons (800–900°C), results in a H+, Cl− and sodic enrichment, and in the sequential leaching of Fe (at less than 700°C), then Ca and Mg (between 600 and 500°C) from minerals of the primary magmatic diorite assemblage: titanomagnetite-ilmenite, plagioclase (An70–40), augite, hypersthene. As a consequence of the cationic leaching, the lower mobility of silica and aluminium and the enrichment in sodium, residual altered dioritic rocks present a retromorphic mineral assemblage evolving down to boundary conditions of the greenchist-amphibolite facies (450°C). Fe, Mg and Ca are carried in cationic form associated with Cl− anions, toward cooler rocks where they are precipitated. The deposition (between 550 and 450°C) of magnetite, followed iron-mineralization paragenesis, and occurred in fractured zones located both in altered diorites and contact andesites.
TL;DR: In this paper, the structural transformation of α-Fe 2 O 3 to Fe 3 O 4 was investigated by Mossbauer effect spectroscopy and the results were consistent with a primarily physical origin for the transformation.
TL;DR: In this paper, a method is presented to calculate ab initio exchange constants and spin-wave excitations of multi-sublattice magnetic structures on the basis of total-energy calculations of incommensurate magnetic structures.
Abstract: A method is presented to calculate ab initio exchange constants and spin-wave excitations of multi-sublattice magnetic structures on the basis of total-energy calculations of incommensurate magnetic structures. Here the exchange energies, dispersion curves and Curie temperature for magnetite (Fe3O4) are obtained and compared with experimental results.
TL;DR: In this paper, a p(1 × 1) reconstruction of the Fe 3 O 4 (001) surface has been observed for epitaxial thin films of magnetite, which has not been observed before for magnetite.
TL;DR: In this article, the modified increment method is applied to the calculation of oxygen isotope fractionation in different structures of magnetite and the results show that, at isotopic equilibrium, the magnetite of a spinel-type structure has the very similar behavior of oxygen atom partitioning to hematite, but it is considerably depleted in 18 O relative to the magnetites of an inverse spinel type structure.
TL;DR: In this paper, the authors reported a method for preparing chromium-doped magnetite by heating chromiumdoped iron(III)hydroxoacetates (IHA), prepared both by coprecipitation and by impregnation, which provides the production of the catalyst in its active form avoiding the step of activation in industrial processes.
Abstract: Chromium-doped magnetite is the traditional catalyst used in the high temperature shift reaction in industrial processes. This work reports a method for preparing this solid, by heating chromium-doped iron(III)hydroxoacetates (IHA), prepared both by coprecipitation and by impregnation. This method provides the production of the catalyst in its active form avoiding the step of activation in industrial processes. It was noted that the presence of chromium affects magnetite formation as well as its characteristics. IHA containing chromium produces magnetite at higher temperature than does the pure IHA. Chromium-doped magnetites have higher surface areas, are less crystalline, have lower Fe(II)/Fe(III) ratios and are catalysts more active than those obtained from plain IHA. These effects depend on the preparation method of the precursor. It was also found that the impregnated sample is made of aggregates of a few crystals whereas the coprecipitate is made of smaller polycrystalline particles. In addition, the coprecipitation method leads to a more even distribution of chromium in solids. These observations are consistent with the best performance shown by the catalyst prepared by coprecipitation.
TL;DR: This article showed that basalt is oxidized in gas mixtures with CO number densities approximately equal to those at the surface of Venus and showed that hematite and Fe 3+ in pyroxene are produced in the oxidized basalt.
TL;DR: In this article, the authors describe several types of mineralization from the upper part of the Bushveld complex as well as the processes leading to their formation, including Vanadium and Ti-bearing magnetite layers, which occur throughout the entire Upper Zone of the intrusion, have formed in response to magma mixing events, which resulted either from a breakdown of densely stratified liquid layers or an influx of a small volume of new magma.
TL;DR: In this paper, a chemical shift of about 5 eV was observed between ferrous and ferric ions in the XANES absorption spectra of FeO and Fe2O3 where the Fe ions coordinate in regular octahedra.
Abstract: A chemical shift of about 5 eV was observed between ferrous and ferric ions in the XANES absorption spectra of FeO and Fe2O3 where the Fe ions coordinate in regular octahedra. Experimental f’e was estimated based on the cross‐section data in the 108 eV region at the Fe K edge using the Cromer and Liberman’s equation. The result shows that the maximum difference in f’e between Fe2+ and Fe3+ is about 2.5 at the longer wavelength side of the Fe2+K edge. The XANES spectrum of Fe3O4 (magnetite) lies between those of FeO and Fe2O3. Synchrotron x‐ray‐diffraction data of a magnetite crystal were measured on a vertical‐type four‐circle diffractometer and used for evaluation at seven wavelengths of 1.7415, 1.7420, 1.7425, 1.7431, 1.7452, 1.7499, and 1.7567 A. The difference‐Fourier technique promises the possibility of mapping valence differences for atoms of the same atomic species in a mixed‐valence crystal.
TL;DR: In this article, the authors measured the saturation magnetization of FerOo-MgAlrO4 solid solution at 4.4 K and in fields up to 12 T.
Abstract: Magnetic hysteresis properties of the FerOo-MgAlrO4 solid solution have been measured at 4.4 K and in fields up to 12 T. The trend in saturation magnetization, M\"(4.4 K), as a function of composition is consistent with the cation distribution model of Nell and Wood (1989) and suggests tlllat M, (4.4 K) changes sign at a composition of 30 mol9o FerOo. The change in sign of M\"(4.4 K) at this composition is correlated with an anomalous peak in coercivity of over 700 mT. This effect has been explained in terms of fluctuations in both composition and the degree of nonconvergent cation order, which lead to fine-scale magnetic domains with antiferromagnetic exchange coupling.
TL;DR: In this paper, precipitation studies for Fe(II) in the NH 3 (NH 4 ) 2 SO 4 O 2 system were carried out using a factorial design of experiments.
TL;DR: In this article, a system of small magnetic particles embedded in a nonmagnetic matrix were prepared by high energy ball milling, and in situ chemical reactions were used to control the properties of the product.
Abstract: Systems of small magnetic particles embedded in a nonmagnetic matrix were prepared by high energy ball milling. Besides carefully chosen milling conditions, in situ chemical reactions were used to control the properties of the product. Nanocomposites of iron particles in metal oxides (Al 2 O 3 and ZnO), and magnetite particles in copper metal were prepared by reaction milling. The samples were characterized by X-ray diffraction and magnetic methods. A few hours of ball milling resulted in the completion of most chemical changes. Iron nanoparticles were formed with lattice strains of about 0.005; coercivities up to 400 Oe were achieved. The magnetization of the iron particles is 25–40% less than that expected for bulk iron.