Showing papers on "Magnetite published in 1993"

The effect of oxidation on the Verwey transition in magnetite

Abstract: At the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tv in the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tv by 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non-stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero-field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 1012 change in particle volume does not materially affect the remanence transition at Tv but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tv provides a sensitive test for maghemitization in soils, sediments and rocks.

Magnetite formation by a sulphate-reducing bacterium

Abstract: BACTERIAL production of magnetite (Fe3O4)1 makes an important contribution to iron biomineralization and rcmanent magnetization of sediments2,3 Accurate magnetostratigraphy, reconstruction of the Earth's past magnetic-field behaviour and extraction of environmental information from the geomagnetic record depend on an understanding of the conditions under which bacterial magnetite is formed In aquatic sediments, the process is thought to be restricted to a zone between the levels at which nitrate and iron reduction occur4 In sulphate-reducing habitats, deeper in the sediment, the presence of H2S reduces iron oxyhydroxides to iron sulphides5,6 Thus magnetite would not be expected to form under such reducing conditions5,7 We report here the isolation and pure culture of a dissimilatory sulphate-reducing bacterium, designated RS-1, which can synthesize intracellular magnetite particles RS-1 is a freshwater anaerobe which is also capable of extracellular iron sulphide precipitation This isolate illustrates the wider meta-bolic diversity of magnetic bacteria and suggests the presence of a novel mechanism of magnetic biomineralization The discovery of such bacteria may also explain why large quantities of magnetite have been observed in sulphate-rich, oil-bearing, sedimentary deposits8–11 In addition, these results significantly enlarge the environments in which biogenic magnetite may be expected to occur and have important implications regarding the evolution of the ability to synthesize magnetite

Geochemical control of magnetite dissolution in subarctic lake sediments and the implications for environmental magnetism

Abstract: Geochemical and mineral magnetic studies of lake sediments from the Karsa valley in north Sweden demonstrate that down-core variations in magnetic properties have been determined by post-depositional magnetite dissolution rather than by sediment source changes. A low sediment accumulation rate during times of climatic amelioration combined with a higher organic carbon content produced intense reducing conditions with almost complete magnetite loss. An increase in the sedimentation rate and a reduction in the organic carbon content, primarily due to glacier reformation, allowed magnetite preservation. Thus the mineral magnetic profiles record lake sediment palaeoredox conditions brought about by climate change. This new interpretation of mineral magnetic transformations in freshwater sediments may have far reaching implications for other mineral magnetic studies concerned with the alternation between glacial (stadial) and interglacial (interstadial) episodes during the Quaternary period.

Thermally altered palagonitic tephra: A spectral and process analog to the soil and dust of Mars

Abstract: Six palagonitic soil samples (PH-1 through PH-6) which were collected at 30-cm intervals from a lava slab on Mauna Kea, Hawaii, are studied. The samples present an alteration sequence caused by heating during emplacement of molten lava over a preexisting tephra cone. Techniques employed include visible and near-IUR spectroscopy, Moessbauer spectroscopy, and magnetic analysis. The four samples closest to the slab (PH-1 through PH-4) were strongly altered in response to heating during its emplacement; their iron oxide mineralogy is dominated by nanophase ferric oxide. The sample adjacent to the slab (PH-1) has a factor of 3 less H2O and contains crystalline hematite and magnetite in addition to nanophase ferric oxide. It is argued that localized thermal alteration events may provide a volumetrically important mechanism for the palagonitization of basaltic glass and the production of crystalline ferric oxides on Mars.

Abiotic Reduction of 4-Chloronitrobenzene to 4-Chloroaniline in a Dissimilatory Iron-Reducing Enrichment Culture

Abstract: 4-chloronitrobenzene (4-Cl-NB) was rapidly reduced to 4-chloroaniline with half-lives of minutes in a dissimilatory Fe(III)-reducing enrichment culture. The initial pseudo-first-order rate constants at 25 degrees C ranged from 0.11 to 0.19 per minute. The linear Arrhenius correlation in a temperature range of 6 to 85 degrees C and the unchanged reactivity after pasteurization indicated that the nitroreduction occurred abiotically. A fine-grained black solid which was identified as poorly crystalline magnetite (Fe(3)O(4)) by X-ray diffraction accumulated in the enrichments. Magnetite produced by the Fe(III)-reducing bacterium Geobacter metallireducens GS-15 and synthetic magnetite also reduced 4-Cl-NB. These results suggest that the reduction of 4-Cl-NB by the enrichment material was a surface-mediated reaction by dissimilatory formed Fe(II) associated with magnetite.

Thermal generation of ferromagnetic minerals from iron‐enriched smectites

Abstract: SUMMARY In recent years remagnetization of orogenic belts has been explained by fluid migration through rocks undergoing deformation. A laboratory study of remagnetization is presented in which varying amounts of iron (0-13.5 weight per cent Fe,O,) are adsorbed onto smectite surfaces. All smectite samples contain structural Fe (111) which is located in octahedral sites and is thermally stable up to 700°C. An increase in the amount of iron adsorbed onto the clay surface leads to the formation of ferric nanophases in which parts are magnetic. Mineralogical changes that occur during thermal treatment between room temperature and 700 "C were monitored using electron spin resonance (ESR), bulk susceptibility, acquisition of isothermal remanent magnetization (IRM) and Curie temperature analysis. After heating the samples to 250"C, a new ferrimagnetic phase is created as indicated by ESR and IRM acquisition. ESR spectra, IRM acquisition and Curie analyses suggest that magnetite is the predominant phase that is being created. These grains continue to be created and grow with heating up to 500°C. Above this temperature a decrease in the intensity of the IRM at 1T suggests that the phase is being transformed into haematite. The thermal experiments on iron-loaded smectites show that surfaceinduced processes can lead to the formation of new magnetic minerals under conditions characteristic of low-grade metamorphism.

Origin of magnetite responsible for remagnetization of Early Paleozoic limestones of New York State

Abstract: Scanning electron microscope and Scanning transmission electron microscope observations of thin sections and separates of the Devonian Onondaga and Helderberg limestones and Ordovician Trenton limestone in New York state allow us to identify three types of magnetite: pseudoframboids, nonspherical magnetite, and fine-grained magnetite. Magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes. Fine-grained magnetite, consisting of aggregates of one or more rounded single crystals, approximately 2000 A (200 run) in diameter, could not be observed in thin sections of New York carbonates due to its small size but was found in magnetic extracts. Hysteresis measurements of magnetic extracts verify that fine-grained magnetite is capable of carrying remanent magnetizations. However, pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain-sized crystals that also can be carriers of the remagnetization. These data, in combination with observations from other localities, collectively imply that the Alleghenian remagnetization is carried by the fine-grained magnetite, although pseudoframboidal and nonspherical magnetite may also contribute. Thus the Alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite. Crystallization of magnetite was mediated by fluids, with dissolution and crystallization activated by stress during the Alleghenian Orogeny. The origin of such fluids is unknown, but they may have originated through crustal scale fluid migration. Tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of the fluids.

Reduction of supported iron oxide studied by temperature-programmed reduction combined with mössbauer spectroscopy and X-ray diffraction

Abstract: The reduction of iron oxide supported on alumina, anatase and rutile and the interactions between iron oxides and these supports have been studied using temperature-programmed reduction, Mossbauer spectroscopy and X-ray diffraction. The intermediates formed during reduction depend strongly on the nature of the support, the iron loading and the reduction conditions used. Several iron species such as bulk α-Fe2O3, superparamagnetic α-Fe2O3 and surface iron–aluminium oxides are found in the alumina-supported iron oxide samples. The iron–aluminium species can be formed either by incorporation of Al3+ into the structure of iron oxide in acidic solution during the preparation of the samples or by incorporation of Fe2+ into the structure of the alumina support under high-temperature reduction. For the as-prepared Fe/TiO2 samples, the anatase and rutile have only weak interactions with the iron oxide as only bulk α-Fe2O3 species have been detected. However, some intermediates such as (1 –x)FeTiO3–xFe2O3 solid solution, surface iron–titanium oxide and bulk FeTiO3 phase can be formed during reduction because of the reducibility and mobility of titanium ions on the surface. Surface titanium ions can act as an electron-transfer medium for the reduction of Fe3+ to Fe2+ and can even be substituted for Fe3+ ions in the octahedral sites of magnetite formed during the reduction. In all cases, when the loading of iron oxide is low, Fe3+ and Fe2+ ions are stable during reduction due to the formation of iron oxide–support intermediates by strong interactions between iron oxides and the supports.

Carbon dioxide decomposition into carbon with the rhodium-bearing magnetite activated by H2-reduction

Abstract: The CO2 decomposition into carbon with the rhodium-bearing activated magnetite (Rh-AM) was studied in comparison with the activated magnetite (AM). The Rh-AM and the AM were prepared by flowing hydrogen gas through the rhodium-bearing magnetite (Rh-M) and the magnetite (M), respectively. The rate of activation of the Rh-M to the Rh-AM was about three times higher than that of the M to the AM at 300 °C. The reactivity for the CO2 decomposition into carbon with the Rh-AM (70% CO2 was decomposed in 40 min) was higher than that with the AM (30% in 40 min) at 300 °C. The Rh-M was activated to the Rh-AM at a lower temperature of 250 °C, and the Rh-AM decomposed CO2 into carbon at 250 °C. On the other hand, the M was little activated at 250 °C.

Cryptic grain-scale heterogeneity of oxygen isotope ratios in metamorphic magnetite.

Abstract: Oxygen isotope ratios measured by ion microprobe in magnetite from granulite-facies marble of the Adirondack Mountains, New York, range from +2 to +11 per mil (standard mean ocean water) across a single grain that measures 3 millimeters by 5 millimeters. Low values are concentrated in irregular domains near the grain boundary but also occur in the grain's interior. In contrast, grains 1 millimeter in diameter that are from a second nearby sample show no significant heterogeneity, except within 10 micrometers of the grain boundary. These data, including large gradients of up to 9 per mil per 10 micrometers, provide important new constraints on the nature and origins of intragrain isotopic heterogeneity and on oxygen isotope thermometry. The differences between these magnetite grains result from contrasting mechanisms of isotope exchange with fluids after the peak of regional metamorphism. Volume diffusion of oxygen through the crystal structure of magnetite contributed to isotope exchange in the rims of small grains, but larger grains are crosscut by healed cracks that are not readily detected and that short-circuited diffusion.

Magnetic properties of copper‐magnetite nanocomposites prepared by ball milling

Abstract: Small particles of Fe3O4, embedded in a Cu matrix, have been prepared both by direct milling of a mixture of Cu and Fe3O4 and by reaction milling of CuO and Fe. It is shown that the most significant changes in phase composition, magnetization, coercivity, remanence, and switching field distribution take place during the first 2 h of milling. Both processes provide small (about 25 nm) semihard magnetite particles after 1 h, with Hc up to 350 Oe and squareness of 0.3. Prolonged milling causes partial oxidation of magnetite to α‐Fe2O3 and the presence of superparamagnetic magnetite particles, both reducing the magnetization of the composites. Conclusions based on magnetic measurements about the details of the technological process and microstructural changes are in very good agreement with results of x‐ray diffraction measurements.

An electrochemical investigation of the dissolution of magnetite in acidic electrolytes

Abstract: The electrochemical behavior of magnetite was investigated in perchloric acid and ethylenediaminetetra-acetic acid (EDTA). The potential dependence of the total dissolution flux is similar for both acids; the form of this dependence indicates that ion-transfer reactions are rate determining. In EDTA-containing electrolytes, equilibrium is established by electron-transfer between the magnetite and the Fe 2+ /Fe 3+ species liberated by dissolution. This process defines the open-circuit potential of magnetite (E oc ) in this electrolyte. A similar process does not determine E oc in perchloric acid; the nature of the processes defining this potential in perchloric acid remains uncertain

Methanation of CO2 with H2-reduced magnetite

Abstract: The methanation reaction of CO2 was studied with H2-reduced magnetite. A high conversion ratio of about 0.9 (in 30 min of the reaction time) with a selectivity of nearly 100% was obtained at 300°C and at 0.1 MPa for H2-reduced magnetite which had been prepared by passing H2 gas for 1–5 h at 300°C. From the results of X-ray diffractometry and Mossbauer spectroscopy, and from chemical analysis of the deposited carbon, H2-reduced magnetite is considered to decompose adsorbed CO2 into carbon, and to incorporate the oxygen of the CO2 into the spinel-type structure of the magnetite, associated with oxidation of the Fe2+ ion into Fe3+ ion in the magnetite. The high conversion ratio in the methanation reaction is considered to come from a higher reactivity of the elementary carbon deposited on the surface of the H2-reduced magnetite.

Effects of ELF magnetic fields on biological magnetite.

Abstract: The effects of 60 Hz magnetic fields of 5 [mu]T (50 mG) or less on biological structures holding magnetite (Fe[sub 3]0[sub 4]) are shown to be much smaller than that from thermal agitation; hence such interactions cannot be expected to be biologically significant. Various species have been shown to manufacture magnetite, which is used in some cases in conjunction with the earth's magnetic field to determine direction ([Frankel, 1986; Kirschvink and Kobayashi-Kirschvink, 1991].) This magnetite is usually found in sets of single domain grains, about 500[angstrom] in diameter, where all grains are magnetized in the same direction. Typically, such grains are enclosed by a membrane and are called magnetosomes. Hence, the possibility that weak ELF magnetic fields may cause biological effects in humans by the interaction of the field with magnetosomes must be considered.

Examples and genetic significance of the formation of iron oxides in the Nigerian banded iron-formations

Abstract: Ore microscopic studies reveal two main parageneses in the banded iron-formations of Nigeria. In the low-grade metamorphic schist belts of northern Nigeria, a magnetitic paragenesis comprising magnetite, silicates (grunerite and garnet), and quartz is developed. Magnetite which sometimes contains carbonate inclusions is markedly martitized. In contrast, the higher-grade metamorphic terrains of central Nigeria exhibit a different paragenesis consisting of hematite (including specularite) and quartz. Here, minerals of the magnetitic paragenesis only occur as relics. The protolith of these banded iron-formation occurrences envisioned as carbonate-containing sediments, with high concentrations of Fe and Si, and lower contents of Ca, Mg, Al (and also Mn where they are associated with gondite) underwent both submarine weathering and metamorphic changes in their evolution. During submarine weathering, sheet silicates and porphyroblasts of Fe-Mn-(Mg-Ca)-carbonate solid solutions, were formed. At the outset of a regional metamorphic episode, grunerite, garnet and porphyroblastic magnetite were developed. Magnetite formed at the expense of carbonate and sheetsilicates but was later martitized under post-metamorphic conditions. In the course of a later heterogeneous tectono-metamorphic event, martitized magnetite was transformed as follows: under low-grade metamorphism, as observed in the northern Nigerian schist belts, recrystallization into coarse-grained martite occurred, while at the higher grades of metamorphism in central Nigeria, recrystallization into hematite and, ultimately, specularite, took place. This relationship between magnetite and hematite has also been observed in many other banded iron-formations from different parts of the world, thus underscoring its widespread significance. Magnetite crystallizes first at the expense of carbonate and silicate minerals and hematite is subsequently derived from it directly or generally through martitization. This metamorphic phenomenon contradicts the common assumption that magnetite and hematite in banded iron-formations are invariably the products of direct precipitation from solution, in response to changes in environmental Eh/pH or different (reducing/oxidizing) diagenetic alterations of precipitated ferric hydroxide.

Preparation and characterization of fine magnetite particles from iron(III) carboxylate dissolved in organic solvent

Abstract: The preparation of fine, crystaline magnetite from nonaqueous iron(III) carboxylate solution at elevated temperatures was investigated. On treatment with water, the iron carboxylate solution precipitated pure hematite (α-Fe 2 O 3 ) at 200°C and a mixture of hematite and magnetite (Fe 3 O 4 ) at 245°C. In the absence of water, a heat treatment of the organic solution at 245°C and 0.45 MPa for 60 min resulted in the formation of magnetite alone. The resulting magnetite was found to be crystaline particles with size ca. 0.1 μm and be free from contamination by the organic starting material

Formation and Transformation of Magnetite (Fe3O4) on Steel Surfaces Under Continuous and Cyclic Water Fog Testing

Abstract: Formation and transformation of magnetite on two selected low-alloy structural steels were studied using cyclic and continuous water fog tests. It is shown that continuous wetting of steel...

Thermal Decomposition of the Iron(III) Hydroxide and Magnetite Composites of Poly(vinyl alcohol). Preparation of Magnetite and Metallic Iron Particles.

Abstract: Upon the precipitation of iron(III) hydroxide (Fe(OH)3) and magnetite in the presence of poly(vinyl alcohol) (PVA) at R < 3, PVA is bound to the precipitates in very high yields, where R = [PVA polymer residue]/[Fe ion]. The thermal decomposition of the Fe(OH)3–PVA composite at R = 1.0 by heating at ca. 700 °C in nitrogen for 10 min or more affords almost pure iron metal particles of ca. 200 A in diameter; almost no residues originating from the PVA remain. Before formation of the metallic iron, magnetite particles are produced as its precursor in the 250—650 °C temperature range, where the particle size increases with increasing temperature from ca. 80 up to ca. 350 A in diameter, although these iron and magnetite products are not uniform, but are agglomerate or sintered particles. In these processes, the initially bound PVA effectively functions as a reducing agent during its own decomposition.