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Showing papers on "Magnetite published in 1987"


Journal ArticleDOI
19 Nov 1987-Nature
TL;DR: The GS-15 organism as mentioned in this paper is not magnetotactic, but reduces amorphic ferric oxide to extracellular magnetite during the reduction of ferric iron as the terminal electron acceptor for organic matter oxidation.
Abstract: The potential contribution of microbial metabolism to the magnetization of sediments has only recently been recognized. In the presence of oxygen, magnetotactic bacteria can form intracellular chains of magnetite while using oxygen or nitrate as the terminal electron acceptor for metabolism1. The production of ultrafine-grained magnetite by magnetotactic bacteria in surficial aerobic sediments may contribute significantly to the natural remanent magnetism of sediments2–4. However, recent studies on iron reduction in anaerobic sediments suggested that bacteria can also generate magnetite in the absence of oxygen5. We report here on a sediment organism, designated GS-15, which produces copious quantities of ultrafine-grained magnetite under anaerobic conditions. GS-15 is not magnetotactic, but reduces amorphic ferric oxide to extracellular magnetite during the reduction of ferric iron as the terminal electron acceptor for organic matter oxidation. This novel metabolism may be the mechanism for the formation of ultrafine-grained magnetite in anaerobic sediments, and couldaccount for the accumulation of magnetite in ancient iron formations and hydrocarbon deposits.

921 citations


Journal ArticleDOI
TL;DR: In this article, the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide and the surface area of the magnetite in anoxic marine sediments.

499 citations


Journal ArticleDOI
TL;DR: On the basis of the agreement of theory with data for solutions of small paramagnetic complexes, large magnetite particles, and liver containing low‐density polymer‐coated magnetite agglomerates, it is argued that the theory is sufficiently reliable so that, e.g., for ferritin, it appears that diffusion through intracellular gradients determines 1/T2.
Abstract: Since 1/T2 of protons of tissue water is generally much greater than 1/T1 at typical imaging fields, small single-ion contrast agents--such as Gd(DTPA), which make comparable incremental contributions and therefore smaller fractional contributions to 1/T2 compared to 1/T1--are not as desirable for contrast-enhancement as agents that could enhance 1/T2 preferentially. In principle, such specialized agents will only be effective at higher fields because the field dependence (dispersion) of 1/T1 is such that it approaches zero at high fields whereas 1/T2 approaches a constant value. The residual 1/T2 is called the "secular" contribution and arises from fluctuations in time--as sensed by the protons of diffusing solvent or tissue water molecules--of the component of the magnetic field parallel to the static applied field. For solutions or suspensions of sufficiently large paramagnetic or ferromagnetic particles (greater than or equal to 250 A diameter), the paramagnetic contributions to the relaxation rates satisfy 1/T2 much greater than 1/T1 at typical imaging fields. We examine the theory of secular relaxation in some detail, particularly as it applies to systems relevant to magnetic resonance imaging, and then analyze the data for solutions, suspensions, or tissue containing ferritin, erythrocytes, agar-bound magnetite particles, and liver with low-density composite polymer-coated magnetite. In most cases we can explain the relaxation data, often quantitatively, in terms of the theory of relaxation of protons (water molecules) diffusing in the outer sphere environments of magnetized particles. The dipolar field produced by these particles has a strong spatial dependence, and its apparent fluctuations in time as seen by the diffusing protons produce spin transitions that contribute to both 1/T1 and /T2 comparably at low fields; for the larger particles, because of dispersion, the secular term dominates at fields of interest. On the basis of the agreement of theory with data for solutions of small paramagnetic complexes, large magnetite particles, and liver containing low-density polymer-coated magnetite agglomerates, it is argued that the theory is sufficiently reliable so that, e.g., for ferritin--for which 1/T2 is unexpectedly large--the source of its large relaxivity must reside in nonideal chemistry of the ferritin core. For blood, it appears that diffusion through intracellular gradients determines 1/T2.

360 citations


Journal Article
TL;DR: In this article, the authors studied the quaysfayalite-iron equilibria in the temperature range 1000-1400 and 1050-1300 K, respectively, using an electrochemical technique.
Abstract: The quartz-fayalite-iron and quartz-fayalite-magnetite equilibria have been studied in the temperature range 1000-1400 and 1050-1300 K, respectively, using an electrochemical technique. The results are in excellent agreement with the calorimetric data on fayalite (Robie et al., 1982). For magnetite, the results demonstrate conclusively that there is no zero-point entropy and that the earlier heat-content measurements of Coughlin et al. (1951) should be preferred to the measurements by differential scanning calorimetry of Gronvold and Sveen (1974) above the Curie point. For quartz-fayalite-iron,

268 citations


Journal ArticleDOI
05 Jun 1987-Science
TL;DR: The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals.
Abstract: The discrepancy between the magnetic hysteresis properties of magnetite crystals that are precipitated from solution ( 0.3 micrometer) is not an inherent property of magnetite but is caused by the highly stressed state of crushed material and by adhering finer fragments. The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals. Coercive forces of these narrowly sized crystals follow a power law over a wide size range (0.1 micrometer to 1 millimeter) as predicted by theory. Dislocation etch pits show similar dislocation densities for hydrothermally grown (3 x 1010 meter -2) and natural (1 x 1010 meter-2) magnetite crystals. Hysteresis parameters of hydrothermally grown crystals are similar to those of natural crystals but are about one-fifth of those for crushed grains.

179 citations


Journal ArticleDOI
08 Apr 1987-Nature
TL;DR: In this article, the formation of fine-grained authigenic magnetites near the commonly observed brown-tan-green colour boundary, which marks the transition from Fe-oxidizing to Fe-reducing conditions, was investigated.
Abstract: The resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition. If authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition, then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals. Using palaeo- and rock-magnetic and sediment geochemical analyses, we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments. Here, we report evidence of the formation of fine-grained authigenic magnetites near the commonly observed brown–tan-green colour boundary, which marks the transition from Fe-oxidizing to Fe-reducing conditions. We propose that biogenic magnetite, produced by magnetotactic bacteria, forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter. The magnetite is created as a metabolic by-product of the microorganisms' use of iron redox transitions as a source of energy. Active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction.

170 citations


Journal ArticleDOI
29 Jan 1987-Nature
TL;DR: In this article, the authors report palaeomagnetic, rock magnetic, petrographic and geochemical results of a study of samples from Permian spelebthems and gilsonite found in the Ordovician Arbuckle Group in southern Oklahoma.
Abstract: Establishing a relationship between hydrocarbon migration and the precipitation of authigenic magnetite in sedimentary rocks is of significant interest with respect to (1) elucidating mechanisms for remagnetization and establishing the origin of secondary magnetizations residing in magnetite, (2) developing a method to date hydrocarbon migration events by determining the time of remanence acquisition by palaeomagnetic methods, and (3) evaluating whether searching for anomalous concentrations of diagenetic magnetic minerals and/or aeromagnetic anomalies is justified as a relatively inexpensive exploration tool. The direct association of hydrocarbon migration and the precipitation of authigenic magnetite has, however, not been established. In this paper we report palaeomagnetic, rock magnetic, petrographic and geochemical results of a study of samples from Permian spelebthems and gilsonite found in the Ordovician Arbuckle Group in southern Oklahoma. The results indicate that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite.

134 citations


Journal ArticleDOI
TL;DR: Several anaerobic bacteria isolated from the sediments of Contrary Creek, an iron-rich environment, produced magnetite when cultured in combinations but not when cultured alone in synthetic iron oxyhydroxide medium, indicating microbial alteration of the local E(h) and pH conditions.
Abstract: Several anaerobic bacteria isolated from the sediments of Contrary Creek, an iron-rich environment, produced magnetite when cultured in combinations but not when cultured alone in synthetic iron oxyhydroxide medium. When glucose was added as a carbon source, the pH of the medium decreased (to 5.5) and no magnetite was formed. When the same growth medium without glucose was used, the pH increased (to 8.5) and magnetite was formed. In both cases, Fe2+ was released into the growth medium. Geochemical equilibrium equations with Eh and pH as master variables were solved for the concentrations of iron and inorganic carbon that were observed in the system. Magnetite was predicted to be the dominant iron oxide formed at high pHs, while free Fe2+ or siderite were the dominant forms of iron expected at low pHs. Thus, magnetite formation occurs because of microbial alteration of the local Eh and pH conditions, along with concurrent reduction of ferric iron (direct biological reduction or abiological oxidation-reduction reactions).

129 citations


Journal ArticleDOI
TL;DR: In this article, a series of experiments has been carried out to investigate the possible formation of magnetite, Fe3O4, under ambient soil-forming conditions, which was achieved through controlled oxidation of Fe2+ solutions at room temperatures and near neutral pH values.
Abstract: A series of experiments has been carried out to investigate the possible formation of magnetite, Fe3O4, under ambient soil-forming conditions. Rapid and easy synthesis of magnetite was achieved through controlled oxidation of Fe2+ solutions at room temperatures and near neutral pH values. The synthetic products were found to range in size between 0·01–0·07 µm (mean diameter) and hence span the theoretical superparamagnetic-single-domain grain-size boundary.

128 citations


Journal ArticleDOI
01 Jan 1987-Geology
TL;DR: Samples of solid bitumen from the Thornton Quarry and the Cynthia Quarry were found to be strongly magnetic and to have rock magnetic properties suggesting that the magnetizable grains present are magnetite as discussed by the authors.
Abstract: Samples of solid bitumen from the Thornton Quarry (Illinois) and the Cynthia Quarry (Mississippi) were found to be strongly magnetic and to have rock magnetic properties suggesting that the magnetizable grains present are magnetite. Studies of magnetic isolates revealed that magnetite is present primarily as spherical crystal aggregates that appear identical to magnetite spherules isolated from re-magnetized Paleozoic carbonate units from other localities. Organic geochemical analyses of the solid bitumen suggest an origin by microbial attack on what once was liquid crude oil. The occurrence of secondary magnetite as inclusions within solid bitumen suggests a relationship between crude oil biodegradation and development of that mineral in our samples. We infer that secondary magnetite in other geologic environments may be related to the presence of hydrocarbons. The discovery of a natural association of secondary magnetite and hydrocarbons has important implications for paleomagnetism and for petroleum exploration.

120 citations


Journal ArticleDOI
TL;DR: The anisotropy of susceptibility of metamorphic rocks can be due to paramagnetic rock-forming silicates such as amphiboles, chlorites and micas as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, an idealized morphology of the biogenic crystals is based on an elongated cubo-octahedral form comprising a hexagonal prism of {111} and {100} faces capped by ( 111) and (111) faces with associated truncations.
Abstract: Bacterial magnetite particles of anisotropic morphology have been studied by high-resolution transmission electron microscopy. Lattice images of individual crystals are consistent with a well-ordered magnetite cubic inverse spinel structure. The idealized morphology of the biogenic crystals is based on an elongated cubo-octahedral form comprising a hexagonal prism of {111} and {100} faces capped by (111) and (111) faces with associated {111} and {100} truncations. Analysis of many particles of diverse size suggests that crystal growth takes place in two stages. The first stage is associated with the formation of well-ordered, isotropic, single-domain crystals of cubo-octahedral morphology. In this stage the crystal length and width develop concurrently up to a size of 20 nm. The second stage involves the anisotropic growth of the isotropic particles along the [112] direction. A crystal growth mechanism is postulated which involves the specific nucleation of the (111) face on a surrounding organic membrane. Unidirectional growth then occurs by selective suppression of certain crystallographic axes through spatial and chemical constraints induced by the adjacent organic boundary.

Journal ArticleDOI
TL;DR: The magnetic susceptibility of the Helvetic Jurassic black shales, sampled around the Aar and Gothard massifs in the Swiss Alps, is due to iron-bearing silicates, magnetite or pyrrhotite as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, the chemical remanent magnetization (CRM) of equidimensional magnetite to a mixture of 90% hematite and 10% cation-deficient magnetite is jointly controlled by the initial remanence of the parent magnetite and the field H CRM applied during oxidation.

Journal ArticleDOI
TL;DR: In this paper, the presence of magnetotactic bacteria and ultrafine-grained single domain magnetite in carbonate oozes and calcareous laminated sediments from Laguna Figueroa, Baja California, Mexico has been investigated.

Journal ArticleDOI
01 Apr 1987-Fuel
TL;DR: In this article, the decomposition of pyrite in air, 2% oxygen in nitrogen and nitrogen atmospheres, has been studied at temperatures ranging from 300 to 1100 °C.

Journal ArticleDOI
TL;DR: Hysteresis measurements for dispersed single domain (SD) titanomagnetite particles precipitated in silicates, and for dispersed pure magnetites in a large data suite covering more than three decades of grain size are presented in this article.

Journal ArticleDOI
TL;DR: In this article, a new theoretical approach for the optical and dc conductivity is applied to magnetite, based on a small-polaron (SP) model which takes into account also the polaronic short-range order.
Abstract: A recent and new theoretical approach for the optical and dc conductivity is applied to magnetite. It is based on a small-polaron (SP) model which takes into account also the polaronic short-range order. The good agreement between the theoretical fit and our new and thorough optical measurements supports the idea of superposition of SP band and hopping conductivity in magnetite.

Journal ArticleDOI
TL;DR: In this article, temperature and concentration dependences of thermal conductivity of hydrocarbon-based magnetic fluids containing colloidal Fe3O4 particles coated with oleic acid are obtained.

Journal ArticleDOI
TL;DR: In this article, the Nernst partition coefficients of molybdenum between magnetite and two rhyolitic melts have been determined at 800 degrees C, 1 kb of pressure, and at two oxygen fugacities.
Abstract: The partitioning of molybdenum between magnetite (mt) and two rhyolitic melts has been determined at 800 degrees C, 1 kb of pressure, and at two oxygen fugacities. At an oxygen fugacity one-half a log unit above nickel-nickel oxide, D (super mt/melt) Mo = 0.21 + or - 0.08. A decrease in the oxygen fugacity to the graphite-methane buffer is accompanied by an increase in D (super mt/melt) Mo to 0.52 + or - 0.13. The differences in melt composition have no discernible effect.These data are interpreted by writing balanced chemical potential relationships involving independently variable phase components. The Nernst partition coefficients given above are then formulated as a function of these equilibria. Given the observed variation in D (super mt/melt) Mo with oxygen fugacity, and assuming that Fe 2 MoO 4 is the molybdenum-bearing phase component in magnetite, our data suggest that Mo(III) may be present in naturally occurring silicic magmas. Further, we suggest the establishment of critical bulk partition coefficients for ore-forming systems, defined in terms of critical values of the efficiency of removal function defined by Candela and Holland (1986). These preliminary results indicate that the crystallization of ferromagnesian or titanium-bearing phases may be important in reducing the amount of molybdenum available for orthomagmatic-hydrothermal ore formation. Oxygen fugacity may play a large role in the melt-crystal-vapor equilibria ofmolybdenum under magmatic conditions.

Journal ArticleDOI
TL;DR: In this paper, the magnetization of a magnetite colloid prepared by a water/oil micro-emulsion method was measured and the size distribution of colloidal particles is a log-normal type with a mean diameter of 3.6 nm and a standard deviation of σ=1.28.
Abstract: The magnetizations of a magnetite colloid prepared by a Water/Oil microemulsion method were measured. Magnetization curves are not represented by a simple Langevin function, but are given by a complex formula incorporating the surface condition of ultrafine particles. The temperature dependence of the spontaneous magnetization is different from the bulk and is explained by considering the existence of surface magnetism. The size distribution of colloidal particles is a log-normal type with a mean diameter of =3.6 nm and with a standard deviation of σ=1.28 (from an electron micrograph).

Journal ArticleDOI
TL;DR: Instrumental neutron activation analysis was used to determine the concentrations of Ir, Fe, Ni, Co, Cr, and Sc in 16 “iron and 8 “chondritic” magnetic deep sea spherules with sizes of about 200 μm as mentioned in this paper.
Abstract: Instrumental neutron activation analysis was used to determine the concentrations of Ir, Fe, Ni, Co, Cr, and Sc in 16 “iron” and 8 “chondritic” magnetic deep sea spherules with sizes of about 200 μm. With the exception of two iron spheres, high indium concentrations were found in each sphere, thus confirming their extraterrestrial origin. Moreover, the “depth profile” of the residual mass of iridium was monitored in individual spheres by comparing the variations of the intensities of the γ ray lines of iridium, cobalt, and iron during a very gradual sequential polishing. Each sharp but partial discontinuity, observed only in the Ir profiles, reflects the intersection of a polished section with a tiny Ir-rich nugget. These profiles show that such nuggets are much more common in both the iron and the chondritic spheres than previously thought. We found new types of disequilibrated clusters of micrometer-sized nuggets of platinum metals. All types of nuggets, which were characterized with a scanning electron microscope and an energy dispersive X ray spectrometer, seem to be associated with magnetite. The compositions of the nuggets, as well as the fractionation patterns for Ir, Fe, Ni, Co, and Sc in the spheres, suggest that most of the Ir-rich iron spheres originate from iron-rich bodies.

Journal ArticleDOI
TL;DR: In this paper, the reduction of oxygen and hydrogen peroxide (H2O2) into hydroxyl radicals by selected iron compounds in a phosphate buffered solution was studied.
Abstract: The deleterious effects of radicals in biological systems are now admitted. The aim of the present work was to study the reduction of oxygen and hydrogen peroxide (H2O2) into hydroxyl radicals by selected iron compounds in a phosphate buffered solution. Radical detection has been carried out by electron spin resonance spectroscopy (ESR) with “spin trapping”; (5,5‐dimethyl‐1‐pyrroline N‐oxide, DMPO). The following minerals have been used: goethite (FeOOH a), akaganeite (FeOOH β), hematite Fe2O3α), magnetite (Fe3O4), siderite (FeCO3), and crocidolite fibres (Na2Fe3Si8O22(OH)2). Crocidolite and siderite reduced O2 into OH∗. Catalase inhibited this reduction. The reduction occurs at the mineral surface. When H2O2 was added to the buffer solution, goethite was inactive, akaganeite, magnetite, crocidolite and hematite were active and siderite was very active. These reactions could play a role in the mechanisms of toxic injury caused by some solid iron compounds.

Journal ArticleDOI
TL;DR: In this article, the reduction of single crystal iron oxides by hydrogen and hydrogen-argon mixtures was studied in a high voltage transmission electron microscope, where the cell enables a direct observation of the solid during reaction, thus permitting the nucleation and growth of solid reaction products to be observed.
Abstract: An “environmental cell” located in a high voltage transmission electron microscope has been used to study the reduction of single crystal iron oxides by hydrogen and hydrogen-argon mixtures. The cell enables a direct observation of the solid during reaction, thus permitting the nucleation and growth of solid reaction products to be observed. Hematite was reduced at temperatures in the range 387 to 610°C with gas pressures up to 5.3 kP. Reduction with pure hydrogen was considerably faster than when argon was present. Lath magnetite which rapidly transforms to porous magnetite and thence (more slowly) to porous iron was observed. The reduction of magnetite and of wustite single crystals was observed in the temperature range 300 to 514°C using both hydrogen and hydrogen-argon mixtures at gas pressures up to 6.6 kP. Incubation periods were found for magnetite reduction; during these periods faceted pits formed in the oxide. Iron formed in the early stages was epitaxial with the host magnetite; at later stages the epitaxy was lost and fissures frequently formed in the metal. The morphology of the iron differed between the gas mixtures. Disproportionation accompanied the reduction of wustite, producing intermediate polycrystalline magnetite despite reducing conditions. The disproportionation appeared to be promoted by the reduction reaction. For both oxides, reduction in the hydrogen-argon mixture was slower than in pure hydrogen.

Journal ArticleDOI
TL;DR: In this paper, hydrogen-impregnated fenestral dolomites of the Phosphoria Formation were sampled on both flanks of Sheep Mountain Anticline, Wyoming and X-ray diffraction analysis indicates that magnetite is present in the extracts; the spheres are interpreted to be authigenic magnetite.
Abstract: Hydrocarbon impregnated fenestral dolomites of the Phosphoria Formation were sampled on both flanks of Sheep Mountain Anticline, Wyoming. Demagnetization results indicate one dominant component that is either synfolding (Laramide in age) or predates folding (Cretaceous in age). Thermal demagnetization yields stable decay to temperatures between 500°C and 550°C and rock magnetic experiments suggest that the magnetization is dominated by a low coercivity component. Magnetic extracts contain spheres (<15 µ diameter) as well as other authigenic forms. Energy dispersive analysis demonstrates that Fe is the only detectable element present in the spheres and X-ray diffraction analysis indicates that magnetite is present in the extracts; the spheres are interpreted to be authigenic magnetite. The magnetization is interpreted as a chemical remanent magnetization (CRM) residing in this authigenic magnetite. Chemical conditions created by the hydrocarbons may have caused precipitation of the authigenic magnetite and acquisition of the associated CRM.

Journal ArticleDOI
TL;DR: In this article, the authors showed that using 1m HI or 2m NH 4 Cl solutions as mineralizers gave euhedral magnetite crystals from 8 μm to 1 mm in size.


Journal ArticleDOI
TL;DR: Magnetic susceptibility was determined for nine mixed sandstone, siltstone and shale samples collected above a burned coal seam near Kemmerer, Wyoming as discussed by the authors, where magnetite is the predominant ferrimagnetic mineral present.

Journal ArticleDOI
TL;DR: The results of the Lowric-Fuller test indicate that the remanence signal is dominated by PSD or SD carriers as mentioned in this paper, which may explain the magnetic behavior of the ferrian ilmenite.