Showing papers on "Magnetite published in 1989"

Magnetofossils, the Magnetization of Sediments, and the Evolution of Magnetite Biomineralization

Abstract: Magnetite (Fe_3O_4) is one of the most stable carriers of natural remanent magnetization (NRM) in sedimentary rocks, and paleomagnetic studies of magnetite-bearing sediments, such as deep-sea cores and pelagic limestones, have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales. Despite this important role, there is as yet a very poor understanding of how ultrafine-grained (< 0.1 µm) magnetite is formed, transported, and preserved in marine environments. A major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite, formed by magnetotactic bacteria and/or other magnetite-precipitating organisms, is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks. Since these magnetite particles are of biogenic origin, they are termed properly magnetofossils (Kirschvink & Chang 1984).

A Comparison of Magnetite Particles Produced Anaerobically by Magnetotactic and Dissimilatory Iron‐Reducing Bacteria

Abstract: We compare the magnetic properties of fine-grained magnetite produced by two newly isolated anaerobic bacteria, a magnetotactic bacterium (MV-1) and a dissimilatory iron-reducing bacterium (GS-15). Although room-temperature magnetic properties are generally different between the two microorganisms, MV-1 and GS-15 magnetites can be most easily distinguished by the temperature variation of saturation remanence obtained at liquid helium temperatures. Magnetite produced by MV-1 displays a sharp discontinuity in intensity at 100 K related to the Verwey transition. Magnetite produced by GS-15 displays a gradual decrease in intensity with temperature due to the progressive unblocking of magnetization. The differing behavior is due exclusively to different grain size distributions produced by these microorganisms. MV-1 produces magnetite with a narrow grain size distribution that is within the stable single domain size range at room temperature and below. GS-15 produces magnetite with a wide grain size distribution extending into the superparamagnetic (SPM) size range. Our results show that a substantial fraction of particles produced by GS-15 are SPM at room temperature.

Magnetofossil dissolution in a palaeomagnetically unstable deep-sea sediment

Abstract: MAGNETOFOSSILS1, the fossil remains of bacterial magneto-somes2, are found in various deep-sea sediments, and have been linked to the preservation of stable natural remanent magnetization in many of them1,3–5. They have also been extracted and identified from lithified carbonates of Jurassic5 and Precambrian6 age, showing that magnetotactic bacteria have been a sedimentary source of fine-grained magnetite for much of geological time. Some clay-rich deep-sea sediments, however, do not record a stable remanent magnetization, for unknown reasons. Here we report results from a high-resolution transmission-electron-microscope study on samples of this type collected by the Deep Sea Drilling Project (DSDP); the material contains a complex mixture of single-domain magnetic minerals. Well-preserved magnetofossils show the same crystal structures as those found in magnetosomes from recent bacteria7–10, whereas others in these same preparations display a wide range of dissolution, corrosion and aggregation effects. Rock magnetic measurements are consistent with the presence of these alterations in many DSDP sediments, including those that preserve reliable palaeomagnetic directions. As we were unable to fina authigenic magnetic minerals in our sample, and as the magnetic fraction is dominated by well preserved magnetofossils, we suggest that the poor preservation of the magnetization is a result of diagenetic interactions between the magnetofossils and the clay minerals in the matrix.

Magnetite and magnetotaxis in microorganisms

Abstract: Magnetotactic bacteria from freshwater and marine sediments orient and navigate along geomagnetic field lines. Their magnetotactic response is based on intracellular, single magnetic domains of ferrimagnetic magnetite, which impart a permanent magnetic dipole moment to the cell.

Encapsulated superparamagnetic particles

Abstract: Stable, encapsulated superparamagnetic magnetite particles having a narrow particle size distribution with average particle diameters in the range of from about 50 Å to about 350 Å are prepared by forming an aqueous dispersion of magnetite particles having the above particle size characteristics in the presence of a surfactant, coacervating a mixture of gelatin and a carboxyl containing hydrophilic polymer such as gum arabic to form a thin coating of coacervate on the magnetite particles and crosslinking the coacervate coating with a gelatin hardener such as glutaraldehyde.

Effect of Oxygen Potential on Mineral Formation in Lime-fluxed Iron Ore Sinter

Abstract: In industrial iron ore sintering, the raw material is heated in a reducing atmosphere and cooled in an oxidizing atmosphere. In order to study the characteristics of mineral formation under different regimes of heating and cooling in industrial sintering, small tablet specimens made from powdered materials were sintered in a tube furnace under controlled gas atmospheres. The results obtained are summarized as follows.In the heating stage acicular calcium ferrite can be produced from the reaction of hematite and the flux below 1 180°C. With an increase of temperature, the calcium ferrite crystal size increases and at higher temperature the calcium ferrite transforms to magnetite and silicate melt. During the cooling stage, the magnetite tends to react with the silicate melt and oxygen to generate calcium ferrite at a medium oxygen partial pressure (around 1×10-2 atm). Reoxidized hematite is formed from magnetite at a higher oxygen potential (e.g., 5×10-2 atm). The magnetite (produced in the heating stage) is preserved at a low oxygen potential. Acicular calcium ferrite may also be generated from magnetite ore at high oxygen potential (e.g., PO2>5×10-3 atom) during the heating stage of sintering.Calcium ferrite produced from the reaction of hematite and the flux in heating stage is a form of calcium diferrite (CaO·2Fe2O3) with some substitution of Si and Al. The composition of calcium ferrite generated from magnetite depends on the amounts of SiO2 and CaO from silicate melt reacting with magnetite. The basicity (mole ratio of CaO/SiO2) of silicate melt consumed in the reaction is around 2.

Sintering Conditions for Simulating the Formation of Mineral Phases in Industrial Iron Ore Sinter

Abstract: In industrial iron ore sintering, the raw material is heated in a reducing atmosphere and cooled in an oxidizing atmosphere. In order to study the effects of gas atmosphere in industrial sintering, small tablet specimens containing powdered commercial iron ore, limestone, quartz and kaolin were heated in controlled gas atmospheres to examine the effects of gaseous atmosphere, heating temperature and cooling condition on the formation of minerals in sinter. The results obtained are summarized as follows.In the heating stage of laboratory sintering, with a decrease of partial pressure of oxygen, the magnetite content increases and hematite content decreases. The calcium ferrite content is found also to decrease at the low sintering temperature (1 210°C). However, at a higher sintering temperature (1 255°C), a medium oxygen potential (5×10-3 atm) produces the most calcium ferrite. In the air cooling stage, magnetite may react with the silicate melt and oxygen to generate a large amount of columnar calcium ferrite.A typical microstructure of the bond composed of columnar calcium ferrite, granular magnetite grain and glassy silicate in a normal industrial sinter can be simulated reasonably by heating a specimen to 1 255°C for 4 min in the gaseous mixture CO=1%, CO2=24% and N2=75% and then cooling it slowly in air.

The nature of the active phase of the Fe/K-catalyst for dehydrogenation of ethylbenzene

Abstract: The industrial catalyst for high temperature dehydrogenation of ethylbenzene based on iron and potassium oxides undergoes, under reaction conditions, essentially a transformation into magnetite, Fe3O4, and a mixture of ternary oxides containing trivalent iron, viz. K2Fe22O34 and KFeO2. The latter compound constitutes the outside of the catalyst particles and is indeed the catalytically active phase.

A study of the relationship between hydrocarbon migration and the precipitation of authigenic magnetic minerals in the Triassic Chugwater Formation, southern Montana

Abstract: To test the hypothesis that hydrocarbons can cause the precipitation of authigenic magnetite, hydrocarbon-saturated, bleached, and unaltered samples of the Chugwater red beds were examined, using petrographic, rock-magnetic, and paleomagnetic techniques. Samples were collected from the crest of Red Dome in Carbon County, Montana, where the Chugwater is saturated with hydrocarbons and bleached around fractures. The fractures formed during Laramide deformation and provided conduits for oil migration. The red beds contain Triassic and rotated Triassic magnetizations, which reside in hematite. The magnetization of the oil-saturated sandstones predominantly resides in magnetite, and there is some rock-magnetic evidence for pyrrhotite. The intensity of the natural remanent magnetization (NRM) in these sands is at least an order of magnitude less than that observed in the red beds. A stable magnetic direction could not be isolated from the oil-saturated sandstones. Petrographic studies indicate that both detrital and authigenic magnetite are present in the oil-saturated sandstones. The bleached specimens possess intermediate magnetic characteristics and contain abundant secondary authigenic specular hematite. These specimens contain an early Tertiary magnetization that resides in the authigenic hematite. The chemical conditions created by the hydrocarbons caused the dissolution of the hematite, which "uncovered" and relatively enhanced the magnetization residing in detrital magnetite. It is also likely that hydrocarbons caused the precipitation of some authigenic magnetite and pyrrhotite. The early Tertiary magnetization in the bleached sandstones is interpreted to be of chemical origin and a byproduct of hydrocarbon migration at Red Dome. The magnetization was probably acquired when the dissolved iron from the red beds precipitated as specular hematite. The results of this study have implications for understanding hydrocarbon-induced diagenesis and remagnetization mechanisms, and for aeromagnetic hydrocarbon exploration.

Magnetic recording medium

Abstract: PURPOSE:To maintain an excellent erasing characteristic and to enhance coercive force by coating a resin compsn. contg. the acicular magnetic iron oxide particle powder the surfaces of which are coated with the double layers consisting of the lower layer which is a Co compd. layer and the upper layer which is a specific ferrite layer onto a substrate. CONSTITUTION:The resin compsn. contg. the acicular magnetic iron oxide particle powder the surfaces of which are coated with the double layers consisting of the lower layer which is the Co compd. layer and the upper layer which is the spinel type ferrite layer contg. Mn and Zn or any among Mn, Ni and Zn is coated on the substrate. Acicular maghemite particles, acicular magnetite particles and the particles contg. 1 or >=2 kinds among Co, Ni, Si, Al, Zn, P, etc., therein are used for the iron oxide particles. The ferrite layer is 1.0-10.0wt.% by the weight of Mn.Zn, Ni.Zn or Mn.Ni.Zn with respect to the weight of the acicular iron oxide particles coated with the Co compd. on the particle surfaces and the ratios of Mn.Ni and Mn.Ni are 0.1-80atm.% of Mn, Ni and Zn.

Magnetism and transmission electron microscopy of Fe-Ti oxides and pyroxenes in a granulite from Lofoten, Norway

Abstract: Granulites from southwest Lofoten, Norway, often carry intense and stable components of natural remanent magnetization (NRM). Coercivities in excess of 100 mT and blocking temperatures up to 580°–645°C are common. Several phases in these rocks can carry stable components of NRM. Clinopyroxenes (augite with exsolved pigeonite and orthopyroxene) usually contain abundant Fe-Ti oxides, the majority of which are too small to be studied conclusively with visible light. Additionally, these rocks typically contain coarse-grained (∼100–1000 μm) exsolution intergrowths of rhombohedral Fe-Ti oxides and coarse-grained magnetite. We have studied the magnetism and mineralogy of the titanohematite and the inclusion-bearing augites in one granulite sample using transmission electron microscopy (TEM) and magnetic experiments to understand better the remanence properties of naturally occurring Fe-Ti oxides and their relationships to silicate minerals. The coarse-grained titanohematite has a coercivity of 350 mT and a laboratory saturation remanence that unblocks at 570°–600°C. The hysteresis characteristics of this titanohematite are typical of much smaller ∼10μm-sized single domains, which apparently reflects the presence of exsolved ilmenite lamellae, as small as <10 nm in thickness. These lamellae presumably inhibit (1) domain wall nucleation and (2) movement of walls that manage to nucleate. The inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560°C. Magnetite, hematite, and ilmenite are present in the augite, and intergrowth of the rhombohedral phases, revealed by TEM, persists down to the 20-nm scale. The distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure, while that of the rhombohedral Fe-Ti oxides is strongly controlled by silicate microstructure, with intergrowths of these oxides commonly decorating lamellae of pigeonite, orthopyroxene, and clinoamphibole within the augite. The NRM of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m.y. ago. Paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event. In addition to the included oxides, the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites: pigeonite lamellae are not parallel to the b-crystallographic axis, pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur. Rare earth elements are concentrated within inclusions that reside in the augite.

Biogenic and lithogenic magnetic minerals in Atlantic and Pacific deep sea sediments and their paleomagnetic significance

Abstract: The carrier of the natural magnetization of deep sea sediments was characterized by mineralogical, electron microscopic, and rock magnetic investigations. Magnetic single domain (SD) and pseudo single domain (PSD) particles which are most important for the stable remanent magnetization were separated from the magnetic »coarse fraction« and concentrated as magnetic »fine fraction«. The magnetic coarse fraction consists of lithogenic magnetite and titanomagnetite, which often contains exsolution-lamellae of ilmenite. Both minerals are partially maghematized and occur isolated in the sediment or embedded in rock particles, in regionally different concentrations. The magnetic fine fraction consists of lithogenic magnetite and titanomagnetite and biogenic magnetite (magnetofossils = fossil bacterial magnetosomes), the latter generally maghematized.

Mechanism of formation of magnetite from ferrous hydroxide in aqueous corrosion processes

Abstract: The stoichiometric conditions for the formation of ferrous hydroxide Fe(OH)2, by mixing Fe2+ ions with caustic soda NaOH, leads by oxidation to magnetite, irrelevant of the foreign anions, e.g. Cl− or SO42−, as demonstrated from Mossbauer spectroscopy. The electrochemical potential Eh and pH value of the initial conditions correspond to the drastic change from basic to acidic medium, observed when varying the initial Fe2+/OH− ratio. Mossbauer analysis of the end products of oxidation at various temperatures shows that magnetite is only obtained at stoichiometry at very low temperature, but extends off stoichiometry at higher temperatures. The mechanism of formation of magnetite through an intermediate compound is discussed.

The adsorption of uranium(VI) onto a magnetite sol

Abstract: Adsorption of uranium(VI) species onto a magnetite sol was studied onto a magnetite sol was studied in the presence of bicarbonate ion in the pH range 7 to 9. The initial uranium uptake decreases as the pH increases, and an anionic hydroxy carbonate species is likely involved. At the highest pH and at uranium(VI) concentrations above 70 μmole · dm−3, a precipitate forms on the surface of the magnetite. A few studies were carried out on adsorption of uranium(VI) onto the same sol but in the presence of sodium chloride and in the pH range of 3 to 6. A small initial adsorption was noted at lower pH values, but this increased considerably when the pH reached 5.7. These adsorption results are roughly similar to those observed for adsorption of uranium(VI) onto a hematite sol when adsorption is compared on a unit surface area basis.

The role of chlorine in the formation of magnetite skarns

Abstract: (1989). THE ROLE OF CHLORINE IN THE FORMATION OF MAGNETITE SKARNS. International Geology Review: Vol. 31, No. 1, pp. 63-71.

Characterization and Dissolution of Deposited Magnetite Films on AISI 304 Stainless Steel Surfaces in EDTA and EDTA/Oxalate Solutions

Abstract: The dissolution of deposited magnetite layers on AISI 304 stainless steel (SS) surfaces has been studied in acidic (pH=2.6 to 3.0) ethylenediaminetetraacetic acid (EDTA) and EDTA/oxalate solutions at 80 to 90 C. Oxide layers were grown on SS coupons in 1.0 mol·L−1 NaOH at 275 C in SS-lined titanium autoclaves for 14 days. These layers consisted of a base layer of composition Ni0.1CrFe0.9O4 and a deposited upper layer of crystals of NixFe3−xO4 (with Ni <10% by weight). Dissolution experiments were performed in thermostatted glass cells under potential-controlled and freely dissolving conditions. The concentrations of dissolved iron, chromium, and nickel were analyzed by inductively coupled plasma spectrometry. Cyclic voltammetric and anodic polarization experiments were performed on clean SS disks to determine the corrosion characteristics in EDTA/oxalate solutions. These experiments showed that dissolution of deposited magnetite films occurred via a reductive dissolution process providing the ele...

Identification of the Magnetic Poles on Strong Magnetic Grains from Meteorites Using Magnetotactic Bacteria

Abstract: Magnetotactic bacteria (north seeking bacteria) have been used to identify the magnetic S pole of iron-nickel grains selected from St. Severin LL6 chondrite. The results indicate that the bacteria are sensitive magnetic sensors which can be used to detect not only the S pole in the grains but also the directions of lines of magnetic force radiated from the grains. The magnetic coercive force and the stability of natural remanent magnetization can also be measured with the bacteria by applying a steady magnetic field. These methods can in principle be applied to terrestrial rocks having relatively strong natural remanent magnetization. Thus, the magnetotactic bacteria can give useful information for rock magnetism and paleomagnetism as a bio-magnetometer.Combining the method of south seeking bacteria and Bitter pattern analyses using colloidal magnetite particles, complex magnetization structures on the surface of Fe-Ni grains from the St. Severin meteorite have been revealed, which is important for an understanding the chondrite magnetism.

Sphere-like magnetite particles and a process for producing the same

Abstract: Disclosed herein are sphere-like magnetite particles showing an apparent density of from 0.40 to 1.00 g/cm 3 and a high thermo-stability and containing from 0.1 to 5.0 atomic % of Si to Fe, and a process for producing the sphere-like magnetite particles.

Source of Anomalous Magnetization in Area of Hydrocarbon Potential: Petrologic Evidence from Jurassic Preuss Sandstone, Wyoming-Idaho Thrust Belt

Abstract: The Jurassic Preuss Sandstone, which crops out in the central part of the Wyoming-Idaho thrust belt on trend with a hydrocarbon-producing region to the south, has been previously identified as the source of anomalous magnetization in the area. Elsewhere, anomalous magnetization in sedimentary rocks near hydrocarbon accumulations has been attributed to hydrocarbon-engendered magnetic minerals, but magnetization of the Preuss is controlled by detrital magnetite. Evidence of a detrital origin for magnetite includes (1) concentration of magnetite grains along laminations containing other heavy minerals, (2) the presence of exsolved ilmenite, hematite, and spinel in the magnetite grains, and (3) titanium contents typical of igneous-derived magnetite. That detrital magnetite is responsible for the anomalous magnetization in the Preuss is further indicated by the systematic eastward decrease in magnetite abundance corresponding to a similar eastward decrease in magnetic susceptibility and remanent magnetization of the unit. Petrologic and vitrinite reflectance studies indicate a complex low-temperature (<150°C or 302°F) diagenetic history for the Preuss. Nevertheless, preservation of detrital magnetite, the presence of diagenetically early ferric oxide minerals, and the absence of sulfide minerals all indicate that the Preuss has not experienced sulfidic-reducing conditions common in areas of hydrocarbon seepage. The marine carbon isotopic composition of calcite that cements most Preuss sandstones (^dgr13C values ranging from -2.47 to 1.48^pmil is evidence that carbonate diagenesis also was not influenced by hydrocarbons. The results of this multidisciplinary study of the Preuss underscore the importance of similar studies when evaluating the sources of aeromagnetic anomalies in areas of hydrocarbon potential.

Relation of the Verwey transition in magnetite to an order―disorder transition of strongly correlated electrons

Abstract: The principal features of the Verwey transition in magnetite have been simulated by adopting elements of order-disorder theory. In certain limiting cases we obtain the model of Strassler and Kittel which had previously been used to rationalize the electronic and thermodynamic properties of magnetite. According to the present microscopic model the discontinuous Verwey transition in magnetite is driven by a change in a highly correlated electron system with temperature from a charge-ordered small-polaron state associated with local lattice deformations to a disordered state in which electrons resonate between Fe2+ and Fe3+ ions located on the octahedral cationic sites.

Dendritic magnetite and miniature diapir-like concentrations of apatite: Two magmatic features of the Kiirunavaara iron ore

Abstract: Dendritic forms of magnetite are more common in the Kiirunavaara deposit than previously realized. They comprise columnar magnetite and two different varieties of platy magnetite. Besides forming lenticular bodies, the columnar magnetite occurs in two widespread ore types: in apatite-poor ore, and as a fringe around apatite concentrations in skeleton ore (an ore type characterized by platy magnetite dendrites). These concentrations, which locally have a diapir-like shape, appear to be products of local differentiation in an ore ‘melt’. Platy magnetite also occurs as lenses, superficially resembling the lenticular bodies of columnar magnetite. A magmatic (subvolcanic) origin of the Kiirunavaara deposit is the simplest explanation for the new observations.