Showing papers on "Magnetite published in 1986"

High-Pressure crystal chemistry of spinel (MgAl2O4) and magnetite (Fe3O4): Comparisons with silicate spinels

Abstract: High-pressure crystal structures and compressibilities have been determined by x-ray methods for MgAl2O4 spinel and its isomorph magnetite, Fe3O4. The measured bulk moduli, K, of spinel and magnetite (assuming K′=4) are 1.94±0.06 and 1.86±0.05 Mbar, respectively, in accord with previous ultrasonic determinations. The oxygen u parameter, the only variable atomic position coordinate in the spinel structure (Fd3m, Z=8), decreases with pressure in MgAl2O4, thus indicating that the magnesium tetrahedron is more compressible than the aluminum octahedron. In magnetite the u parameter is unchanged, and both tetrahedron and octahedron display the 1.9 Mbar bulk modulus characteristic of the entire crystal. This behavior contrasts with that of nickel silicate spinel (γ-Ni2SiO4), in which the u parameter increases with pressure because the silicon tetrahedron is relatively incompressible compared to the nickel octahedron.

Magnetotactic bacteria and single-domain magnetite in hemipelagic sediments

Abstract: Remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time-scale, study geomagnetic reversals and secular variation, and measure the rates of seafloor spreading. Although these sediments may contain different magnetic minerals, magnetite is the most commonly observed and magnetically stable phase, and its size, shape and post-depositional fate affect the magnetic remanence of the sediments. Biogenic magnetites are single-domain, with a high natural magnetic remanence (NRM), and have been suggested as a significant source of magnetic remanence in marine sediments. We have studied surface sediments from the Santa Barbara Basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra-fine-grained, single-domain magnetite. Using a novel extraction technique, transmission electron microscopy and SQUID magnetometry, we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments.

Raman Spectroscopy of Thin Corrosion Films on Iron at 100 to 150 C in Air

Abstract: Laser Raman spectroscopy (LRS) has been applied for the detection and characterization of thin corrosion films formed on iron in air at a temperature range from 100 to 150 C. In situ ellipsometric measurements have also been conducted for quantitative estimations of the film growth kinetics. It is found that (1) the oxidation of iron in dry air leads to the formation of a surface oxide film composed primarily of magnetite and (2) the water vapor in air accelerates the formation of hematite. The ratio of magnetite to hematite in the surface oxide film appears to increase with (1) increasing water vapor pressure, (2) ascending temperature, and (3) extending oxidation time.

Carbon isotope evidence for a magmatic origin for Archaean gold-quartz vein ore deposits

Abstract: Sediments from three sites in the Santa Barbara Basin were examined with a 160X power light microscope and TEM equipment to characterize the magnetostatic bacteria (MB) in the samples Both the free magnetite and the crystals in the MB in the samples had lengths from 40-60 nm in length and increased in size from one end to the next An intact magnetosome was also observed Scanning the sediments with saturation isothermal remanent magnetization (SIRM) and altering field demagnetization techniques using a SQUID magnetometer yielded coercivity spectra which showed that the primary remanence carrier in the sediments was single domain magnetite Although it is expected that the predominance of the bacterial magnetite component will decrease with depth in the open ocean basin, single-domain bacteria as old as 50 Myr have been observed in oceanic sediments

Regional variations in spinel compositions: An important key to the Cretaceous/Tertiary event

Abstract: Spinel found in spherules from Cretaceous/Tertiary boundary sediments exhibits a wide range in composition and is distinguished from typical igneous spinel by high Mg, Al, and Ni, relatively low Ti and Cr, and high Fe2O3/FeO. Solid-solution compositions range from nearly pure magnetite to magnesioferrite to relatively pure MgAl2O4 spinel. The Ni-spinel trevorite is also a common component. A regional variation in composition is also observed: spinel from two North Pacific sites has higher Mg and Al and lower Ni and Fe than spinel from two European sites and one South Atlantic site. The most probable source of the spinel is crystallization from molten silicate droplets produced from a major impact event. The regional variations may provide a key to locating the impact site(s).

Extensive deposition of banded iron formations was possible without photosynthesis

Abstract: Precambrian banded iron formations (BIFs) consist of alternating layers of silica and iron minerals such as haematite, magnetite and siderite1,2, but there is controversy as to the origin of the iron. According to one point of view, iron was precipitated from sea water containing Fe(II) ions in solution3,4. Because the photodissociation of water vapour in the Precambrian atmosphere would have been too slow to generate enough of the most probable oxidizing agent, oxygen5, this process would have had to wait for the evolution of organisms producing oxygen. Cairns-Smith6 and Braterman et al.7 alternatively suggested abiotic photodissociation of FeOH+. Through the use of a Precambrian ocean model, I demonstrate here that such a process can quantitatively account for the formation of known BIFs.

Partial oxidation process with magnetic separation of the ground slag

Abstract: Synthesis gas is produced by the partial oxidation of a feedstock comprising petroleum coke and/or heavy liquid hydrocarbon fuel containing sulfur and having an ash that contains nickel, vanadium and silicon. An iron-containing or an iron and calcium-containing additive is introduced into the reaction zone along with the feed. The additive combines with at least a portion of the iron and nickel constituents and sulfur found in the feedstock to produce a liquid phase washing agent that collects and transports at least a portion of the vanadium-containing oxide laths and spinels and other ash components out the reaction zone. Slag and other particulate matter is separated from the effluent gas stream and ground to produce a heterogeneous mixture of magnetic and non-magnetic solid particles. The particles have various degrees of magnetic susceptibility and are thereby separated into the following three separate portions of magnetic particles: (1) particles of an Fe-Ni alloy, and a minor amount of magnetite particles; (2) particles of an oxysulfide phase of iron and nickel, a minor amount of particles of vanadium compounds, and particles of iron silicate and calcium silicate; and (3) particles of vanadium-containing spinels, and particles of metal silicates. The first and third portions are sent to a metal refiner to recover nickel and vanadium respectively. The second portion of magnetic particles having an increased Fe/V wt. ratio is recycled through the gasifier.

Defects and Cation Diffusion in Magnetite (VI): Point Defect Relaxation and Correlation in Cation Tracer Diffusion

Abstract: Chemical diffusion coefficients have been derived from experiments on the point defect relaxation in magnetite. Based on data from preceding studies on the nonstoichiometry of magnetite and on the iron tracer diffusion of this oxide, the chemical diffusion coefficients are analyzed with regard to elementary diffusion processes and the correlated motion of tracer cations.

Defects and Cation Diffusion in Magnetite (VII): Diffusion Controlled Formation of Magnetite During Reactions in the Iron‐Oxygen System

Abstract: Parabolic rate constants describing the kinetics of the exclusively diffusion controlled growth of magnetite scales during simple reactions in the iron-oxygen system are derived on basis of preceding studies on the defect structure and on the transport properties of this oxide. Furthermore, experimental results for the formation of magnetite by solid state reaction between the iron oxides wustite and hematite are reported and compared with calculated data.

Increase in Low-temperature Reduction of Iron Ore Sinter due to Hematite-alumina Solid Solution and Columnar Calcium Ferite

Abstract: Direct observation of lattice images by TEM of magnetite reduced from hematite made clear ultramicroscopic partial distortion due to solid solution of Al2O3. Furthermore, the effect of solid solution of Al2O3 with hematite on strain of magnetite, which had reduced grains of the hematite with a mixture gas of CO-CO2, was estimated by line-broadenings of peaks of X-ray diffraction. It revealed that increasing amounts of Al2O3 increased the strain of magnetite.Fracture strength of the minerals in sintered ore was estimated quantitatively by Vickers indentation, indicating crack propagation-arrest characteristics. Fracture toughness of both glassy silicate and calcium ferrite resulted in values of the same order with a little effect of Al2O3. Calcium ferrite, however, had the lowest value for “critical load”, indicating crack initiation characteristic among all the minerals tested.An experimental study of the crystallization mechanism of skeletal hematite indicated that magnetite coexisting with liquid silicate decomposed, melted and was oxidized at the falling stage of sintering.

Structural and magnetic characterization of the lithiated iron oxide LixFe3O4

Abstract: The Rietveld profile‐analysis method is used to investigate the x‐ray diffraction pattern of lithiated Fe3O4. It is shown that, after exposure to air, pure magnetite coexists with a lithium‐inserted LixFe3O4 phase. The Mossbauer spectra at 300 and 4.2 K have been used to estimate the lithium content of the sample, the pure magnetite concentration, and the iron distribution over the available 16c and 16d sites of the spinel structure. Magnetization measurements from 4.2 to 120 K with an external magnetic field up to 150 kOe have been used to obtain the saturation magnetic moment, the magnetic anisotropy constants, and the susceptibility. It is concluded that a noncollinear spin structure should be present in Li0.5Fe3O4. These results indicate that there is no room‐temperature extrusion of iron even for x→2.0, but that on exposure to air LixFe3O4 samples with x>0.5 are oxidized at room temperature by delithiation.

Preparation of thin films in the system γFe 2 O 3 - Fe 3 O 4 for recording media by spray pyrolysis of organometallic solutions using an ultrasonic pump

Abstract: Thin films of magnetic spinel iron oxides were prepared by pyrolysis of an ultrasonically generated aerosol of organometallic compounds The bases of the method and experimental procedure are described Polycrystalline films of good adherence and homogeneity were obtained Lattice parameters were observed to be modified by film substrate strain interaction and to depend strongly on the hydrogen content The oxidation state of iron was discussed in regard to the experimental growing conditions Corresponding magnetic properties were investigated Pure magnetite Fe 3 O 4 films with H_{c} = 400 Oe and M_{s} = 400 EMU/cm3and gamma ferric oxides films with H_{c} = 400 Oe and M_{s} = 250 EMU/cm3have been obtained without post deposition heat treatment Annealing magnetite films in air resulted in a considerable increase of their coercivity (up to 800 Oe)

Characterisation of soils by mineralmagnetic measurements

Abstract: Processes of iron mineral authigenesis, diagenesis, and dissolution, in interaction with primary inputs of iron minerals, act to produce vertical differentiation of soil magnetic properties. Thus, magnetic iron oxides, in common with other iron forms, may both respond to and reflect soil forming processes. Investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes. Evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance, in several areas of environmental study: for example, the observed magnetic variation can be used to discriminate between individual soils and soil horizons, for the purposes of soil surveying, sediment ‘tagging’ and tracing, and empirical modelling of source-sediment linkages. This paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance. The effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters (including magnetic susceptibility, frequency dependent susceptibility, and anhysteretic and saturation remanences). Contrary to current pedological thought, but in accord with other, magnetically-based, studies (e.g., Mullins), the presence of magnetite/maghemite (typically of superparamagnetic-single domain grain size) within soils has been found to be an extremely widespread phenomenon. Its contemporary neoformation within the soil environment is inferred. However, the input of artificially-generated ferrimagnetic material is indicated for one of the magnetically ‘enhanced’ soils examined here, and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic, processes of magnetic enhancement are inferred. From the data presented, discrimination of industrially-derived magnetic particles appears possible, on the basis of their very low levels of frequency dependent susceptibility (χfd), low anhysteretic remanence (ARM), and dissociation from the finer, clay-sized (< 2 μm) components of the soil. Not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation. Eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content; variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons. Within gleyed (waterlogged) soils, processes of magnetite dissolution appear to be grain size-specific; preferential reduction of those grains of ultrafine (superparamagnetic) and fine (single domain) magnetic grain size is indicated. Where soil forms a major contributor to catchment sediment loads, this has significance in the area of source-sediment modelling. Soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments; further, only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record.

Thermally activated viscous remanence in some magnetite‐ and hematite‐bearing dolomites

Abstract: A Brunhes-age viscous remanent magnetization (VRM) carried by magnetite and hematite in the El Paso and Montoya dolomite formations has unblocking temperatures significantly higher than those predicted by single-domain theory. The samples appear to contain a substantial fraction of pseudo-single domain magnetite, suggesting transdomain processes may be responsible for the stable VRM. A strong inverse correlation between the occurrence of high unblocking temperatures and high isothermal remanence fractions acquired above 0.3 Tesla suggests that the hematite grain size may also be important in determining the range of observed temperatures. Larger grains are inferred to have lower coercivities but higher unblocking temperatures. An older, reverse-polarity component is also observed, and appears to be a record of late Cretaceous-Tertiary tectonic activity.

Mössbauer Spectroscopic Study of the Iron Mineralogy of Post-Glacial Marine Clays

Abstract: Three post-glacial marine clays from eastern Canada and one marine clay from Japan have been studied by M6ssbauer spectroscopy to ascertain their iron mineralogy. Small amounts of hematite (in two samples) and magnetite (in one sample) were found in the Canadian clays, and hematite was detected in the Japanese clay. The major spectral components were ferrous and ferric doublets, consistent with X-ray powder diffraction results that show chlorite, mica, and amphibole in the Canadian samples and smectite in the Japanese sample. Citrate-dithionite extraction removed hematite and most of the magnetite from these samples. Acid-base extraction also removed chlorite and some mica from the Canadian samples. Samples treated by these extractions had appreciably lower geotechnical yield stresses at given water contents.

Magnetite in the radular teeth of chitons

Abstract: To understand the process by which living organisms synthesize magnetite, we measured the Mossbauer spectra on the teeth at different maturation stages along the radula of chitons. Our results show that magnetite is progressively mineralized from hydrous ferric oxide precursor in the early stage of maturation. As the denticles become fully mineralized, a part of magnetite is oxidized to maghemite and the excess of iron supply may be used for the formation of lepidocrocite.

A model for the biological precipitation of Precambrian iron-formation

Abstract: A biological model for the precipitation of Precambrian iron formations is presented. Assuming an oxygen deficient atmosphere and water column to allow sufficient Fe solubility, it is proposed that local oxidizing environments, produced biologically, led to precipitation of iron formations. It is further suggested that spheroidal structures about 30 mm in diameter, which are widespread in low grade cherty rion formations, are relict forms of the organic walled microfossil Eosphaera tylerii. The presence of these structures suggests that the organism may have had a siliceous test, which allowed sufficient rigidity for accumulation and preservation. The model involves precipitation of ferric hydrates by oxidation of iron in the photic zone by a variety of photosynthetic organisms. Silica may have formed in the frustules of silica secreting organisms, including Eosphaera tylerii. Iron formates formed, therefore, by a sediment rain of biologically produced ferric hydrates and silica and other organic material. Siderite and hematite formed diagenetically on basin floors, and subsequent metamorphism produced magnetite and iron silicates.

Process for the production of black pigments based on iron

Abstract: A process for the production of black iron oxide pigments having a magnetite structure from iron ion-containing acidic waste water which comprises precipitating the iron ions with alkaline substances in a first stage at ambient temperature to produce a raw slurry and in a second stage, concentrating the raw slurry adjusted to a pH of from 5.0 to 8.0 with acidic substances and then ripening the concentrated slurry at temperatures above 70° C.

Magnetite exsolution in almandine garnet

Abstract: A B S T R A C T. Three almandine-rich metamorphic garnets have been studied by analytical electron microscopy and electron microprobe analysis. Electron microprobe analyses with total Fe calculated as Fe 2 + show that there are no significant departures from stoichiometry due to the presence of Fe 3§ in any of the garnets studied. However, in the transmission electron microscope (TEM) all the garnets were found to contain myriad spherical, iron-rich particles up to 400/~ in diameter. Microdiffraction techniques have revealed that the particles are a cubic spinel phase, consistent with magnetite. There is no crystallographic relationship between the host garnet and the particles, a rare situation for exsolution processes. The presence of such particles is interpreted in terms of the exsolution of magnetite from almandine garnet during cooling. This can apparently occur at temperatures below 550 ~ The size of the particles is a qualitative indicator of the cooling rate of the rock, but is also dependent on the original Fe a + content of the host garnet.