Showing papers on "Magnetite published in 1992"

Magnetite biomineralization in the human brain.

Abstract: Although the mineral magnetite (Fe3O4) is precipitated biochemically by bacteria, protists, and a variety of animals, it has not been documented previously in human tissue. Using an ultrasensitive superconducting magnetometer in a clean-lab environment, we have detected the presence of ferromagnetic material in a variety of tissues from the human brain. Magnetic particle extracts from solubilized brain tissues examined with high-resolution transmission electron microscopy, electron diffraction, and elemental analyses identify minerals in the magnetite-maghemite family, with many of the crystal morphologies and structures resembling strongly those precipitated by magnetotactic bacteria and fish. These magnetic and high-resolution transmission electron microscopy measurements imply the presence of a minimum of 5 million single-domain crystals per gram for most tissues in the brain and greater than 100 million crystals per gram for pia and dura. Magnetic property data indicate the crystals are in clumps of between 50 and 100 particles. Biogenic magnetite in the human brain may account for high-field saturation effects observed in the T1 and T2 values of magnetic resonance imaging and, perhaps, for a variety of biological effects of low-frequency magnetic fields.

Magnetoferritin: in vitro synthesis of a novel magnetic protein

Abstract: The iron storage protein ferritin consists of a spherical polypeptide shell (apoferritin) surrounding a 6-nanometer inorganic core of the hydrated iron oxide ferrihydrite (5Fe2O3.9H2O). Previous studies have shown that the in vitro reconstitution of apoferritin yields mineral cores essentially identical to those of the native proteins. A magnetic mineral was synthesized within the nanodimensional cavity of horse spleen ferritin by the use of controlled reconstitution conditions. Transmission electron microscopy and electron diffraction analysis indicate that the entrapped mineral particles are discrete 6-nanometer spherical single crystals of the ferrimagnetic iron oxide magnetite (Fe3O4). The resulting magnetic protein, "magnetoferritin," could have uses in biomedical imaging, cell labeling, and separation procedures.

Topographic and Magnetic-Sensitive Scanning Tunneling Microscope Study of Magnetite

Abstract: The topographic and magnetic surface structure of a natural single crystal of magnetite (Fe304), a common mineral, has been studied from the submicrometer scale down to the atomic scale with a scanning tunneling microscope having nonmagnetic tungsten as well as ferromagnetic iron probe tips. Several different (001) crystal planes were imaged to atomic resolution with both kinds of tips. A selective imaging of the octahedrally coordinated Fe B-sites in the Fe-O planes, and even a selective imaging of the different magnetic ions Fe2+ and Fe3+, has been achieved, demonstrating for the first time that magnetic imaging can be realized at the atomic level.

Measurement and Modeling of Thermal Conductivity for Dense Iron Oxide and Porous Iron Ore Agglomerates in Stepwise Reduction

Abstract: Thermal conductivities of dense pure hematite, magnetite and wustite measured using the laser flash method as reference value have been summarized in the form of an empirical equation k=1/(AT+B). Wustite shows an almost constant and the relatively low thermal conductivity due to the lattice imperfection. Thermal resistivities, 1/k, of three iron oxides appear to change linearly as a function of temperature up to the Tammann temperature.Effective thermal conductivities of fired, nonfired pellets and sinter, reduced into magnetite, wustite and metallic iron by CO-CO2 or H2 gas, have also been systematically measured in the temperature range from room temperature to 1273 K. The porosity of samples was found to change from 20 to 62% by the reduction from hematite to metallic iron. Measured effective thermal conductivity values of these iron ore agglomerates are remarkably smaller than those of dense materials due to the existence of pore. Effective thermal conductivities of metallic iron and hematite strongly depend on temperature. There is no significant difference in the effective thermal conductivities of reduced samples at the same reduction degree.The measured effective thermal conductivity values have been well-explained by the modified unit cell model originally proposed by Luikov when considering the structure composed of core part and connecting part in the solid phase. This model also clearly identify the distinction of solid structure in samples originated from the difference in preparation and reduction degree.

Coal cleaning process

Abstract: Fine particle coal is beneficiated in specially designed dense medium cyclones to improve particle acceleration and enhance separation efficiency. Raw coal feed is first sized to remove fine coal particles. The coarse fraction is then separated into clean coal, middlings, and refuse. Middlings are comminuted for beneficiation with the fine fraction. The fine fraction is deslimed in a countercurrent cyclone circuit and then separated as multiple fractions of different size specifications in dense medium cyclones. The dense medium contains ultra-fine magnetite particles of a narrow size distribution which aid separation and improves magnetite recovery. Magnetite is recovered from each separated fraction independently, with non-magnetic effluent water from one fraction diluting feed to a smaller-size fraction, and improving both overall coal and magnetite recovery. Magnetite recovery is in specially designed recovery units, based on particle size, with final separation in a rougher-cleaner-scavenger circuit of magnetic drum separators incorporating a high strength rare earth magnet.

Investigation of the magnetic properties of iron oxide nanoparticles used as contrast agent for MRI.

Abstract: Superparamagnetic iron oxide particles, a new class of contrast agents for MRI, are extremely good enhancers of proton relaxation. However, the development of such particle systems has resulted in a wide range of preparations whose physico-chemical properties differ greatly. We have conducted a set of physical experiments: X ray diffraction analysis, relaxivity measurements, susceptibility determinations, and thermomagnetic cycling on different preparations of superparamagnetic particles. Our results demonstrate a good correlation between susceptibilities measured in liquid samples at room temperature and the R2/R1 ratio. Susceptibility measurements between liquid nitrogen temperature and room temperature show three different types of behavior dependent on the size of iron oxide crystals. Comparison of heating and cooling curves from strong field thermomagnetic cycles provides information about the maghemite/magnetite crystal content. The information on magnetic properties reported in this study may help to characterize and to select these materials for use as MRI contrast agents.

A method to study alterations of magnetic minerals during thermal demagnetization applied to a fine-grained marine marl (Trubi formation, Sicily)

Abstract: SUMMARY The method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization (IRM) during stepwise thermal demagnetization. The procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals involved in the alterations. It distinguishes between thermal decay due to unblocking of the remanence and demagnetization due to alterations of magnetic minerals. As a result, the initial magnetic mineralogy can also be determined. The method is demonstrated with samples of fine-grained marine marls from the Pliocene Trubi formation in Sicily (Italy). Single domain (SD) magnetite is the dominant remanence carrier in this sediment. A first alteration during heating to only 145°C is accompanied by a considerable reduction of coercivities higher than 0.1 T. This alteration is thought to be due to the reduction of stress in superficially maghemitized SD magnetite grains. Between 390 and 480°C grains with magnetite-like properties are produced, most likely by oxidation of pyrite in the sediment. At about 560°C the breakdown of iron-bearing silicates also produces magnetic grains. The magnetic grains formed during both alteration processes have low blocking temperatures. Demagnetization above 510°C is not only due to unblocking of the IRM. Alteration of magnetite to haematite proves to be an important demagnetization mechanism.

A stable carbon-based organic magnet

Abstract: The amorphous-like carbon compound prepared by direct pyrolysis of 1,2-diaminopropane is found to be a stable strong magnet, the apparent saturation magnetization (Ms) of which is 10.5 emu g–1(0.022 µB per carbon atom), being comparable to ca. one-third of magnetite (Fe3O4, 30.7 emu g–1).

Photodissolution of iron oxides. IV. A comparative study on the photodissolution of hematite, magnetite, and maghemite in EDTA media

Abstract: The thermal and 254-nm photochemical dissolution reactions of magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) suspended in EDTA aqueous solutions were compared. γ-Fe2O3 and Fe3O4 are...

Iron-alumina nanocomposites prepared by ball milling

Abstract: Small magnetic particles of iron embedded in an insulating alumina matrix have been prepared by ball milling, either by direct milling of a mixture of iron and alumina powders or indirectly by ball-milling enhanced displacement reaction between magnetite and aluminum metal. The average particle size could be reduced to the 10-nm range as indicated by X-ray diffraction linewidths and SEM (scanning electron microscopy). The change in the saturation magnetization and the coercivity relates to the change of the phase composition, decrease of the particle size, and accumulation of internal stresses. >

Adsorption of CO2 on oxygen-deficient magnetite : adsorption enthalpy and adsorption isotherm

Abstract: The adsorption isotherm and the enthalpy of adsorption of CO2 on oxides of oxygen-deficient magnetite have been studied by adsorption techniques. Oxygen-deficient magnetite was prepared by flowing H2 gas through magnetite powder at 300 °C. Adsorption of CO2 onto oxygen-deficient magnetite was studied in the temperature range 150–300 °C. We found that the adsorption can be expressed by the Langmuir dissociative isotherm for three fragments: one carbon atom and two oxygen ions. Deposition of carbon after the adsorption reaction suggests that reduction of surface carbon seems to be involved in the adsorption reaction. The high reactivity for the reduction of CO2 to carbon is considered to come from such a reactive site where an electron is readily donated to the carbon of the CO2 molecule and the oxygen in the CO2 molecule is readily incorporated into a lattice point in the form of O2–. Electron hopping between the Fe2+ and Fe3+ ions in the spinel structure of the magnetite would facilitate the donation of an electron at the adsorption site. The distorted spinel structure of the surface of the H2-reduced magnetite, where the oxygen site is defected, would facilitate the incorporation of the oxygen of CO2.

Trace element composition of detrital magnetite from coastal sediments of northwestern Japan Sea for provenance study

Abstract: Typomorphic varieties of magnetite in coastal sediments of the NW Japan Sea were distinguished by their contents of 11 trace elements: Ti, Mn, Cr, V, Ni, Co, Zr, Sn, Zn, Pb, Cu. Magnetite from principal magmatic complexes differing in composition and crystallization conditions were used as standard samples. The trace elements show that magnetite in sediments of the inlets near river mouths corresponds to that in rocks occurring in the basin of the rivers flowing into these inlets. Magnetite corresponding to that in rocks exposed on the coast and forming active cliffs indicates that these rocks supply abrasion material in the coastal zone. Different magnetite composition in modern nearshore sediments from that of relict shelf deposits is due to the more extensive provenance at sea-leve low-stands during cold phases of the Late Pleistocene.

The calculated solubility of platinum and gold in oxygen-saturated fluids and the genesis of platinum-palladium and gold mineralization in the unconformity-related uranium deposits

Abstract: Thermodynamic calculations on the solubility of platinum and gold indicate that saline (1 m NaCl), fluids saturated with atmospheric oxygen can transport geologically realistic concentrations of platinum-group-elements (PGE), gold, and uranium as chloro-complexes. A number of calculations involving fluid-rock interaction suggest that the oxygen-saturated fluids flowing through rocks containing quartz, muscovite, kaolinite, magnetite and hematite, initially oxidize any magnetite to hematite, allowing subsequent batches of ore fluids to retain their high oxidation state. During their migration through the aquifer, the oxidizing fluids would move the oxidation-reduction interface deeper into the aquifer, leaching and redepositing platinum and gold. The redissolution of earlier precipitated platinum and gold depends on the fluid/ rock ratio and the associated increase in the oxidation state. Therefore, lowering of fluid/rock ratios and/or mixing of the oxidized fluids with a large amount of reduced fluid will precipitate uranium, PGE, and gold. It is suggested that this model can explain the genesis of gold and PGE mineralization in the unconformity-related uranium deposits of the Alligator Rivers Uranium Field in the Northern Territory, Australia.

Evolution of PGE mineralization in hortonolitic dunites of the Mooihoek and Onverwacht pipes, Bushveld Complex

Abstract: Platinum group minerals (PGM) and associated silicates, oxides and sulfides from four samples of platiniferous hortonolitic dunites of the Mooihoek and Onverwacht pipes, Bushveld Complex, show the following PGM assemblages: 1) (Pt, Fe) alloy similar to isoferroplatinum in composition, syngenetic with olivine; 2) sperrylite, syngenetic with clinopyroxene; 3) tetraferroplatinum, laurite, hollingworthite, unnamed mineral (Rh, Ru)AsS, syngenetic with titanomagnetite; 4) tetraferroplatinum (?), sperrylite, geversite, hollingworthite, platarsite, cabriite, rustenburgite, sobolevskite (?) and unnamed (Pd, Pt)4(Cu, Fe)2(Sn, Sb)3, (Pt, Ir)3Sb, (Rh, Ir, Pt)SbS, Rh(Sb, Bi)S, and Pt(Bi, Sb), syngenetic with late amphiboles, magnetite, chlorite, ilmenite, and pentlandite. Fine (less than 0.1 mm) drop-like sulfide inclusions, consisting of troilite, ferruginous chalcopyrite, ferruginous pentlandite, and encircled by micrograins of magnetite, ilmenite, amphiboles, and biotite, were found in nonbroken grains of olivine. The inclusions' bulk composition is that of high-temperature sulfide phase in equilibrium with high-iron silicate melt, under conditions of low sulfur fugacity. The temperatures at which different mineral assemblages were formed were studied in a sample of wehrlite, rich in titanomagnetite, and the following stages were identified: 1) a high temperature (980–1020°C) stage measured using two-pyroxene thermometry; and 2) a high to medium temperature stage, measured using exsolution textures of titanomagnetite aggregates. This stage occurs in three stages: first, exsolution in the magnetite-ulvospinel-spinel system (separation of Zn-Cr-hercynite, 800–1000°C); later, in the magnetite-ulvospinel system (separation of ulvospinel, about 500°C); and finally oxidizing exsolution with the genesis of ilmenite (590–410°C with −log fO2 being 21 and 33, respectively). Evolutionary features of mineral assemblages suggest that hortonolitic dunites underwent, partly at least, a magmatic stage and a lengthy stage of subsolidus autometamorphic transformations under the influence of metal-bearing fluid. Low solubility of platinoids in silicate and low-sulfur sulfide melts probably was the reason for, on the one hand, direct crystallization of PGM from melt, unrelated to separations of primary sulfide phase, and, on the other hand, concentration of platinum group elements (PGE) in fluid. The PGE-rich fluid generated numerous PGM assemblages both at the stage of crystallization of residual melt which concentrated iron and titanium (inclusions in titanomagnetite) and as the result of intense medium-temperature autometasomatic reworking of primary hortonolitic dunites (simultaneously with minerals of the late chlorite-magnetite-amphibole assemblage).

Adsorption of Radioactive Ions 137Cs+, 85Sr2+, and 60Co2+ on Natural Magnetite and Hematite

Abstract: Natural magnetite and hematite have been used as granular sorbents for 137Cs+, 85Sr2+, and 60Co2+ at tracer concentration levels in aqueous solutions of constant pH (range 2–10) at 25°C. The kinetics of adsorption, up to the first 60 to 90 min, followed a first-order equation. At pH 6–8 about 50% Cs, 30% Co, and 18% Sr is removed from the solution with magnetite and 78% Co with hematite. The difference in the sorption capabilities of magnetite and hematite is discussed in terms of crystal structures of these oxides.

Magnetizing Reduction of Iron Ores

Abstract: The efficiency of reduction-roasting/magnetic separation of hematite, goethite and any extremely finegrained iron ore, which is non-responsive to conventional processing techniques, is reviewed. Roasting equipment, roasting mechanism, superior results over conventional techniques, and certain advantages are highlighted. Previous works on solid state diffusion theory and gas diffusion theory, in the direction of explaining the mechanism of iron ore reduction are indicated. The transformation of hematite to magnetite, wustite, and metallic iron, with a reductant, at elevated temperature, is established by XRD analysis.

Stress, grain size, and magnetic stability of magnetite

Abstract: Existing data of thermal variations of bulk coercivity Hc for various magnetite samples are shown to be linearly proportional to λ/Ms in the temperature range from the Verwey transition to the Neel temperature of magnetite, where λ is some appropriate magnetostriction constant and Ms is the saturation magnetization. This clearly indicates that the temperature stability of bulk coercivity in these magnetite samples is stress controlled, as concluded early by Hodych (1982, 1986) based on Hc data measured at low temperatures. However, no existing theories of bulk coercivity for multidomain grains are satisfactory at explaining this seemly simple λ/Ms dependency of Hc. Although other complexities may exist, we suggest a mechanism in which internal stress associated with dislocations in these samples not only causes domain wall pinning but also determines the anisotropy of domain walls. The theory thus developed is consistent with the observed temperature dependency of Hc. We further suggest that the temperature dependency of bulk coercivity of all magnetite grains is predominantly controlled by stress associated with dislocations, with a few exceptions such as for single-domain (or nearly single-domain) grains for which the stability is primarily controlled by grain shape.

Oxidation kinetics of fayalite (Fe2SiO4)

Abstract: Single crystals of fayalite (Fe2SiO4) have been oxidized either in the hematite or the magnetite stability field to investigate the kinetics and mechanisms of oxidation. For samples heated in air at 770° C, a two-phase region composed of fine-grained iron oxide and silica phases formed as the reaction front moved into the sample, and an iron oxide layer formed external to this two-phase region. The presence of the single-phase oxide layer coating the specimens indicates that oxidation occurs by the migration of iron from the fayalite to the gas-solid interface rather than by the movement of oxygen in the opposite direction. For oxidation in air, the kinetics followed a parabolic growth law, with the rate of oxidation limited by the diffusion of iron from the internal reaction front to the gas-solid interface through the iron oxide. When fayalite was oxidized in the magnetite stability field, using a CO/CO2 gas mixture at 1030° C, oxidation was controlled by the reaction at the gas-solid interface, yielding an oxidation rate considerably slower than that predicted for diffusion-controlled growth of the oxide layer.

Storage diagenesis versus sulphide authigenesis: possible implications in environmental magnetism

Abstract: Experiments involving the gradual drying out of controlled mixtures of soil and organic lake sediment during storage at room temperature show that this leads to a loss of magnetic susceptibility and isothermal remanence greatly in excess of the initial values for the sediment components of the mixtures. We conclude that during storage in the moist state, soil-derived, fine-grained, ferrimagnetic iron oxides (magnetite and/or maghemite) are transformed to paramagnetic and/or imperfect antiferrimagnetic minerals. The imperfect anti-ferromagnetic component of the initial mixtures, which probably includes goethite, appears to survive and may even increase during storage. The experimental results compare well with the previously documented effects of storing wet sediment from the site, Peckforton Mere, Cheshire, U.K., over a comparable time interval. We conclude that transformation of fine grained ferrimagnets during ‘storage diagenesis’ may be responsible for many of the examples of loss of magnetic susceptibility and remanence attributed by other authors solely to the oxidation of an iron sulphide such as greigite. Only where greigite is positively identified is it valid to infer a contribution from it to the magnetic properties of lake sediments: loss of susceptibility or remanence during storage is not alone a sufficient basis for such an inference. Early drying of samples will normally avoid the effects of storage diagenesis; and recent sediment samples so treated will, where greigite formation, bacterial magnetite and magnetite dissolution are insignificant, provide a valid basis for source identification on the basis of magnetic properties.

High-temperature hydrogen sulfide removal from coal-derived gas by iron ore

Abstract: Six kinds of hematite and two kinds of magnetite were examined as solid sorbents of H 2 S for high-temperature desulfurization. The hematite samples were more reactive than the magnetite samples, and reactivity of the sample of iron ore depended on the specifiic ore. The hematite ores were confirmed to be effective sorbents of H 2 S in H 2 O-poor and/or H 2 -rich gas. The reactivities of the samples of the hematite were decreased by the partial reduction to Fe 3 O 4 with the simulated coal-derived gas (in the absence of H 2 S). Impurities and thermal history of an iron ore seemed to affect its reactivity

Magnetic Characteristics of Sedimentary Rocks from the Tsengwen-chi and Erhjen-chi Sections in Southwestern Taiwan

Abstract: Four basic magnetic parameters (κ, SIRM, Hcr, MDF(subscript SIRM))and two thermal magnetic analyses were investigated in this study to characterize the magnetic properties of magnetite-, pyrrhotite- and greigite-bearing rocks of the Tsengwen-chi and Erhjen-chi sections. These rocks have very similar behaviors in the acquisition of isothermal remanent magnetization (IRM) in spite of bearing with different magnetic minerals. Because of the difference in magnetic mineral concentration and grain size, greigite-, pyrrhotite-, and magnetite-bearing rocks have relatively high, intermediate and low parameter values (κ, SIRM, Hcr, MDF(subscript SIRM)), respectively. Thermomagnetic analysis in a nitrogen atmosphere has further revealed that each magnetic mineral has its own diagnostic magnetization curves and that greigite could have transformed to pyrrhotite and magnetite, pyrrhotite to magnetite, and magnetite to hematite. Bulk magnetic susceptibility has also shown remarkable drop in greigite-bearing rocks below 320℃, while there are minor and almost noneffective susceptibility changes in pyrrhotite- and magnetite-bearing rocks below 400℃, respectively.

Crystallization of ferric hydroxide into spinel by adsorption on colloidal magnetite

Abstract: Interaction between “ferric hydroxide” and colloidal magnetite in alkaline medium has been characterized by transmission electron microscopy, X ray powder diffraction, Mossbauer spectroscopy, and kinetics of dissolution in acidic medium. An adsorption reaction takes place immediately. It involves an interfacial electron transfer which starts magnetite oxidation and makes ferric hydroxide transform into a spinel layer growing epitactically on the particle surface.

Maghemite in Icelandic Basalts

Abstract: Curie temperatures indicating non-titaniferous magnetite are common in Icelandic basalts of all ages, especially Tertiary ones. Yet, microprobe analyses of such samples have shown high titanium in the magnetite. To resolve this paradox, and the mechanism at work, the magnetic mineral fraction of eight basalt samples with Js-T curves characteristic for pure magnetite was subjected to a multi-disciplinary analysis including Mossbauer spectroscopy and X-ray diffraction. In most of the samples titanium in the magnetite, as analysed with the microprobe, ranged between 16 and 28 wt.%, indicating submicroscopic solvus exsolution in the titanomagnetite, beyond the power of resolution for the microprobe. More unexpectedly in view of the reversible Js-T curves, Mossbauer spectroscopy showed appreciable proportion of maghemite in the magnetic fraction. A three-stage mechanism is proposed for the formation of the mineral assemblages observed: (1) limited high-temperature oxyexsolution; (2) solvus exsolution during low-temperature hydrothermal alteration; and (3) maghemitization of the magnetite. Finally, the maghemite may transform to hematite with time. It is concluded that maghemite is much more common in Icelandic rocks than hitherto believed.

Rheology and stability of magnetite dense media

Abstract: The efficiency of the dense medium separation process is known to depend on the rheology and stability of the medium. In particular, the medium should exhibit a low viscosity and a high settling stability. Despite this knowledge, little information existed on these medium properties. The lack of information stems partially from the difficulties associated with measuring the rheological properties of unstable suspensions. In order to measure these properties, it was necessary to design a rheometer for settling suspensions. Once this was achieved, the rheology and stability of magnetite suspensions were characterized and the influences of various medium parameters on these properties were investigated. Settling experiments revealed that magnetite dense media exhibit bulk zone settling properties that are characterized by the presence of (from top to bottom): a supernatant, a transition zone, a constant density zone and a sediment. The constant density zone was found to have a solids content that was the same as that of the initial suspension. Test results indicated that the suspension mudline settled at approximately the same rate as the constant density zone and should therefore provide a good indication of the media stability. Based on knowledge of the settling properties of magnetite suspensions, a rheometer fixture was designed that could be used to measure the rheological properties of such suspensions. The fixture is an elongated double gap concentric cylinder cup and bob arrangement that positions the bob in the constant density zone of the settling suspension during