Showing papers on "Magnetite published in 1983"

Diagenetic magnetite carries ancient yet secondary remanence in some Paleozoic sedimentary carbonates

Abstract: Many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite. When such magnetizations are observed, it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition. However, paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the Helderberg and Bonneterre carbonates (United States) indicate that the magnetite present in these rocks is most likely of diagenetic (i.e., postdepositional) origin.

Diagenesis of Metals Chemically Complexed to Bacteria: Laboratory Formation of Metal Phosphates, Sulfides, and Organic Condensates in Artificial Sediments

Abstract: Cells of Bacillus subtilis, when suspended in a 5mM metal solution, bind metals tenaciously to their cell walls. These metal-loaded cells, when mixed with a synthetic sediment and put under laboratory conditions to simulate low-temperature sediment diagenesis, nucleate the formation of a mixed assemblage of crystalline metal phosphates, metal sulfides, and polymeric, metal-complexed, organic residues. The sequential series of diagenetic events leading to the formation of authigenic mineral phases was followed by transmission electron microscopy and energy-dispersive X-ray analysis. The minerals quartz (SiO/sub 2/) and calcite (CaCO/sub 3/) were employed in the synthetic sediment. Crystalline magnetite (Fe/sub 2/O/sub 3/) and elemental sulfur were added as redox buffering agents to ensure anoxic conditions. Quartz and magnetite appeared unreactive throughout the experimental conditions. Elemental sulfur interacted with the metal-loaded cells, affected both the eventual chemistry and crystal habit of the metal phosphates, and formed a variety of crystalline metal sulfides. Calcite raised the pH of the fluid phase of the sediment, which influenced phosphate mineralization and inhibited metal sulfide genesis.

Morphology and structure of biogenic magnetite particles

Abstract: Blakemore1 found aquatic bacteria that swim along magnetic lines of force. Such bacteria have small particles of magnetite (magnetosomes) within them2,3 of various shapes3–5. We describe here the morphology and structure of bacterial magnetosomes investigated by high-resolution electron microscopy. From direct observation of various kinds of lattice images, the particles are determined to be single crystals with a hexagonal prism shape truncated by {111} planes. The lattice spacings measured agree with those of magnetite.

Redution Kinetics of Hematite and Magnetite Pellets Containing Coal Char

Abstract: The rate of reduction of hematite and magnetite pellets containing coal char in a nitrogen atmosphere and at the temperatures between 800°C and 1200°C was investigated. The reduction rate was found to increase with increasing temperatures, and increasing calcium oxide content of the pellets. The reduction was not stepwise. That is, iron, hematite, magnetite and wustite were detected by X-ray analysis as coexisting phases during early stages of the reduction process. Large temperature differences were found between the core and surface of the pellets, indicating that the reduction process occurred under non-isothermal conditions.Calculated values for the apparent activation energy ranged between 38kcal/mol to 51kcal/mol. These values are lower than those reported by previous workers, but still within the accepted range for chemically controlled processes. The rate of the carbon gasification reaction and the rate of heat transfer from the furnace wall to the reacting pellet were found to exert the strongest influence on the overall rate of the process. Considering the observed non-isothermal conditions, the highly endotermic nature of the carbon gasification reaction and the large increase in the reduction rate with increasing temperature, it appeared that the overall process was heat transfer controlled.

Heterogeneous electron transfer as a pathway in the dissolution of magnetite in oxalic acid solutions

Abstract: The dissolution of metal oxides is a process of importance in several fields such as hydrometallurgy, passivity of metals, and cleaning of boilers and metal surfaces in general. Oxalic acid is one of the most effective reagents for dissolution of magnetite under mild acid conditions. Magnetite is the oxide that confers passivity to steel surfaces. In the present communication, the more salient features of the mechanism of dissolution of magnetite by oxalic acid solutions are discussed with special focus on the role played by ferrous ions in the process. Oxalate plays an unique role among complexing carboxylic ligands in the dissolution of magnetite; it not only facilitates the electron-transfer reaction but also mediates in a relatively fast dissolution during the initial induction period (the induction period is much shorter than in the case of the dissolution of magnetite by ethylenediaminetetraacetic or nitrilotriacetic acid). This unique role has been used in the development of a very efficient scale removal formulation used in the decontamination of nuclear power plants.

55Fe diffusion in magnetite crystals at 500° C and its relevance to oxidation of iron

Abstract: A computer controlled sputter-sectioning apparatus is described which is particularly useful for self diffusion studies at “low” temperatures (D in the range 10−12 to 10−19cm2 sec−1). The technique has been applied to measure the diffusion coefficient of 55Fe in the magnetite lattice as a function of oxygen activity at 500° C. The results are in broad agreement with an extrapolation of pre-existing high temperature diffusion data (T>900°C). The low temperature data have been used to estimate the rate constant for magnetite formation on iron in CO2 + 1 vol% CO at 500° C and this is found to be 250 times smaller than the experimentally measured value. This disagreement is probably attributable to diffusion along oxide grain boundaries during oxidation. The rate constant for the formation of magnetite during the oxidation of iron in oxygen has also been calculated from the diffusion data and is compared with the measured rate constant for temperatures between 325 and 1100°C.

The point of zero charge and adsorption properties of natural magnetite

Abstract: In this paper we report the point of zero charge (pHzpc) of natural magnetite and its adsorptive properties with regard to alkali metal ions. It has been found that pHzpc of freshly ground, untreated magnetite is 6.5, decreasing, after treatment with HCl, to the value of pH 3.8. This discrepancy might be explained as a consequence of possible structure changes on the magnetite surface. Adsorption properties of magnetite with respect to Li+, Na+ and K+ ions in the concentration and pH ranges varying from 0.1 to 1.0 mol·dm−3 and 6.5 to 10, respectively, were examined. For the used chloride media used, the adsorption sequence obtained is Na+>K+>Li+.

Mössbauer emission spectroscopy study of cobalt-surface-doped acicular magnetite particles

Abstract: Mossbauer emission spectroscopy was used to study cobalt deposited by alkaline precipitation on the surface of acicular magnetite particles. The samples closely resemble the cobalt-surface-doped particles currently used in magnetic recording tapes. A portion of the cobalt was found to be incorporated into a composition approximating that of cobalt ferrite (CoFe 2 O 4 ); this is the material responsible for increasing the coercivity of the particles. In samples having 0.33 and 3.3 percent (by weight) deposited cobalt, 85 percent and 34 percent, respectively, of the cobalt was found in CoFe 2 O 4 . The cobalt not incorporated into CoFe 2 O 4 was found in oxides and hydroxides, mainly in Co(OH) 2 and (probably) Co 3 O 4 . The amount of cobalt found in the form of CoFe 2 O 4 is correlated with the increase in coercivity that results from deposition. The possible origins of the uniaxial magnetic anisotropy of surface-doped oxide particles are discussed.

Thermodynamics of the system iron-aluminum-oxygen between 1073 K and 1573 K

Abstract: Equilibrium oxygen pressures of the univariant ternary coexistences hematite + spinel + alumina, alloy + wustite + spinel, and alloy + spinel + alumina were determined at temperatures in the range 1123 to 1423 K using solid state electrochemical cells. The compositions of alloys and oxide phases of this Fe-Al-0 system that equilibrated with one another at 1273, 1423, and 1573 K were determined. These results and those from the literature are used to determine stabilities of the ternary oxide phases as a function of oxygen pressure and composition by constructing equilibrium oxygen pressure diagrams at 1073, 1173, and 1273 K.

Structural Changes Occurring during Reduction of Hematite and Magnetite Pellets Containing Coal Char

Abstract: The structural changes that occurred during the reduction of hematite and magnetite pellets containing coal char, in the temperature range between 900°C and 1200°C, were studied.The behavior of the pellets was complex. Significant temperature gradients within the pellet were attained during the process, and the reduction was accompanied by catastrophic swelling of the pellets between 900°C and 1000°C, while shrinkage was observed at 1100 and 1200°C. Observed microstructural features such as intragranular porosity and cracking of oxide grains resulted from severe lattice disturbances occurring upon transformation of hematite to magnetite.The catastrophic swelling observed for hematite and magnetite pellets 900 and 1000°C was due to the filamentary or whisker growth of iron outward from the wustite surfaces. The whisker formation appears to be due to a changing reducing potential of the gas phase within the pellet coupled with the presence of calcium oxide on the wustite surface. The shrinkage reported at 1100 and 1200°C is accounted for in terms of sintering of iron filaments.The pellets strength was found to decrease between 900°C and 1000°C, and increased at higher temperatures. The weakness at low temperatures was due to the absence of bonding between iron filaments or whiskers. The high strength obtained at higher temperatures (1100 and 1200°C) was produced by sintering of iron filaments.

Rate processes and structural changes in gaseous reduction of hematite particles to magnetite

Abstract: Carol Lake hematite particles were reduced to magnetite at 600 and 1000 °C using CO + CO2 mixtures. The rate of reduction was measured in gravimetric tests and structural changes followed by optical microscopy, BET surface area, and mercury porosimetry. At 600 °C, the reduction of each grain approximately followed the shrinking core model, and fine pores were created in the magnetite produced. The volume of pores was constant at a porosity level of 8.8 pct, but the average pore size depended on the rate of oxygen removal, and finer pores were obtained under conditions of fast reduction. The reaction followed a different path at 1000 °C and proceeded by sideway thickening of a finite number of magnetite lamellae formed parallel to each other in each grain. Reduced particles showed reentrant surface depressions and cracks, but no porosity. Reaction mechanisms were postulated to relate these structural features to the progress of the reaction. Examination of the reaction steps indicated that the separation of oxygen from solid surfaces was likely to be the rate determining step at both temperatures. This was reflected in rate measurements by a strong dependence on CO pressure while the influence of oxygen activity (as represented by CO2/CO ratio) was of secondary importance at 1000 °C and negligible at 600 °C. A detailed analysis of reaction rates could not be made, however, because the particles were of a wide range of sizes and their structure changed during reduction.

Oxide-formation characteristics in the early stages of oxidation of Fe and Fe-Cr alloys

Abstract: Electron microscopy investigations have been conducted on the oxides formed on Fe and Fe-Cr alloys at elevated temperatures (700–800‡C) and at low oxygen partial pressures (∼10−3Pa). Oxide nucleation and growth on chromium-rich iron alloys are significantly different from that for pure iron. On pure Fe and on Fe-3%Cr, wustite and magnetite particles nucleate and grow out of the surface, while on the higher Cr-containing alloys (≥9 wt. % Cr) the spinel oxide (Fe, Cr)3O4 nucleates and grows into the metal. The differences in oxide formation in the early stages of oxidation are explained in terms of the diffusion of different species being rate-controlling and in terms of rapid diffusion, for example, at the metal-oxide interface.

Synthesis of iron-dispersed carbons by pressure pyrolysis of divinylbenzene-vinylferrocene copolymer

Abstract: Versatile carbons with finely dispersed iron were synthesized by pressure pyrolysis of a copolymer prepared from divinylbenzene and vinylferrocene at temperatures below 680‡ C and pressures of 125 MPa. The pyrolysis conditions of the copolymer were found to influence the final morphology of carbons to give fibrils, spheres and polyhedra. The resulting carbons contained uniformly fine particles of cementite (Fe3C) which were less than 30 nm in size, whereas the magnetite was dispersed in the carbon matrix by pressure pyrolysis in the presence of water. Highly dispersed cementite in carbon was found to decompose into metallic iron by further heat treatment above 850‡ C. Porous spherulitic carbons were also synthesized by heat treatment of magnetite containing carbon spherulites.

Studies of magnetite deposition from a flowing suspension

Abstract: The deposition of magnetite from flowing water suspensions is a problem particularly important in boiler plant. Measurements have been made of the rate of deposition of magnetite particles from suspension in demincraliscd water pumped through vertical aluminium tubes of 19 mm bore over a range of temperatures I2-75°C and Reynolds number 7900 to 88500 under isothermal conditions. Bulk concentrations of magnetite particles (of the order of 2 μm diameter) were in the range 200-600 ppm. It would appear that deposition is mass transfer controlled. The initial deposilion rate for a given temperature and concentration varies as the suspension velocity to the power 0.73. The measured asymptotic deposition quantity varies as the suspension velocity to the power −0.66. The conclusions imply a sticking probability for the magnetite on to the aluminium lube walls as 1 for the experimental conditions.

Discrete single-domain and pseudo-single-domain titanomagnetite particles in silicic glass of an ash-flow tuff

Abstract: Single-domain and pseudo-single-domain titanium-poor magnetite grains are present in silicic vitrophyre glass of an ash-flow tuff from western Nevada. Conventional transmission electron microscopy and analytical electron microscopy corroborate inferences, made solely from rock magnetic and remanence data, that the thermoremanent magnetization in the vitrophyre is carried by such grains. The magnetite grains vary in size, shape, and distribution throughout the glass. They probably crystal lized during violent eruption and welding of the ash flow, at temperatures well in excess of 600 °C.

Designing HGMS matrix arrays for selective filtration

Abstract: High Gradient Magnetic Separators typically use a random array of wire matrix elements such as stainless steel wool or expanded metal for the magnetic capture of fine particles. Difficult mineral separation tasks may require systematic and controlled matrix construction to allow more selective separation of one material over another. Of interest here is the dependence of the magnetic capture force on the spacing and shielding of wire matrix elements and how such design parameters lead to a more selective matrix array. A laboratory technique developed in our group provides good confirmation of computer modeling studies. We use arrays of nickel or iron wires, spaced from 6 to 30 times the wire radius. Magnetite filled epoxy is added to the volume around the wire matrix. The wires are magnetized to saturation and the dispersed magnetite moves away from the negative gradients above and below the magnetized wires. The computer studies correlate to the magnetite patterns observed.

The conditions for the formation of lath and porous magnetite on reduction of hematite in H2/H2O gas mixtures

Abstract: Etude realisee en vue de confirmer l'hypothese suivant laquelle la morphologie de la magnetite obtenue est fonction du potentiel d'oxygene dans le gaz reducteur

Iron oxide minerals in soils of the Ha'apai group, Kingdom of Tonga

Abstract: The iron oxide mineralogy of three soils from the Ha'apai Group (c. latitude 19°50?S., longitude 174°30' W.), Kingdom of Tonga, has been investigated. The Group consists mainly of an archipelago of low coral atolls. Soils are formed in the andesitic tephra which overlie the coral. Accessions of marine salts are high and give rise to high soil pH values (>7) and high concentrations of exchangeable cations. The soils are dark reddish brown to reddish brown in colour, with limited colour differentiation with soil depth, and they are not hqdromorphic. The results of study of whole soils and size fractions by chemical extraction, X-ray diffraction, differential thermal analysis, Moessbauer spectroscopy, and by measurement of variation of magnetic attraction following heating to various temperatures, lead to the following conclusions: iron oxide minerals comprise 7-12% by weight of the soils; the predominant clay size iron oxide is very poorly crystalline aluminium-substituted lepidocrocite (a-Fe 0.8Al 0.2,OOH); ferrihydrite is probably also present in the clay fraction; lepidocrocite and ferrihydrite together account for 50-60% of the total iron in the soils studied; magnetite, probably partially oxidized towards maghemite, is present, particularly in sand and silt fractions, and accounts for about 10% of the total iron in the soils. The remainder of the total iron occurs in unweathered minerals in the silt and sand fractions and in the structure of halloysite in the clay fraction. The common soil iron oxides, goethite and hematite, have not been detected. Although the occurrence of lepidocrocite is usually linked with hydromorphic soil conditions, this report, along with others, suggest that it is also found under non-hydromorphic conditions.

Vertical Chemical Gradients in a Single Grain of Magnetite from the Bushveld Complex, South Africa

Abstract: HIGHLY compatible trace elements are the most sensitive indicators of fractionation in magmas and variation in abundances of such trace elements in volcanic suites or in cumulate minerals from layered complexes may be used to place constraints on fractionation mechanisms. A potential area of application of these concepts in layered complex formation is in the use of Cr abundances in magnetite. The exact value for the parti t ioning of Cr between magnetite and silicate liquid is unknown (it is probably quite dependent on oxygen fugacity) but is almost certainly in excess of 100 (Lindstrom, 1976; Leeman et al., 1978). In the upper zone of the Bushveld Complex there exist virtually pure, monomineral ic layers of magnetitite (Willemse, 1969; Molyneux, 1974; yon Gruenewaldt, 1973), one of which (the 'Main Seam') ranges from 1.5 to 2.5 m in thickness (Willemse, 1969). Such a layer provides an excellent opportunity for geochemically testing fractionation hypotheses and also for determining mechanisms of formation of

Vertical magnetic recording medium and its production

Abstract: PURPOSE: To obtain a magnetic recording medium which is particularly inexpensive and durable without using Co by forming a thin film magnetic material consisting essentially of iron oxide having vertical magnetic anisotropy directly or via a thin film consisting of high permeability material on a nonmagnetic base plate. CONSTITUTION: A thin film 1 which consists of iron oxide selected from Fe 3 O 4 , FeO x (4/3