Showing papers on "Magnetite published in 1981"

Dissolution of Iron Oxides and Oxyhydroxides in Hydrochloric and Perchloric Acids

Abstract: The dissolution of synthetic magnetite, maghemite, hematite, goethite, lepidocrocite, and ak- aganeite was faster in HCI than in HCIO4. In the presence of H § the C1- ion increased the dissolution rate, but the 004- ion had no effect, suggesting that the formation of Fe-Cl surface complexes assists dissolution. The effect of temperature on the initial dissolution rate can be described by the Arrhenius equation, with dissolution rates in the order: lepidocrocite > magnetite > akaganeite > maghemite > hematite > goe- thite. Activation energies and frequency factors for these minerals are 20.0, 19.0, 16.0, 20.3, 20.9, 22.5 kcal/ mole and 5.8  1011, 1.8  101~ 7.4  10 7, 5.1  101~ 2.1  101~ 3.0 x 1011 g Fe dissolved/mZ/hr, re- spectively. The complete dissolution of magnetite, maghemite, hematite, and goethite is well described by the cube-root law, whereas that of lepidocrocite is not.

Relationship between median destructive field and remanent coercive forces for dispersed natural magnetite, titanomagnetite and hematite

Abstract: Summary. Remanent acquisition curves, remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well-defined grain-size fractions between 5 and 250 μm of natural magnetite, titanomagnetite and hematite. From these curves, the remanent acquisition coercive force H'cr, the remanent coercive force Hcr and the median destructive field of IRM H½I were determined. Theoretically these parameters should be the same for an assembly of non-interacting, homogeneously distributed, randomly oriented single-domain grains. For a given hematite specimen H'cr, Hcr and H½I have about the same value in spite of the strong grain-size dependence of these parameters. For each specimen of magnetite and titanomagnetite the value of H'cr is larger than Hcr which again is larger than H½I. However, the ratios H'cr/Hcr and H½I/Hcr appear to have a (different) constant value. An interesting relationship which appears to hold for dispersed magnetite, titanomagnetite or hematite grains between 5 and 250 μm, independently of grain-size, quantity and packing density of the magnetic material, is:

Nature and origin of hematite in the Moenkopi Formation (Triassic), Colorado Plateau: A contribution to the origin of magnetism in red beds

Abstract: Petrographic studies show that hematite is present in the Moenkopi Formation in at least five and possibly six forms: (1) microcrystalline hematite, (2) crystals of specular hematite, (3) polycrystalline and monocrystalline grains, (4) grains of partly hematitized ilmenite, (5) grains composed of primary ilmenite-hematite intergrowths, and (6) ultrafine pigment. The microcrystalline hematite and crystals of specular hematite are unequivocally authigenic. They form cement in interstitial and secondary voids, and they have replaced detrital iron-bearing silicate minerals. Furthermore, microcrystalline hematite is superimposed on other authigenic cementing minerals such as potassium feldspar, calcite, dolomite, and quartz, and in some cases it has replaced authigenic pyrite. In addition, both microcrystalline and specularite crystals are common daughter products of intrastratally altered biotite grains. Thermodynamic considerations coupled with studies of hematite-magnetite relationships in modern sediments indicate that most of the hematite in the polycrystalline grains, and probably the monocrystalline grains as well, was formed authigenically by post-depositional replacement of detrital grains of magnetite. The ilmenite probably has similarly altered in situ to hematite. The only hematite of unquestionable detrital origin in the red beds is the hematite in the ilmenite-hematite intergrowths (‘tiger striped’ grains) and that in monocrystalline detrital grains containing rutile exsolution platelets, both of which are products of high-temperature processes. With the exception of the ultrafine pigment, each of the above forms is coarser grained than the superparamagnetic threshold for hematite, and therefore each contributes components of remanent magnetism to the rocks. Inasmuch as most of the hematite varieties represent authigenic products of intrastratal alterations that require considerable geologic time, we conclude that the red bed remanence is largely chemical remanent magnetization (CRM) acquired over long time intervals. The pigment in the Moenkopi red beds consists partly of authigenic ultrafine red iron oxide and partly of translucent microcrystalline hematite. The ultrafine red iron oxide may or may not be hematite, but even if it is, the grain size probably lies below the paramagnetic threshold for hematite. Much of the pigment, therefore, may not contribute greatly to the remanent magnetism in the rocks.

Chemistry of rock-forming minerals of the Cretaceous-Paleocene batholith in southwestern Japan and implications for magma genesis

Abstract: Petrographic descriptions and electron microprobe analyses of minerals are presented for 35 specimens from seven suites chosen to examine the transition from magnetite series to ilmenite series granitoids along two transects across the Cretaceous-Paleocene Inner Zone batholith of southwestern Japan. Regularities in chemical compositions of amphiboles, biotites, and feldspars suggest that fundamentally similar processes produced the magmas that formed the two series. Constant or decreasing Fe/(Fe+Mg) for biotites and amphiboles with increasing host rock silica content, coupled with the absence of early formed magnetite and sphene, suggest that magnetite series rocks may have become oxidized during crystallization near the level of intrusion, through the processes of second boiling and differential loss of hydrogen. For the Daito-Yokota, magnetite series suite, Fe/(Fe+Mg) for biotites decreased from 0.48 to 0.37 as SiO2 content of the host rock increased from 55.3 to 75.5 wt %; for an ilmenite series suite from the Takanawa Peninsula, Fe/(Fe+Mg) for biotites increased from 0.51 to 0.77 with an increase in host rock SiO2 from 53.4 to 75.5. Detailed consideration of amphibole chemistry shows predominance of edenitic and tschermakitic substitution schemes as well as coupling between substitutions of Ti in octahedral sites and AlIV. Interrelations between amphibole and biotite chemistry show that Fe/(Fe+Mg) and Mn contents can be interpreted in terms of equilibration, whereas Ti content cannot. The chemistry of chlorites correlates well with that of biotites; primary and secondary muscovites are distinct in composition. Plagioclase in all studied suites shows igneous zoning appropriate to host rock composition; perthitic alkali feldspars in all samples have lost albite component, and temperatures based on the two-feldspar geothermometer are low. The biotite-apatite geothermometer is also inoperative for this group of samples because fully fluorinated apatites typically occur in biotites of modest F content. Whereas magnetites have reequilibrated, analyses of ilmenites for the representative Daito-Yokota and Takanawa suites corroborate biotite compositional data and suggest that fO2 probably differed by 2 to 3 orders of magnitude during crystallization of silica-rich magnetite and ilmenite series granites. Whole-rock chemistry supports mineral chemistry in suggesting that the studied granitoids have crystallized from magmas generated in a lower crustal environment in which mantle-derived magmas partially melted source materials with igneous characteristics.

Microstructural changes on the reduction of hematite to maanetite

Abstract: The microstructures resulting from the reduction of hematite to magnetite have been examined for a wide range of temperatures and gas conditions. A transition in product morphology from plate or lath magnetite to porous magnetite was found to occur at a constant free energy difference between the reducing gas mixture and the hematite over a range of reaction temperatures. Direct observations of lath nucleation and growth are reported and show the self-accelerating effect of the Fe2O3 → Fe3O4 transformation. The limits of porous growth are discussed in terms of established theories of discontinuous precipitation and a mechanism for the formation of lath magnetite is proposed.

Electronic Conduction and Thermopower of Magnetite and Iron‐Aluminate Spinels

Abstract: Electrical-conductivity and thermoelectric-power measurements of magnetite and iron-aluminate spinels at high temperatures confirm a small-polaron model of electronic conduction. Electrical properties corroborate the previously established cation distribution and defect model. Iron cations become randomly distributed between sublattices at the highest temperature. An n-to-p transition therefore occurs at the 50%Fe3O4-50%FeAl2O4 composition. Cation distribution and defect equilibria appear to be independent of composition.

Redistribution of ore elements during serpentinization and talc-carbonate alteration of some Archean dunites, Western Australia

Abstract: Serpentinization of barren and weakly mineralized Archean dunites in the Yilgarn Block, Western Australia, occurred without any apparent loss of major components, including the important ore elements Ni and Fe. Sulfur addition during initial serpentinization resulted in about 30 percent of original silicate Ni being redistributed to newly formed sulfide by reactions similar to: olivine + water = lizardite + brucite and lizardite + brucite + S = lizardite + brucite + magnetite + heazlewoodite + water + H 2 . Serpentinization at a higher temperature or metamorphism of the lizardite-bearing assemblage formed antigorite serpentinite in which up to 60 percent of bulk rock Ni is incorporated in sulfide form: lizardite + brucite + magnetite + heazlewoodite + CO 2 + S = antigorite + magnesite + magnetite + Ni sulfide + water + H 2 .Complete CO 2 metasomatism resulted in talc-carbonate rocks with still more Ni redistributed to sulfides, and the formation of some Fe sulfide: antigorite + magnesite + magnetite + Ni sulfide + CO 2 + S = talc + magnesite + pentlanditc + Fe sulfide + or - magnetite + water + O 2 . In areas where low S activity precluded sulfide formation, Ni was strongly enriched in magnetite in serpentinites or talc in talc-carbonate rocks.Preexisting disseminated Ni sulfides of magmatic origin were significantly upgraded by serpentinization. Up to 30 percent of sulfide Ni in ores averaging about 0.7 percent Ni is of metamorphic origin, and this proportion may be higher in talc-carbonate ores. Fe sulfides of metamorphic origin form up to 20 percent of the total sulfide fraction in talc-carbonate-altered disseminated Ni ores, so that about half the sulfide content of these ores may be metamorphically derived by redistribution of elements originally held in olivine.

Contribution of Magnetite to Oxalate-Extractable Iron in Soils and Sediments from the Maumee River Basin of Ohio1

Abstract: Magnetic separations of whole soil and sediment samples from the Maumee River Basin (Ohio) commonly produced small but significant quantities of detrital magnetite. These magnetites were soluble in acid solutions of ammonium oxalate and were also more reactive than a standard magnetite sample of comparable particle size. Differential thermal analyses suggested that the enhanced reactivity of the soil and sediment magnetites may be due to altered grain surfaces. Since detrital magnetite is a common constituent of the sand and silt fractions of many soils and sediments, care should be exercised when evaluating “amorphous” iron data obtained by any of the popular oxalate procedures.

Changes of Mineral Phases during the Sintering of Iron Ore-Lime Stone Systems

Abstract: In order to elucidate the fundamental aspects of the sintering processes of self-fluxing sinters, the mineral formation processes of mixtures of iron ore and lime stone with or without coke were studied. After heating between 1200° and 1350°C the sinters were examined by microscopy, X-ray diffraction and EPMA.The results indicated that the mineral formation processes of the systems were fundamentally the same as those of 80%Fe2O3 CaO-SiO2 system reported earlier. They were as follows:(1) Molten calcium ferrite was formed at the beginning.(2) Gangue minerals dissolved into the melt.(3) Iron oxides were precipitated. They were either or both of hematite and magnetite, depending on the oxygen potential or the SiO2 content of the melt.(4) At higher temperatures, solid iron oxides and a melt coexisted, and characteristic microstructure was formed during solidification depending on the basicity.On the basis of the above results, the microstructures and the formation processes of self-fluxing sinters with various basicities were discussed.

Ferromagnetic crystals (magnetite?) in human tissue

Abstract: In recent years, a variety of animals have been found which are able to synthesize the ferromagnetic mineral magnetite (Fe3O4). Lowenstam (1962) originally recognized biogenic magnetite in the radular teeth of a primitive marine mollusc, the chiton (Polyplacophora), and since then it has been identified as a precipitate in several magnetically sensitive organisms, including honey bees (Gould, Kirschvink & Deffeyes, 1978), homing pigeons (Walcott, Gould & Kirschvink, 1979) and in magnetotactic bacteria (Frankel, Blakemore & Wolfe, 1979). Zoeger, Dunn & Fuller (1980) also report a localized concentration of magnetite in dolphin heads, although magnetosensory behavioural experiments have not as yet been done on them. Magnetite is biologically unique because it is both ferromagnetic and conducts electricity like a metal; consequently it interacts strongly with magnetic and electric fields. Due to the numerous industrial and research environments which expose people to artificially intense electromagnetic conditions, it is of importance to know whether or not this material might exist in human tissue. Kirschvink & Gould (1980) have argued that there are probably one or more non-sensory metabolic functions for magnetite from which specialized magnetoreceptors could have evolved; consequently one might expect to find small amounts of magnetite in all animals, including humans. In an attempt to partially answer this question, I searched for magnetic remanence in four intact human adrenal glands which had been removed during autopsy and were frozen quickly in non-magnetic containers. Results of this analysis are shown on Fig. 1. Indeed, there is a measurable amount of high-coercivity ferromagnetic material present which appears to be finely disseminated throughout the tissue. Between 1 and 10 million single-domain magnetite crystals per gram would be necessary to account for the observed magnetic remanence. Although these measurements do not uniquely identify the crystal phase as magnetite, no other ferromagnetic minerals have ever been observed as biologic precipitates. Positive identification, of course, awaits the development of magnetic separation techniques capable of isolating and purifying these submicroscopic crystals. Barnothy & Sumegi (1969) have shown that mouse adrenals are particularly prone to degeneration in moderately strong magnetic fields; this effect might be due to the presence of magnetite.

The effects of nucleation and growth on the reduction of Fe2O3 to Fe3O4

Abstract: The kinetics of reduction of hematite powder to magnetite in CO-CO2 gas mixtures at temperatures between 500 and 663 °C have been measured. The reactions are described in terms of a simple nucleation and growth model. The chemical reaction rate constants for the reduction of hematite to magnetite are obtained and the nucleation frequencies of magnetite on hematite are calculated for a range of temperatures and oxygen partial pressures. A possible technique for the improvement of the “reducibility” of dense hematite ores is suggested.

Synthetic rhombohedral magnetite-pigment

Abstract: In a process to produce a synthetic rhombehedral magnetite comprising the steps of: A. Contacting ferrous chloride solution having an Fe++ concentration of from about 0.9 to 2.4 moles per liter with a stoichiometric amount of carbonate ion; B. heating the mixture to a temperature of from about 70°-90° C.; C. aerating the mixture to oxidize the iron to magnetite having a Fe++ / total Fe++ and Fe+++ ratio of from about 0.25-0.38; and D. recovering the magnetite so produced, the improvement which comprises providing the carbonate in the form of finely divided particles of an average size of less than 3.5 microns. The process wherein said magnetite is calcined at a temperature of from 650°-925° C. in the presence of oxygen to produce alpha ferric oxide is claimed and a synthetic rhombohedral magnetite having a BET surface area of greater than about 13 m2 /g and an average particle size of less than about 0.08 microns as measures along the long axis is also claimed.

A study of the reduction of hematite to magnetite using a stabilized zirconia cell

Abstract: Batches hematite particles were reduced to magnetite by hydrogen in small scale fluidized bed, from which samples of partially reduced ore could be extracted for metallographic studies or surface area measurements. A stabilized zirconia cell was used to monitor the gas phase composition at eight locations in the bed. With this device a complete picture of the consumption of hydrogen was established that could detect small changes in the efficiency of the reduction reaction. Approximate rates of reduction could also be calculated. Hematite reduction took place in three stages; an initial period in which the efficiency of the reaction increased with time, followed by a period of relatively constant reduction efficiency, and finally after about 85 to 90 pct transformation a period of falling efficiency. Wustite did not form while hematite was present. The efficiency of the reaction increased when the hematite particles were coated with platinum and decreased when the particles were coated either with silica or with a naturally occurring dirt film. A kinetic and morphological analysis indicated that the reaction was rate controlled by a surface reaction at the bottom of pores in the magnetite. These pores do not appear to penetrate to the receding hematite interface. The initial increase in the efficiency of the reaction was attributed to the developing pore structure of the magnetite which increased the area for reaction.

A composite magnetic developer

Abstract: A dry composite magnetic developer comprises a dry blend of (A) developer particles comprising a nonpulverizing agglomerate of cubic particles of magnetite having a number average particle size of 1 to 10 microns as measured by an electron microscope dispersed in a binder resin medium and (B) developer particles comprising magnetite particles having a particle size of 0.2 to 1 µm in a binder resin medium at an (A)/ (B) weight ratio of from 95/5 to 10/90.