Showing papers on "Magnetite published in 1979"

Magnetite in Freshwater Magnetotactic Bacteria

Abstract: A previously undescribed magnetotactic spirillum isolated from a freshwater swamp was mass cultured in the magnetic as well as the nonmagnetic state in chemically defined culture media. Results of Mossbauer spectroscopic analysis applied to whole cells identifies magnetite as a constituent of these magnetic bacteria.

Optical properties of magnetite (Fe3O4)

Abstract: The reflectivity of magnetite (Fe3O4) has been measured in the spectral energy range between 003 and 12 eV The optical constants have been determined by means of a Kramers-Kronig analysis The interband transition spectrum can be interpreted by O 2p to Fe4s transitions and be Fe3dn to 3dn-14s orbital promotion processes including final-state effects of the atom-like 3dn-1 configurations

Grain size limits for pseudosingle domain behavior in magnetite: Implications for paleomagnetism

Abstract: A theoretical model of grain size dependence of domain transitions in magnetite is presented. This domain model is used to predict the grain size range for pseudosingle domain (PSD) behavior in magnetite. For cubic magnetite particles, the single domain (SD) to two-domain (TD) transition occurs at ∼800 ± 200 A, the TD-three-domain transition occurs at ∼ 1500 A, and the PSD-"true" multidomain (MD) transition occurs at \sim 8\mu m. These results are in fair agreement with experimental results within a factor of 2. Our results indicate that the grain size range for the canonical case of a PSD grain (a TD grain with a single 180° wall) or "PSARK" (subdomain magnetic moment) ranges from ∼700 A for cubic particles of magnetite to \sim3.2 \mu m for a length: width ratio of 5:1. Comparison with grain size distributions of magnetite grains in igneous rocks suggest that the PSARK can be the major contributor to the remanence of these grains.

Aeromagnetic Detection of Diagenetic Magnetite over Oil Fields: GEOLOGIC NOTES

Abstract: High-wave-number magnetic anomalies measured as part of an airborne magnetic survey over the Cement oil field, Oklahoma, are interpreted as reflecting abundant near-surface magnetite formed by the reduction of hydrated iron oxides and/or hematite as a direct result of petroleum microseepage. Airborne geophysical methods provide an economically sound way to prospect for resources (Craib, 1972; Maxim and Cullen, 1977). In this report we suggest a correlation between high-wave-number anomalies observed on profiles from an airborne magnetic survey over the Cement oil field, Oklahoma, and the near-surface diagenetic formation of magnetite as a direct result of hydrocarbon microseepage from underlying reservoirs. This is especially interesting because, for most petroleum exploration applications, magnetic anomalies originate in crystalline basement rocks beneath the sedimentary cover. The Cement oil field is located in the southeast part of the Anadarko basin in Caddo and Grady Counties. The Cement structure is a northwest-southeast doubly plunging asymmetric anticline that has two distinct domes (East Cement and West Cement); it is bounded on the north flank by a large thrust fault. Oil and gas production is from Permian sandstone and Pennsylvanian carbonate and clastic rocks which range in depth from about 600 to 2,270 m. The reservoirs at Cement were shown previously to have undergone long-continued microseepage resulting in diagenetic alteration of the shallow overburden (Donovan, 1974; Donovan and Dalziel, 1977). Flight-line profiles of airborne measurements (flown at 120 m) of the earth's total magnetic field over the Cement area display anomalous high-wave-number peaks suggesting a shallow source (Fig. 1). Because the magnetic susceptibility of sedimentary rocks is largely a measure of their magnetite content, we conducted a systematic search for magnetite in borehole cuttings collected during the development of the oil field. Ferromagnetic material was separated magnetically from crushed samples taken from cuttings from the uppermost 300 m of five boreholes and composited at ~30-m intervals. Samples from depths less than about 80 m were not preserved. X-ray diffraction of the separated material confirmed it to be magnetite. Over the anticline, the amount of magnetite in the rocks appears to increase near the surface (Figs. 2, 3). We suggest that the slow but long-continued leakage from a petroleum or natural gas reservoir of hydrocarbons and/or associated compounds causes a chemical reducing environment in the overlying rocks and the consequent reduction of hydrated ferric oxides and hematite to form magnetite. Hematite and hydrated ferric oxides are common as grain coatings and bonding agents in clastic sedimentary rocks. Many ferric oxides are stable in the oxidizing zone, but in reducing environments they undergo reduction and release the more soluble ferrous iron in solution. A relatively large loss of iron through reduction and dissolution results in rocks that appear "bleached" or tinted. However, not all ferric iron may undergo reduction and dissolution. We postulate, schematically, that a sequence of a teration may take place as follows: hydrated ferric oxides ^rarr hematite (Fe2O3) ^rarr magnetite (Fe+2 Fe2+3 O4) Comparison of the Cement aeromagnetic data with aeromagnetic data from the smaller but End_Page 245------------------------------ Fig. 1. A, Selected segments of magnetic profiles from Cement oil field aeromagnetic survey. Cement structure is located on northeast flank of Anadarko basin and magnetic profiles probably reflect rise of magnetic basement to north (compare with map of Jones and Lyons, 1964). High-wave-number peaks are readily observable in profiles over crest of structure presumably where maximum formation of diagenetic magnetite took place. For comparison, 214Bi/208T1 profile (flight line ML34) from airbornegamma radiation data collected simultaneously with magnetics is also shown (bold line). Anomalously high 214Bi/208T1 ratios along crest owe their origin to late diagenetic uranium mineralization associated with petroleum microseepage (McKay and Hyden, 1 56; Donovan et al, 1975; Al-Shaieb et al, 1977). B, Oil field structure contoured on top of Hoxbar Formation (Pennsylvanian) modified from Donovan (1974), flight-line segments for magnetic profiles, and boreholes sampled for magnetite. Contour interval is 1,000 ft (~333 m). End_Page 246------------------------------ equally densely developed Davenport oil field of Lincoln County, Oklahoma, indicates that the phenomenon is not due to cultural interferences (pipelines, storage tanks, casing, etc), because no high-wave-number anomalies are seen in the Davenport data. Supporting evidence for the diagenetic formation of magnetite exists at Cement. The evenly red-colored Permian Rush Springs Sandstone is remarkably bleached at the surface, in a pattern that closely coincides with the area of oil and gas production; within this tinted area it is relatively depleted of iron. Along the crest of the surface-expressed anticline, the normally friable Rush Springs is strongly impregnated with carbonate cements (mostly calcite), whose C13/C12 isotopic ratios indicate a petroleum-derived carbon source. A thin overlying gypsum bed has been converted to calcite with similarly indicative C13/C12 ratios (Donovan, 1974). Pirson (1975) proposed that magnetoelectric effects may occur at the interface between the chemically reduced rock overlying a seeping hydrocarbon deposit and the oxidized rock surrounding the deposit, that is, an active process occurs whereby a giant fuel cell generates magnetoelectric effects. The phenomenon we describe has a different origin; namely, the diagenetic production of a ferromagnetic mineral that is a "fossil" indicator of microseepage. Search elsewhere for anomalous concentrations of diagenetically produced materials could be useful to explorationists. Fig. 2. Relations between magnetite (plotted as weight percent of total sample) and drilled depth for five boreholes at Cement oil field. Position of wells is shown in Fig. 1B. Samples were composited from drill cuttings collected ~15 m above and below plotted center point. Fig. 3. Probable range of magnetite with depth in near-surface rocks at Cement oil field along and near anticlinal crest. Only data from four boreholes along crest are plotted. End_Page 247------------------------------

Mössbauer effect and magnetic studies of secondary iron oxides in soils

Abstract: Summary Simple, rapid and non-destructive measurements of magnetic properties (magnetic susceptibility, saturation isothermal remanent magnetization and coercivity of isothermal remanence) coupled with more time consuming and sophisticated analyses such as thermomagnetic and Mossbauer effect studies, provide a basis for identifying forms of iron oxide present in the soil. At two of the sites studied, Caldy Hill, Merseyside, and Llyn Bychan, N. Wales, the secondary ferrimagnetic oxide formed in surface soil as a result of recent forest fires is shown to be non-stoichiometric magnetite approximating to the formula Fe2. 904. No evidence for the presence of maghemite was found in any of the soil samples from these sites or in soils from the Annecy region of S.E. France.

Diffuse reflectance spectra and optical properties of some iron and titanium oxides and oxyhydroxides

Abstract: The reliability and utility of diffuse reflectance spectra are briefly but critically reviewed. The results of measurements of diffuse reflectance over the wavelength range 200 < λ < 2500 nm are reported for wustite, magnetite, hematite, maghemite, ilmenite, ulvospinel, and α-FeO · OH (goethite), β-FeO 7sd OH, γ-FeO · OH (lepidocrocite), and δ-FeO · OH. The spectra have been assigned by reference to simplified molecular-orbital energy-level diagrams derived from recent SCF-Xα calculations. The specular reflectances reported in the Quantitative Data File (Henry, 1977) are related to the diffuse reflectance spectra in a rational way. Minerals that absorb strongly throughout the visible display little dispersion of specular reflectance, and their powders are dark (wustite, magnetite, ilmenite, ulvospinel); those that absorb much more strongly in the near ultraviolet than in the visible have specular reflectances that decrease monotonically from blue to red according to a simple dispersion relation derived by combining the Sellmeier dispersion and Fresnel reflexion equations; their powders are strongly coloured (hematite, maghemite, lepidocrocite, goethite) and their optical anisotropy is closely related to crystal structure.

Magnetite-sulfide-metal complexes in the Allende meteorite

Abstract: A model of liquid immiscibility is presented that seemingly accounts for the sulfide-oxide-metal complexes that are present in olivine-rich chondrules in the Allende meteorite. The four major assemblages that are identified are: (1) magnetite + Ni-Fe metal; (2) magnetite + troilite + Ni-Fe metal; (3) magnetite + troilite + pentlandite + Ni-Fe metal; and (4) troilite + or - pentlandite. Specific attention is focused on oxide-metal associations and experimental data confirm earlier suggestions that magnetite results from the oxidation of an initially high-Fe-content metal alloy. Oxidation decreases the modal abundance of the Fe metal and this is accompanied by substantial increases in Ni contents which reach a maximum of approximately 70 wt % Ni. The proposed oxidation mechanism is entirely consistent with condensation of Fe-metal + olivine (Fa5) that subsequently reequilibrated at lower temperatures. Although the sulfide constituents could also have formed by the reaction of Fe-Ni metal + gaseous H2S, sulfide immiscibility under increased conditions of partial O2 pressure is the preferred process.

A model for the deposition of Cornwall-type magnetite deposits

Abstract: Cornwall-type magnetite deposits, as exemplified by the deposit at Cornwall, Pennsylvania, are found at the contact between diabase and carbonate rocks. Contact temperatures are estimated at 600 degrees to 670 degrees C. Ore emplacement occurred after the diabase was fractured and at lower temperature ( approximately 500 degrees C). We propose a depositional model based on convecting, supercritical aqueous chloride solutions. Acquisition of FeCl 2 is accomplished at depth (hot end of the cell) as a two-stage process. First HCl is formed by conversion of muscovite to K-feldspar and then Fe-minerals such as magnetite or pyrite are dissolved to yield FeCl 2 . The FeCl 2 -rich solutions circulate to the cold end of the cell, where they come in contact with CaCO 3 . Magnetite and pyrite are precipitated and the solutions become enriched in CaCl 2 and CO 2 , some of which may descend with the return flow.Using recent data on the free energies of formation of supercritical aqueous KCl, FeCl 2 , and CaCl 2 , it has been possible to calculate fluid compositions. The main variables are T, f (sub H 2 ) , and X (sub H 2 O) (or X (sub CO 2 ) ). Making reasonable assumptions, the mFeCl 2 levels are appropriately high and there seems to be no difficulty in transporting large amounts of magnetite or pyrite by this process. Support for the model is found in the ubiquitous K-metasomatism.

Metamorphism of iron-formation; parageneses in the system Fe-Si-C-O-H

Abstract: Through the use of phase diagrams previously described for the systems C-O-H and Fe-C-O-H and the mineral assemblages reported from metamorphosed iron-formation, a series of isobaric, isothermal diagrams showing phase relations in the system Fe-Si-C-O-H has been constructed. They indicate that in the presence of CO 2 -rich fluids the magnetite + quartz field is bounded by a stability field for siderite and, in the presence of H 2 O-rich fluids, by a field for hydrous Fe silicate (grunerite, minnesotaite, or greenalite). The siderite field expands with progressively lower temperatures and under greenschist facies conditions hydrous Fe silicates are stable only in extremely H 2 O-rich fluids. The lowest oxygen fugacity under which magnetite + quartz is stable is either that of the quartz-magnetite-fayalite buffer, or, if fayalite is unstable, that of the invariant point involving hydrous Fe silicate, quartz, magnetite, and siderite. Since another invariant point containing hydrous Fe silicate, quartz, siderite, and graphite occurs at low oxygen fugacities, this indicates that graphite should play no role in the metamorphism of iron-formations which contain quartz and magnetite.By assuming that quartz and magnetite are always present, phase assemblages for the system Fe-Si-C-O-H can be projected onto an isobaric T-X (sub CO 2 ) plot which is contoured for oxygen fugacity. This shows that if fluid composition is internally controlled, the only primary mineral assemblages which will not produce the assemblage quartz-magnetite-fayalite (or quartz-magnetite-ferrohypersthene for Mg-bearing systems) at the highest grades of metamorphism are hematite-siderite + or - magnetite, where hematite exceeds siderite in abundance, and hematite-quartz + or - magnetite. It also indicates that, during prograde metamorphism, the fluid coexisting with a siderite-silicate iron-formation will be buffered to moderate or high X (sub CO 2 ) as siderite and quartz react to silicates. At higher temperatures the fluid will be enriched in H 2 O again as grunerite breaks down to fayalite (or ferrohyperthene) + quartz. Due to the buffering of the fluid composition, the assemblage grunerite-fayalite (ferrohyperthene)-quartz-magnetite may be stable over a wide range of temperatures, a fact which could explain its widespread occurrence in nature.

Dilution stable water based magnetic fluids

Abstract: A dilution stable water based magnetic fluid is provided by dispersing magnetic particles in water with the aid of a C10 -C15 aliphatic monocarboxylic acid. The magnetic particles are preferably particles of magnetite, prepared by precipitation of dissolved iron chloride salts from aqueous solution by the use of ammonium hydroxide. The preferred acids used in preparing the dispersions are dodecanoic (C12) and tridecanoic (C13).

On the Multiple Orientation Relationships Between Hematite and Magnetite

Abstract: Lattice fringe and diffraction observations of hematite/ magnetite interfaces are described: elongated laths of magnetite, with (111)M and ({\bar 1}11)M parallel to (0001)H and, less frequently, (211)M and ({\bar 2}11)M parallel to (0001)H, were found. In general, the interfaces are incoherent as shown by the presence of steps, which are probably transformation dislocations. The magnetite is invariably multiply twinned, showing steps which may be twinning dislocations. A theoretical analysis of the possible secondary and ternary orientation relationships due to multiple twinning explains one of the two further less-frequent relationships indicated by texture goniometry. Such higher-order relations are not topotaxic and their identification and interpretation is a general problem for texture goniometry.

Heterogeneous phase reactions of Martian volatiles with putative regolith minerals

Abstract: The chemical reactivity of several minerals thought to be present in Martian fines is tested with respect to gases known in the Martian atmosphere. In these experiments, liquid water is excluded from the system, environmental temperatures are maintained below 0°, and the solar illumination spectrum is stimulated in the visible and UV using a Xenon arc lamp. Reactions are detected by mass spectrometric analysis of the gas phase over solid samples. No reacions were detected for Mars nominal gas over sulfates, nitrates, chloride, nontronite clay, or magnetite. Oxidation was not observed for basaltic glass, nontronite, and magnetite. However, experiments incorporating SO2 gas - an expected product of volcanism and intrusive volatile release - gave positive results. Displacement of CO2 by SO2 occurred in all four carbonates tested. These reactions are catalyzed by irradiation with the solar simulator. A calcium nitrate hydrate released NO2 in the presence of SO2. These results have implications for cycling of atmospheric CO2, H2O, and N2 through the regolith.

Magnetite alteration in deeply weathered adamellite

Abstract: Summary Magnetite grains in deeply weathered granitic rocks in Western Australia altered to haematite pseudomorphs which usually had magnetite cores. No evidence was found for maghemite development either as a separate phase or in solid-solution with magnetite.

A new approach to the synthesis of cobalt modified iron oxide

Abstract: The new approach to the synthesis of cobalt modified iron oxide with an autoclave in a hydrogen atmospher is reported. The acicular particles of magnetite used for this technique are the following three kinds: (1) The magnetite made by usual preparation method. (2) The magnetite made from goethite whose surface was treated with SiO 2 . (3) The magnetite obtained by annealing of the magnetite in (2). The product has a black acicular crystalline structure. The analysis of magnetic measurements proved that the surface localization of cobalt compounds is very important in order to improve the magnetic stability.

Microstructures in titanomagnetites as guides to cooling rates of a Swedish intrusion

Abstract: Iron-titanium oxides from Taberg in Sweden exhibit under the electron microscope several microstructural features not visible when studied by optical microscopy, including homogeneous nucleation of magnetite ss from ulvospinel ss , and the spinodal decomposition of magnetites ss . Such microstructures have promise as indicators of cooling rates.

Petrology of the San Leone magnetite skarn deposit (S.W. Sardinia)

Abstract: The San Leone magnetite skarn deposit consists of lens-shaped bodies interlayered in a Silurian quartzo-pelitic formation metamorphosed by Hercynian granodiorite. Iron oxide deposits of this type can result from iron being introduced into a limestone by metasomatism in the vicinity of an intrusive. On the other hand, iron-rich sedimentary carbonates can be transformed into oxides in the vicinity of an intrusive. San Leone appears to be a deposit where both phenomena have operated. A two-stage model is proposed to explain the evolution of the deposit.1. The lens-shaped deposits consisted initially of iron-rich carbonate sediments with quartz-pelitic and/or pyroclastic intercalations. Diffusion across layering, taking place at the time of metamorphism, at about 550 degrees C and low pressure led to the formation of monomineralic (mainly hedenbergite) or oligomineralic layers. This required local mobility of silicon and iron. The state of the iron oxidation appears to have remained unchanged during this metamorphic stage. Resulting minerals include hedenbergite, andradite, grandite, epidote, vesuvianite, calcite, and magnetite.2. The hydrothermal stage, taking place at about 450 degrees C and also at low pressure, consisted of two kinds of phenomena: alteration of hedenbergite and other metamorphic silicates into quartz, magnetite, ferrotremolite, ilvaite, babingtonite, fluorite, calcite, and scheelite and replacement of the remaining calcite by magnetite. Iron was introduced into the lenses by infiltration, presumably along bedding planes. The silicates show an unusually high iron content, especially the amphibole which appears to represent the purest ferrotremolite yet reported from nature.

Temperature Dependence of Magneto-Electric Effect of Magnetite (Fe 3 O 4 ) at about 10 K

Abstract: Magneto-electric effect was measured in a temperature range between 4.2 and 30 K on a plate of Fe 3 O 4 cut parallel to (110) referred to the cubic axes. The voltage as high as several tens of volts was observed to be induced across a 1.45 mm thick specimen when the magnetization is rotated in the (110) plane at 4.2 K. The effect changes drastically as the temperature is increased towards 10 K, at which a specific heat anomaly had been reported.

Kinetics of the Displacement Reaction between Fe and CU2O at Temperatures between 800 and 1050 C.

Abstract: The products of the displacement reaction between solid iron and Cu2O in the temperature range of 800 to 1050°C are solid copper, wustite and magnetite. The product oxide in contact with the iron phase is in the form of rods embedded in a matrix of copper. A thin layer of copper separates the oxide rods from the Cu2O phase. The rates of growth of the product phases are controlled by solid-state counterdiffusion of oxygen and iron through the product phases, except for the initial stage of the reaction. A previously developed kinetic model for the diffusion-controlled growth of the product phases is employed to calculate the rates of growth of each of the product phases. Theoretical values of growth rates are in excellent agreement with experimental values for temperatures above 950°C. Below 950°C, the mechanism for the growth of the magnetite phase changes, which yields marked differences between the experimental and theoretical rates of growth. The mechanism for the displacement reaction below 950°C has not been determined.

Natural convective mass transfer rates between solid magnetite and molten mattes

Abstract: A twin balance apparatus capable of continuously weighing both the solid and liquid phase was constructed to measure the magnetite-matte reaction. The rate of magnetite consumption during the reaction with iron sulfide increased from 2.5 to 10.8 g Fe3O4/min with increasing temperature from 1473 to 1623 K. In the magnetite-(Fe-S-O) melt reactions, the dissolution rate decreased from 5.4 to 1.5 g Fe3O4/min with increasing oxygen content in the bulk liquid phase. The rate approached zero when the bulk phase contained approximately 32 at. pct O which was close to the magnetite saturation limit. The rate of reaction between magnetite and liquids of the FeS-Cu2S binary system decreased from 5.4 to 0.2 g Fe3O4/min with increasing Cu2S content from 0 to 60 mol pct Cu2S. Examination of the reaction rate data of the Fe-S-O system in conjunction with a magnetite pellet profile study and oxygen analyses in the matte showed that the dissolution mechanism was one of diffusion enhanced by natural convection. The results of reacting magnetite with FeS-Cu2S melts suggested that the same mechanism operated. The industrial significance of this investigation is discussed briefly in relation, to the problem of the presence of solid magnetite in copper smelting processes.