Showing papers on "Magnetite published in 1991"
TL;DR: In this paper, a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200°C on the basis of the modified increment method.
323 citations
TL;DR: Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2-8 μm and precision in the range of 1 sigma as discussed by the authors.
Abstract: Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 μm and precision in the range of 1‰ (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of δ18O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe3O4 in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with δ18O=8.9±1‰ SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9‰ depleted in δ18O. These low δ18O values increase in smooth gradients across the outer 10 μm of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 μm diameter magnetites which average 1.2‰ lower in δ18O than coarse magnetites due to low δ18O rims. Conventional analysis of coexisting calcite yields °18O=18.19, suggesting that bulk Δ18O (Cc-Mt)=9.3‰ and yielding an apparent equilibration “temperature” of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H2O) indicates that diffusion is faster in magnetite and modelling of the low δ18O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H2O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low δ18O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.
107 citations
Journal Article•
90 citations
83 citations
TL;DR: The Northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite as mentioned in this paper, and the magnetic fabric of this sedimentary rock, as defined by the anisotropy of magnetic susceptibility, is controlled by bedding compaction but is inverse (i.e., the maximum susceptibility axes are normal to the bedding plane).
Abstract: The Northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite. The magnetic fabric of this sedimentary rock, as defined by the anisotropy of magnetic susceptibility, is controlled by bedding compaction but is inverse (i.e., the maximum susceptibility axes are normal to the bedding plane). This inverse fabric is attributed to the dominating presence of siderite. The rocks were incrementally heated to 600°C, and the magnetic fabric was measured at room temperature and at near liquid nitrogen temperature after each heating step; cooling the samples in liquid nitrogen enhances the paramagnetic contribution to the magnetic fabric. After heating the samples above 250°C, the room temperature magnetic fabric became normal with the minimum susceptibility axes perpendicular to the bedding plane. The low-temperature magnetic fabric remains inverse until the samples are heated to about 500°C. After heating to higher temperatures a normal fabric was observed at both room temperature and low temperature. Rock magnetic evidence shows that magnetite is created during the heating. The start of mineralogical changes in the rock was detected by electron spin resonance spectroscopy, powder X ray diffraction, and acquisition of isothermal remanence. Berthierine starts to break down at approximately 250°C and is totally oxidized by 500°C. The creation of magnetite in the heating experiments corresponds with this mineralogical change. The change in the room temperature magnetic fabric is associated with the decomposition of berthierine, part of which alters to magnetite. The new magnetite phase has a normal magnetic fabric which overprints the inverse magnetic fabric of siderite.
80 citations
TL;DR: In this article, a significant oxygen isotope fractionation is obtained between hematite and magnetite, which is attributed to the quantitative redox of iron in the experiments, that can in turn explain the nearly identical isotope composition of oxygen in coexisting hematites and magnetites pairs from the banded iron-formation in the Iron Quadrangle, Brazil.
Abstract: Thermodynamic oxygen isotope factors for hematite and magnetite are calculated using the modified increment method and incorporating the theoretical and experimental reduced partition function ratios determined for quartz and water in the temperature range 0 to 1200 o C. The obtained oxygen isotope fractionation factors between hematite, magnetite and water are expressed as 10 3 ln α Fe2O3-H2O =2.69×10 6 /T 2 -12.82×10 3 /T+3.78, 10 3 ln α Fe3 O 4- H 2 O=3.02×10 6 /T 2 -12.00×10 3 /T+3.31, and those between quartz and hematite or magnetite are expressed as 10 3 ln α Si O 2-FeO3 =1.55×10 6 /T 2 +9.05×10 3 /T-4.82, 10 3 ln α SiO2-Fe3O4 =1.22×10 6 /T 2 +8.22×10 3 /T-4.35. The present results are well in agreement with existing theoretical, experimental and empirical calibrations involving magnetite and hematite. A significant oxygen isotope fractionation is obtained between hematite and magnetite, whith hematite being depleted in 18 O relative to magnetite. This matches with the observation from Precambrian iron-formation in the Hamersley Range, Australia. The rule of effect of oxidation state on the oxygen isotope fractionation seems not to work for the two iron oxides. Previous experimental calibration of magnetite-water system by reduction of hematite to magnetite is concordant with the present result for hematite-water system. This is attributed to the quantitative redox of iron in the experiments, that can in turn explain the nearly identical isotope composition of oxygen in coexisting hematite and magnetite pairs from the banded iron-formation in the Iron Quadrangle, Brazil
64 citations
TL;DR: In this article, the authors investigated the crystallization process of an Fe2O3 40, CaO⋅SiO2 60wt% glass and revealed principles for obtaining ferrimgnetic and bioactive glass-ceramics useful as thermoseeds for hyperthermia of cancer.
Abstract: The crystallization process of an Fe2O3 40, CaO⋅SiO2 60wt% glass was investigated as well as magnetic properties of the crystallized products, in order to reveal principles for obtaining ferrimgnetic and bioactive glass-ceramics useful as thermoseeds for hyperthermia of cancer. When the glass was heat-treated up to temperatures 700° to 950°C, magnetite particles were homogeneously precipitated in a CaO⋅SiO2-based glassy and/or crystalline matrix. The diameter and content of the magnetite increased from 6 to 30nm and 10 to 36wt%, respectively, with increasing temperature of the heat treatment from 700° to 950°C. Above 1000°C, a part of the magnetite was converted into hematite (α-Fe2O3). The glass showed maxima in saturation magnetization and coercive force, 32emu/g and 500 Oe, respectively, when heat-treated up to 950°C. The variation of the magnetization with heat treatment temperature could be well quantitatively interpreted in terms of the content of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. The coercive forces of the magnetite-containing glass-ceramics were much higher than those of the magnetite powders of the corresponding particle sizes which were prepared from aqueous solutions.
60 citations
TL;DR: In this paper, chemical and microbiological processes involved in generating anomalous magnetization related to hydrocarbon accumulations, including hydrocarbon seepage environments, are discussed, and the formation and destruction of magnetic mineral assemblages can be predicted.
Abstract: (Aero‐)magnetic anomalies have been reported from several commercial hydrocarbon accumulations. However, the processes responsible for such anomalies are relatively poorly understood. This paper conceptually discusses chemical and microbiological processes involved in generating anomalous magnetization related to hydrocarbon accumulations, including hydrocarbon seepage environments. Based on thermodynamic criteria and microbiologic activity, the formation and destruction of magnetic mineral assemblages can be predicted. Under the influence of hydrocarbons, magnetite and pyrrhotite are the most important magnetic minerals formed, and the most abundant magnetic mineral destroyed is hematite. Hence, the invasion of hydrocarbons may result in “positive,” “absent,” or “negative” magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion, depending upon the amounts of authigenic magnetite and pyrrhotite formed relative to the amounts of hematite destroyed. Magnetism may be generated al...
51 citations
TL;DR: In this article, it was shown that magnetite predominates as the oxidation end product at the characteristic point B, where R = 0.5, at all temperatures; this confirms the proposed mechanism of formation of magnetite from ferrous hydroxide.
46 citations
TL;DR: In this article, a TEM study reveals that the spinel consists of aggregates of closely spaced spherules of 10-30 nm in diameter, which is in complete agreement with rock magnetic determinations.
Abstract: Between 20 and 50 per cent of the total iron in low-Ca fly ashes resides in low-substitution magnetite spinel with Curie temperatures in between 565 and 590°C. A TEM study reveals that the spinel consists of aggregates of closely spaced spherules of 10 – 30 nm in diameter, which is in complete agreement with rock magnetic determinations. The low-field magnetic susceptibility correlates extremely well with total iron and fairly well with specific heavy metals. Measurement of the magnetic susceptibility may serve as a rapid tool for the evaluation of the speciation of these heavy metals in fly ash.
44 citations
TL;DR: The Onondaga Formation along a Rochester-Syracuse (New York) transect has a marked eastward increasing trend in K/Al ratio, which correlates with an increase in authigenic magnetite content and degree of diagenetic illitization as discussed by the authors.
Abstract: Whole-rock chemical analyses indicate that the Onondaga Formation along a Rochester-Syracuse (New York) transect has a marked eastward increasing trend in K/Al ratio, which correlates with an increase in authigenic magnetite content and degree of diagenetic illitization. An eastward increase in the content of some authigenic minerals including K-feldspar, quartz, and Fe-rich chlorite in the rocks is indicated by petrographic and X-ray diffraction studies. These observations suggest a link between remagnetization and potassic alteration of Fe-bearing silicates in the Onondaga. Two hypothetical reactions are proposed in which magnetite authigenesis is related to the breakdown of Fe-bearing smectite in the presence of potassium. Products of these reactions include authigenic K-feldspar, illite, quartz and chlorite, as well as magnetite. Our results suggest that remagnetization was triggered by a basin-wide migration of K-rich fluids during the Alleghenian Orogeny.
TL;DR: In this article, the mechanism of magnetite formation from an aqueous alkaline slurry of Fe(OH) 2 was shown to involve the sequence dissolution-oxidation-nucleation on Fe 3 O 4.
TL;DR: In this paper, a process to remove iron from bauxite ore is described in which the ore is first finely crushed, calcined, and contacted with a reducing gas to convert iron present as very weakly magnetic goethite or hematite to magnetite or elemental iron.
TL;DR: In this article, the reduction/oxidation of a high loading iron catalyst supported on γ-Al203 has been studied using dynamic X-ray diffraction (DXRD) in situ determination of changes in the lattice parameters as magnetite was reduced to iron indicated that aluminum was unevenly incorporated into the matrix of the supported iron oxide particles, leading to a form of metal-support interaction and manifested by reduction rates which are much slower than for unsupported magnetite.
TL;DR: The syenitic Murdock Creek intrusion, of late Archean age, lies within one of the major lode gold camps of the world, at Kirkland Lake, Ontario as mentioned in this paper.
Abstract: The syenitic Murdock Creek intrusion, of late Archean age, lies within one of the major lode gold camps of the world, at Kirkland Lake, Ontario There is a strong spatial association between syenitic intrusions, gold mineralization, and carbonatization Hydrothermal fluids that carried the gold had an unusually high oxygen fugacity ($$f_{o_{2}}$$) Notwithstanding a wide range in rock compositions within the intrusion (from clino-pyroxenite to alkali-feldspar syenite), clinopyroxene and biotite have consistently low Fe/(Fe + Mg) and high $$Fe^{3+}/(Fe^{2+} + Fe^{3+})$$ ratios These compositional features indicate a deficiency of $$Fe^{2+}$$ in the magma during crystallization Together with the presence of abundant, early-formed magnetite and titanite, this is interpreted to result from an intrinsically high magmatic $$f_{o_{2}}$$ that remained constant during pluton evolution ($$f_{o_{2}} \approx 10^{-12} bars$$) Mineral compositions, proportions of crystallizing mineral phases, and textural features i
TL;DR: A regional aeromagnetic anomaly centred around Roan, Northern Vestranden, Western Gneiss Region, Central Norway, is caused by the presence of high-grade orthogneisses and metabasites with 1.5-1 mm amoeboid grains surrounded by plagioclase and K-feldspar as discussed by the authors.
01 Jan 1991
TL;DR: In this article, the pore size distribution of the KM 1 catalyst varies with reduction temperature, and the surface area of the catalyst reduced at higher temperature is smaller than of that reduced at a lower temperature.
Abstract: This chapter may be opened by two statements from the scientific literature
“The active catalyst for the ammonia synthesis is alpha iron with small amounts of oxidic additives The quality of the final catalyst is crucially influenced by the activation process which is the reduction of magnetite to metallic iron It is important to minimise the concentration of the reaction product water which is a catalyst poison”(1)
“ shows how the pore size distribution of the KM 1 catalyst varies with reduction temperature The peak of small pores represents a pore system formed by the reduction of magnetite, while the system of larger pores is formed by the reduction of wustite the surface area of the catalyst reduced at higher temperature is smaller than of that reduced at a lower temperature”(2)
TL;DR: In this article, the corrosion behavior of carbon steel in high-temperature water, and the structure and composition of the oxide film were examined as functions of dissolved oxygen concentration (DO), temperature (T), and corrosion time (t).
Abstract: The corrosion behavior of carbon steel in high-temperature water, and the structure and composition of the oxide film were examined as functions of dissolved oxygen concentration (DO), temperature (T), and corrosion time (t). The total amount of iron corroded (WT) was differentiated into the amounts of iron ions in the oxide (WF) and dissolved into the water (WD). The total rate of corrosion (rT), the rate of iron dissolution (rD), and the rate of accumulation of iron in the oxide (rF) were obtained by differentiating the time variations in WT, WF, and WD. The structure and composition of the oxide film were examined by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission and scanning electron microscopy. In general, rT increases with increasing DO and T, rD shows T- and DO-dependent minimum, and there is serious localized corrosion at high DO above 500 ppb. Oxide films consist of magnetite except at T=60°C, DO=50 to 200 ppb where a thin layer of hydrous ferric oxide ...
TL;DR: The structure of finely dispersed iron powders obtained by electrodeposition with oleic acid as a stabilizer and isophytol as an adsorptive modifier has been studied by X-ray photoelectron, gamma resonance and infrared spectroscopies as mentioned in this paper.
TL;DR: In this article, the light absorption spectra of magnetite fine particles of about 7 nm in mean diameter dispersed in magnetic fluid were measured as functions of photon energy, E, from 0.5 eV through 1.8 eV and of particles' temperature from 294 K up to 383 K.
Abstract: Light absorption spectra of magnetite fine particles of about 7 nm in mean diameter dispersed in magnetic fluid were measured as functions of photon energy, E , from 0.5 eV through 1.8 eV and of particles' temperature from 294 K up to 383 K. The light absorption coefficient α of the particles is smaller than that of the bulk magnetite. With increasing temperature, α decreases for E lower than 1.378 eV, but it increases for E higher than 1.378 eV. In addition, the spectra are shifted towards higher energy region by 0.136 eV compared with that of the bulk magnetite. If concentration of oscillator strengths on the bottom of 4s band of iron ion is assumed, the energy shift can be interpreted in terms of the quantum size effect of the fine particles, and the effective mass of the 4s electron is evaluated to be 0.37 times as much as that of a bare electron.
TL;DR: In this article, a limestone with pseudo-single-domain (PSD) pure magnetite is given an isothermal remanent magnetization (IRM), and samples are deformed triaxially at 200 MPa (2 kbar) confining pressure at room temperature and at a strain-rate of 10 −5 s −1.
TL;DR: The bulk interaction between iron(III) oxide and titanium(IV) oxide occurs at temperatures in excess of 773 K and the phase transition from anatase phase to rutile phase is observed to occur approximately 200 K lower in the presence of iron than when it is absent.
TL;DR: In this article, the authors examined four deep-sea sediments by TEM and found biogenic magnetites for the first time from the Japan Sea and the western Pacific Ocean, and the shapes, sizes, and chemical and crystallographic characteristics of the magnetite grains were described.
Abstract: Examining four deep-sea sediments by TEM, biogenic magnetites were found, for the first time from the Japan Sea and the western Pacific Ocean. Octahedral type of biogenic magnetite was dominant in the specimen from the Pacific Ocean and teardrop-shaped magnetite was predominant in the specimen from the Japan Sea. Shapes, sizes, and chemical and crystallographic characteristics of the magnetite grains were described. In an HREM image of the specimen from the Japan Sea it was clarified that a teardrop-shaped magnetite grain is straightly elongated along the a-direction, or a little curved along the longitudinal direction
Patent•
17 Jun 1991
TL;DR: In this paper, a process for improving the concentration of nonmagnetic heavy minerals using a gravity-magnetic type separator, by the addition of a magnetic mineral or phase, such as magnetite, ilmenite, or iron filings, to the feed slurry is described.
Abstract: A process for improving the concentration of non-magnetic heavy minerals using a gravity-magnetic type separator, by the addition of a magnetic mineral or phase, such as magnetite, ilmenite, or iron filings, to the feed slurry. The addition of the magnetic material prior to feeding the gravity-magnetic separator results in an increase in the recovery of the non-magnetic heavy minerals in the feed, such as rutile, zircon, gold, tin, tungsten, etc.
TL;DR: A mineralogic, chemical, and rock-magnetic investigation was conducted in Lake Greifen, a moderate-sulfate containing eutrophic lake, in order to assess iron mineral diagenesis in a sedimentary sequence deposited during different tropic states and varying redox conditions as mentioned in this paper.
Abstract: A mineralogic, chemical, and rock-magnetic investigation was conducted in Lake Greifen, a moderate-sulfate containing eutrophic lake, in order to assess iron mineral diagenesis in a sedimentary sequence deposited during different tropic states and varying redox conditions. The sedimentary sequence is composed of 0.25 m of eutrophic anoxic varved sediments underlain by transitional laminated marls, deposited as the lake changed from oxic to dysaerobic conditions, and then a basal unit of pre-eutrophic bioturbated marls. Grain size, morphology, and composition of magnetite observed by TEM suggest that single-domain biogenic magnetite is currently produced within the upper anoxic varved sediments. Similarly characterized fine-grained magnetites observed in the pre-eutrophic sediments indicate magnetite authigenesis prior to eutrophication. Low values of NRM, susceptibility, SIRM, and ARM in the anoxic varved sediments below the zone of magnetite production indicate the rapid destruction of magnetite. Iron speciation analyses, interstitial water results, and the ARM:SIRM ratio indicate that reductive dissolution is progressive. These results suggest that (1) rapid destruction of fine-grained magnetite occurs in high productivity, low-sulfate containing anoxic sediments following dissolution of ferric phases; and (2) magnetite preservation reflects redox conditions that, in Lake Greifen, are determined by productivity. These findings differ from marine reductive diagenetic models inmore » which magnetite dissolution is considered insignificant, or slow, in the absence of high sulfide concentrations.« less
TL;DR: The chromium content of this magnetite could be at least that of the stainless steel substrate (x≥0·5), and this substituted magnetite coexists with a partially oxidised magnetite as mentioned in this paper.
Abstract: The iron oxide which predominates inferritic stainless steel conversion coatings for photothermal conversion is a chromium substituted magnetite (Fe2+Fe3+2−x Cr3+ x )O2−4: the chromium content of this magnetite could be at least that of the stainless steel substrate (x≥0·5). This substituted magnetite coexists with a partially oxidised magnetite. The thermal treatment of these coatings induces theformation of the corundum type phase α Fe2O3 by oxidation of the substituted magnetitefrom 175°C. At higher temperatures, FeCr2O4 and α Fe6−yCryO9 appear.MST/1216
TL;DR: In this article, the reduction of haematite to magnetite was completed even at 150 °C in concentrated CsOH solution (1.0 mol l−1).
Abstract: Haematite (α-Fe2O3) was reduced to magnetite (Fe3O4) under hydrothermal conditions, by the use of an autoclave into which hydrogen gas was able to be admitted. The stable reduced conditions for magnetite precipitation were produced even at 100 °C. In acid and neutral mineralizers, the rate of haematite reduction was small below 300 °C and increased with an increase in reaction temperature above 300 °C. Magnetite crystals obtained in these solutions were large and faceted octahedrons in shape. Alkaline mineralizers except NaOH solution accelerated the reduction at low temperatures below 300 °C and gave small magnetite crystals. The reduction of haematite to magnetite was completed even at 150 °C in concentrated CsOH solution (1.0 mol l−1).
TL;DR: In this article, the thermal stability of ferritic stainless steel conversion coatings for selective solar absorbers was studied in air and under low oxygen pressure (partial vacuum 2 × 10 -2 Pa).