Showing papers on "Hematite published in 1992"
TL;DR: In this paper, the authors investigated the role of hard limestone on the pedoclimate of terra rossa and found that the processes of rubification and vermiculitization could have taken place at the same time.
Abstract: Terra rossa samples were taken from the B horizons of soil profiles and from cracks within limestone in Italy. The average annual temperature (AAT) of the sites ranged from 8.4 to 20.3°C and the average annual precipitation (AAP) from 511 to 3113 mm, with either a 5–6 month water deficit or a large water surplus. Goethite and hematite were identified in all the samples. Under a moist (> 1700 mm AAP) and cool (13°C AAT) climate, a xeric, hematitic pedoenvironment was preserved by the well-litified carbonate rock. Hematite occurred in trace amounts, even with an AAT of 8.4°C and an AAP of 3300 mm, confirming the specific role of the hard limestone on the pedoclimate of terra rossa. The lowest mean crystallite dimension of goethite and hematite was found in the samples from the wettest sites, and in these samples hematite was nearly free of Al substitution. Rubification in terra rossa appeared to be due to the specific pedoenvironment. The hematite cannot be considered a relict phase formed under another climate. Illite and kaolinite were the main clay minerals in samples from xeric sites whereas more weathered clays, such as Al-interlayered vermiculite, occurred in cool, moist sites. We postulate that the processes of rubification and vermiculitization could have taken place at the same time. The effects of layer charge magnitude and location on expansion were represented by an energy change (expansion energy: ΔEr) during the hydration and solvation processes. Plots of basal spacings versus ΔEr show a reasonable relationship; the spacings generally decrease stepwise as the value of ΔEr increases. The basal spacings of K-samples with glycerol solvation, Na-saturated and K-saturated samples at 100% RH are apt to contract stepwise with increasing value of ΔEr. For these samples, the figures showing the relationship between each expanded phase and the charge characteristics are obtained from the isoquants of ΔEr, given the boundary of the expanded phases. A behavior test using these figures may be combined with the Greene-Kelly test to estimate the amount and the location of the layer charge of common smectites.
313 citations
01 May 1992
TL;DR: In this article, spindle-type hematite particles of narrow size distribution were coated with uniform layers of silica by hydrolysis of tetraethylorthosilicate (TEOS) in 2-propanol.
Abstract: Spindle-type hematite particles of narrow size distribution were coated with uniform layers of silica by hydrolysis of tetraethylorthosilicate (TEOS) in 2-propanol. The effects of the reaction time and initial concentrations of ammonia and water on the thickness of the silica shell were investigated and the rate of the coating process, in terms of the TEOS concentration, was evaluated.
270 citations
TL;DR: In this article, the dissolution of 39 soils derived from various parent materials in south-western Australia have been studied using a variety of techniques using a Cube Root Law and by Kabai's equation.
Abstract: SUMMARY
Iron oxides from 39 soils derived from various parent materials in south-western Australia have been studied using a variety of techniques. Goethite and hematite were the only two Fe oxides present. The goethite/(goethite+hematite) ratio ranged from 0.18 to 1.0, and was highest in soils on acidic igneous rocks, decreasing for soils on alluvial and mafic parent materials. In a few soils derived from acid rocks only goethite was present. The redness rating of soils increased linearly with increasing amount of hematite. Al substitution in goethite ranged from 13 to 35 mol%, with higher values for soils on acid igneous rocks (median value = 26 mol%) than for soils on mafic (19 mol%) and alluvial (17 mol%) parent materials. Substitution of A1 in hematite ranged from 4 to 23 mol%, and was greatest in soils on mafic parent materials (median value = 12 mol% A1). A1 substitution in hematite was about half of that in associated goethite. The dehydroxylation temperature for goethite increased linearly with increasing A1 substitution. Goethite and hematite had similar crystal sizes (c. 20 nm), and both were in the form of irregular plates.
Dissolution with 1 M HCl of iron oxides concentrated from the soils by 5 M NaOH digestion could be described both by the Cube Root Law and by Kabai's equation. Only one straight line was obtained for the dissolution data using Kabai's equation for samples containing both goethite and hematite, in contrast to the results of other workers. Major proportions of the Co, Cr, Cu, Mn, Ni and Zn in the soils were concentrated with the iron oxides, and the dissolution kinetics of these elements indicate that some may be present in the structure of the iron oxides.
209 citations
TL;DR: In this article, pseudocubic particles of hematite (mean edge length 1.65 μm) with an exceedingly narrow size distribution (coefficient of variation 6.0%) have been prepared from highly condensed ferric hydroxide gel of approximately 1 mol dm−3 with a nominal excess concentration of ferric ions of 0.10 mol −3 through aging at 100°C for 8 days.
177 citations
TL;DR: In this paper, the effects of adsorbed fulvic acid, a naturally occurring organic acid, on the kinetics of hematite aggregation and on the resulting structure were investigated.
107 citations
Journal Article•
97 citations
TL;DR: In this paper, Li et al. measured the thermal conductivities of pure hematite, magnetite, and wustite using the laser flash method as reference value and summarized them in the form of an empirical equation k = 1/(AT+B).
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.
94 citations
TL;DR: The erect is explained to result from electronic quadrupole transitions in the anomalous scattering from iron and demonstrates that diffraction can be a sensitive probe of the electronic environment at specific atomic sites in the crystal.
Abstract: An unexpected resonant enhancement in x-ray diffraction occurs at a forbidden reflection in the antiferromagnet hematite. This fifty fold increase in the diffracted signal, 10 eV below the iron K edge, exhibits a rotation in the x-ray polarization and a 60 o azimuthal symmetry as the crystal is rotated about the reflection vector. The erect is explained to result from electronic quadrupole transitions in the anomalous scattering from iron and demonstrates that diffraction can be a sensitive probe of the electronic environment at specific atomic sites in the crystal
91 citations
TL;DR: In this article, the presence of a small number of OH-groups in the products of the decomposition up to 900-1000°C was supposed to hinder the formation of perfect hematite structure.
Abstract: Goethite, hematite and intermediate products of goetite thermal decomposition were studied by IR and Raman spectroscopy to identify these products used as catalysts of some chemical reactions. The presence of a small number of OH-groups in the products of the decomposition up to 900–1000° C was supposed to hinder the formation of perfect hematite structure. The hypothesis concerning C3v6space group of protohematite indistinguishable from D3d6space group of hematite by X-Ray diffraction was suggested. This hypothesis explains both the additional lines in IR and Raman spectra compared to hematite spectra and the same position of peaks in X-Ray diffraction picture.
79 citations
TL;DR: In this paper, the thermal and 254-nm photochemical dissolution reactions of magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (α-Fe 2O3) suspended in EDTA aqueous solutions were compared.
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...
57 citations
TL;DR: A combination of methods, including separation, crystallo-optical techniques, SEM, TEM, X-ray, etc, were used to characterize the phase mineralogy, chemical composition, microstructure and some genetic phase pecularities in solid waste products from coal burning fly ashes, bottom ashes and lagooned ashes from the burning of Bobov Dol and East Maritza coal at thermoelectric power plants, respectively as discussed by the authors.
TL;DR: In this article, the authors studied terra rossa samples from the B horizons of soil profiles and from cracks within limestone in Italy, and found that a xeric, hematitic pedoenvironment was preserved by the well-litified carbonate rock.
Abstract: Terra rossa samples were taken from the B horizons of soil profiles and from cracks within limestone in Italy. The average annual temperature (AAT) of the sites ranged from 8.4 to 20.3°C and the average annual precipitation (AAP) from 511 to 3113 mm, with either a 5–6 month water deficit or a large water surplus. Goethite and hematite were identified in all the samples. Under a moist (> 1700 mm AAP) and cool (13°C AAT) climate, a xeric, hematitic pedoenvironment was preserved by the well-litified carbonate rock. Hematite occurred in trace amounts, even with an AAT of 8.4°C and an AAP of 3300 mm, confirming the specific role of the hard limestone on the pedoclimate of terra rossa. The lowest mean crystallite dimension of goethite and hematite was found in the samples from the wettest sites, and in these samples hematite was nearly free of Al substitution. Rubification in terra rossa appeared to be due to the specific pedoenvironment. The hematite cannot be considered a relict phase formed under another climate. Mite and kaolinite were the main clay minerals in samples from xeric sites whereas more weathered clays, such as Al-interlayered vermiculite, occurred in cool, moist sites. We postulate that the processes of rubification and vermiculitization could have taken place at the same time.
TL;DR: In this paper, the difference in the sorption capabilities of magnetite and hematite is discussed in terms of crystal structures of these oxides, and the kinetics of adsorption, up to the first 60 to 90 min, followed by a first-order equation.
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.
TL;DR: In this paper, the efficiency of reduction-roasting/magnetic separation of hematite, goethite and any extremely fine-grained iron ore, which is nonresponsive to conventional processing techniques, is reviewed.
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.
TL;DR: In this paper, spectral shape and absorption band positions were analyzed for the detection of small amounts of crystalline hematite (αFe2O3) on Mars, and it was shown that the 2.6- to 0.7-μm ferric absorption band varies across the surface at the 1-2% level, with bright regions typically having a deeper band; and many dark regions and a few isolated bright regions are perhaps more spectrally heterogeneous than once thought.
TL;DR: In this article, the effects of the key variables on the selective flotation of Mt. Weld phosphate ore were studied with the aim of optimizing the flotation process to yield a concentrate of the following specifications: >36% P2O5, P 2 O 5 (Fe 2 O 3 + Al 2 O3 ) >10 at a P 2O5 recovery of >70%.
01 Nov 1992
TL;DR: In this article, 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 and 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.
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.
TL;DR: Sulfate ion adsorption and desorption experiments carried out on synthetic goethite and hematite and natural hematitic oxides show that sulfate ion adaption is a highly irreversible reaction as discussed by the authors.
Abstract: Sulfate ion adsorption and desorption experiments carried out on synthetic goethite and hematite and natural hematite show adsorption to be a highly irreversible reaction. All oxides showed an increase in sulfate ion adsorption with decrease in pH. Only a small fraction of sorbed sulfate was desorbable after 48 hr, and only at a pH of 3. Extreme irreversibility of sulfate sorption on these common soil minerals suggests that adsorbed sulfate is more immobile in watersheds than previously considered and that recovery models which inherently assume reversibility may need to be modified.
01 Aug 1992
TL;DR: In this article, the effects of citrate ions on the formation of hematite particles on a forced hydrolysis reaction of ferric chloride solution during a long period of aging were examined.
Abstract: Effects of citrate ions on the formation of hematite particles on a forced hydrolysis reaction of ferric chloride solution during a long period of aging were examined. The cubic and rhomboidal hematite particles with narrow size distribution were produced through the phase transformation from β-FeOOH to hematite in the presence of various amounts of citrate ions.
TL;DR: In this paper, six kinds of hematite and two kinds of magnetite were examined as solid sorbents of H 2 S for high-temperature desulfurization.
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
TL;DR: In this paper, the authors analyzed 9 red and brown closely associated soils developed from highly calcareous materials in central Tuscany, Italy, to determine: the mineralogical properties of the iron oxides, the relationship between redness and hematite content, and the relation between parent material and iron oxide mineralogy.
TL;DR: In this paper, selected Brazilian Oxisols were sampled and submitted to high-gradient magnetic separation (HGMS) to study the iron oxide-clay mineral associations, which indicated a strong interaction between kaolinite and iron oxides, which would indicate a low estimation of their surface areas obtained by the difference method using BET-N2 data.
Abstract: Selected Brazilian Oxisols were sampled and submitted to high-gradient magnetic separation (HGMS) to study the iron oxide-clay mineral associations. The soils, derived from four different parent materials, have mineralogy dominated mostly by hematite, goethite, and kaolinite. Gibbsite appears in most soil samples. The high-gradient magnetic separation showed good separation for some soils, as indicated by color differentiation and iron oxide segregation between magnetic and nonmagnetic fractions. Soils that showed a somewhat low surface area for the iron oxides associated with high phosphate adsorption were poorly separated by HGMS. This suggests a strong interaction between kaolinite and iron oxides, which would indicate a low estimation of their surface areas obtained by the difference method using BET-N2 data. A relative concentration of anatase and rutile in the magnetic portion of some of the samples was attributed to the presence of Fe, either as coatings on the crystals or within the structure of these two minerals.
TL;DR: In this paper, the thermal decomposition of natural pyrite (cubic, FeS2) has been investigated using X-ray diffraction and 57Fe Mossbauer spectroscopy.
Abstract: Thermal decomposition of natural pyrite (cubic, FeS2) has been investigated using X-ray diffraction and57Fe Mossbauer spectroscopy. X-ray diffraction analysis of pyrite ore from different sources showed the presence of associated minerals, such as quartz, szomolnokite, stilbite or stellerite, micas and hematite. Hematite, maghemite and pyrrhotite were detected as thermal decomposition products of natural pyrite. The phase composition of the thermal decomposition products depends on the temperature, time of heating and starting size of pyrite crystals. Hematite is the end product of the thermal decomposition of natural pyrite.
TL;DR: The firing color of clays seems to be influenced by clay micro-structure as discussed by the authors, and the clays appear to have a dark brown color on firing when exposed to Na, whereas grains of parallel structure remain yellow.
TL;DR: In this paper, 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.
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.
TL;DR: In this paper, the authors used microprobe analyses of coexisting titanomagnetite-ilmenite phases to identify the origin of the heavy opaque minerals of Cox's Bazar beach sands.
TL;DR: In this article, the shape sensitivity of a dispersed hematite Mossbauer spectrum to the way of particle packing is detected experimentally, and it is shown that reversible particle transition from superparamagnetic to superferromagnetic state due to correlation bounds arising between magnetization vectors of neighboring particles.
TL;DR: In this paper, the adsorption of ovalbumin, γ-globulin, and lysozyme on uniform spherical hematite and chromium hydroxide particles in aqueous media has been studied as a function of the pH at a constant ionic strength.
Abstract: The adsorption of ovalbumin, γ-globulin, and lysozyme on uniform spherical hematite and chromium hydroxide particles in aqueous media has been studied as a function of the pH at a constant ionic strength. The uptake of ovalbumin and γ-globulin was greatest at their isoelectric points and differed little at 10−2 and 10−3 mol dm−3 NaNO3. The adsorption of lysozyme was strongly influenced by the ionic strength.
TL;DR: In this article, the influence of surface interaction on wet high gradient magnetic separation (WHGMS) for hematite and quartz particles (<10 μm) was discussed by computing the net interaction energies for the particle-particle and particle-matrix element systems at various pH values.
TL;DR: The Rapid Thermal Decomposition of precursors in Solution (RTDS) process is described in this article as an approach to nanoscale particle production, which involves a brief (< 2 seconds) exposure of an aqueous solution containing dissolved metal oxides to hydrothermal conditions amenable to particle nucleation and growth.
Abstract: The Rapid Thermal Decomposition of precursors in Solution (RTDS) process is described as an approach to nanoscale particle production. The RTDS method involves a brief (< 2 sec) exposure of an aqueous solution containing dissolved metal oxide precursors to hydrothermal conditions amenable to particle nucleation and growth. Particle growth is terminated by passing the solution through a pressure restrictor and into a cooled collection region. Initial RTDS results involving the production of iron, titanium, and zirconium oxide powders from aqueous solutions are described. Powders generated were characterized using transmission electron microscopy, powder X-ray diffraction, BET surface area analysis, and Mossbauer spectroscopy. Iron oxide (hematite phase) having crystallite sizes ranging from a few nanometers to tens-of-nanometers was produced (rom 0.1 M Fe(NO3) 3 and Fe(NH4)(SO4) 2 solutions. Crystallite size in the hematite powders was found to be dependent on the temperature to which the solution...