Showing papers on "Hematite published in 1987"

Effect of silicate species on the transformation of ferrihydrite into goethite and hematite in alkaline media

Abstract: The transformation of ferrihydrite to goethite and/or hematite in alkaline media is strongly retarded by the presence of silicate species. These species probably stabilize ferrihydrite by adsorbing on the particles of ferrihydrite and linking them into an immobile network. At concentrations low enough for the transformation to proceed, silicate species promote the formation of hematite and hinder the nucleation of goethite. The presence of silicate species modifies the morphology of both reaction products. Hematite forms ellipsoidal single crystals, commonly displaying outgrowths of goethite. Silicate species in solution appear to enhance the development of the (021) faces of goethite, probably by preferential adsorption on these faces; at high levels of silicate species, goethite crystals adopt a pseudohexagonal habit. This morphology has not been observed previously for goethite.

The Kinetics and Reaction Mechanism of the Goethite to Hematite Transformation

Abstract: A complete mechanism for the transformation goethite to hematite based upon the results of thermogravimetric, transmission electron microscope and X-ray diffraction investigations is presented. A porous microstructure and hematite crystallites in twin orientation are found to develop during transformation. For the main part of the transformation, and at higher temperatures, the reaction is controlled by a two-dimensional phase boundary. Activation energies of 169 ± 8 kJ/mole (for an ore mineral) and 154 ± 15 kJ/mole (for a recent sedimentary goethite) were obtained for this part of the transformation. At early stages and lower temperatures, the mechanism is one of proton/iron transfer across the reaction interface. Important goethite characteristics are grain size, shape, crystallinity and excess water content. The activation energy is found to depend upon temperature and degree of dehydration.

The Reductive Dissolution of Synthetic Goethite and Hematite in Dithionite

Abstract: The reductive dissolution by Na-dithionite of 28 synthetic goethites and 26 hematites having widely different crystal morphologies, specific surfaces and aluminium substitution levels has been investigated. For both minerals the initial dissolution rate per unit of surface area decreased with aluminium substitution. At similar aluminium substitution and specific surface, goethites and hematites showed similar dissolution rates. These results suggest that preferential, reductive dissolution of hematite in some natural environments, such as soils or sediments, might be due to the generally lower aluminium substitution of this mineral compared to goethite.

Discrimination of iron alteration minerals in visible and near‐infrared reflectance data

Abstract: Field and laboratory studies were conducted to relate the mineralogy of rock and soil samples containing hematite (α-Fe2O3), goethite (α-FeOOH), and jarosite (KFe3(SO4)2(OH)6) to their reflectance properties in visible and near-infrared wavelengths. Field reflectance measurements of regolith containing one or more of these minerals were made in the Goldfield, Nevada, mining district with a four-channel radiometer. The mineralogy and reflectance properties of regolith samples collected from these field measurement sites were then determined by laboratory studies. Mossbauer spectroscopy was used to identify the iron minerals present and their relative proportions. The reflectance spectra of samples containing hematite and goethite were characterized by a reflectance minimum near 900 nm. The location of this minimum could be accounted for by the relative proportions of hematite and goethite present in the samples. Under certain conditions it may be feasible to estimate the relative proportions of hematite and goethite on the earth's surface by determining the location of this near-infrared reflectance minimum in high spectral resolution remote sensing data. In addition, the field measurements suggest that the ratio of band passes at 852 and 982 nm may be used to distinguish qualitatively among hematite, goethite, and jarosite. Thus maps of the distribution of these minerals may be derived from scanner images acquired in those band passes.

Iron oxides in lateritic soils from Western Australia

Abstract: SUMMARY Lateritic soils developed from dolerite contained larger amounts of goethite and haematite than those developed from granite. The goethite/(goethite + haematite) ratio in granitic soils ranged from 0.55 to 1 and from 0.29 to 0.83 in doleritic soils. Maghemite ranged in abundance from 0 to 10% and it only occurred in duricrust. Mole % Al substitution ranged from 16 to 33% in goethite and from 2.5 to 10% in haematite and was similar for both granitic and doleritic soils. Al substitution in maghemite was <5%. A significant, positive correlation (P<0.01) occurred between Al substitution in goethite and the amount of gibbsite in the soil. The dehydroxylation temperature of goethite ranged from 292 to 334°C and was positively correlated with the mole % Al substitution. Goethite crystals varied in size from 16 to 26 nm and haematite crystals from 18 to 59 nm. Goethite and haematite crystals occurred as aggregates of subrounded platy crystals. Iron oxides obtained by NaOH treatment contained much of the minor element contents of the soils; mean concentrations (μg g−1) were: Zn 19.9, Cu 31, Mn 68, Ni 140, Co 32, Cr 394 and V 696. These minor elements were most abundant in iron oxides derived from dolerite, but were not preferentially associated with goethite or haematite.

Occurrence and transformations of iron and manganese in a colluvial terra rossa toposequence of northern Italy

Abstract: A soil toposequence in NE Italy was studied, which consists of a terra rossa on Cretaceous limestone on the upper slope grading downwards into a colluvial fan with terra rossa material and finally into alluvial river sediments. It is postulated that the red colluviated terra rossa material has come under a moister hydroregime which provided reducing conditions. Because hematite of the terra rossa dissolved preferentially over goethite, as shown by quantitative Fe oxide mineralogy, soil color changed from 2.5YR to 7.5YR. The soils contained two types of concretions, red ones with a low Feo/Fed ratio and a high content of hematite and low content of Mn-oxides and black ones with a high Feo/Fed ratio, a small amount of hematite and abundant Mn oxides. The red concretions are therefore considered as inherited from an earlier period of pedogenesis whereas the black ones are neoformed in the present pedoenvironment. This is further supported by the lower Al-for-Fe substitution of goethite in the black concretions as compared to a higher Al substitution in the goethite inherited from the terra rossa.

Preparation and properties of coated, uniform, inorganic colloidal particles: I. Aluminum (hydrous) oxide on hematite, chromia, and titania

Abstract: Spherical alumina (hydrous) oxide particles of narrow size distribution were prepared by first hydrolyzing aluminum sulfate solutions at elevated temperatures in the presence of urea The system was quenched before any solids were formed Dilution of the prepared solutions at room temperature led to sol formation The same procedure produced surface coatings on admixed, preformed solid particles Uniform hematite, chromium hydroxide, and titania were used as core materials The thickness of the coatings could be varied by proper adjustment of the concentrations of particles, aluminum salt solutions, and reaction time

Variations in the properties of iron oxides within individual specimens of lateritic duricrust

Abstract: The morphologically distinct materials in Darling Range lateritic duricrust (ie loose and cemented pisoliths, concretions, matrix, pisolith coatings and void coatings) exhibit goethite/(goethite + hematite) ratios ranging from 015 for individual pisoliths to 10 for void coatings Mole % Al substitution ranged from 20 to 34% in goethite and from 2 to 15% in hematite Goethite and hematite in pisoliths and concretions were mostly highly Al substituted Al substitution in goethite was positively related (P < 001) to Al substitution in hematite Al substitution in maghemite was less than 5% Goethite crystals ranged in size from 130 to 260 A Hematite crystals ranged from 140 to 520 A, and were systematically smaller in pisoliths Crystal size of goethite and hematite decreased with increasing Al substitution Hematite crystals were usually about 50% larger than goethite crystals in the same sample, and crystal sizes of goethite and hematite were positively correlated (P < 001) Goethite and hematite occurred as aggregates of subrounded platy crystals Differences in the properties of goethite and hematite between morphologically distinct materials in single hand specimens are indicative of the complex history of these duricrusts

Production of hydroxyl radicals by iron solid compounds

Abstract: The deleterious effects of radicals in biological systems are now admitted. The aim of the present work was to study the reduction of oxygen and hydrogen peroxide (H2O2) into hydroxyl radicals by selected iron compounds in a phosphate buffered solution. Radical detection has been carried out by electron spin resonance spectroscopy (ESR) with “spin trapping”; (5,5‐dimethyl‐1‐pyrroline N‐oxide, DMPO). The following minerals have been used: goethite (FeOOH a), akaganeite (FeOOH β), hematite Fe2O3α), magnetite (Fe3O4), siderite (FeCO3), and crocidolite fibres (Na2Fe3Si8O22(OH)2). Crocidolite and siderite reduced O2 into OH∗. Catalase inhibited this reduction. The reduction occurs at the mineral surface. When H2O2 was added to the buffer solution, goethite was inactive, akaganeite, magnetite, crocidolite and hematite were active and siderite was very active. These reactions could play a role in the mechanisms of toxic injury caused by some solid iron compounds.

Investigation of iron oxide reduction by TEM

Abstract: An “environmental cell” located in a high voltage transmission electron microscope has been used to study the reduction of single crystal iron oxides by hydrogen and hydrogen-argon mixtures. The cell enables a direct observation of the solid during reaction, thus permitting the nucleation and growth of solid reaction products to be observed. Hematite was reduced at temperatures in the range 387 to 610°C with gas pressures up to 5.3 kP. Reduction with pure hydrogen was considerably faster than when argon was present. Lath magnetite which rapidly transforms to porous magnetite and thence (more slowly) to porous iron was observed. The reduction of magnetite and of wustite single crystals was observed in the temperature range 300 to 514°C using both hydrogen and hydrogen-argon mixtures at gas pressures up to 6.6 kP. Incubation periods were found for magnetite reduction; during these periods faceted pits formed in the oxide. Iron formed in the early stages was epitaxial with the host magnetite; at later stages the epitaxy was lost and fissures frequently formed in the metal. The morphology of the iron differed between the gas mixtures. Disproportionation accompanied the reduction of wustite, producing intermediate polycrystalline magnetite despite reducing conditions. The disproportionation appeared to be promoted by the reduction reaction. For both oxides, reduction in the hydrogen-argon mixture was slower than in pure hydrogen.

The formation of maghemite and hematite from lepidocrocite and goethite in a Cambisol from Corsica, France†

Abstract: The influence of a recent forest fire on the iron oxide phases, present in the A and the B horizon of a brown earth from Corsica, was examined by X-ray diffraction and chemical analysis. In the B horizon goethite and lepidocrocite, in the A horizon maghemite and hematite were identified. From the Al/(Al+Fe) ratio, which was determined for all four minerals by XRD, it was concluded that goethite was the precursor of hematite and lepidocrocite the precursor of maghemite respectively. The proportion of maghemite formed in the A horizon was calculated from the difference in Fed between the untreated sample and that free of maghemite. Because of substantial dissolution of clay minerals even after 5 citrate-bicarbonate treatments, it was not possible to determine Ald correctly.

An Overview of Iron Precipitation in Hydrometallurgy

Abstract: Iron is commonly eliminated from hydrometallurgical process streams by oxidation to Fe3+, followed by hydrolysis–precipitation of a filterable iron compound. Current practice focusses on the precipitation of four species: iron hydroxide, the jarosites, FeO.OH and hematite. Although the precipitation of ferric hydroxide normally is avoided because of its poor filtration properties, the key role of Fe(OH)3 in the removal of the last traces of dissolved iron and in the elimination of objectionable impurities, such as germanium in zinc circuits, is discussed. The precipitation of jarosite–type compounds is widely used to reject iron and sulphate in a readily filterable form, and new applications of this technology are reviewed. The ability of jarosites to incorporate impurities such as silver or arsenic is discussed. The precipitation of α FeO.OH (goethite) in sulphate media requires that the concentration of Fe3+ be

Standard potentials (Eo) of iron(III) oxides under reducing conditions

Abstract: Partial reduction of iron(III) oxides with hydrogen in the presence of a platinum catalyst leads to an equilibrium state after 4–20 h. From the measured Eh, pH, and Fe2+ concentration conditional standard potentials can be calculated using the formula Eo (volt) = Eh + 0.059 lg(Fe2+) + 0.18 pH which indicate the stability of Fe oxides against reduction. The reduceability decreases following the order ferrihydrite > lepidocrocite > hematite > goethite. The difference between hematite and goethite was more pronounced than that predicted from thermodynamic data.

The FeMn ore deposits of Urucum, Brazil: An oxygen isotope study

Abstract: The Fe Mn ore deposits of Urucum are located close to the Brasilian-Bolivian border. They appear to be considerably younger (Late Proterozoic) than most other banded iron ore formations. The Mn ores occur in three or four well-defined beds. Oxygen isotope determinations have been carried out in Fe-poor and Fe-rich layers and on a few Mn ores. Due to the extremely small grain size, a conventional mineral separation was impossible. However, since the banded iron ores consist of two components only (SiO 2 and Fe 2 O 3 ), a determination of the Fe content allows a calculation of the oxygen isotope composition of the pure end-members. The so calculated δ 18 O-values vary between +21.3 and +26.4‰ for quartz and between +0.8 and +6.5‰ for hematite. Isotope equilibrium may have been reached in some but not all samples. Calculated temperatures vary between 250° and 280°C, depending upon which calibration curve is used. Such temperatures are higher than indicated by maximum burial depths. The δ 19 O-values appear to reflect diagenetic and burial metamorphic conditions rather than primary conditions.

Zn Mn Ilmenite in the Kuiqi Granite from Fuzhou, Fujian Province, East China

Abstract: Zn-Mn ilmenite occurs as a principal constituent of the miarolitic cavities in the Kuiqi granite from Fuzhou. In the cavities potassium feldspar, albite, quartz, fluorite, aegirine and arfvedsonite also occur with accessory zircon, magnetite, hematite and sphalerite. Zn-Mn ilmenite forms a trigonal platy euhedral crystal up to 30 mm in length. Its chemical composition ranges from 37.3 to 63.8 mol.% FeTiO3 (ilmenite molecule), 61.1 to 22.4 mol.% MnTiO3 (pyrophanite molecule) and 0.6 to 15.3 mol.% ZnTiO3 (Znmetatitanate molecule). ZnO content ranges from 0.34 wt.% to 7.63 wt.%. Zonal structure is noticeable in the Zn-Mn ilmenite. FeTiO3 and ZnTiO3 molecules increase towards the crystal rim, while MnTiO3 molecule decreases towards the rim. Unit cell parameters of the rim and the core area = 5.092(6)A,c = 14.08(2)A,V = 316.1A3 anda = 5.106(7)A,c = 14.02(3)A,V = 316.6A3, respectively. Coexisting minerals, except for arfvedsonite and sphalerite, are very low in ZnO content. It is suggested that complete isomorphous replacement between FeTiO3 MnTiO3 and ZnTiO3 may be possible. Oxygen fugacity conditions for crystallization of Zn-Mn ilmenite are considered to be in the vicinity of the magnetite-hematite and quartz-fayalite-magnetite buffers.

Diagenesis of iron in proglacial sand deposits of late- and post-Weichselian age

Abstract: Iron diagenesis was studied in Quaternary sand deposits of a temperate climate using an integrated approach of sedimentary petrography and groundwater chemistry. The iron-bearing minerals, amphiboles, pyroxenes, magnetite, and ilmenite are all extensively altered. Quantitatively, amphiboles and pyroxenes are by far the most important. Goethite and lepidocrocite are the most important of the iron oxyhydroxide weathering products, and they coat quartz and feldspars as well as amphiboles and pyroxenes. However, lepidocrocite is absent in the unsaturated zone. Hematite is only present associated with magnetite and ilmenite. Total iron contents of the sediments range from 0.3-0.9 percent, with 30-80 percent as coatings on quartz and feldspar grains, 25-60 percent in the amphibole and pyrox ne fraction, and only 2-15 percent in the magnetite and ilmenite fraction. Therefore, amphiboles and pyroxenes are the most important sources of iron for these diagenetic processes. The groundwater has a pH between 5.5 and 6.0 and contains 10-30 µM Fe2+ and a low total of dissolved solids. Subtraction of the contribution of the dissolved solids from atmospheric precipitation indicates higher pH and HCO3-, SiO2, Ca2+, Na+m and Fe2+ concentrations due to mineral reactions in the aquifer. Stability relationships for iron-bearing minerals under the assumption of anoxic conditions show strong undersaturation for iron-bearing silicates, while undersaturation for magnetite and ilmenite is much less. The saturation state for magnetite is strongly dependent on the iron oxide phase formed by incongruent dissolution. The surfaces of iron silicates and magnetite are highly etch-pitted, indicating that mineral dissolution is controlled by surface reactions rather than by diffusion through alteration products. The rate of mineral alteration is slow in comparison with groundwater flow. Thus, diagenetic changes occur in the deposit as a whole rather than as a diagenetic front advancing through the deposit. The abundance of FeOOH on all detrital grains suggests that this proglacial deposit could develop into a red bed during further diagenesis. This possibility should be considered in the paleoclimatic interpretation of red beds.

High-temperature oxidation of Fe-2 1 4 Cr-1Mo in oxygen

Abstract: The oxidation of a 2 1 4 Cr-1Mo steel in dry flowing oxygen has been studied in the temperature range 550–700°C for periods of up to 100 hr. A detailed low-resolution microstructural investigation revealed a layered oxide consisting of a very fine-grained and finely pored innermost layer of doped spinel, a central columnar-grained relatively coarsely pored layer of magnetite, and an outer fine-grained hematite layer with fine pores and covered with whiskers of α-Fe2O3. This structure is compared with previous results on Fe and model Fe-Cr alloys, as are the kinetics of the oxidation reaction.

Mössbauer spectroscopic and X-ray diffraction study of the thermal decomposition of natural siderite and goethite

Abstract: Thermal decomposition of natural siderite and goethite has been studied using Mossbauer spectroscopy and X-ray diffraction. Hematite and magnetite were the principal compounds formed during high temperature treatment of siderite. Natural goethite transformed to hematite at high temperatures. The crystal structures, stoichiometry and the nuclear magnetic properties of Fe-oxides formed are discussed.

Comparative thermogravimetric study of chlorinations of hematite and wustite

Abstract: The reactions of hematite and wustite with Cl2, COCl2, CO+Cl2 and gaseous CCl4 were studied by thermogravimetry, X-ray diffraction and computer-assisted thermodynamic calculations. During the chlorination of hematite, only gaseous products were formed. The apparent activation energies were calculated from the isothermal TG curves. The chlorination of wustite involves the formation and transformation of a series of solid chloride products. FeCl2(s), FeCl3(s) and FeOCl(s) were found as intermediates during chlorination with Cl2 or Cl2+CO, while only FeCl2(s) was detected in reactions with COCl2 oe CCl4. The chlorinations of the iron oxides with various agents were compared, and the differences in reactivity sequences were established.

Morphology and porous texture of iron fine particles with relation to their magnetic properties

Abstract: The effects of silica film as thin as 0.4 nm on the variation of morphology of particles, in the process of reduction of microporous haematite particles with hydrogen gas, were investigated by means of gas adsorption, electron microscopy, X-ray diffraction and magnetic measurements. Microporous haematite particles were prepared by decomposition of the silica-coated goethite particles. A silica film of 0.4 nm thickness was coated on the surface of the goethite particles (length: 400 nm, width 25 nm). In the process of reduction, collapse and sintering of the particles were markedly suppressed by coating with ultra-thin silica film. In the reduced particles, iron crystallites were separated from each other forming iron particles of large porosity. By introducing heat-treatment of the silica-coated haematite particles at 700° C in air prior to reduction, highly acicular iron particles having the crystallites grown along the elongated direction of the particles were obtained. A marked increase in the coercivity of the iron particles from 1075 to 1530 Oe was attributed to the increase of the shape anisotropy.