Showing papers on "Hematite published in 1991"

Solubility and dissolution of iron oxides

Abstract: In most soils, FeIII oxides (group name) are the common source of Fe for plant nutrition. Since this Fe has to be supplied via solution, the solubility and the dissolution rate of the Fe oxides are essential for the Fe supply. Hydrolysis constants and solubility products (Ksp) describing the effect of pH on FeIII ion concentration in solution are available for the well-known Fe oxides occurring in soils such as goethite, hematite, ferrihydrite. Ksp values are usually extremely low ((Fe3+)·(OH)3=10−37−10−44). However, for each mineral type, Ksp may increase by several orders of magnitude with decreasing crystal size and it decreases with increasing Al substitution assuming ideal solid solution between the pure end-members. Based on such calculations a poorly crystalline goethite with a crystal size of 5 nm may well reach the solubility of ferrihydrite. The variations in Ksp are of relevance for soils because crystal size and Al substitution of soil Fe oxides vary considerably and can now be determined relatively easily. The concentration of Fe2+ in soil solutions is often much higher than that of Fe(III) ions. Therefore, redox potential strongly influences the activity of FeII. At a given pH and Eh, the activity of FeII is higher the higher Ksp of the FeIII oxide and thus also varies with the type of Fe oxide present. Besides the solubility, it is the dissolution rate which governs the supply of soluble Fe to the plant roots. Dissolution of Fe oxides takes place either by protonation, complexation or, most important, by reduction. Numerous dissolution rate studies with various FeIII oxides were conducted in strong mineral acids (protonation) and they have shown that besides the Fe oxide species, crystal size and/or crystal order and substitution are important determinative factors. For example, in soils, small amounts of a more highly soluble meta- or instable Fe oxide such as ferrihydrite with a large specific surface (several hundred m2g−1) may be essential for the Fe supply to the plant root. Its higher dissolution rate can also be used to quantify its amount in soils. Ferrihydrite can be an important component in soils with high amounts of organic matter and/or active redox dynamics, whereas highly aerated and strongly weathered soils are usually very low in ferrihydrite. On the other hand, dissolution rates of goethites decrease as their Al substitution increases. Much less information exists on the rate of reductive and chelative dissolution of Fe oxides which generally simulate soil conditions better than dissolution by protonation. Here again, type of oxide, crystal size and substitution are important factors. Organic anions such as oxalate, which are adsorbed at the surface, may weaken the Fe3+-O bonds and thereby increase reductive dissolution. As often observed in weathering, the dissolution features of the crystals appear to follow zones of weakness in the crystal.

Conducting polymer composites

Abstract: Electrically conducting polypyrrole coatings on inorganic cores were obtained by deposition of pyrrole on catalytically active particles. The finely dispersed core materials were hematite (polyhedral and spindle-type), silica coated with hematite, and cerium(IV) oxide. The degree of polymer coverage could be controlled by varying the aging time and the properties of the carrier particles. The so prepared coated powders were examined by transmission electron microscopy and further characterized by elemental and thermogravimetric analyses, as well as by electrophoresis, X-ray diffraction, and conductivity measurements. The conductivities were comparable to those reported for pure polypyrrole and they increased with pressure.

Coupling adsorption and particle aggregation: laboratory studies of "colloidal pumping" using iron-59-labeled hematite

Abstract: 59 Fe-labeled, spherical hematite particles not retained by filters because of their size coagulated to form aggregates, which were filter-retained. Batch systems containing hematite particles and a suite of radionuclides showed that, as the hematite coagulated, colloid-adsorbed radionuclide species were transferred from the colloid pool to the filter-retained particle pool. Sorptive equilibrium was rapidly achieved; thus, the rate-limiting step for the appearance of the radionuclides in the filter-retained particle pool was coagulation

Concentration of iron oxides from soil clays by 5 m NaOH treatment: the complete removal of sodalite and kaolin

Abstract: A B S T R A C T: The K~mpf & Schwertmann (1982) procedure for concentrating iron oxides in soil clays by dissolution of kaolin and gibbsite by boiling for 1 h in 5 M NaOH may not dissolve all kaolin, and also results in the precipitation of sodalite. For the complete dissolution of kaolin in kaolin-rich soil clays a boiling time of 2 h in 5 M NaOH was required. The large amounts of sodalite produced were not removed by the prescribed single wash in 0-5 M HC1. Oxalate soluble AI contents of iron oxide concentrates were sometimes very high and dithionite Fe contents were very low both in concentrates containing sodalite, and in those for which sodalite was not detected by XRD, but where a previously unsuspected amorphous sodalite-like phase may have been present. Complete removal of precipitated sodalite was achieved by two extractions with 0.5 M HCI at 25~ for 20 rain. This modified procedure does not alter the Al-substitution and crystal size of goethite, hematite and maghemite as determined by XRD measurements.

Relationships between aggregation and iron oxides in Terra Rossa soils from southern Italy

Abstract: Summary Thirteen samples from six Terra Rossa profiles of southern Italy were studied to determine the influence of Fe oxides in aggregation. Removal of all the Fe oxides by the citrate-bicarbonate-dithionite treatment resulted, in all samples, in a decrease in the proportion of the silt plus sand fraction and an increase in the clay fraction. The change in the particle size distribution (PSD) was highly correlated with the total Fe oxide content and with the hematite content but not so well correlated with the goethite content. When the samples were treated with acid NH4-oxalate to remove the poorly crystalline Fe oxides, there was a decrease in the proportion of the sand fraction and, in general, an increase in the proportion of silt and little or no increase in the proportion of clay; the change in PSD was better correlated with the ratio oxalate-extractable Si/oxalate-extractable Fe than with any other single property. This indicates that silica may contribute to aggregation, probably by forming bridges between Fe oxides and other soil particles.

Phase transformations during heating of llmenite concentrates

Abstract: llmenite concentrates were heated in argon and oxygen in the temperature range 700 °C to 1000 °C to study the behavior of the pseudorutile phase and other changes which occur. Pseudorutile does not persist in argon or oxygen in the temperature range studied. In argon at 700 °C, pseudorutile decomposes into hematite and rutile, while at 1000 °C, it combines with ilmenite to form ferrous-ferritic pseudobrookite solid solution. A new phase “Fe2O3-2TiO2” was identified as an intermediate product during the heating of ilmenite or pseudorutile in oxygen. This compound decomposes into hematite and rutile below 800 °C and to pseudobrookite and rutile above 800 °C. The sequence of reactions during the heating of ilmenite and pseudorutile in oxygen is proposed.

Oxygen isotope fractionation in hematite and magnetite: A theoretical calculation and application to geothermometry of metamorphic iron-formations

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

Crystallization of (FeO, Fe2O3)-CaO-SiO2 Glasses and Magnetic Properties of Their Crystallized Products

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.

Sulfate Ion Binding on Goethite and Hematite

Abstract: The nature of the sulfate bond to iron oxides (goethite and hematite) is assessed by infrared spectroscopy, differential thermal analysis, thermal gravimetric analysis, and the ratio of protons consumed (or hydroxyls released) per sulfate ion sorbed. The sulfate ion bond is strong. It is proposed th

Extraction of iron oxides from sediments using reductive dissolution by titanium (III)

Abstract: A new iron oxide dissolution method designed to measure the abundance of “free” Fe oxide phases and associated elements in soils and sediments has been tested. The method employs a ternary complex of Ti(III), citrate, and ethylenediaminetetraacetate (EDTA) as a reductant and bicarbonate as a proton acceptor. The Ti(III)-citrate-EDTA-HCO3 method dissolved more synthetic amorphous ferric oxide and goethite, but less synthetic hematite, than the dithionite-citrate-HCO3 method of Mehra and Jackson. The production of acidity by the dissolution indicated that Ti(IV) is hydrolyzed to TiO2 during the extractions. The heated dithionite method dissolved 3–6 times more Al from kaolinite and nontronite standard clays than room temperature dithionite, and 4–6 times more Al than the Ti(III)-citrate-EDTA-HCO3 method. Furthermore, the release of Fe from the clay mineral samples consistently and rapidly reached a plateau during multiple extractions by the Ti(III)-citrate-EDTA-HCO3 method, indicating that a well-defined Fe oxide fraction was removed. Fe released by the dithionite method continued to increase with each extraction, suggesting that some release of structural Fe occurred. Tests on two natural sediments and one heavy mineral fraction from the Miocene Cohansey Sand in the New Jersey Coastal Plain suggested that the Ti(III)-citrate-EDTA-HCO3 method removed Fe oxides more effectively and more selectively than the dithionite method. The selectivity of the Ti(III)-citrate-EDTA-HCO3 method is enhanced by rapid extractions at room temperature and low free ligand concentrations.

Mössbauer study of the thermal decomposition of lepidocrocite and characterization of the decomposition products

Abstract: The lepidocrocite (γ-FeOOH) to maghemite (γ-Fe2O3), and the maghemite to hematite (α-Fe2O3) transition temperatures have been monitored by TGA and DSC measurements for four initial γ-FeOOH samples with different particle sizes. The transition temperature of γ-FeOOH to γ-Fe2O3 and the size of the resulting particles were not affected by the particle size of the parent lepidocrocite. In contrast, the γ-Fe2O3 to γ-Fe2O3 transition temperature seems to depend on the amount of excess water molecules present in the parent lepidocrocite. Thirteen products obtained by heating for one hour at selected temperatures, were considered. Powder X-ray diffraction was used to qualify their composition and to determine their mean crystallite diameters. Transmission electron micrographs revealed the particle morphology. The Mossbauer spectra at 80 K and room temperature of the mixed and pure decomposition products generally had to be analyzed with a distribution of hyperfine fields and, where appropriate, with an additional quadrupole-splitting distribution. The Mossbauer spectra at variable temperature between 4.2 and 400 K of two single-phase γ-Fe2O3 samples with extremely small particles show the effect of superparamagnetism over a very broad temperature range. Only at the lowest temperatures (T⩽55 K), two distributed components were resolved from the magnetically split spectra. In the external-field spectra the ΔmI=0 transitions have not vanished. This effect is an intrinsic property of the maghemite particles, indicating a strong spin canting with respect to the applied-field direction. The spectra are successfully reproduced using a bidimensional-distribution approach in which both the canting angle and the magnetic hyperfine field vary within certain intervals. The observed distributions are ascribed to the defect structure of the maghemites (unordered vacancy distribution on B-sites, large surface-to-bulk ratio, presence of OH- groups). An important new finding is the correlation between the magnitude of the hyperfine field and the average canting angle for A-site ferric ions, whereas the B-site spins show a more uniform canting. The Mossbauer parameters of the two hematite samples with MCD104 values of respectively 61.0 and 26.5 nm display a temperature variation which is very similar to that of small-particle hematites obtained from thermal decomposition of goethite. However, for a given MCD the Morin transition temperature for the latter samples is about 30 K lower. This has tentatively been ascribed to the different mechanisms of formation, presumably resulting in slightly larger lattice parameters for the hematite particles formed from goethite, thus shifting the Morin transition to lower temperatures.

Reflectance Spectroscopy of Palagonite and Iron-Rich Montmorillonite Clay Mixtures: Implications for the Surface Composition of Mars

Abstract: Because of the power of remote sensing reflectance spectroscopy in determining mineralogy, it was used as the major method of identifying possible mineral analogs of the Martian surface. A summary of proposed Martian surface compositions from reflectance spectroscopy before 1979 was presented. Since that time, iron-rich montmorillonite clay, nanocrystalline or nanophase hematite, and palagonite were suggested as Mars soil analog materials.

Effects of phosphate on iron oxide dissolution in ethylenediamine-N,N,N'N'-tetraacetic acid and oxalate

Abstract: The content of poorly crystalline iron oxides, mainly ferrihydrite, in soils is commonly determined by the oxalate method and less commonly by the ethylenediamine-N,N,N',N'-tetraacetic acid (EDTA) method (Schwertmann, 1964; Schwertmann et al., 1982; Borggaard, 1981, 1988). Comparable amounts of i ron are dissolved by the two methods from many soils (Borggaard, 1988), and both ligands presumably act via surface complexat ion (Chang and Matijevic, 1983; Zinder et al., 1986). Recently, however, various differences in the d i s s o l u t i o n m e c h a n i s m s have been suggested (Borggaard, 1990). In contrast to EDTA, oxalate is rather unstable and decomposes, for example, in the presence of light (Schwertmann, 1964). Decomposi t ion ofoxala te leads to the formation ofiron(II) , which may accelerate the dissolution o f iron oxide (Cornell and Schindler, 1987). Furthermore, EDTA, having four carboxyl groups, has been postulated to be adsorbed onto iron oxide surfaces by the formation of tetranuclear surface complexes at room temperature (Borggaard, 1990). Such surface complexes may retard or inhibit iron oxide dissolution because several bonds must be broken before the i ron-EDTA complex can be released (Chang and Matijevic, 1983). Oxalate is bidentate and forms binuclear (and monodentate) surface complexes (Parf i t t e t al., 1977; Cornell and Schindler, 1987), which obviously cannot prevent the dissolution o f crystalline iron oxide (Borggaard, 1982, 1988). I f the surface complexes are different, however, the EDTA and oxalate methods should respond differently i f competing anions, such as phosphate, are present simultanously. Phosphate should increase the dissolution rate in EDTA because it eliminates or suppresses the formation of tetranuclear surface complexes, whereas the competition for adsorpt ion sites should slow down the dissolution in oxalate. The purpose of this paper is to test the hypothesis of different mechanisms for the dissolution of iron oxides in EDTA and oxlate due to different surface complexes. Therefore, adsorpt ion and dissolution properties o f the system iron oxide-EDTA/oxalate with and

Development of magnetic soil from ferroan dolomite

Abstract: An occurrence of maghemitic soil in association with ferroan dolomite in Minas Gerais is characterized by X-ray diffraction, magnetization measurements and Mossbauer spectroscopy. Altered layers on outliers of dolomite, which has composition (Ca1.04Mg0.83Fe0.11Mn0.02)(CO3)2 indicate the weathering sequence: ferroan dolomite → calcite + poorly-crystallized ferric hydroxide → maghemite + hematite + goethite. The spontaneous magnetization of the maghemitic soil (65 wt% Fe2O3) is 8.8 JT−1kg−1; the magnetization of the soil maghemite itself is estimated to be 25±5 JT−1kg−1, which is greatly reduced from the value for pure maghemite because of isomorphous substitution.

Mineral remains of early life on earth? On mars?

Abstract: The oldest sedimentary rocks on Earth, the 3.8‐Ga Isua Iron‐Formation in southwestern Greenland, are metamorphosed past the point where organic‐walled fossils would remain. Acid residues and thin sections of these rocks reveal ferric microstructures that have filamentous, hollow rod, and spherical shapes not characteristic of crystalline minerals. Instead, they resemble ferric‐coated remains of bacteria. Modern so‐called iron bacteria were therefore studied to enhance a search image for oxide minerals precipitated by early bacteria. Iron bacteria become coated with ferrihydrite, a metastable mineral that converts to hematite, which is stable under high temperatures. If these unusual morphotypes are mineral remains of microfossils, then life must have evolved somewhat earlier than 3.8 Ga, and may have involved the interaction of sediments and molecular oxygen in water, with iron as a catalyst. Timing is constrained by the early in fall of planetary materials that would have heated the planet's sur...

The Catalysis of Maghemite and Hematite on the Aldol and the Retro-Aldol Condensation of Acetone

Abstract: The catalytic activities of maghemite on the aldol condensation of acetone and on the retro-aldol condensation of diacetone alcohol are found to be higher than those of hematite. Further, the catalytic activity of maghemite on the formation of mesityl oxide is significantly higher than that of hematite. The active sites are considered to be surface hydroxyl groups which behave as a Bronsted base and acid points. Maghemite has more basic surface hydroxyl groups than does hematite, and far more acidic surface hydroxyl groups than does hematite they are sufficiently strong to make its average surface acidity greater than that of hematite. The basic surface hydroxyl groups of maghemite are speculated to be those which bond to the iron ions of the octahedral sites the acidic ones are those of the tetrahedral sites of the spinel lattice of the surface.

Improvement of the reduction process in p content and other gangues in iron ore and its agglomerates

Abstract: This invention is directed to an inprovement of the process reduction of P contents and other gangues in iron ores and its agglomerates, in which during thermic pretreatment phase ore is subjected to agglomeration (briquetting, pelletizing sintering) before making it undergo the leaching process with hydrochloric acid, hydrochloric gas, chlorides or their by-products. Thermic treatment at 1200 °C causes a structural rearrangement of hematite and goethite (the latter becoming hematite) so that its dissolution with mineral mineral acids becomes difficult. At the same time, there is a migration of associated gangues to the surface of the particles, thus facilitating the attack and its dissolution with mineral acids.