Showing papers on "Hematite published in 1984"

Formation of monodispersed spindle-type hematite particles

Abstract: Procedures have been developed which yield spindle-type colloidal hematite particles of narrow size distribution by forced hydrolysis of ferric chloride solutions at elevated temperatures. It was shown that small additions of phosphate or hypophosphite ions to such systems before aging had a significant effect on the resulting particle shape. Specifically, the axial ratio of the precipitated solids depended on the concentration of these anions in the original solutions.

High temperature stability of maghemite (γ-Fe2O3)

Abstract: The temperature stability of acicular maghemite (γ-Fe2O3) was studied between 20°C and 600°C by x-ray diffraction and magnetic measurements. Synthetic monodomain maghemite was found to be stable with respect to hematite (α-Fe2O3) at temperatures as high as 500°C, inverting to α-Fe2O3 between 510-660°C. However, a complete inversion of the γ to α form for the sample was not achieved even at 660°C, the final product was a two-phase mixture, with 65% of the original γ-Fe2O3 unchanged. Two conclusions are made: (a) the absence of maghemite in a natural sample cannot be proven if heating has been restricted to moderate temperatures (300°-400°C) and (b) the true Curie point of pure maghemite is close to 645°C.

Iron oxide mineralogy of placic horizons

Abstract: SUMMARY The iron oxide mineralogy of 15 placic horizons from Germany, New Zealand, Belgium and U.K. was determined by differential X-ray diffraction (DXRD). All samples examined contained ferrihydrite and goethite, lepidocrocite occurred in 10, and six contained non-DCB-reducible, lithogenic hematite. Ferrihydrite was most readily identified in ‘untreated minus oxalate-treated’ DXRD patterns. Al-substitution of goethites ranged from 4 to 13 mole %. Mean crystallite dimensions varied from 11 to 21 nm and from 6 to 21 nm for the goethite (110) and (111) lines respectively, from 7 to 51 nm for the lepidocrocite (020) line (lath thickness), and from 55 to 134 nm for the hematite (104) line. These values are typical for goethites from carbon-rich environments with poor crystallization conditions, and for well crystalline lithogenic hematites. The DXRD method is ideally suited to the evaluation of the many selective dissolution techniques used in soil chemistry and mineralogy.

Magnetic remanence and fabric properties of laboratory‐deposited hematite‐bearing red sandstone

Abstract: Redeposition experiments of disintegrated hematite bearing sandstone have confirmed a tangent relationship between inclinations of the DRM and ambient magnetic field. Pronounced variations of DRM-intensities with field inclination are attributed to processes acting during the depositional formation of laminated sediments. Coinciding azimuthal distributions of remanence and maximum susceptibility directions reflect intrinsic properties of flaky hematite grains in which the magnetic moment is parallel to the direction of maximum susceptibility. This parallelism probably depends on magnetic domain configurations.

Mineralogical and chemical properties of weathered magnetite grains from lateritic saprolite

Abstract: SUMMARY Magnetite grains in lateritic saprolite have altered in situ to porous hematite (martite) grains. Hematite occurs as relatively large (∼ 1 μm) platy crystals, triangular arrays of small (0.01–0.05 μm) lathlike crystals and very small (< 100 A) platy anhedral particles. Titanium increased in altered grains and may substitute for Fe3+ in hematite. Mn, V, Cr and Zn remained approximately constant whereas the Ni content decreased. Halloysite crystallized from soil solution within pores resulting in a marked increase in the Si and Al contents of grains.

Morphology and mineralogy of fly ash from a coal-fueled power plant

Abstract: Morphology, chemical composition and mineralogy of fly ash from a coal-fueled power plant have been investigated. Optical and scanning electron microscopic examinations on both untreated particle samples and thin sections have enabled the investigation of internal and superficial microstructure. Chemical and diffractometric analyses performed on fly ash samples, fractionated by means of magnetic and gravimetric separators, associated the particle morphology to the elemental and mineralogical composition. A classification of single particles was achieved as follows: 1) glassy aluminosilicate with avariable composition; 2) spongy carbonaceous; 3) spherical metallic particles constituted of different iron oxide phases (magnetite, hematite, maghemite); 4) spherical rutile particles; 5) spherical lime particles; 6) mineral formless particles (i.e. quartz and mullite). The mineralogical composition of the coal utilized during instack samplings is also given. Lastly a correlation between the crystalline and amorphous particles constituing the fly ash and the mineral matter present in the burnt fuel has been proposed.

Effect of Oxygen Partial Pressure on Oxidation of Iron at 573 K

Abstract: The oxidation of iron was investigated by Rutherford backscattering spectroscopy at 573 K under the oxygen partial pressure from 10−1 to 105 Pa for 3~24 h. In the first stage of oxidation, thickness of the oxide film reached the maximum around 1 Pa. In the second stage the oxidation obeyed the parabolic rate law. From 10−1 to 10−0.5 Pa the parabolic rate constant increased with increasing oxygen partial pressure due to the growth of magnetite, whereas from 1 to 100.5 Pa hematite covered magnetite but the parabolic rate constants were kept still high values possibly due to the presence of grain boundaries. Between 10 and 104.3 Pa the protective hematite coverage over magnetite kept the parabolic rate constants low regardless of oxygen partial pressure. At 105 Pa a sharp increase of the parabolic rate constant was observed due to the simultaneous growth of hematite and magnetite. The oxygen concentration in water for the inhibition of the corrosion may correspond to the oxygen partial pressure for the forma...

The Precipitation of Hematite from Iron-Loaded Versatic Acid Solutions by Hydrolytic Stripping

Abstract: Solutions of Versatic acid containing ferric iron precipitate hematite when reacted with water at temperatures above about 130°C. Factors affecting the rates of precipitation have been investigated. It is shown that precipitation can initiate either by a process of homogeneous self-nucleation or by heterogeneous reaction at the surface of hematite particles in solution. The former process is favored by high loadings of iron in the organic phase and by increases in reaction temperature. It is suggested that differences in the structures of the iron-versatic complexes in the organic phase determine the course of the reaction. The effects of the age of the organic solutions on the rates of precipitation are discussed.

Remanent magnetization of a “paramagnetic” composition in the ilmenite‐hematite solid solution series

Abstract: In the ilmenite-hematite solid solution series, compositions more ilmenite-rich than Ilm(73)Hem(27) are classically thought to be paramagnetic at room temperature. Ilm(80)Hem(20) samples have nevertheless been synthesized that acquire hard saturation remanent and thermoremanent moments. From analysis of AC demagnetization data, the source of the measured remanence is believed to be a single-domain (SD) like material within the IlM(80)Hem(20) grains themselves. On the basis of transmission electron microscope observations, it is suggested that transformation-induced domain boundaries, which in part are enriched in hematite component relative to the bulk composition of the grains, could act as the magnetic carrier of the SD-like remanence.

Morphological and magnetic changes during mechano-chemical transformation of lepidocrocite to hematite

Abstract: The changes taking place during the grinding of lepidocrocite (γ-FeOOH) powder are investigated by means of transmission electron microscopy and Mossbauer effect spectroscopy. The study has shown that small comminuted particles transform directly to superparamagnetic hematite, and that during prolonged grinding they agglomerate to form partially-developed antiferromagnetic larger particles of hematite. A theoretical explanation is suggested, which emphasize also the importance of active molecular surface groups formed on newly exposed surfaces to the process of mechano-chemical agglomeration and crystallization.

Morphology of iron fine particles

Abstract: The process of formation of iron fine particles from goethite microcrystals was investigated by means of powder X-ray diffraction, gas adsorption, and electron microscopy. In the first process, microporous hematite particles were prepared by decomposing goethite particles. The pores formed in hematite particles were slit-shaped (width : 0.9 nm). In the second process, microporous hematite particles were reduced at 250-350°C under the flow of hydrogen gas, and the reduction degree R (%) was determined. In the sample of R=8%, mesopores of 4 - 8 nm were formed by aggregation of slit-shaped micropores, but the skeleton of acicular particles was not significantly altered. With the progress in reduction, a number of crystallites were formed in the parent particles together with growth of pore size. In the sample of R=72%, the crystallites were separated from the parent particles. Finally, the separated crystallites sintered to give granular iron particles having low coercivity of 375 Oe. It was found that alumina or silica gel coated on the surface of goethite particles was remarkably effective for preventing the separation and sintering of crystallites formed in the parent particles. The silica-coated iron fine particles consisting of the crystallites which were as large as single domain particles showed excellent acicular shape and high coercivity of 1600 - 1700 Oe.

Mineralverwitterung, Tonmineralbildung Und Rubefizierung In Terrae Calcis Der Slowakei: Ein Beitrag Zum Paläoklimatischen Aussagewert Von Kalkstein-Rotlehmen In Mitteleuropa

Abstract: Mineral weathering, clay mineral formation and rubefication of Terrae calcis in Slovakia — A contribution to paleoclimatic evidence of limestone Rotlehm (terra rossa) in Central Europe Limestone Rotlehm (Terra rossa) is very often interpreted as a product of intensive mineral weathering and clay formation under subtropic (to tropic) climate. Especially the formation of hematite in soils as the main process of rubefication is generally believed to occur only in warm climates with a seasonal alternation of precipitation and drought. Fossil or relict Terrae rossae in Central Europe are therefore very often considered as witnesses of a corresponding paleoclimate. On the other side many authors consider the Terra rossa as a product of the insoluble residue in limestones which contains material from earlier formed rubefied soils. This viewpoint is known as the residual theory (“Losungs-” or “Ruckstands” theorie). In this case the climate factors for the formation of Terra rossa would be less significant. Besides micromorphological studies mineral optical investigations of the fractions > 2 μm and X-ray investigations of the clay particle fractions 2-0.2 μm and 1. The quantitative composition in the single fractions of the primary minerals and clay minerals of the nine Terrae calcis in comparison with the LR show that the Terrae fuscae of Ludrova(see figs. 1, 10) have a lithologic discontinuity: the clay content of the LR is distinctly higher than in the accompanying Terrae fuscae. For the other Terrae calcis an approximate sedimentarily homogeneity can be assumed. 2. In the Terrae rossae especially near Drevenik (fig. 2), Ivanovce (fig 4) and in “Vo Viniciach” near Levice (fig. 9) and approximately also in the two other Terrae rossae near Levice (fig. 7, 8) no or almost no mineral weathering and clay mineral formation is verifiable. These soils therefore are lithomorphic soils in the sense of the residual theory: their formation was independent of the regional climatic characteristics. 3. In the Terra rossa near Ratnove (fig. 5) the clay content is very high (87%), but the residue of the underlying travertine consists already of 75% clay minerals. Weathering of feldspars and above all of phyllosilicates resulted in a formation of illites and kaolinites. However, the rate of clay formation is low in comparison with interglacial loess soils in the neighbouring Carpathian Basin; the largest part of the clay minerals in this Terra rossa must be regarded as inherited, too. 4. On the other hand, the content of primary minerals > 2 μm in the LR near Kolinany Malok is very high (78%). The clay content of the Terra rossa is about twice as high as in the LR (47% to 22%, fig. 6 K in comparison with 6 L). The weathering of the largest parts of the feldspars and phyllosilicates has resulted in a formation of illites and to a greater extent in a formation of kaolinites. 5. In the Terra fusca from early to middle Pleistocene travertine (fig. 3) only a slight mineral weathering and clay mineral formation is detectable, although the LR contains a larger part of coarse and medium silt (probably loess). For the determination of the iron oxides Mossbauer spectra of the 1. In the Terrae fuscae including their LR of strong brown to reddish yellow colours hematite can not be detected. Goethite is the only iron oxide in these sample pairs. 2. The hematite content determines in nearly all Terrae rossae sample pairs the quite different fraction of red (the hue notation) of the (Munsell) colour. Two exceptions are the Terrae rossae with their LR in Drevenik (fig. 2) and “Vo viniciach” in Levice (fig. 9): in both cases the Terra rossa and its LR have the same or nearly the same colour, but the hematite content in the Terra rossa is quite higher than in the accompanying LR (cf. tab. 2). 3. All Terrae rossae show with respect to their LR an increase of the hematite content, but with remarkable differences. In one of the Terrae rossae in Levice (fig. 8, O-P) the increase of hematite is very pronounced (although no or almost no mineral weathering and clay formation was found). On the other side the increase of the hematite content in the Terra rossa of Ratnovce (fig. 5, G-H) is just detectable (although some mineral weathering especially of phyllosilicates and clay formation has taken place). In the Terra rossa of Kolinany Malok (the only soil with a high mineral weathering and clay formation) the increase of hematite is intermediate (cf. fig. 6, K-L). In the Terra rossa of Ivanovce (fig. 4, E-F) no change in the low hematite content can be observed. But in this exposure a doline filling of a soil sediment (in general 7.5 YR 5/8) is present which contains small fractions of soil with distinct red colour (10 R 4/6). These fractions show a Mossbauer spectrum of pure hematite. These results show that a rubefication in the Terrae rossae could be established, the extent of which is mostly but not always determined by the hematite content and obviously independent of the intensity of weathering; in most of the Terrae rossae no mineral weathering and clay formation was verifiable.

Paleomagnetic dating of the transformation of oolitic goethite to hematite in iron ore

Abstract: We show that the oolitic hematite ores of Birmingham, Alabama, carry a remanence direction stable enough to have survived major folding Nevertheless, this remanence was very likely acquired in the

Stability of monodispersed hematite sols in the presence of citric acid

Abstract: The effect of citric acid on the stability of a hematite sol consisting of uniform spherical particles has been studied as a function of pH and of the acid and sol concentrations. The critical coagulation concentration decreases significantly with increasing pH and so does the critical stabilization concentration due to charge reversal. The phenomena observed are directly related to chemisorption of the citrate ion on metal oxide particles.

Disintegration of synthesized iron oxides by reduction at 550.DEG.C in 30%CO-N2 gas stream.

Abstract: Samples of ternary 80%Fe2O3 CaO-SiO2 systems and 77.6Fe2O3-CaO-SiO2 3%Al2O3 systems were sintered at 1300, 1 350 and 1400°C in air. Their disintegration, caused by the reduction at 550°C for 60min in 30% CO-N2 gas stream, was studied and the following results were obtained.(1) Samples studied contain only granular type hematite, reported as harmless, and yet cracks developed severely through minerals surrounding hematite crystals in some samples especially of the quarternary systems during the reduction, resulting in severe disintegration.(2) The extent of crack development changes depending on the toughness of minerals surrounding hematite crystals and the intercrystal spacings of hematite as well.(3) In the case of samples with low basicity, the toughness of glassy silicate decreases when Al2O3 is added into it or when it crystallizes during the cooling. The intercrystal spacing increases as a result of the increase in size and decrease in number of hematite crystals. In samples being satisfied with both conditions, cracks develop severely resulting in the large disintegration.(4) In the case of samples with high basicity, the intercrystal spacing is so large that the extent of crack development depends mostly on the toughness of minerals surrounding hematite crystals. As the minerals in question increase in size, the toughness decreases resulting in the severe crack development. Al2O3 helps increase amounts of calcium ferrite which increases in size at 1 350°C and over.

Transmitted and Reflected Visible Light Microscopy of Two Bituminous Fly Ashes

Abstract: Fly ashes of high magnetic content taken from two midwestern power plants were examined to determine the mineralogy of the magnetic and nonmagnetic fractions. Fly ash spheres from the magnetic fraction are predominantly composed of ferrite spinel, hematite and silicate glass. The hematite appears to be a replacement product of the original ferrite spinel. Nonmagnetic phases include mullite, lime, small amounts of hematite and silicate glass. Quartz morphology indicates that it did not fuse in the furnace. Mullite and lime have morphologies indicative of crystallization in the furnace. Hematite is bonded to the nonmagnetic particles or as a complete replacement of ferrite spinel spheres.