Showing papers on "Hematite published in 1988"

Estimation of forms of Fe and Al: A review, and analysis of contrasting soils by dissolution and Moessbauer methods.

Abstract: Three methods have been used to extract iron (Fe) and aluminium (Al) from a wide range of soils. The three extractants were pyrophosphate reagent (p), acid-oxalate reagent (o), and dithionite-citrate reagent (d), and each reagent is thought to extract different forms of Fe and Al. The forms of Fe in the soils were studied before and after extraction using Moessbauer spectroscopy. Pyrophosphate-extractable Fe (Fep) does not specifically relate to any particular form of Fe in soils and it should not be used to estimate Fe in humus complexes. In some soils (e.g., Spodosol, Ochrept) Fep arises from Fe in ferrihydrite. In other soils (Ultisol, Hydrandept) Fep arises from goethite and ferrihydrite which is dispersed by pyrophosphate. Alp corresponds to Al in humus complexes and, in most soils, can be used to estimate Al in such complexes. Al and Si in allophane and imogolite were extracted in acid-oxalate reagent, and could be estimated from Alo-Alp and Sio, provided large amounts of ferrihydrite were not present. Ferrihydrite was also extracted in acid-oxalate reagent and, assuming complete extraction, could be estimated by multiplying Feo by the factor 1.7. Dithionite-citrate reagent dissolved goethite and hematite in addition to ferrihydrite. In some soils repeated extractions were required to dissoive most of the goethite and hematite. Quantitative estimates of Fe in goethite and hematite can be made from Fed -Feo. Other techniques, however, such as X-ray diffraction and Moessbauer spectroscopy, were necessary to identify iron oxides positively in most soils.

Influence of ph and organic substance on the adsorption of as(v) on geologic materials

Abstract: The adsorption of As(V) on alumina, hematite, kaolin and quartz has been measured as a function of pH (2 to 10), and As concentrations (10−4 to 10 −8 M; in the alumina and kaolin systems only). The effects of sulfate (0 to 80 mg L−1) and fulvic acid (0 to 25 mg L−1) were studied. The charge of the solid surface and the As speciation in solution (determined by pH) were the most important chemical parameters affecting the sorption behavior. At pH below PZC of the solid, there was a qualitative correlation between the adsorption and the anion exchange capacity of the solid. For hematite at low pH (below 5) there was a reduction of the adsorption possibly related to the formation of positively charged species. The presence of sulfate or fulvic acid reduced the adsorption.

Transformation of Trace Element-Substituted Maghemite to Hematite

Abstract: X-ray powder diffraction (XRD), transmission electron microscopy, infrared spectroscopy, differential thermal analysis, and surface area (BET) measurements were employed to investigate the transformation of microcrystalline maghemite to hematite. At 500°C pure maghemite was completely altered to hematite in 3 hr, whereas maghemites containing small amounts (

Saturation magnetisation, coercivity and lattice parameter changes in the system Fe3O4-γFe2O3, and their relationship to structure

Abstract: This study has characterised the oxidation products of a fine-grained single domain magnetite which was made synthetically by a colloidal method. Changes in the intrinsic magnetic properties (saturation magnetisation, saturation remanence, and coercive force) during progressive oxidation are correlated with lattice parameter changes and an oxidation mechanism. It is proposed that magnetite oxidises to hematite via at least two metastable maghemites. The first of these, formed on low temperature oxidation by the formation of a magnetite/maghemite solid solution, is a face centered maghemite with lattice parameter a= 8.3419±0.0006 A. A second maghemite, produced on oxidation at higher temperatures, has a primitive cubic structure and a lattice parameter a = 8.3505±0.0005 A. Maghemite cation distributions are derived to explain the reduced saturation magnetisations of between 56 and 74 Am2 kg-1 observed, and a maghemite structure containing an increase in tetrahedral Fe3+ ions and up to 3 octahedral vacancies per 32 oxygen unit cell is proposed.

Volcanogenic iron oxide deposits, Cerro de Mercado and vicinity, Durango

Abstract: The iron deposits of the Durango City, Mexico, area were formed by subaerial volcanic processes during a hiatus between two major eruptive cycles emanating from the 30-m.y.-old Chupaderos caldera. The first major eruption of the Chupaderos caldera produced the hematitic rhyolitic ash-flow tuffs of the Aguila Formation. During resurgent doming of the caldera floor, the Cacaria Formation filled the moat around the central dome of the Chupaderos caldera. The lower Cacaria, the Leona Member, consists of extensive rhyolite flow domes, flows, and volcanoclastic tuffs. The various facies of the Mercado Iron Member were deposited on the surface of the Leona Member as well as on the resurgently domed Aquila Formation. A minor amount of quartz latite extrusive activity was concurrent with the eruption of the Mercado Iron Member. Both units preceded the eruption of a second major welded tuff, the Santuario Formation, which incorporated fragments of iron oxides in its base.The Mercado Iron Member of the Cacaria Formation consists of seven facies variations. The Cerro de Mereado iron deposit consists of four facies: (1) a martitc facies--massive to layered coarsely crystalline porous martitc (hematite pseudomorphous after magnetite) at the base, with dike and pluglike extensions downward into the underlying rhyolite, (2) a sandy magnetite facies--unconsolidated, laminated fine-grained sandy magnetite above the martite, (3) a blocky facies--unlaminated sandy magnetite matrix, mixed with blocks of the overlying quartz latite flow, and (4) a mixed iron oxide facies--tabular and dikelike bodies of fine-grained magnetite-hematite intergrowths that cut and cap the system. A satellite deposit at Pena Morada consists of three facies: (5) a breccia facies--dense, fine-grained hematite at the base with included rhyolite porphyry fragments, (6) the layered facies--dense fine-grained hematite layers interlayered with laminated hematite powder, and (7) the powdery hematite facies--a very fine grained cyrstalline hematite powder at the top which is finely laminated. The dense layers of the middle facies (6) exhibit crackled, vesicular, and ropy textures on their upper surfaces. The powdery hematite facies (7) also forms an extensive blanket of laminated, hematite powder referred to as the peripheral deposits. These deposits discontinuously crop out over an area of 300 km 2 around the Cerro de Mercado and Pena Morada deposits, wherever the contact between the Leona Member and the Aguila Formation, and the younger units is exposed. The peripheral deposits vary from 1 m in thickness to little more than staining of the contact. Regionally they thin away from Pena Morada, while locally they are thinnest on palcotopographic slopes.Geologic relationships suggest that the iron deposits formed as a result of a variety of subaerial volcanic processes. The main deposit at Cerro de Mereado apparently resulted from the eruption of an iron magma rich in fluorine, chlorine, carbon dioxide, and water. Sheeted flows and flow breccias formed a volcanic dome above an intrusive feeder system. Iron oxides crystallized as magnetite, with abundant, clear, yellow-green apatite crystals forming concurrently in gas cavities and open breccias. Large volumes of halogen-rich gases streamed up through the iron oxide flows and oxidized the magnetite to hematite (martite) and redeposited the iron leached from the now-porous martite as laminated sandy magnetite in an extensive fumarolic blanket. During the later stages of the cooling process, a quartz latite dike intruded and flowed out over the deposit. Basal blocky flow breccias of the quartz latite mixed with and disrupted the finely laminated texture of much of the sandy magnetite, creating extensive quartz latite breccias with a sandy magnetite matrix. Late-stage hematite-magnetite dikes cut the entire system and fed flows which capped the mound. Lateral to Cerro de Mereado large volumes of iron-rich vapor explosively vented into the atmosphere and crystallized as fine-grained hematite dust which formed an ashlike blanket covering an area greater than 300 km 2 . Flow textures, interlayered with the ashlike hematite, at the base of the Pena Morada deposit, suggest actual flows or welded flows of this material. The occurrence of a maximum thickness of the ashlike hematite at Pena Morada indicates its proximity to a vent. At Cerro de Mercado the volatile-rich nature of the system resulted in extensive replacement of the underlying premineralization rhyolites by a mixture of magnetite and pyroxene. Postminer-alization tuffs overlying the iron ore contain iron oxide fragments at their bases with no alteration.An immiscible iron-rich volatile phase is believed to have evolved from the parent rhyolite magma by the introduction of CO 2 into the magma from carbonate wall rocks. This volatile-rich phase rose to the top of the magma chamber and up through the resurgent floor of the caldera. The fluid is believed to have boiled during its ascent, separating into vapor and liquid phases. At the magmatic temperatures envisioned, water would disassociate and the oxygen would combine with the iron in the liquid phase to form a volatile-rich iron oxide magma which was driven to the surface by a continuing stream of escaping gases. The hydrogen escaped in the vapor phase along with chlorine and fluorine, forming an intensely acid environment capable of carrying significant volumes of iron in the form of iron chloride vapors until reaching the atmosphere where the microcrystalline hematite powder was formed.Comparison of Cerro de Mercado with other apatite-bearing, low titanium iron deposits associated with silicic volcanic systems suggests that this volcanogenic model may be applicable to many of them. The volcanic environment produces a mixture of intrusive, replacement, and sedimentary textures which may explain the heated debates found in the literature over the origin of many of these deposits. These systems include the Kiruna deposits of Sweden, the central Missouri iron deposits, and the Olympic Dam deposit, all of Precambrian age; the Jurassic deposits of northeast Nevada; and the Tertiary deposits of Mexico and Chile.

Some rockmagnetic parameters for natural goethite, pyrrhotite and fine-grained hematite

Abstract: The increasing importance of sediments for paleomagnetic research prompted a study of rockmagnetic parameters of natural goethite, pyrrhotite and hematite Grain-size dependent behaviour of such parameters is poorly known for goethite and pyrrhotite as well as for fine-grained hematite Data of hematite indicate a complex rockmagnetic behaviour Existing data seem to be contradictory in many aspects The rockmagnetic parameters reported in the present thesis for the natural goethite and pyrrhotite samples comprise twelve grain-size fractions from 250 micrometer down to <5 micrometer For the natural hematite sample, which has a low-temperature origin, the <5 micrometer fraction was divided into six grain-size fractions down to <025 micrometer

Crystallization remanent magnetization during the transformation of maghemite to hematite

Abstract: We have studied crystallization remanent magnetization (CRM) in the γFe2O3 → αFe2O3 transformation, starting from synthetic, equidimensional maghemite crystals of single-domain size (median diameter of 245 A). Inversion of maghemite to hematite on heating was indicated by an exothermic peak in differential thermal analysis (DTA) at 555°C. In CRM experiments, samples of dispersed maghemite were heated in zero field to temperatures between 300°C and 604°C, held for 3 hours in a 50-μT field HCRM, and cooled in zero field to 20°C. The intensity of the resulting CRM MCRM increased with increasing conversion to hematite, reaching a maximum of 784 A m−1 (0.784 emu cm−3) after the 556°C run, then decreased to 1.3 A m−1 (0.0013 emu cm−3) after the 604°C run. Maximum CRM intensity and the most rapid inversion indicated by DTA both occurred around 555°C. MCRM partially inverted samples was always parallel to HCRM. Hematite cannot explain the high intensity and moderate coercivity of the CRM. The bulk of the CRM seems to be carried by maghemite and to be acquired viscously rather than by grain growth. In a second set of experiments, CRM was produced in the presence of a preexisting anhysteretic remanent magnetization (ARM) MARM perpendicular to HCRM. The resulting remanence MT was a composite of surviving anhysteretic remanent magnetization ARM plus CRM directed parallel to HCRM, rather than a single vector of intermediate direction. MT can be synthesized by orthogonally combining CRMs measured in the first set of experiments with ARMs heated in zero field. Phase coupling of maghemite and hematite is not necessary to explain the main features of CRM, with or without an m initial remanence.

Chemistry and mineralogy of Fe‐containing oxides and layer silicates in relation to plant available iron

Abstract: This paper is a review of the structures, surface properties and reactions of Fe‐containing minerals (either natural or applied materials) which influence the availability of Fe to plants. The implications of these reactions to soil testing and plant‐growth studies are also discussed. The predominant Fe‐containing minerals in calcareous soils are Fe oxides (principally goethite, hematite and ferrihydrite) and layer silicates. Dissolution of Fe‐containing minerals occurs as a surface reaction. Therefore, the rate of mobilization of Fe under Fe‐stress conditions is dependent on the particle size distribution, crystallinity and concentration of surface Fe hydroxyls of these minerals. It has been shown that availability of indigenous soil Fe to plants is related to the quantity of amorphous Fe oxides in soils. The long‐term success of solid‐phase Fe amendments to soils is dependent on the reactive surface areas (i.e., the concentration of active surface‐Fe sites), the continued exposure of new active...

Shear flocculation and carrier flotation of fine hematite

Abstract: Enhancement of particle size by particle aggregation is one of the methods that can be used to improve recovery during the flotation of fine particles. This paper describes the results of two such processes, shear flocculation and carrier flotation, for enhancing the flotation of fine hematite. The shear flocculation of fine hematite particles with sodium dodecyl sulfate (SDS) collector was investigated as a function of the agitation rate of the suspension. The flotation separation of hematite and quartz is enhanced by this technique. In a second aspect of this investigation, pre-conditioned coarse hematite particles were added to the ore suspension to act as carrier particles for the fine hematite. The combined pulp is agitated to promote the coating of the reagentized coarse hematite with the fine hematite. Carrier flotation not only increase the recovery and grade of the hematite concentrate, but also cut the pre-flotation conditioning period by two-thirds over the flocculation/flotation process where no carrier particles were present.

Structural characteristics of hematite and goethite and their relationships with kaolinite in a laterite from Cameroon : a TEM study

Abstract: TEM investigations on goethite and hematite associated with kaolinite in lateritic weathering profiles (Cameroon) have shown : (I) the variability of goethite habit, in contrast to hematite, according to sampling locality, (2) the occurrence of intermediate phases, primary or resulting from a topotactic transformation of hematite into goethite, (3) intergrowths of hematite and goethite, and (4) epitaxy of goethite upon kaolinite. These data are discussed in terms of growth and genetic relationships between minerals (mineral development and relative stability). Petrological implications are considered.

Magnetic Behavior of Natural Goethite During Thermal Demagnetization

Abstract: Goethite maximum blocking temperatures (60–105°C) are governed by the amount of isomorphous substitution in the goethite lattice. The goethite dehydration reaction (250–350°C) is attended with a concurrent remanence decrease in finely intergrown hematite due to a change of the original hematite structure promoted by the liberated water vapor. This remanence decrease leads to a bending point in the thermal decay curve, which could be easily mistaken for the presence of another magnetic constituent. The magnetic properties of the minerals formed through goethite dehydration (hematite and traces magnetite) strongly depend on their growth history.

Mixed Colloidal Dispersions of Silica and Hematite

Abstract: The colloidal behavior of mixed dispersions containing hematite (diameter=138.9 nm) and silica (diameter=7.0 nm) has been studied by measuring the stability ratio, zeta potential, and particle size. When various particle numbers of silica were added to a constant particle number of hematite, different colloidal behavior was observed, depending on the pH. In a pH region where the two solids were oppositely charged, hematite particles showed a dispersion-coagulation-redispersion process with increasing particle number of silica, resulting in a coating layer of silica particles on the hematite surface. On the other hand, in a region where both solids were negative, the addition of silica enhanced the stability of the hematite.

The mechanism of ferrite formation from iron sulfides during zinc roasting

Abstract: The formation of zinc ferrite (ZnFe2O4) during the fluidized-bed roasting of zinc concentrates presents subsequent processing difficulties both for zinc recovery and for iron separation and disposal. A major source of iron in these concentrates is from the iron sulfides — pyrite and pyrrhotite. This study examined the changes undergone by these iron minerals when roasted together with sphalerite at 1223 K in a fluidizing gas mixture of 3 pct oxygen and 97 pct nitrogen. Optical microscopy and electron microprobe analysis were employed to identify the three stages that lead to ferrite formation and to examine the processes that occur within each stage. The first stage is oxidation of the sulfides to highly vesicular, amorphous magnetite particles containing small amounts of zinc. The second stage involves both densification of these particles by sintering and counterdiffusion of iron and zinc cations to form a continuous phase of homogeneous zinc-rich spinel and a precipitate of hematite. In the third stage, continuation of cation diffusion and increasingPo 2 results in the formation of stoichiometric zinc ferrite. These observations have been interpreted by reference to the established phase relationships that occur in the Zn-Fe-O system, and a detailed, solid state reaction mechanism for the formation of zinc ferrite has been proposed.

Electroconductive iron oxide particles

Abstract: Disclosed herein are electroconductive iron oxide particles having a volume resistivity less than 5×10 6 Ω-cm, and comprising iron oxide particles selected from hematite and maghemite, and SnO 2 particles containing Sb as a solid solution deposited on the surfaces of the iron oxide particles, and a process for producing the same.

Influence of the Original Structure on the Kinetics of Hydrogen Reduction of Hematite Compacts

Abstract: Green compacts of hematite (54% porosity) were sintered in air at 1000°C for 30min and at 1300°C for 180min to produce porous (35% porosity) and dense (8% porosity) compacts, respectively. Hematite compacts were isothermally reduced with hydrogen at 500-1100°C using weight-loss technique as a function of time. The structural changes accompanying sintering and reduction processes were examined with mercury pressure porosimeter and optical and scanning electron microscopes. The reduction of porous and dense samples reveals the presence of reduction rate minimum at 650°C which was attributed to sintering and densification of the freshly reduced iron around the oxide grains. The retardation in the rate at 650°C was increased with the increase in the reduction extents and with decrease in the porosity of the compacts. The computed values of activation energy were correlated with the corresponding reduction mechanisms.

Quantitative Determination of Hematite and Goethite in Lateritic Bauxites By Thermodifferential X-Ray Powder Diffraction

Abstract: An X-ray thermodifferential powder diffraction method for the quantitative determination of goethite and hematite in lateritic bauxites has been developed and consists of measuring the integrated intensities of the 012 line of hematite before and after heating the sample at 900°C and of correcting the obtained values by the X-ray mass absorption coefficient of either the untreated or heated matrix. From the corrected line intensities and the chemical analyses, the amounts of iron to be allocated to goethite and hematite in the untreated samples can be estimated. The actual content of goethite and hematite in a sample is calculated by taking into account the degree of Al substitution in each of these minerals. The method was tested on artificial mixtures of goethite and hematite and subsequently used to analyze 98 auger drill samples from lateritic bauxites of Guinea Bissau. The estimated precision of the determination of goethite and hematite content was ±2% (absolute). The method can not be applied to samples containing < 10% Fe2O3 (on a whole weight basis) unless preconcentration is carried out.

Mössbauer spectroscopic and X-ray diffraction study of the thermal decomposition of Fe(CH3COO)2 and FeOH(CH3COO)2

Abstract: Thermal decomposition of iron(II) acetate, Fe(CH3COO)2, and iron(III) acetate hydroxide, FeOH(CH3COO)2, has been studied using57Fe Mossbauer spectroscopy and X-ray diffraction. Samples were thermally treated in air atmosphere between 150°C and 1000°C. The formation of maghemite 'γ-Fe2O3, and hematite, α-Fe2O3, is discussed. Hematite appears as the final decomposition product.