Showing papers on "Hematite published in 1990"

Origin of rare earth element-enriched hematite breccias at the Olympic Dam Cu-U-Au-Ag deposit, Roxby Downs, South Australia

Abstract: Hematite breccias, the host rocks to Cu-U-Au-Ag ore at the Olympic Dam deposit, occur as steeply dipping, northwest-striking, dikelike bodies within fractured granite. The breccia complex has a strike length of over 5 km and extends to depths greater than 1 km. Both the deposit as a whole and the individual breccia bodies are zoned from weakly brecciated, sericitized and hematized granite on the margins, through heterolithic breccias, to hematite-quartz microbreccia at the center. Hematite occurs as euhedral laths and fine-grained aggregates in the matrices of all breccia types, in veins and fragments, as pseudomorphs after feldspar, and rarely as pseudomorphs after siderite and magnetite. Relict magnetite is rare, and most iron oxide was deposited as hematite. Heterolithic breccias include fragments of earlier breccia and veins. Minor sedimentary rocks, including bedded hematite and laminated barite, also are present as clast components. In the upper portion of the deposit, volcaniclastic conglomerate and siltstone occur in a fault-bounded block approximately 500 X 500 m in plan view.Copper-iron sulfides occur as interstitial grains (most common), microveinlets, and rare clasts. They are most abundant in heterolithic breccias and are zoned laterally from chalcocitebornitc to chalcopyrite-pyrite. Zonal boundaries are subparallel to steep breccia contacts. Copper-iron sulfides commonly are intergrown with fluorite and locally rim corroded quartz grains. The breccias are highly enriched in light (LREE) and heavy rare earth elements (HREE) (avg = approximately 5,000 ppm REE in hematite-rich rocks). Total LREE content and La/Lu ratios are correlated with hematite abundance; maximum La values (10,000 X chondrite) occur in hematite-quartz microbreccias from the geographic center of the deposit. Five hydrothermal REE phases have been identified: bastnaesite, florencite, monazite, xenotime, and britholite(?). They occur in all breccia types and in a variety of habits: disseminated in quartz-sericite matrices of granite breccia, in quartz-sericite veins, interstitial to hematite laths, intergrown with barite in laminated sedimentary rocks, and as inclusions in hematite and in sulfide grains.The hematite breccias formed by progressive hydrothermal brecciation and iron metasomatism of the granite host. Minor sedimentary components preserved as blocks and fragments within the upper portion of the deposit point to a near-surface environment for the breccia complex. Textural relations indicate that most of the copper was introduced late relative to hematization and brecciation. Based on spatial relations between barren zones, oxidized sulfide grains, and high-grade chalcocite ore, some supergene redistribution of copper probably occurred. The abundance of REE in hydrothermal phases, REE enrichment of altered relative to unaltered wall rock, concentration of REE in the center of the system, and variable slopes of chondrite-normalized patterns, combined with the lack of evidence of intrusion of unusual magmas at the present level of exposure, attest to extensive transport and deposition of REE by hydrothermal fluids responsible for hematization and breccia formation. This study demonstrates hydrothermal REE mobility on a scale previously undocumented and suggests that REE may be particularly mobile in F-rich hydrothermal systems.

Chemical aspects of iron oxide coagulation in water: laboratory studies and implications for natural systems.

Abstract: Initial coagulation rates of colloidal hematite (α-Fe2O3) particles (diameter less than 0.1 µm) were measured experimentally in well-defined laboratory systems at constant temperature. The relative stability ratio,W, was obtained at various ionic strengths in NaCl medium and at pH values in the range from 3 to 12. ExperimentalW values ranged from 1 to 104 in various systems. The results delineate the roles ofspecific andgeneralized coagulation mechanisms for iron oxides. Among the specifically-interacting species (ΔG ads 0 >ΔG coul 0 ) studied were phosphate, monomeric organic acids of various structures, and polymeric organic acids. The critical coagulation-restabilization concentrations of specifically-interacting anions (from 10−7 to 10−4 molar) can be compared with the general effects of non-specific electrolyte coagulants (10−3 to 10−1 molar). The laboratory results are interpreted with the help of a Surface Complex Formation/Diffuse Layer Model (SCF/DLM) which describes variations of interfacial charge and potential resulting from variations of coagulating species in solution. Comparison of these laboratory experiments with observations on iron behavior in estuarine and lake waters aids in understanding iron removal mechanisms and coagulation time scales in natural systems.

Observational evidence of crystalline iron oxides on Mars

Abstract: A series of new spectral observations of Mars was obtained at Mauna Kea Observatory in the 04-10-micron wavelength range during the extremely favorable 1988 opposition, which yielded new spectral reflectance and relative reflectance data for a number of distinct spots on the Martian surface at 500-600 km spatial resolution The new spectra revealed absorptions at 062-072 micron and at 081-094 micron, both seen clearly for the first time These absorption features are interpreted as Fe(3+) electronic transition bands that indicate the presence of crystalline ferric oxide or hydroxide minerals on the Martian surface

Some aspects concerning the characterization of iron oxides and hydroxides in soils and clays

Abstract: A review of the systematic Mossbauer studies on the most encountered iron oxides and hydroxides is given in which the qualitative and quantitative aspects, helpful in the characterization of natural smaples, are emphasized. The present possibilities of Mossbauer spectroscopy in soil characterization are further illustrated from some examples of natural soils.

Preparation and magnetic properties of uniform hematite platelets

Abstract: Plate-like hematite particles of narrow size distribution were prepared by aging basic ferric salt solutions in the presence of EDTA and KNO3 at elevated temperatures. Under the influence of the geomagnetic field these particles agglomerate flat on the cell surface. The orientation can be rapidly changed to an upright position by employing a perpendicular external magnetic field of moderate strength. The latter effect can be used to estimate the magnetization of the particles.

Magnetic properties of natural goethite.III, Magnetic behaviour and properties of minerals originating from goethite dehydration during thermal demagnetization

Abstract: SUMMARY The magnetic behaviour during stepwise demagnetization of a set of artificial samples, containing well-defined natural goethite from five localities (chemical composition, crystallite size and grain size are known) is reported. Differential Scanning Calorimetry measurements indicated that the goethites converted to haematite between 260" and 360°C. In this temperature range a small decrease in initial susceptibility and an additional remanence decay leading toward an extra bending point in the thermal decay curve are observed. Because titanomagnetite or pyrrhotite are absent in the samples, this extra bending point is tentatively interpreted as being due to recrystallization of the minor haematite already present in the original goethite concentrates, facilitated by water made available through the goethite/haematite conversion. At much higher temperatures, at some 600 "C, a self-reversal of the remanence is observed in most specimens. The haematite derived from the goethites between 260" and 360 "C appeared to be prone to further alteration at successive higher demagnetization temperatures. Chemical alterations in the magnetic mineral suite, starting at temperatures from 380 "C upward-depending on the grain size of the original goethite-were inferred from an increase in initial susceptibility. The susceptibility behaviour showed an appreciable variation, due to the creation of varying trace amounts of magnetite, which appeared to be already present after the 400°C step. After the final 685°C demagnetization step the magnetic mineralogy was usually dominated by magnetite. The observed variation is thought to be the result of local within-specimen differences in availability of the amount and possibly also the composition of the vapour phase. The vapour phase is due to chemical reactions between the matrix constituents: waterglass and calcite, leading to a C02/H20 vapour phase. Reducing capacity, necessary for the magnetite formation, is envisaged to be created by decomposition of trace amounts of organic matter at relatively low temperatures, up to some 400°C. At higher temperatures (over 550°C) decomposition of traces of ferrous iron bearing clayminerals presumably donates the ferrous iron. Differences in the magnetite/haematite ratio as well as in the properties of both minerals were assessed in some detail with more or less routine rockmagnetic methods: acquisition of the isothermal remanent magnetization at room temperature (in fields up to 11.2MAm-' or 14T), AF demagnetization and low-temperature cycling of the saturation remanence. This was done after the 400 "C step to study the magnetic properties of the newly formed haematite and after the 685°C step to evaluate the properties of the magnetite and more evolved haematite. After the 400°C step the haematite appeared to be very fine-crystalline and magnetically extremely hard. Its saturation remanence is considerably lower than that of well-crystalline haematite. After the 685 "C step its hardness has decreased and its saturation remanence slightly increased. Chemical differences between the

Origins of Marslike spectral and magnetic properties of a Hawaiian palagonitic soil

Abstract: Results are presented on spectral, Moessbauer, static magnetic, petrographic, and compositional data for a Hawaiian palagonitic soil from Mauna Kea (HWMK1). It was found that reflectivity spectra of size separates smaller than 20 microns resemble spectra for Martian bright regions, while spectra of larger size separates show characteristics in common with spectra for Martian dark regions. Data on the HWMK1 soil are consistent with the partitioning of iron among the following minerals: olivine, titanomagnetite, hematite, and a superparamagnetic colored ferric oxide. These mineralogies are heterogeneously distributed within the soil with respect to both particle type and soil-particle diameter. The strongly magnetic titanomagnetite is associated with black particles and is responsible for the magnetic nature of the soil, while the colored weakly magnetic superparamagnetic ferric oxide and minor hematite are associated with orange particles.

Matrix effects for reflectivity spectra of dispersed nanophase (superparamagnetic) hematite with application to Martian spectral data

Abstract: The effect of the matrix on reflectivity spectra of nanophase (superparamagnetic) hematite (np-Hm) dispersed within it was studied over the 350–2200-nm wavelength range. Np-Hm is characterized by particle diameters less than ∼10 nm and has properties distinctly different from larger diameter hematite particles (bulk-Hm). Data were obtained for four series of powder samples which have different matrix properties and variable concentrations of np-Hm dispersed within discrete powder particles. The matrix materials are two size ranges of each of two compositions which have different internal scattering characteristics. Reflectivity data show that matrix properties have a large influence on the reflectivity spectra of pigmentary np-Hm. Samples with the same Fe2O3, content can have np-Hm absorption edges characterized by very different positions (color) and consequently, very different slope (R600/R450) and curvature ((R600)×(R450)/(R550)2) indices, where, for example, R600 is the reflectivity at 600 nm. Conversely, samples with equivalent absorption edges can have very different Fe2O3 concentrations. For all samples, ranges of slope and curvature indices are 0.99–8.55 and 0.50–2.38, respectively. Although it may be possible to place limits, quantitative relationships between positions of ferric absorption edges and Fe2O3 concentrations are unreliable without knowledge of matrix properties of the system under consideration. As an example, the curvature index is a measure of relative crystallinity (relative proportion of bulk-Hm) for a series of samples containing relatively increasing proportions of bulk-Hm over np-Hm only at constant Fe2O3 concentration and constant matrix effects. In the absence of the stated conditions, variations in the curvature index can result from any combination of crystallinity, matrix effects, and pigment (hematite) concentration. It is possible to match the Fe2O3 concentration, magnetic properties, and spectral data for Martian surface material with a laboratory mixture whose only ferric-bearing phase is hematite. What made this possible is a matrix material with suitable optical scattering properties and differences in optical and magnetic properties of np-Hm and bulk-Hm. Most of the hematite is present as strongly magnetic np-Hm, but some bulk-Hm is required to give a shallow ∼860-nm band. Hematite is thus very likely present on Mars and may even be the dominant ferric mineralogy.

Hematite nanospheres of possible colloidal origin from a precambrian banded iron formation.

Abstract: Exceptionally small spheres (nanospheres) of hematite (diameters between 120 and 200 nanometers) occur in the Marra Mamba Iron Formation of the Hamersley Basin, Australia. The nanospheres are clustered into small aggregates and may have formed by structural ordering and dehydration of colloidal iron hydroxide particles. Individual spheres consist of numerous thin, curved hematite platelets surrounding a central void that is approximately half the diamter of the sphere; this texture suggests that they formed by a volume reduction of the original colloidal particles by ∼12.5%. The occurrence of hematite nanospheres supports the hypothesis that some ofthe iron was deposited colloidally during the development ofbanded iron formations, approximately 2.5 billion years ago.

Infrared vibrations of hematite formed from aqueous- and dry-thermal incubation of Si-containing ferrihydrite

Abstract: Ferrihydrite samples having Si/Fe molar ratios ranging from 0 to 1 were synthesized by the reaction of Fe2(SO4)3 and Na2SiO3 with NaOH to an equilibrium pH of 8.2. Hematite formed by incubating the ferrihydrite (Si/Fe molar ratios -0.10 did not transform to hematite following the aqueous-thermal treatment. The ferrihydrite samples were thermally treated for 2 hr at consecutive 100*C intervals from 100 ~ to 8000C. The Si-free ferrihydrite transformed at 300"C to poorly crystalline hematite. Transmission electron microscopic analyses indicated that the hematite consisted of aggregates of spheroidal particles of 20-80 /~ cross sections. Broad IR bands were observed at 529 and 452 cm-l; however, after heating the sample to 800~ the particle cross sections increased to about 150--600 A, and additional IR bands were present at 378 and 325 cm -~. The differences in the IR patterns of hematite formed from ferrihydrite at 300 ~ and 800~ were probably due to increases in particle size and aggregation and improved crystallinity of the hematite particles following the higher temperature treatment. The hematite formed by the thermal transformation of the ferrihydrite having a 0.01 Si/Fe molar ratio was also spheroidal, and IR vibrations were present at about 528 and 443 cm -~. An increase in the temperature of the thermal treatment, however, did not result in additional IR bands. Differences in the IR vibrations of hematite formed during aqueous- and dry-thermal treatments of the ferrihydrite samples were probably due to differences in the particle size and morphology of the product. The Si content, due to its effect on particle size of the precursor and the prevention of sintering and particle growth of hematite, influenced the IR pattern of the product. Particle morphology and IR spectroscopy may therefore be useful indicators of the precursor of hematite and the conditions of hematite formation in soil.

Converting jarosite residues into compact hematite products

Abstract: Sodium jarosite is readily converted into hematite by hydrothermal reaction at temperatures greater than 220°C. Although the initial acid and ferric ion concentrations must be kept low to avoid the unwanted formation of Fe(SO4)(OH), the conversion reaction is unaffected by modest concentrations of ZnSO4, FeSO4 or Na2SO4. Hematite seeding is desirable to promote the reaction and to stabilize the reaction system. The hematite conversion product will likely contain ∼0.5% Zn and ∼2% SO4; most of the arsenic in the jarosite will remain with the hematite.

Scanning and transmission electron microscope observations of magnetite and other iron phases in Ordovician carbonates from east Tennessee

Abstract: Previous paleomagnetic observations for the carbonates of the Lower Ordovician Knox Group have indicated that ancient magnetizations in these rocks are of the same age as the late Paleozoic Alleghenian Orogeny. Rock magnetic properties strongly suggest magnetite as the carrier of the magnetization, but the textural and crystalline characteristics, sizes, morphologies, and mineral associations of these magnetites are poorly known. We have examined magnetic extracts and iron oxides in thin sections with scanning (SEM) and scanning/transmission (STEM) electron microscope techniques to determine whether the observed iron-oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers. Several different forms of magnetite in limestones and dolomites, which in places are host to Mississippi-Valley type deposits, are documented by imaging and energy-dispersive analysis using SEM and STEM, by X ray diffraction and electron diffraction patterns using STEM. The magnetite is either spherical with a dimpled surface or nonspherical in the form of void-filling single grains or grain aggregates. Most of the iron oxides have the composition of pure end-member magnetite, but occasional titanomagnetite and hematite, including rare zincian hematite, have been observed (only in limestone). Wherever found in thin section, nonspherical magnetites occur in association with secondary dolomite, potassium-feldspar, illite, and quartz. Some iron oxides have, in fact, inclusions of K-feldspar and quartz. Some of the magnetite (spherical and nonspherical) is polycrystalline; this implies that the larger observed grains may consist of single domains or pseudo-single domains. This provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single-domain (but interacting?) subgrains, on the basis of remanent coercivities and blocking temperatures. We interpret the pure end-member magnetite to be authigenic, having formed at approximately the same time as the K-feldspars, which in nearby areas have yielded late Paleozoic radioisotopic ages (278–322 Ma). The Knox carbonates therefore are inferred to carry a chemical remanent magnetization. Iron-rich clays or original iron-titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution-precipitation processes. These processes require rock-fluid interactions which are thought to be related to migrating connate brines during the Alleghenian Orogeny.

Phase transformations and stress relaxation in γ′-Fe4N1-x surface layers during oxidation

Abstract: Compound layers composed of γ′-Fe4N1-x on α-Fe substrates were oxidized in oxygen-containing gas atmospheres at 603 K. The microstructural changes were analyzed applying X-ray diffraction, dilatometry, and light, scanning electron, and transmission electron microscopy. Compositional changes were traced, in particular, with scanning Auger electron spectroscopy. Dual-phase, Fe3O4 (magnetite) and α-Fe2O3 (hematite), oxide layers formed at the compound layer surface. At the interface between the oxide and nitride layers, e-Fe2N1=x nucleated because of a local nitrogen enrichment caused by the conversion of γ′ nitride into oxide. The volume of each of the three phases formed (hematite, magnetite, and e nitride) increased with the square root of oxidation time, indicating solid-state diffusion-controlled layer growth. No evidence was obtained for the existence of γ′ or e oxynitrides. Within the γ′ layer, ferrite precipitated during oxidation as a consequence of an iron supersaturation of γ′ nitride due to a production temperature higher than the oxidation temperature. During this iron precipitation, stress relaxation occurred, as was concluded from X-ray diffractometric and dilatometric analyses. The stress relaxation was rate-controlled by nitrogen diffusion in γ′ nitride. The present findings were also used to explain microstructural changes during (commercial) oxidation of e-Fe2(N,C)1-x compound layers.

Production of hematite pigments in the form of platelets

Abstract: Iron oxides in the form of platelets are produced by hydrothermal treatment of aqueous suspensions containing iron(III) hydroxide or oxide hydrates in the presence of alkali metal hydroxides and of aluminum and/or silicon compounds in two stages. The normality of the alkali is from 0 2 to 2 N in the first stage and is increased by from 0.2 to 2.5 in the second stage. The two-stage process according to the invention makes it possible to produce hematite pigments which are in the form of platelets, have increased luster, larger particle diameter, improved dispersibility and color hues which can be adjusted as desired.

Transformation of akaganeite into goethite and hematite in alkaline media

Abstract: The conversion of akaganeite to goethite and/or hematite in alkaline media has been followed by X-ray powder diffraction and transmission electron microscopy (TEM). The rate of transformation fell and the amount of hematite in the product increased as the [OH−] decreased to < 1 M. Kinetic studies and TEM indicated that the transformation involved dissolution of akaganeite followed by reprecipitation of goethite and/or hematite. The rate-determining step was the dissolution of akaganeite.

The reduction of zinc from slags by an iron-carbon melt

Abstract: An experimental investigation was undertaken to study the mechanism of reduction of zinc from slags in the presence of a carbon-saturated iron melt. Batch tests were performed at 1400 °C, and the variation of the zinc and iron concentration in the slag during reduction was determined by sampling the slag at intervals during the test. In graphite crucibles, zinc in slags containing iron was reduced faster than zinc in iron-free slags, both when an iron bath was present and when it was absent. Zinc was reduced faster from slags containing iron when an iron bath was present than when an iron bath was absent. The dominant mechanism of reduction of zinc from slags containing iron appears to be the reaction of Zn2+ ions with Fe2+ ions to form zinc vapor and Fe3+ ions. When an iron bath is present, the Fe3+ ions are reduced back to Fe2+ predominantly by reaction with iron from the bath. Mass transfer of Fe3+ ions in the slag appears to be the rate-controlling step. Reduction of iron from slag by carbon occurred in parallel with the reduction of zinc, and whether there was a net increase or decrease of iron in the slag depended on the relative rates of production and consumption of iron. Lead and copper in the slag were reduced to low levels. The lead volatilized and the copper dissolved in the alloy.

The Intrinsic Equilibrium Constants of the Surface Hydroxyl Groups of Maghemite and Hematite

Abstract: The intrinsic equilibrium constants of the surface hydroxyl groups, which describe the ionization of surface hydroxyl groups and the complexation of ions of a KCl electrolyte, were calculated with the surface charge densities determined by the potentiometric titration, and with the surface site densities evaluated by the surface hydroxyl group densities. The values of the intrinsic ionization constants, Kalint and Ka2int of maghemite, are higher than those of hematite, and the differences of these values have a tendency for ΔpKalint<ΔpKa2int The values of the intrinsic complexation constants, *KK+int and *KCl-int, of maghemite are higher than those of hematite. These results could be attributed to the differences of the electron densities of the oxygen atoms of the surface hydroxyl groups, which are linked to those in the bulk those of maghemite are lower than those of hematite. Also, the differences become smaller with increasing number of bonding protons.

Method of reprocessing jarosite-containing residues

Abstract: For reprocessing of jarosite-containing residues to provide usable products, the jarosite-containing residues and concentrates which contain zinc sulfide are stirred in dilute sulfuric acid, which contains 40 to 100 g/l free sulfuric acid, in an autoclave under an oxygen partial pressure of at least 1,000 kPa and at a temperature from 130° to 170° C., the iron contents and the zinc contents of the residue and of the concentrate which contains zinc sulfide are substantially dissolved, flashing is effected and is succeeded by a separation of solution and solids, the solution is fed to the leaching circuit of a plant for the electrolytic production of zinc with precipitation of iron as hematite, the elemental sulfur is separated from the solids and the hematite residue is supplied to a further utilization.

EFFECT OF CYSTEINE AND MANGANESE ON THE CRYSTALLIZATION OF NONCRYSTALLINE IRON(III) HYDROXIDE AT pH 8

Abstract: To provide a greater understanding of the crystallization of iron oxides under natural aqueous conditions, the combined effect of an inorganic ion (in 2+) and a reducing organic ligand (L-cysteine) on the conversion of noncrystalline ferric hydroxide to goethite and/or hematite was investigated at pH 8. At cysteine: Fe ratios -> 0.2, L-cysteine caused noncrystalline iron(III) hydroxide to transform rapidly into goethite at pH 8; in the absence of the organic ligand, hematite was the predominant reaction product. The presence of Mn (>-9 mole %) in the cysteine-ferric hydroxide system retarded crystallization and reduced the goethite-promoting effect of cysteine. Polarographic measurements showed that the adsorption of cysteine on noncrystalline iron(III) hy- droxide was immediately followed by the oxidation ofcysteine to the disulfide with simultaneous reduction of a proportion of the interracial ferric ions. The partly reduced noncrystalline iron(III) hydroxide dissolved at pH 8 more rapidly than the original material, thus facilitating the formation of goethite. In Mn(II)- noncrystalline iron(III) hydroxide coprecipitates, the interfacial oxidation/reduction reaction with cysteine (and hence the partial reduction of the noncrystalline phase) was reduced, due to replacement of some interfacial Fe(III) by non-reducible Mn. At pH 8, uptake of Mn by Clystalline iron oxides was low (< 5 mole %). Mn precipitated preferentially as pure Mn phases, either rhodochrosite (in NaHCO3 buffer) or hausmannite (in NH4C1/NH3 buffer).