Showing papers on "Hematite published in 1993"

Quantitative mineralogical analysis using the Rietveld full-pattern fitting method

Abstract: Ansrnlcr Quantitative phase analysis of a number of multicomponent standard and natural mineral mixtures has been done using an adaptation of the Rietveld method. Binary mixtures (most 50:50 by weight) of corundum with quartz, hematite, ilmenite, magnetite, biotite, analcime, mordenite, or clinoptilolite were analyzed,using digital powder X-ray diffraction (XRD) data. In addition, a suite of standard mixtures of hematite and corundum, a natural feldspar mixture, the G-l standard granite, two natural bauxite samples, and a mixture of biogenic carbonate minerals were also analyzed. Quantitative information was extracted from refined individual scale factors and unit-cell voiumes (derived from refined unit-cell parameters), obtained with a Rietveld refinement program modified to analyze up to ten phases. The quantitative results for standard mixtures were within 2.50/o (absolute) of the true values, with the exception of the hematite, ilmenite, and magnetite mixtures. Results for the latter mixtures using CuKa data were severely afected by microabsorption, but analysis of the hematite mixtures using FeKa radiation gave results with absolute errors <2010. Results for the G-l granite and the natural feldspar agreed well with optically determined modes, and the method facilitated separation of the significant overlaps in the pattern of the carbonate minerals. Quantitative mineralogical analysis by the Rietveld method has several significant advantages over conventional methods of quantitative analysis. The method uses all intensity data in a pattern rather than a few of the most intense reflections, partially compensating for preferred orientation and extinction. In addition, standard dala are calculated for each phase during analysis, overcoming the troublesome requirement of obtaining standards representative of the materials in an unknown. It is also possible to gain a wealth of information from each sample in addition to amounts of phases. Because some of the most troublesome systematic errors, including sample displacement and zero-point shift, can be refined, the method yields unit-cell parameters of an accuracy comparable with that obtained when using an internal d-value standard. The method should find a wide application in geology, including in modal analysis and compositional determinations of individual mineral components using unit-cell parameter systematics.

Local structure of ferrihydrite and feroxyhite by exafs spectroscopy

Abstract: Synthetic 2-line and 6-line ferrihydrite and feroxyhite samples prepared from ferric salt solutions have been investigated by EXAFS spectroscopy. All these materials have been found to be short-range ordered, consisting of Fe octahedra linked by comers, edges, and faces. Their local structures are related to those of well-crystallized (oxyhydr)oxides, and the absence of hkl reflections in some samples is attributed to the small size of coherent scattering domains. The presence of face sharings indicates that these materials have structural similarities with hematite. Based on Fe-Fe distances and the analysis of the static disorder, it has been concluded that the local structure of feroxyhite is close to that of hematite, whereas ferrihydrite has common structural features with both hematite (αFe203) and cdβFeOOFI. The local structure of ferrihydrite thus differs from that of aqueous Fe polymers obtained by the partial hydrolysis of ferric nitrate and chloride solutions. Differences of local structures among hydrous Fe oxides and aqueous polymers have been interpreted on the basis of a room temperature stability phase diagram established for well-crystallized (oxyhydr)oxides.

Photoreduction of iron oxyhydroxides in the presence of important atmospheric organic compounds

Abstract: The photolytic reduction of amorphous iron hydroxide [am-Fe(OH)_3], lepidocrocite (γ-FeOOH), goethite (α-FeOOH), hematite (α-Fe_2O_3), and natural iron-containing aerosol particles in the presence of formaldehyde, formate, acetate, oxalate, and butyrate has been investigated. Important parameters in the photoreduction experiments are the pH, wavelength of the irradiating light, nature of the electron donor, characteristics of the iron phase. The present results show that the fastest rates of photoreduction of Fe(III) to Fe(II) are achieved with am-Fe(OH)_3 as the electron acceptor and formate as the electron donor. Maximum rates of photoreduction were observed at 330 nm with a continuous decrease to 405 nm. Natural iron-containing aerosol particles show photochemical behavior similar to am-Fe(OH)_3 and γ-FeOOH. These results suggest that a significant fraction of the reactive atmospheric iron in urban aerosol could be present as am-Fe- (OH)_3 and γ-FeOOH. Ambient iron-containing aerosol particles with oxalate as the electron donor resulted in a significant photochemical production of H_2O_2.

Ultrasmall Metal Oxide Particles: Preparation, Photophysical Characterization, and Photocatalytic Properties

Abstract: This paper gives an overview of the author's activities in the research of extremely small metal oxide particles in recent years. In particular, the synthesis of transparent colloidal solutions of extremely small zinc oxide, titanium dioxide, hematite, and titanium/iron mixed oxide particles (2 nm < d 20 nm) in water, ethanol, and 2-propanol is described. Quantum (Q)-size effects are observed during particle growth and at me final stages of synthesis. A simple molecular orbital (MO) picture is presented for the qualitative interpretation of these effects, while quantitative calculations have been carried out using a quantum mechanical model developed by Brus. The photophysical properties of the particles have also been investigated extensively. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band to lower-lying traps. Excess negative charge on the particles, resulting from either deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers, causes a blue-shift in the electronic absorption spectrum, which is explained by electrostatic and MO models. The zero point of charge (pHzpc) of the aqueous colloidal suspensions has been determined by several independent methods. While zinc oxide, titanium dioxide, and titanium/iron mixed oxide particles exhibit considerable photocatalytic activity (as illustrated for the reduction of molecular oxygen and the oxidation of various halogenated carboxyl acids), hematite particles are only found to oxidize S(IV) under bandgap illumination to a reasonable extent (ϕ < 0.3). A mechanism involving surface-bound molecules and free radical intermediates is presented to explain these differences in reactivity.

Adsorption of Cu, Pb, Zn, Co, Ni, and Ag on goethite and hematite; a control on metal mobilization from red beds into stratiform copper deposits

Abstract: A variety of evidence suggests that the fluids forming stratiform and red-bed copper deposits derived their Cu, Ag, and other metals from adjacent red sandstones and shales. Copper and Ag are soluble in moderately concentrated chloride brines at intermediate oxidation states, but published data suggest that Cu is strongly adsorbed on goethite and hematite at pH near 7 and temperatures of 0 degrees to 100 degrees C. Also, Pb, Zn, Co, and Ni are soluble in chloride brines, but are only present in some red-bed Cu deposits. To examine variability in relative adsorption of 0.5 mg/l Ag, Co, Cu, Ni, Pb, and Zn, adsorption on goethite (195 m 2 /l) has been measured as a function of pH (5-9), Cl (super -) (0, 1M, 4M,), temperature (25 degrees C, 50 degrees C), and redox state (air, Fe (super +2) -goethite). Under oxidized conditions at 25 degrees C and 1M Cl, the order of decreasing adsorption is Cu-Pb-Zn-Co-Ni-Ag, with Cu and Pb almost completely adsorbed at pH values greater than 6. However, in 1M Cl at 25 degrees C with Eh buffered by Fe (super +2) -goethite, the order of decreasing adsorption is Pb-Zn-Co-Ni-Cu(-Ag?), and less than 50 percent of the Cu is adsorbed at pH 7. An exploratory experiment with 3M NaCl + 1M Na 2 SO 4 at 25 degrees C with about 4,000 m 2 /g of hematite as adsorbent and 75 mg/l Cu, Ag, Zn, Pb, and Co and 40 mg/l Fe (super +2) was conducted to approximate conditions in a red-bed pore fluid during diagenesis. An appreciable fraction of the Cu and Ag remained in solution at pH 7, but Pb, Zn, and Co were completely adsorbed. Based on this data, the observed range of metal associations (Cu, Cu-Ag, Cu-Co, Cu-Zn-Ag, etc.) in red-bed and stratiform copper deposits seems explainable by variations among districts in pH, Eh, temperature, major element content of pore fluid, and Fe oxide character of the diagenetic environment. Similar adsorption phenomena may account for varying metal ratios in other low-temperature ore deposits and in noneconomic metal enrichments.

Transport of humic matter-coated hematite in packed beds

Abstract: Column filtration experiments, using quartz grains as the bed media, were conducted to study the deposition kinetics of hematite particles in the presence of three wellcharacterized humic substances. Two pH conditions, 5.2 and 7.4, were selected such that the humic matter was adsorbed to an originally positive and negative hematite surface, respectively. Background NaCl concentrations ranged from 0.015 to 0.1 M. The particle deposition rates, expressed in terms of initial experimental attachment efficiencies (α exp ), depended on the magnitude of both electrostatic and steric repulsive interaction energies

Thermally altered palagonitic tephra: A spectral and process analog to the soil and dust of Mars

Abstract: Six palagonitic soil samples (PH-1 through PH-6) which were collected at 30-cm intervals from a lava slab on Mauna Kea, Hawaii, are studied. The samples present an alteration sequence caused by heating during emplacement of molten lava over a preexisting tephra cone. Techniques employed include visible and near-IUR spectroscopy, Moessbauer spectroscopy, and magnetic analysis. The four samples closest to the slab (PH-1 through PH-4) were strongly altered in response to heating during its emplacement; their iron oxide mineralogy is dominated by nanophase ferric oxide. The sample adjacent to the slab (PH-1) has a factor of 3 less H2O and contains crystalline hematite and magnetite in addition to nanophase ferric oxide. It is argued that localized thermal alteration events may provide a volumetrically important mechanism for the palagonitization of basaltic glass and the production of crystalline ferric oxides on Mars.

Kinetics and mass transfer of pseudomorphic replacement: Application to replacement of parent minerals and kaolinite by Al, Fe, and Mn oxides during weathering

Abstract: Parent rock minerals are universally replaced by kaolinite and oxides of Al and Fe (and, occasionally, Mn) during lateritic weathering. The kaolinite is itself also replaced by the oxides. The replacement is pseudomorphic and globally involves enormous volumes of parent rock. For each parent mineral in a weathering profile, the replacement horizon is systematically overlain by one in which the parent mineral exhibits dissolution voids. This spatial association of textures forms through two coupled local reactions, a congruent dissolution above and a pseudomorphic replacement below. The dissolution of a grain is driven by H[sup +]s supplied from above, and it releases aqueous Al and/or Fe. In turn, this Al (or Fe) travels a short distance downward and drives the replacement of another grain by gibbsite (or hematite). Any silicate grain that becomes pseudomorphically replaced by gibbsite or hematite requires import of Al or Fe, unless the porosity of the replacing aggregate is very high. Combining the condition for pseudomorphic recplacement (that the replacing mineral grows about as fast as the replaced mineral dissolves) with reaction rate expressions leads to predicting windows in local water chemistry within which pseudomorphic replacement is possible but outside of which it is not. The modelmore » predicts replacement rates, and correctly accounts for the systematic occurrence of a horizon of partially replaced grains followed upward in the weathering profile by a horizon where grains exhibit dissolution voids but no more replacement. 41 refs., 5 figs., 1 tab.« less

The nanophase iron mineral(s) in Mars soil

Abstract: A series of surface-modified clays containing nanophase (np) iron oxide/oxyhydroxides of extremely small particle sizes, with total iron contents as high as found in Mars soil, were prepared by iron deposition on the clay surface from ferrous chloride solution. Comprehensive studies of the iron mineralogy in these "Mars-soil analogs" were conducted using chemical extractions, solubility analyses, pH and redox, x ray and electron diffractometry, electron microscopic imaging, specific surface area and particle size determinations, differential thermal analyses, magnetic properties characterization, spectral reflectance, and Viking biology simulation experiments. The clay matrix and the procedure used for synthesis produced nanophase iron oxides containing a certain proportion of divalent iron, which slowly converts to more stable, fully oxidized iron minerals. The clay acted as an effective matrix, both chemically and sterically, preventing the major part of the synthesized iron oxides from ripening, i.e., growing and developing larger crystals. The precipitated iron oxides appear as isodiametric or slightly elongated particles in the size range 1-10 nm, having large specific surface area. The noncrystalline nature of the iron compounds precipitated on the surface of the clay was verified by their complete extractability in oxalate. Lepidocrocite (gamma-FeOOH) was detected by selected area electron diffraction. It is formed from a double iron Fe(II)/Fe(III) hydroxy mineral such as "green rust," or ferrosic hydroxide. Magnetic measurements suggested that lepidocrocite converted to the more stable maghemite (gamma-Fe2O3) by mild heat treatment and then to nanophase hematite (alpha-Fe2O3) by extensive heat treatment. After mild heating, the iron-enriched clay became slightly magnetic, to the extent that it adheres to a hand-held magnet, as was observed with Mars soil. The chemical reactivity of the iron-enriched clays strongly resembles, and offers a plausible mechanism for, the somewhat puzzling observations of the Viking biology experiments. Their unique chemical reactivities are attributed to the combined catalytic effects of the iron oxide/oxyhydroxides and silicate phase surfaces. The reflectance spectrum of the clay-iron preparations in the visible range is generally similar to the reflectance curves of bright regions on Mars. This strengthens the evidence for the predominance of nanophase iron oxides/oxyhydroxides in Mars soil. The mode of formation of these nanophase iron oxides on Mars is still unknown. It is puzzling that despite the long period of time since aqueous weathering took place on Mars, they have not developed from their transitory stage to well-crystallized end-members. The possibility is suggested that these phases represent a continuously on-going, extremely slow weathering process.

Mineralogy of three slightly palagonitized basaltic tephra samples from the summit of Mauna Kea, Hawaii

Abstract: The paper characterizes the mineralogy and spectral properties of three slightly palagonitized basaltic tephra samples collected near the summit of Mauna Kea in order to contribute to the basis for inferring mineralogy and processes for Martian surface materials. The mineralogy of size fractions of these samples is examined by diffuse reflectance and FIR spectroscopy, optical microscopy, X-ray diffraction, Moessbauer spectroscopy, magnetic analysis, EMPA, TEM, and SEM. For the 20-1000 micron size fraction, sample HWMK11 (red) is essentially completely oxidized and has a hematite (Ti-hematite) pigment dispersed throughout the silicate matrix. Sample HWMK12 (black) has the lowest proportion of ferric-bearing phases and is thus least weathered. For HWMK11, the amount of hematite is essentially constant, and mica is present only in the coarse clay-sized fraction; smectites are low in structural Fe.

Effect of Raw Material Composition on the Mineral Phases in Lime-fluxed Iron Ore Sinter

Abstract: In this study small tablet specimens of powdered commercial iron ores mixed with fluxes and kaolin were heated in a reducing atmosphere and then cooled slowly in air to simulate industrial sintering. The effect of different ore and flux compositions on the phases found in the sinter bond was studied.An increase in basicity (CaO/SiO2) favoured the formation of calcium ferrite and densification of the bond at a low sintering temperature. The amount of calcium ferite decreased as the Al2O3 content decreased and the amount of reoxidized hematite produced on cooling decreased. A decrease in Al2O3 content promoted a densification of the bond.An increase in MgO content, introduced by addition of dolomite, decreased the amount of calcium ferrite slightly. When MgO was added in the form of serpentine the calcium ferrite content increased considerably. Increase in MgO markedly reduced the amount of reoxidized hematite but produced a less dense bond.Generally the same composition and sintering temperature produced similar mineral compositions in the bond independent of the iron ore used. The only exception was when an ore with coarse dense hematite was used, in which case the bond contained some unreacted hematite. The porosity of the bond did vary with the type of ore used. Limonitic ore produced a bond with closed pores and less reoxidized hematite at lower temperature and basicity than did hematite ores.

Light scattering by spherical particles with hematite and goethitelike optical properties: Effect of water impregnation

Abstract: The interpretation of reflectance spectra (through remote sensing or laboratory research) requires spectral reflectance models that can be inversed to determine the more fundamental physical properties of these surfaces. One of the main hypotheses in reflectance models is that a soil or a powder are generally considered as a collection of particles or aggregates with averaged optical properties. Verification of this hypothesis is of great interest. This paper deals with the study of the scattering and absorption of light by a single mineral particle where diameter ranges from 0.01 to 50 μm. The particles are assumed to be spherical, and their complex refractive index is assumed to be equal to that of hematite or goethite (ordinary indices). These two minerals are responsible for the nonlinear and wavelength-dependent behavior of lateritic soils reflectance spectra upon moistening. The absorption and scattering efficiencies, single scattering albedo, scattering diagram, and opening angle were computed for a single sphere. Different scenarios were considered: first, a sphere in the air, second, a sphere in water, and third, a sphere surrounded by a water layer of different thickness. All the computed spectra (400–700 nm) are classified into three diameter groups (0.01–0.1, 0.1–2, and 2–50 μm). The behavior of the calculated spectra (second and third scenarios) depends both on the particle diameter and the wavelength. A simplified Hapke's model was then used to simulate the reflectance spectra of powders of hematite and goethite. The main result is the existence of an optical elementary arrangement cell which is for the soil what a unit cell is for a crystal.

Adsorption of a hydrophobic bacterium onto hematite: Implications in the froth flotation of the mineral

Abstract: The present study reports on the relationship between adsorption ofMycobacterium phlei onto hematite and flotation of the mineral. From light and scanning electron microscopy, contact angle and electrophoretic mobility observations it was found thatM. phlei is more negatively charged than hematite, that it readily accumulates onto the mineral and that it functions as a flotation collector for the mineral with optimum flotation taking place at about pH 2.5.