Showing papers on "Hematite published in 1997"
TL;DR: In this paper, the laser power dependence of the spectra of oxides and oxyhydroxides was investigated by using Raman microscopy, and it was shown that increasing laser power causes the characteristic bands of hematite to show up in the spectrum of most of the compounds studied.
Abstract: Hematite (α-Fe2O3), magnetite (Fe3O4), wustite (FeO), maghemite (γ-Fe2O3), goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and δ-FeOOH were studied by Raman microscopy. Such compounds have already been studied by Raman spectroscopy, but there are some disagreements in the reported data. Here, Raman microscopy was employed to investigate the laser power dependence of the spectra of these oxides and oxyhydroxides. Low laser power was used for the reference spectra in order to minimize the risks of spectral changes due to sample degradation. The results obtained show that increasing laser power causes the characteristic bands of hematite to show up in the spectra of most of the compounds studied whereas the hematite spectrum undergoes band broadening and band shifts. © 1997 John Wiley & Sons, Ltd.
2,569 citations
TL;DR: In this article, anin situattenuated total reflection (ATR) and Fourier transform infrared (FTIR) method was used to measure sulfate adsorption as a function of aqueous sulfate concentration and pH.
378 citations
TL;DR: In this article, the influence of adsorbed humic acid, solution Ca2+ concentration, and adaption of trace metals (Cu2+, Pb2+) on the transport and deposition kinetics of colloidal hematite particles in a sandy soil matrix was investigated.
Abstract: Understanding colloid release, transport, and deposition in natural heterogeneous porous media is a prerequisite for evaluating the potential role of colloids in subsurface contaminant transport. In this study, we investigate the influence of adsorbed humic acid, solution Ca2+ concentration, and adsorbed trace metals (Cu2+, Pb2+) on the transport and deposition kinetics of colloidal hematite particles (α-Fe2O3; 122 nm diameter) in a sandy soil matrix. A short-pulse chromatographic technique was used to measure colloid deposition rate coefficients and collision efficiencies (α). At pH 5.7, pure hematite was positively charged and deposited rapidly (α ≈ 1) even at low electrolyte concentrations (10-4 M CaCl2). Adsorption of humic acid to the hematite caused reversal of surface charge from positive to negative. As a result, colloid deposition rates were decreased by approximately 2 orders of magnitude (α ≈ 0.01). Deposition rates of humic-coated hematite colloids strongly increased with increasing Ca2+ conce...
250 citations
TL;DR: In this paper, isolated nanometric particles (D < 30 nm) of γ-Fe2O3 in a silica matrix have been prepared by heating at 400 °C the gel formed in the hydrolysis of an ethanol solution of Fe(NO3)3·9H2O and tetraethylorthosilicate (TEOS).
Abstract: Isolated nanometric particles (D < 30 nm) of γ-Fe2O3 in a silica matrix have been prepared by heating at 400 °C the gel formed in the hydrolysis of an ethanol solution of Fe(NO3)3·9H2O and tetraethylorthosilicate (TEOS). However, when FeCl3·6H2O was used as precursor, well-developed hematite particles were obtained in the final composite. This different behavior was already manifest in the initial gels. Thus, the gel obtained from iron nitrate salt shows a compact appearance as a result of its higher degree of network connectivity (polymeric gel) whereas the one from the iron chloride appears more loose and highly hygroscopic (colloidal gel). In addition, small superparamagnetic nuclei are formed during the hydrolysis and condensation of the gel obtained from the iron nitrate salt. The γ-Fe2O3 nanoparticle formation takes place through a reduction−oxidation reaction which occurs during the burning of the organic species trapped inside the gel pore. The growth mechanism of the γ-Fe2O3 nanoparticles in the ...
186 citations
TL;DR: In this article, the chemical and mineralogical composition of Upper Cretaceous Apulian karst bauxites and their textural components, i.e. ooids and matrix, has been studied.
137 citations
TL;DR: In this article, the effect of goethite, hematite, and maghemite on aqueous photoreduction of divalent mercury with organic acids (oxalate, formate and acetate) is investigated.
113 citations
TL;DR: Wang et al. as mentioned in this paper used a silica gel crystal growth technique to produce pyrite under conditions approximating those commonly occurring in anoxic marine sediments, and found that the presence of marine organic matter (freeze-dried plankton) caused an increase in the sulfidization rate of goethite and a major decrease in the rate of Pyrite formation.
112 citations
TL;DR: In this paper, a redness index RI H calculated from Helmholtz chromatic co-ordinates ( lambda d, P e, Y) and correlated to hematite content is found.
Abstract: Data on mineralogy and diffuse reflectance spectra, obtained from 56 samples of Brazilian lateritic soils, were used to establish quantitative relationships between spectral parameters and iron oxides contents (hematite and goethite). A redness index RI H calculated from Helmholtz chromatic co-ordinates ( lambda d, P e, Y) and correlated to hematite content is found. A good estimate of the ratio Hematite/ (Hematite + Goethite) is given by the dominant wavelength lambda which takes into account a large proportion of the tint detected on the soil sample. Based on these laboratory results, radiometric indices were elaborated by combining only those parts of the visible spectrum that correspond to Thematic Mapper (TM) channels: (i) a hematite index I Hm to estimate hematite concentration; and (ii) a ferric index I Fe to estimate the ratio hematite/(hematite goethite). The hematite index, applied to a TM image of the area around Brasilia, allowed hematite contents in the surface horizons of lateritic ...
99 citations
TL;DR: In this paper, the effect of temperature, oxalate concentration and pH on hematite dissolution was studied under various experimental conditions, and it was found that the dissolution process is much faster in an inert atmosphere under visible light.
84 citations
TL;DR: In this article, the authors investigated the oxidation of α-Fe and ǫ-Fe2N1−z at 573 K and 673 K in O2 at 1 atm.
Abstract: The oxidation of α-Fe and ɛ-Fe2N1−z at 573 K and 673 K in O2 at 1 atm was investigated by thermogravimetrical analysis, X-ray diffraction, light-optical microscopy, scanning electron microscopy and electron probe X-ray microanalysis. Upon oxidation at 573 K and 673 K, on α-Fe initially α-Fe2O3 develops, whereas on ɛ-Fe2N1−z initially Fe3O4 develops. In an early stage of oxidation the oxidation rate of ɛ-Fe2N1−z appears to be much larger than of α-Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for ɛ-Fe2N1−z than for α-Fe due to the porous structure of ɛ-Fe2N1−z as prepared by gaseous nitriding of iron. The development of a magnetite layer in-between the hematite layer and the α-Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for α-Fe and ɛ-Fe2N1−z become about equal, indicating that on both substrates the oxide growth is controlled by the same rate limiting step which is attributed to short-circuit diffusion of iron cations. Oxidizing ɛ-Fe2N1−z increases the nitrogen concentration in the remaining ɛ-iron nitride, because the outward flux of iron cations, necessary for oxide growth, leads to an accumulation of nitrogen atoms left behind.
81 citations
01 Aug 1997
TL;DR: Extension of the model to describe phosphate adsorption in a mixed system, such as the kaolinite-hematite system, using the surface protonation-dissociation constant of hematite was qualitatively successful.
Abstract: The constant capacitance model was used to describe phosphate adsorption on hematite, kaolinite, and a kaolinite-hematite system (k-h). The model assumes a ligand exchange mechanism and considers the charge on both adsorbate and adsorbent. The model is shown to provide a quantitative description of phosphate adsorption on these, including the effect of varying pH values. The computer program Ma-Za 2, a program that fits equilibrium constants to experimental data using an optimization technique, was used to obtain optimal values for the anion surface complexation constants on hematite, kaolinite, and a kaolinite-hematite system, while the PC program Ma-Za 1 in Q-Basic language was used for the application of the constant capacitance model. The model represented adsorption of phosphate anions well over the entire pH range studied (3.8-9.0). The main advantage of the model is its ability to represent changes in anion adsorption occurring with changes in pH. Extension of the model to describe phosphate adsorption in a mixed system, such as the kaolinite-hematite system, using the surface protonation-dissociation constant of hematite was qualitatively successful. In mixed system the model reproduced the shape of the adsorption isotherms well over the pH range 3.8-9.0. However, phosphate adsorption was overestimated. The hematite and the kaolinite-hematite system were synthesized and identified by X-ray, NMR, and FT-IR spectroscopy.
TL;DR: The utility of bioprocessing in the beneficiation of the above minerals through bioflotation and bioflocculation is demonstrated.
TL;DR: In this paper, the precipitation of hematite from ferric chloride media was systematically investigated in a series of autoclave experiments and it was shown that the filterability of the fine precipitated hematites is greatly improved by the presence of hemetite seed.
TL;DR: A probable mechanism for the formation of polycrystalline particles has been suggested in order to explain the above trends in magnetic behavior.
TL;DR: In this paper, the positions of the ferric electronic transitions near 430, 500, 630, and 860 nm were measured near 141, 210, and 300 K for four red and well-crystalline powders of hematite used as commercial pigments, two samples of volcanic tephra from Mauna Kea volcano, three samples of palagonitic tephras from the same location that contain nanophase ferric oxide as their dominant pigment and two mixtures of the two types of pigmenting phases.
Abstract: Reflectivity spectra (visible and near IR) were measured near 141, 210, and 300 K for four red and well-crystalline powders of hematite (red hematite) used as commercial pigments, two samples of volcanic tephra from Mauna Kea volcano that contain red hematite as their dominant pigment, three samples of palagonitic tephra from the same location that contain nanophase ferric oxide as their dominant pigment, and two mixtures of the two types of pigmenting phases. Relative proportions of red hematite and nanophase ferric oxide were determined by Mossbauer spectroscopy. For samples containing red hematite as the dominant pigment, the positions of the ferric electronic transitions near 430, 500, 630, and 860 nm are essentially independent of temperature, but their widths decrease with decreasing temperature. This decrease results in a well-defined minimum for the band at 630 nm at low temperatures and in significant increases in reflectivity in spectral regions near 1050 and 600 nm. For example, the reflectivity ratios R 600 /R 530 and R 600 /R 860 both increase by a factor as large as ∼l.4 between 300 and 140 K. The spectral features from nanophase ferric oxide in samples of palagonitic tephra are nearly independent of temperature. Spectral data of Martian bright regions that are characterized by a shallow band minimum near 860 nm, a reflectivity maximum near 740 nm, a distinct bend near 600 nm, and a shallow absorption edge from ∼400 to 740 nm are attributed to the presence of nanophase ferric oxide plus subordinate amounts of red hematite. The 600-, 740-, and 860-nm features are associated with red hematite. Because the reflectivity of red hematite at 600 nm is strongly dependent on temperature and because this wavelength is in the red part of the visible spectrum, the color of the Martian surface may vary as a function of its temperature. A conservative upper limit for the red hematite content of the optical surface of Mars is 5%.
TL;DR: In this article, a field method was developed for the measurement of the hematite:goethite ratio in Tertiary ooidal ironstones, locally named channel iron deposits, from the Hamersley region of Western Australia using visible to near-infrared (400 to 1000 nm) refiectance spectrometry.
Abstract: Empirical relationships and a field method have been developed for the measurement of the hematite:goethite ratio in Tertiary ooidal ironstones, locally named channel iron deposits, from the Hamersley region of Western Australia using visible to near‐infrared (400 to 1000 nm) refiectance spectrometry. The hematite:goethite ratio is important in the characterisation of these iron deposits as Al, P, water and Si are deleterious components commonly associated with goethite. The channel iron deposits typically comprise iron oxy‐hydroxides with less than 1% Fe2+ (present in maghemite or kenomagnetite), less than 8% Al3+‐substitution and with a mean crystal dimension of approximately 20 nm. The natural variations in the hematite:goethite ratio of the channel iron deposits were modelled using laboratory mixtures of pure hematite and goethite. The resultant spectral mixing trends produced consistent relationships with the hematite:goethite ratio, especially for the wavelength of the 6A1?4T1 crystal field absorpti...
TL;DR: In this paper, the authors studied the complexation of hematite, goethite, hematites, gibbsite, and boehmite in pre-dried systems, and found that two major modes of HA bonding are operative, viz., cation bridges forming oxide-M-HA links and direct bonding of HA to coordination centers at the oxide surface.
Abstract: Complexation of HA (and FA) by goethite, hematite, gibbsite, and boehmite was studied in pre-dried systems. Hematite showed the highest fixation of HA at various oxide: HA ratios and at all pH ≥ 7.0. A gradual reduction in HA/FA fixation from 2.0 to 10.0 was observed for all minerals except gibbsite, which showed a very sharp decrease at pH > 7.0 and a maximum at pH 5.0. Exchangeable cations have a remarkably dissimilar influence on HA complexation by the four minerals. Thus, various cationic forms of gibbsite showed a drastic loss of HA fixation capacity compared with the original (pronated) surface, whereas in boehmite, the reverse behavior was observed. Most of the metal ion-substituted hematites showed excellent HA fixation, but goethites revealed a mixed trend. It is inferred that two major modes of HA bonding are operative in hematite and goethite, viz., cation bridges forming oxide-M-HA links and direct bonding of HA to coordination centers at the oxide surface; forces of such bonding are strongest in hematite. In boehmite, cation bridging is the major interactive mode, whereas in gibbsite, HA fixation occurs primarily to coordination centers at the surface.
TL;DR: An infrared study of the chemical changes of synthetic schwertmannite is made in this article, which normally is a coal oxidation byproduct formed by thiobacillus ferrooxidan bacteria.
Abstract: An infrared study is made of the chemical changes of synthetic schwertmannite, which normally is a coal oxidation by-product formed by thiobacillus ferrooxidan bacteria. Schwertmannite, a class of minerals, is a metastable compound that transforms to jarosite, hematite, and/or basic iron sulfate at relatively low temperatures, and is analogous in structure to basic iron sulfate, but with a -Fe-O– cage. A proposed formula for schwertmannite is Fe4O4(OH)2 • AN(2/e) * nH2O, where AN is the anion and e is its charge, where sulfate can be replaced with other anions, a possible catalyst. The facile conversion of schwertmannite to a black or yellow material might make a useful raw material for the pigment industry.
TL;DR: Ferrihydrite reduction, as determined by increasing cubic unit-cell edge lengths between 0.832 and 0.840 nm, increased with reductant concentration and heating time, providing excess reductants remained.
Abstract: Heating experiments were carried out with ferrihydrite, in the presence of organics, to gain more insight into the intermediate formation of a ferrimagnetic cubic phase often observed in soils as the result of fire. Without an organic reductant, no intermediate cubic phase was formed when ferrihydrite was transformed into hematite, regardless of whether the ferrihydrite DTA curve showed one or two exothermic peaks. Addition of glucose or charcoal as reductant caused the initial formation of a cubic magnetite and/or magnetite/maghemite phase in samples heated above 300°C in both air and N2 atmospheres. Ferrihydrite reduction, as determined by increasing cubic unit-cell edge lengths between 0.832 and 0.840 nm, increased with reductant concentration and heating time, providing excess reductant remained. Following consumption of the reductant, FeII may be partially or completely reoxidized, forming maghemite and then hematite. The occurrence of maghemite, but not of magnetite, in soils where forest fires have occurred suggests that sufficient oxygen was available, while temperatures remained elevated, to oxidize magnetite to maghemite.
TL;DR: In this paper, the effect of phosphate on the crystallization rate, nature, and morphology of iron oxides prepared from ferrihydrite in the laboratory was investigated using X-ray diffraction, scanning force microscopy, transmission electron microscopy (TEM), and color.
Abstract: We used X-ray diffraction (XRD), scanning force microscopy (SFM), transmission electron microscopy (TEM), and color to investigate the effect of phosphate on the crystallization rate, nature, and morphology of iron oxides prepared from ferrihydrite in the laboratory. Synthesis was performed at two temperatures (323 and 373 K) and two pH values (9 and 12) from ferric nitrate, for P/Fe atomic ratios ranging from 0 to 2.5%. The presence of phosphate retarded crystallization, tended to favor hematite over goethite, and markedly influenced the morphology of the goethite crystals formed at high pH. Application of SFM in the deflection mode was useful to investigate the morphology of the small goethite crystals, with careful attention paid to operating conditions; in particular, sharp silicon probes were found to produce fewer artifacts than coarser silicon nitride ones. At low P/Fe ratios (,0.2%), the goethite crystals were thin, elongated, multidomain laths; at high P/Fe ratios (.1.5%), star-shaped, twinned crystals were produced. All the theoretical shapes, derived from the assumption that star-shaped crystals result from the epitaxial growth of goethite on a hematite core, were observed by SFM and TEM. The presence of such hematite nuclei was supported by XRD, selected-area electron diffraction, color, and preferential dissolution of the samples in HCl, because it is known that hematite dissolves faster than goethite in acid. With increasing P/Fe ratio, the arms of the star-shaped crystals became shorter. This was likely due to the higher density of P-adsorbing pairs of singly coodinated OH groups on terminal {021} faces relative to prismatic {110} arm faces.
TL;DR: In this article, the separation of ultrafine ( < 10 μm) hematite from quartz using sodium oleate as a selective flocculant has been investigated.
15 Dec 1997
TL;DR: The site densities of the surface of different oxides and hydroxides have been calculated from crystallographic data and the values of site density are adduced.
Abstract: The site densities of the surface of different oxides and hydroxides have been calculated from crystallographic data. The following values for the common habits are recommended: 2.7 nm-2 (Corundum), 2.3 nm-2 (Boehmite and Gibbsite), 2.2 nm-2 (Hematite and Rutile), 2.1 nm-2 (Lepidocrokite), 1.9 nm-2 (Diaspore), and 1.7 nm-2 (Goethite). The probable deviations due to the variations in habits are about +/-0.2 nm-2, except Rutile (+/-0.4 nm-2). For the concrete faces the values of site density are adduced. Copyright 1997 Academic Press.
TL;DR: In this article, high-energy pulsed argon ions that are generated during the focus phase of a dense plasma focus (DPF) device are used for the first time to achieve a complete phase of magnetite from hematite thin films.
TL;DR: The authors reproduce the compositions of magnetite and ferrian ilmenite found in Pinatubo dacite by doing crystallization experiments at 780 degrees C, 2.2 kbar, and NNO + 1.7 (+ or -0.2) log units.
Abstract: The oxidation state of the 1991 Pinatubo dacite and many other oxidized silicic arc volcanic rocks has been overestimated. We reproduce the compositions of magnetite and ferrian ilmenite found in Pinatubo dacite by doing crystallization experiments at 780 degrees C, 2.2 kbar, and NNO + 1.7 (+ or -0.2) log units. The extra components in igneous ilmenite, notably geikielite, expand the solvus in a way that is not correctly accommodated in current formulations of the iron titanium oxide thermobarometer.
TL;DR: In this article, a comparative study on the kinetics and mechanism of mechanochemical activation of magnetite (Fe 3 0 4 ) and maghemite (γ-Fe 2 O 3 ) has been carried out.
Abstract: A comparative study on the kinetics and mechanism of mechanochemical activation (by high energy grinding) of magnetite (Fe 3 0 4 ) and maghemite (γ-Fe 2 O 3 ) has been carried out. It has been established that during the activation the crystal lattice is distorted and phase and structure transformations occur. The phase compositions of intermediate and final products are registered by Mossbauer spectroscopy and X-ray analysis. It was found that the end product of mechanochemical activation of Fe 3 O 4 and γ-Fe 2 O 3 is highly dispersed hematite. The kinetics of phase changes are determined for the two iron oxides which are characterized by different valency states and cation distributions in the spinel lattice. The different rates of phase transformations have been explained by a phonon mechanism of energy dissipation for maghemite and a mixed phonon-electron mechanism for magnetite.
27 May 1997
TL;DR: Mossbauer spectroscopy was applied to characterize the valence states Fe(II) and Fe(III) in sedimentary minerals from a core of the Peru Basin this article.
Abstract: Mossbauer spectroscopy was applied to characterize the valence states Fe(II) and Fe(III) in sedimentary minerals from a core of the Peru Basin. The procedure in unraveling this information includes temperature-dependent measurements from 275 K to very low temperature (300 mK) in zero–field and also at 4.2 K in an applied field (up to 6.2 T) and by mathematical procedures (least-squares fits and spectral simulations) in order to resolve individual spectral components. The depth distribution of the amount of Fe(II) is about 11% of the total Fe to a depth of 19 cm with a subsequent steep increase (within 3 cm) to about 37%, after which it remains constant to the lower end of the sediment core (at about 40 cm). The steep increase of the amount of Fe(II) defines a redox boundary which coincides with the position where the tan/green color transition of the sediment occurs. The isomer shifts and quadrupole splittings of Fe(II) and Fe(III) in the sediment are consistent with hexacoordination by oxygen or hydroxide ligands as in oxide and silicate minerals. Goethite and traces of hematite are observed only above the redox boundary, with a linear gradient extending from about 20% of the total Fe close to the sediment surface to about zero at the redox boundary. The superparamagnetic relaxation behavior allows to estimate the order of magnitude for the size of the largest goethite and hematite particles within the particle-site distribution, e.g. ∼170 A and ∼50 A, respectively. The composition of the sediment spectra recorded at 300 mK in zero-field, apart from the contributions due to goethite and hematite, resembles that of the sheet silicates smectite, illite and chlorite, which have been identified as major constituents of the sediment in the <2 μm fraction by X-ray diffraction. The specific “ferromagnetic” type of magnetic ordering in the sediment, as detected at 4.2 K in an applied field, also resembles that observed in sheet silicates and indicates that both Fe(II) and Fe(III) are involved in magnetic ordering. This “ferromagnetic” behavior is probably due to the double-exchange mechanism known from other mixed-valence Fe(II)–Fe(III) systems. A significant part of the clay-mineral iron is redox sensitive. It is proposed that the color change of the sediment at the redox boundary from tan to green is related to the increase of Fe(II)–Fe(III) pairs in the layer silicates, because of the intervalence electron transfer bands which are caused by such pairs.
TL;DR: A microbial consortium enriched from surface water draining the Lac du Bonnet granitic batholith in Manitoba, Canada, was able to mediate the transformation of 11 % of the Fe in a sample of magnetite into hematite within three weeks as discussed by the authors.
TL;DR: In this article, the authors studied the oxidation of CO on silica-supported hematite (Fe2O3) by the step-response method in a tubular fixed-bed reactor, at temperatures ranging between 270 and 350 °C.
Abstract: The oxidation of CO on silica-supported hematite (Fe2O3) was studied by the step−response method in a tubular fixed-bed reactor, at temperatures ranging between 270 and 350 °C. The oxidation process appeared to proceed through two stages. Firstly, oxygen atoms adsorbed on the surface of hematite react with gas phase CO according to an Eley−Rideal mechanism. Once that adsorbed oxygen has been consumed to some extent, surface oxygen from the lattice of iron oxide is removed in a second stage involving CO adsorption and CO reactive desorption steps, thus generating surface oxygen vacancies. Further reduction of hematite proceeds through diffusion of subsurface oxygen into surface oxygen vacancies. On this basis, a kinetic model was developed, which quantitatively describes the transient behavior of the oxidation process. The activation energies and pre-exponential factors of the rate constants and characteristic subsurface oxygen diffusion time could be determined.
TL;DR: In this article, a porcelain boat was reduced and carburized at 823∼1223K by H2-CO mixtures under low sulfur pressures where metallic iron remained stable.
Abstract: Particles of hematite ores in a porcelain boat were reduced and carburized at 823∼1223K by H2-CO mixtures under low sulfur pressures where metallic iron remained stable.Within major of these various conditions nearly one hour reaction could convert their particles completely to iron carbides such as mostly Fe3C. It was found that traces of gaseous sulfur makes iron carbides enough stable rather than free carbon or metallic iron. Their conversion yields were insensitive to inlet H2/CO mole ratio and ore type. Total sulfur contents in products obtained for lower sulfur pressures were as low as conventional reduced irons.
TL;DR: In this article, a wide range of spinel compositions in the Fe3O4-γFe2O3 series has been identified in magnetic Brazilian soils, and the structural stability and magnetic properties of selected Brazilian pedodomains are discussed, distinguishing those produced from mafic rocks (tuffite, basalt), from those produced in non-mafic lithologies (such as steatite), where inherited magnetite may be exceptionally stable in the soil.
Abstract: Tropical soils often contain high amounts of iron oxides. Hematite (αFe2O3) and goethite (αFeOOH) are the most widespread iron oxides, but magnetite (Fe3O4) and maghemite (γFe2O3) occur in magnetic pedons. A wide range of spinel compositions in the Fe3O4-γFe2O3 series has been identified in magnetic Brazilian soils. Isomorphic substitution of mainly Ti4+, Al3+ and Mg2+, but also of Cr3+ and Mn2+ and other minor elements for iron are related to changes in their structural stability and magnetic properties. Magnetic iron oxides of selected Brazilian pedodomains are discussed, distinguishing those produced from mafic rocks (tuffite, basalt), where primary magnetite transforms to maghemite, from those produced in non-mafic lithologies (such as steatite), where inherited magnetite may be exceptionally stable in the soil.