Showing papers on "Hematite published in 2004"

Jarosite and hematite at Meridiani Planum from Opportunity's Mössbauer spectrometer

Abstract: Mossbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil, olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematite-rich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.

Spectroscopic Evidence for Fe(II)−Fe(III) Electron Transfer at the Iron Oxide−Water Interface

Abstract: Using the isotope specificity of 57Fe Mossbauer spectroscopy, we report spectroscopic observations of Fe(II) reacted with oxide surfaces under conditions typical of natural environments (i.e., wet, anoxic, circumneutral pH, and about 1% Fe(II)). Mossbauer spectra of Fe(II) adsorbed to rutile (TiO2) and aluminum oxide (Al2O3) show only Fe(II) species, whereas spectra of Fe(II) reacted with goethite (α-FeOOH), hematite (α-Fe2O3), and ferrihydrite (Fe5HO8) demonstrate electron transfer between the adsorbed Fe(II) and the underlying iron(III) oxide. Electron-transfer induces growth of an Fe(III) layer on the oxide surface that is similar to the bulk oxide. The resulting oxide is capable of reducing nitrobenzene (as expected based on previous studies), but interestingly, the oxide is only reactive when aqueous Fe(II) is present. This finding suggests a novel pathway for the biogeochemical cycling of Fe and also raises important questions regarding the mechanism of contaminant reduction by Fe(II) in the presenc...

Mineralogy at Meridiani Planum from the Mini-TES Experiment on the Opportunity Rover.

Abstract: The Miniature Thermal Emission Spectrometer (Mini-TES) on Opportunity investigated the mineral abundances and compositions of outcrops, rocks, and soils at Meridiani Planum Coarse crystalline hematite and olivine-rich basaltic sands were observed as predicted from orbital TES spectroscopy Outcrops of aqueous origin are composed of 15 to 35% by volume magnesium and calcium sulfates [a high-silica component modeled as a combination of glass, feldspar, and sheet silicates (approximately 20 to 30%)], and hematite; only minor jarosite is identified in Mini-TES spectra Mini-TES spectra show only a hematite signature in the millimeter-sized spherules Basaltic materials have more plagioclase than pyroxene, contain olivine, and are similar in inferred mineral composition to basalt mapped from orbit Bounce rock is dominated by clinopyroxene and is close in inferred mineral composition to the basaltic martian meteorites Bright wind streak material matches global dust Waterlain rocks covered by unaltered basaltic sands suggest a change from an aqueous environment to one dominated by physical weathering

Mineralogy at Gusev Crater from the Mossbauer spectrometer on the Spirit Rover

Abstract: Mossbauer spectra measured on Mars by the Spirit rover during the primary mission are characterized by two ferrous iron doublets (olivine and probably pyroxene) and a ferric iron doublet (tentatively associated to nanophase ferric iron oxide) Two sextets resulting from nonstoichiometric magnetite are also present, except for a coating on the rock Mazatzal, where a hematite-like sextet is present Greater proportions of ferric-bearing phases are associated with undisturbed soils and rock surfaces as compared to fresh rock surfaces exposed by grinding The ubiquitous presence of olivine in soil suggests that physical rather than chemical weathering processes currently dominate at Gusev crater

Influence of Natural Organic Matter and Ionic Composition on the Kinetics and Structure of Hematite Colloid Aggregation: Implications to Iron Depletion in Estuaries

Abstract: The stability and aggregation behavior of iron oxide colloids in natural waters play an important role in controlling the fate, transport, and bioavailability of trace metals. Time-resolved dynamic light scattering experiments were carried out in a study of the aggregation kinetics and aggregate structure of natural organic matter (NOM) coated hematite colloids and bare hematite colloids. The aggregation behavior was examined over a range of solution chemistries, by adjusting the concentration of the supporting electrolyteNaCl, CaCl2, or simulated seawater. With the solution pH adjusted so that NOM-coated and bare hematite colloids were at the same zeta potential, we observed a significant difference in colloid stability which results from the stability imparted to the colloids by the adsorbed NOM macromolecules. This enhanced stability of NOM-coated hematite colloids was not observed with CaCl2. Aggregate form expressed as fractal dimension was determined for both NOM-coated and bare hematite aggregates ...

Raman spectroscopy of Fe-Ti-Cr-oxides, case study: Martian meteorite EETA79001

Abstract: Raman spectral features of chromite, ulvospinel, magnetite, ilmenite, hematite, and some of their solid solutions are presented. Although most Fe-Ti-Cr-oxides produce relatively weak Raman signals compared to oxyanionic minerals, sufficient information can be extracted from their spectra to identify the end-member mineral phases as well as some information about compositional variations in solid solutions. Correlations between Raman spectral features and mineral chemistry are used to interpret the Raman data of Fe-Ti-Cr oxides found during Raman point-count measurements on rock chips of Martian meteorite EETA79001, as an analog to Mars on-surface planetary investigations. In general, ulvospinel, magnetite, and chromite end-members are readily distinguished by their Raman spectral patterns, as are ilmenite and hematite. In the low signal-to-noise (S/N) spectra generally obtained from the Raman point-count procedure, the position and shape of the strongest peak of Fe-Ti-Cr oxides in the region 660–680 cm−1 (A1g mode) is the most useful for discriminating Fe3+-Ti-Cr-Al substitutions in the magnetite-ulvospinel, ulvospinel-chromite, and chromite-spinel series, but minor peaks in the range 300–600 cm−1 also assist in discrimination. These spectral features are useful for investigating the variability among Fe-Ti-Cr-Al oxide solid solutions in natural samples. In EETA79001, a Martian basaltic meteorite, most of the oxide grains (as measured with the electron microprobe) are ulvospinel, chromian ulvospinel, and chromite, but ilmenite, titanian chromite, and titanomagnetite are also observed. The Fe-Ti-Cr-oxides identified by Raman point-count include end-member ilmenite, low-Al chromite-spinel solid solutions, ulvospinel-magnetite solid solutions, and more complex chromite-spinel-ulvospinel-magnetite solid solutions; the latter exhibit a wide range of main peak positions and broadened peak widths that may reflect structural disorder as well One Raman spectrum suggests end-member magnetite, and one spectrum from a different rock chip appears to be that of non-terrestrial hematite, reflecting local oxidizing alteration, which has not been observed previously in this meteorite. These results show that analyses done in an automated mode on the surface of an unprepared Martian rock sample can provide useful constraints on the Fe-Ti-Cr oxide mineralogy present and on compositional variations within those minerals, including an indication of oxygen fugacity.

Iron oxides and light absorption by pure desert dust: An experimental study

Abstract: [1] Theoretical computations based on Mie theory indicate that absorption of light by desert dust aerosols should be highly sensitive to their content in iron oxides (hematite, goethite, etc.). A selective extraction method that has been developed recently now allows quantification of these minerals in aerosol samples less than 500 μg in mass. Thus it is now possible to assess experimentally the part played by iron oxide minerals on dust absorption properties. In this paper we present an adaptation of Bond et al.'s [1999] method for measuring mineral dust mass absorption efficiencies and deriving single scattering albedo values at two wavelengths (325 and 660 nm). It consists in measuring simultaneously the aerosol mass concentration with a TEOM microbalance, its scattering properties in the visible spectrum with a 3 wavelength nephelometer, and attenuation at the 2 aforementioned λ with a dual-wavelength aethalometer. At first the method is applied to nonabsorbing (iron oxide-free) aerosols in order to check the magnitude of the “apparent absorption” due to scattering artifacts. In good agreement with Bond et al.'s results for visible wavelengths, it is found that 2% of scattering is misinterpreted as absorption. This proportion is also found to be practically insensitive to the aerosol size distribution. After these preliminary measurements the method is applied to aerosols generated by shaking three natural soil samples collected in one of the main Chinese dust source (Gobi desert), in northern Sahara (Tunisia), and in the Sahel (Niger). For these aerosols, mass absorption efficiencies are found to range between 10−2 and 2 10−2 m2 per gram of aerosol at 660 nm and to increase linearly with the iron oxide content at the rate of 0.56 m2 per gram of iron oxide. Owing to the larger absorbing potential of iron oxides at short wavelengths, mass absorption efficiencies at 325 nm are about 6 times larger than at 660 nm. At this last wavelength the single scattering albedo (SSA) is found to decrease from 0.97 for Chinese and Tunisian aerosols to 0.95 for the Niger one that also happens to have the largest iron oxide content (6.5% in mass). At 325 nm the SSA is much lower for the three aerosols (∼0.80) than at 660 nm. These values are similar to recent results obtained close to major mineral dust sources by inversion of Sun photometer or satellite data. Finally, simple computations performed for conditions that prevail at regional scale in the vicinity of important dust sources show that, even when mineral dust is mixed with strongly absorbent particles such as black carbon (BC), the effect of iron oxides on light absorption is in the same order of magnitude as the one of BC.

Long-term in vitro transformation of 2-line ferrihydrite to goethite/hematite at 4, 10, 15 and 25°C

Abstract: 2-line ferrihydrite stored in water at ambient temperatures from 4 to 25°C and at ten different pH values between 2.5 and 12 for up to 10–12 y transformed to both goethite and hematite at all temperatures and pH values except at pH 12 where only goethite was formed. The rate and degree of transformation (20–100%) increased with increasing pH and temperature. The hematite/(hematite+goethite) ratio varied between 0 and ∼0.8, increased with increasing temperature and showed a strong maximum at pH 7–8 which increased from 0.1–0.2 at 4°C to 0.7–0.8 at 25°C. The maximum coincides with the zero point of charge of ferrihydrite where its solubility and, thus, its via-solution transformation rate to goethite are minimal. We assume, therefore, that in this pH-range the (slower) via-solution transformation to hematite can more efficiently compete with that to goethite.

Reaction sequences in the formation of silico-ferrites of calcium and aluminum in iron ore sinter

Abstract: Complex silico-ferrites of calcium and aluminium (low-Fe form, denoted as SFCA; and high-Fe, low-Si form, denoted as SFCA-I) constitute up to 50 vol pct of the mineral composition of fluxed iron ore sinter. The reaction sequences involved in the formation of these two phases have been determined using an in-situ X-ray diffraction (XRD) technique. Experiments were carried out under partial vacuum over the temperature range of T=22 °C to 1215 °C (alumina-free compositions) and T=22 °C to 1260 °C (compositions containing 1 and 5 wt pct Al2O3) using synthetic mixtures of hematite (Fe2O3), calcite (CaCO3), quartz (SiO2), and gibbsite (Al(OH)3). The formation of SFCA and SFCA-I is dominated by solid-state reactions, mainly in the system CaO-Fe2O3. Initially, hematite reacts with lime (CaO) at low temperatures (T ∼ 750 °C to 780 °C) to form the calcium ferrite phase 2CaO·Fe2O3 (C2F). The C2F phase then reacts with hematite to produce CaO·Fe2O3 (CF). The breakdown temperature of C2F to produce the higher-Fe2O3 CF ferrite increases proportionately with the amount of alumina in the bulk sample. Quartz does not react with CaO and hematite, remaining essentially inert until SFCA and SFCA-I began to form at around T=1050 °C. In contrast to previous studies of SFCA formation, the current results show that both SFCA types form initially via a low-temperature solid-state reaction mechanism. The presence of alumina increases the stability range of both SFCA phase types, lowering the temperature at which they begin to form. Crystallization proceeds more rapidly after the calcium ferrites have melted at temperatures close to T=1200 °C and is also faster in the higher-alumina-containing systems.

Thermally stable hematite hollow nanowires.

Abstract: Thermally stable hematite (alpha-Fe(2)O(3)) hollow nanowires were synthesized by a vacuum-pyrolysis route from beta-FeOOH nanowires for the first time. The products can catalyze the oxidation of almost 100% carbon monoxide at 320 degrees C, exhibiting excellent catalytic performances despite their small BET surface area.

Formation of the hematite-bearing unit in Meridiani Planum: Evidence for deposition in standing water

Abstract: [1] The most plausible models for the origin and evolution of a unique geologic unit in Meridiani Planum, Mars, are low-temperature precipitation of Fe oxides/oxyhydroxides from standing water, precipitation from circulating fluids of hydrothermal origin, or the thermal oxidation of magnetite-rich ash. Analysis of Odyssey Thermal Emission Imaging System (THEMIS) infrared and visible images, together with MGS TES, MOLA, and MOC data, has provided additional insight into the Meridiani region. The hematite at Meridiani was most likely derived from a Fe oxyhydroxide precursor such as goethite, is mixed with basalt as the major component, occurs as a thin layer meters to <200 m thick, and is thermophysically distinct from units immediately above and below. Remnants of a hematite-poor unit lie directly above the hematite layer, indicating that hematite formation was sharply confined vertically. The hematite unit appears to embay preexisting channels and occurs only as outliers within closed crater basins, suggesting that it was deposited in a gravity-driven fluid, rather than as a dispersed air fall. The hematite unit lieswithinatopographictroughover � 3/4ofitscircumference,withtheremainingperimeter <150mlowerinelevation.Oxidationofashduringemplacementisunlikelygivenagoethite precursor and basalt as the major component. Hydrothermal alteration does not account for the confined vertical extent of the hematite layer over large distances and across disconnected outliers. The preferred model is the deposition of hematite or precursor Fe oxyhydroxides in water-filled basins, followed by dehydroxylation to hematite in lowtemperature diagenesis. This model accounts for (1) the uniform deposition of a thin hematite-bearing unit over an area � 150,000 km 2 in size; (2) the transition from hematiterichtohematite-poorunitsoverlessthan � 10mverticaldistance;(3)thedistinctdifferences from the underlying layers; (4) goethite as the precursor to hematite; (5) the embayment relationships; (6) the occurrence of remnants of the hematite-bearing unit in isolated craters surrounding the main deposit; (7) the lack of other hydrothermal minerals; and (8) the presence of coarse-grained, low-albedo basalt, rather than ash, as the major component. The occurrence of unweathered olivine, pyroxene, and feldspar throughout the equatorial region provides strongevidencethat extensive aqueousweathering hasnotoccurred onMars. Thus the presence of a small number of bodies of standing water appears to represent brief, localized phenomena set against the backdrop of a cold, frozen planet. INDEX TERMS: 5410 Planetology: Solid Surface Planets: Composition; 6225 Planetology: Solar System Objects: Mars; 5464 Planetology: Solid Surface Planets: Remote sensing; 3672 Mineralogy and Petrology: Planetary mineralogy and petrology (5410); KEYWORDS: hematite, Meridiani, standing water

Transformation of magnetite to hematite and its influence on the dissolution of iron oxide minerals

Abstract: Deformed rocks of the Itabira Iron Formation (itabirites) in Brazil show microstructural evidence of pressure solution of quartz and iron oxides; it appears that magnetite was dissolved and hematite precipitated. The dissolution of magnetite seems to be related to its transformation to hematite by oxidation of Fe2+ to Fe3+. The transformation of magnetite to hematite occurs along {111} planes, and results in the development of hematite domains along {111} that are parallel to the foliation. The difference in volume created by the transformation of magnetite to hematite and the shear stress acting on the interphase boundaries allow fluids to migrate along these planes. The dissolution of magnetite involves the hydrolyzation of the Fe2+—O bonds at interphase boundaries of high normal stress. The high fugacity of oxygen in the fluid phase promotes the reaction of Fe2+ (in solution) with oxygen. Fe2+ ions oxidize to Fe3+ and precipitate as hematite platelets with their longest axes oriented parallel to the direction of maximum stretching. The transformation of magnetite to hematite during deformation plays an important role in the fabric evolution of the iron formation rocks. The transformation along {111} creates planes of weakness that facilitate fracturing. The fracturing plus the dissolution result in a reduction of magnetite grain size, and the oriented precipitation results in layers of hematite platelets. These processes produce a new fabric characterized by a penetrative foliation and lineation.

Nature and origin of lamellar magnetism in the hematite-ilmenite series

Abstract: Grains consisting of finely exsolved members of the hematite-ilmenite solid-solution series, such as are present in some slowly cooled middle Proterozoic igneous and metamorphic rocks, impart unusually strong and stable remanent magnetization. TEM analysis shows multiple generations of ilmenite and hematite exsolution lamellae, with lamellar widths ranging from millimeters to nanometers. Rock-magnetic experiments suggest remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths, yet is stronger than can be explained by canted antiferromagnetic (CAF) hematite or coexisting paramagnetic (PM) Fe-Ti-ordered ( R 3) ilmenite alone. In alternating field demagnetization to 100 mT, many samples lose little remanence, indicating that the NRM is stable over billions of years. This feature has implications for understanding magnetism of deep rocks on Earth, or on planets like Mars that no longer have a magnetic field. Atomic-scale simulations of an R 3 ilmenite lamella in a CAF hematite host, based on empirical cation-cation and spin-spin pair interaction parameters, show that contacts of the lamella are occupied by “contact layers” with a hybrid composition of Fe ions intermediate between Fe2+-rich layers in ilmenite and Fe3+-rich layers in hematite. Structural configurations dictate that each lamella has two contact layers magnetically in phase with each other, and out of phase with the magnetic moment of an odd non-self-canceling Fe3+-rich layer in the hematite host. The two contact layers and the odd hematite layer form a magnetic substructure with opposite but unequal magnetic moments: a lamellar “ferrimagnetism” made possible by the exsolution. Because it is confined to magnetic interaction involving the moments of just three ionic layers associated with each individual exsolution lamella, lamellar magnetism is unique and quite distinct from conventional ferrimagnetism. Simulation cells indicate that the magnetic moments of contact layers are locked to the magnetic moments of adjacent AF hematite layers and are parallel to the basal plane (001). Thus, lamellar magnetism is created at the temperature of chemical exsolution, and is a chemical remanent, rather than thermal remanent, magnetization. However, in thermal demagnetization experiments, too short for lamellar resorption, demagnetization temperatures are those of the CAF hematite, considerably higher than temperatures of original lamellae formation. Internal crystal structure cannot dictate that the contact layers of different lamellae will form magnetically in phase with each other to give the highest net magnetic moment, but magnetic moments of lamellae can be made to form in phase by the external force of the magnetizing field at the time of exsolution. A thesis of this paper is that an external magnetic field can dictate the magnetic moments and hence the chemical location of ilmenite lamellae in a hematite host, and that once in place, neither the location nor the magnetic moment will be easily disturbed. In an ilmenite host, the external magnetic field cannot control the chemical location of a hematite lamella, which is dictated by the enclosing ilmenite, but once lamellae have formed, the field can dictate their magnetic moments. These moments, however, are not locked chemically to the host, resulting in lower coercivity. The effectiveness of the external force in single crystals is dictated by their orientation with respect to the magnetizing field. In grains with (001) oriented parallel to the field, it would be effective in producing in-phase magnetic moments and very strong remanence. In grains with (001) normal to the field, the field would be less effective in producing in-phase magnetic moments, hence producing weak remanence. The most intense lamellar magnetism per formula unit occurs with in-phase magnetization, high lamellar yields, and the largest number of lamellae per unit volume (i.e., smallest lamellar size). Compared to the magnetic moment per formula unit ( M pfu) and magnetic moment per unit volume ( M V) of end-member magnetite ( M pfu = 4 μB, M V = 480 kA/m) and hematite ( M pfu = 0.0115 μB, M V = 2.1 kA/m), results for some atomic models reasonably tied to natural conditions are M pfu = 0.46–1.36 μB and M V = 84–250 kA/m.

The origin of hematite in high-grade iron ores based on infrared microscopy and fluid inclusion studies: the example of the conceição mine, quadrilátero ferrífero, brazil

Abstract: Petrographic and textural analysis combined with fluid inclusion studies by infrared microscopy of high-grade (>65% Fe) hematite ore samples from the Conceicao deposit, in the northeastern part of the Quadrilatero Ferrifero, Brazil, indicate a complex process of oxidation and mineralization during two orogenic events, each developed under different conditions and involving distinct fluids. The earliest mineralization formed massive magnetite-rich orebodies under relatively reducing conditions in the early stages of the Transamazonian orogeny. Magnetite was oxidized (martitized) with the development of porous hematite crystals (hematite I). Possibly during this stage, new hematite crystals were also formed from low-temperature, low- to medium-salinity fluids, as indicated by two-phase fluid inclusions. The origin of these fluids is still uncertain but tentatively interpreted as being modified surface water. The fluids were transported along normal faults and fractures during post-tectonic collapse following the Transamazonian orogeny (2.1–2.0 Ga) and creation of the dome-and-keel structural pattern of the Quadrilatero Ferrifero. These solutions were also likely responsible for the initial oxidation of the iron formations and the development of hematite I. Subsequent uplifted hot basement rocks or post-tectonic plutons were probable heat sources for the regional metamorphism and development of a granoblastic fabric of hematite II grains in the iron formations and high-grade orebodies. However, the ore was only partially recrystallized, as several relics of the early magnetite, martite, and hematite are still preserved in the granular hematite II crystals. During the Brasiliano-Pan-African orogeny (0.8–0.6 Ga), high-salinity fluids, with temperatures varying from ~120° to a maximum of approximately 350°C, penetrated the iron formations along shear zones, crystallizing initially tabular and thereafter platy hematite crystals (hematite III and specularite) forming schistose orebodies. Quartz veins that cut across the ore and envelop specularite plates and ore fragments formed from late-stage, high-temperature, and low-salinity fluids containing CO2. These later fluids did not alter the ore. Each of these stages of mineralization produced orebodies with distinct features. Recurrent hydrothermal mineralization is thought to have been responsible for the development of giant, high-grade iron ore deposits in structurally favorable sites. Fold hinges with enhanced permeability and deep faults able to conduct the fluids to the surface, repeatedly over time, should be important targets for exploration of new resources.

Effect of precursor mineralogy on the thermal infrared emission spectra of hematite: Application to Martian hematite mineralization

Abstract: [1] Observations from the Thermal Emission Spectrometer (TES) instrument aboard the Mars Global Surveyor (MGS) spacecraft led to the discovery of two isolated deposits of gray, crystalline hematite located in Meridiani Planum and Aram Chaos and several smaller deposits in Valles Marineris. Several pathways for formation of these hematite deposits have been proposed, involving both aqueous and nonaqueous processes. This work uses the precise shape and position of spectral features in the Martian hematite thermal emission spectrum to constrain hematite formation pathways. Thermal infrared emission spectra, X-ray powder diffraction patterns, Mossbauer spectra, and transmission electron microscope (TEM) photomicrographs were obtained for synthetic and natural hematite samples derived by (1) dehydroxylation of fine-grained goethite and (2) oxidation of magnetite. Collectively, the instrumental analyses show that the mineralogical composition and crystal morphology of precursor samples and the time and temperature conditions under which decomposition to hematite occur determine the crystallinity and crystal morphology of the hematite product. Comparison of laboratory and MGS-TES spectra shows that the Martian hematite spectra correspond closely with a synthetic hematite sample derived by pseudomorphic and topotactic dehydroxylation of goethite at 300°C. Spectra of goethite-precursor samples dehydroxylated at higher temperatures provide increasingly poor fits. Spectra of hematite samples derived by high-temperature thermal oxidation of magnetite are also poorer fits to the Martian hematite spectrum. Thermal emission spectra of goethites heated at lower temperatures are characterized by the spectral signatures of both hematite and goethite and are not consistent with the Martian spectra. The characteristic that distinguishes the synthetic hematite sample with the Mars-like spectral signature from the other synthetic hematite samples is the high proportion of crystal surfaces that are crystallographic {001} faces (c faces) for the former but not the latter. The high proportion of {001} face area results because the largest surface of the lath-shaped hematite particles is the (001) face, as determined by TEM. Thus a possible formation pathway for hematite in Meridiani Planum, Aram Chaos, and Valles Marineris is precipitation of goethite from aqueous solutions as lath-shaped crystals, possibly as a stain, cement, and/or massive deposit, followed by pseudomorphic and topotactic dehydroxylation to hematite at temperatures below ∼300°C.

Fe3-xMnxO4 catalysts: phase transformations and carbon monoxide oxidation

Abstract: In this work, the synthesis, characterization and transformation of Fe3−xMnxO4 (x=0–0.53) spinels have been studied by Mossbauer spectroscopy, powder X-ray diffraction (XRD), thermal analyses (TG and DSC) and temperature programmed reduction (TPR) experiments. Mossbauer spectroscopy and XRD lattice parameters (ao) showed the presence of pure spinel phases with Mn incorporation mainly in the octahedral site replacing Fe2+. Upon thermal treatment in air, the spinels Fe3−xMnxO4 are oxidized to an Mn containing maghemite. At higher temperatures the maghemite is converted to the hexagonal α-Fe2O3 hematite phase. DSC analyses showed that the presence of manganese in the maghemite structure strongly decreases the transition temperature to hematite. The effect of these phase transformations on the catalytic carbon monoxide oxidation was investigated. It has been observed that the presence of Mn on the catalysts Fe3−xMnxO4 does not significantly affect the catalytic activity at lower temperature. On the other hand, at temperatures higher than 200 °C the magnetites are oxidized to maghemite/hematite and a great increase in the catalytic activity is observed. Catalytic experiments with the different iron oxides Fe3O4 (magnetite), γ-Fe2O3 (maghemite) and α-Fe2O3 (hematite) showed that at lower temperatures, magnetite is the most active phase whereas at higher temperatures, hematite showed the highest activity.