Dehydroxylation Of Goethite Research Articles

Thermal analysis of goethite

Differential scanning calorimetry shows two endotherms at 75 and 225°C for synthetic goethite. The latter endotherm is strongly asymmetric on the low temperature side. The endotherms were attributed to the loss of water and the dehydroxylation of the goethite. The temperature of the endotherms and the enthalpy of the phase change were found to be linear functions of the percentage of aluminium substitution into the goethite. High-resolution thermogravimetric analysis of goethite showed three mass loss steps, occurring at ~175, 196 and 263°C. The temperatures of these mass loss steps and the percentage of mass loss were also linearly related to the degree of Al substitution. The use of infrared emission spectroscopy confirmed the temperature of dehydroxylation. The observation of the low temperature dehydroxylation of goethite and its relation to ancient aboriginal cave art is discussed.

Voltammetric Identification of Pedogenic Iron Oxides in Paleosol and Loess

Voltammetry of immobilized microparticles was used to detect low concentrations of goethite and hematite in paleosol and loess samples. The total content of Fe oxides in the natural samples was below 2 wt% and below the detection limit of X-ray powder diffraction. Goethite was distinguished from hematite due to the different electrochemical reactivity of the product of thermal dehydroxylation of goethite (very fine crystalline hematite). The voltammetric analysis was tested using synthetic samples of ferrihydrite, goethite, hematite, magnetite, or Mg-ferrite with SiO2 and a natural sample of smectite with an admixture of goethite. The detection is most sensitive for goethite and hematite (detection limit about 0.1 wt%), and approximately an order of magnitude better than X-ray powder diffraction analysis.

Infrared spectroscopy of goethite dehydroxylation: III. FT-IR microscopy of in situ study of the thermal transformation of goethite to hematite

Fourier transform infrared microscopy has been used to investigate in situ dehydroxylation of goethite to form hematite. The characterisation was based on the behaviour of hydroxyl units, which were observed in the hydroxyl stretching and hydroxyl deformation and water bending regions, and the Fe–O vibrations of the newly formed hematite during the thermal dehydroxylation process. Two hydroxyl stretching modes ( ν 1 and ν 2), and three bending ( ν bending-1, 2, 3) and two deformation ( ν deformation-1, 2) modes were observed for goethite. The characteristic vibration at 916 cm −1 was observed together with the residuals of the ν 1 and ν 2 bands in hematite spectrum. The structural transformation between goethite and hematite through thermal dehydroxylation was interpreted in order to provide criteria that can be used for the characterisation of thermally activated bauxite and their conversion to activated alumina phases.

Infrared spectroscopy of goethite dehydroxylation. II. Effect of aluminium substitution on the behaviour of hydroxyl units

Dehydroxylation of goethite as affected by aluminium substitution was investigated using Fourier transform infrared spectroscopy (FT–IR) in conjunction with X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA). The band intensities of hydroxyl vibrations were indicative of the degree of dehydroxylation and the changes in band parameters due to aluminium substitution were observed. The effect of aluminium substitution on band parameters of FT–IR spectra of goethite and its partially and fully dehydroxylated products, the mixture of goethite/hematite and hematite, were interpreted. The results of this study have confirmed that aluminium substituted goethite is thermally more stable than non-substituted goethite and is in harmony with the results of XRD and DTGA. A larger amount of non-stoichiometric hydroxyl units is associated with a higher aluminium substitution. A shift to a higher wavenumber of bending and hydroxyl stretching vibrations is attributed to the effects of aluminium substitution associated with non-stoichiometric hydroxyl units on the a– b plane relative to the b– c plane of goethite. The results provide information for the characterisation of activated bauxite containing hematite and goethite.

The behavior of hydroxyl units of synthetic goethite and its dehydroxylated product hematite

The behavior of the hydroxyl units of synthetic goethite and its dehydroxylated product hematite was characterized using a combination of Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) during the thermal transformation over a temperature range of 180–270°C. Hematite was detected at temperatures above 200°C by XRD while goethite was not observed above 230°C. Five intense OH vibrations at 3212–3194, 1687–1674, 1643–1640, 888–884 and 800–798 cm −1, and a H 2O vibration at 3450–3445 cm −1 were observed for goethite. The intensity of hydroxyl stretching and bending vibrations decreased with the extent of dehydroxylation of goethite. Infrared absorption bands clearly show the phase transformation between goethite and hematite; in particular, the migration of excess hydroxyl units from goethite to hematite. Two bands at 536–533 and 454–452 cm −1 are the low wavenumber vibrations of FeO in the hematite structure. Band component analysis data of FTIR spectra support the fact that the hydroxyl units mainly affect the a plane in goethite and the equivalent c plane in hematite.

Kinetics of thermal transformations of precipitated magnetite and goethite

The aim of the present study was to determine the kinetic equations for the thermal transformations of precipitated iron oxides and hydroxides, namely for the process of thermal dehydroxylation of goethite and consecutive of hematite crystal structure growth as well as for the oxidation of magnetite to maghemite and its thermal transformation into crystalline hematite. The investigations have been carried out using thermogravimetry (TG/DTG/DTA), X-ray powder diffractometry (XRD) and high temperature powder diffractometry (HT-XRD). This presentation contains the continuation of our earlier works.

Distinguishing Between Surface and Bulk Dehydration-Dehydroxylation Reactions in Synthetic Goethites by High-Resolution Thermogravimetric Analysis

AbstractSynthetic goethites studied by high-resolution thermogravimetry (HRTGA) show variability in surface characteristics and structural stability as a function of aging conditions. Goethites were synthesized at either pH 6 or 11, at temperatures of 40 or 70°C, and in the presence or absence of sorbed Mn or Pb. Data from HRTGA analysis revealed at least four distinct weight-loss events near goethite dehydroxylation that relate to 1) three events involving the evolution of water associated with surface Fe-O functional groups and 2) bulk dehydroxylation of goethite during transformation to hematite. The relative mass of evolved surface and bulk structural water was related to the predominant particle morphology as determined by transmission electron microscopy (TEM). Differentiation of surface and bulk decomposition reactions allowed the identification of bulk structural dehydroxylation. Goethite crystallin-ity, estimated by the bulk dehydroxylation temperature, appeared to depend on the kinetics of crystallization. This trend was most evident for systems aged at pH 11 and 40°C. Greater concentrations of coprecipitated Mn or Pb dramatically improved goethite crystallinity as indicated by higher dehydroxylation temperatures and smaller widths of the (110) Bragg reflection. Comparison of bulk dehydroxylation temperatures for these samples to other preparations suggests that structural defects predominated over the effects of particle size and Mn/Pb substitution in determining goethite thermal stability. A conceptual model is proposed to account for the disparate dehydroxylation profiles displayed by goethites of varying crystallinity.

Re-examination of the kinetics of the thermal dehydroxylation of goethite

The kinetics of thermal dehydroxylation of aluminuous goethites [1] synthesised from a ferrous salt has been re-examined using the general reaction order kinetic law. The utilised data processing was based on the procedures employed by dissolution kinetics. Recalculation of the activation energies EA of the dehydroxylation yielded the values 130, 132, 128, and 123 kJ mol−1 for pure goethite, goethite with 10, 20, and 30 mol% Al substitution, respectively. The values of EA are in a good agreement with those given for goethite in literature. The EA values are linearly related with the chemically bound excess H2O/OH− in the crystal lattice that is apparently influenced by Al substitution.

Quantitative thermogravimetric analysis of haematite, goethite and kaolinite in Western Australian iron ores

A quantitative thermogravimetric method has been developed for the routine analysis of goethite and kaolinite in a Western Australian iron ore sample. Standards were used to construct calibration graphs based on the mass loss due to the dehydroxylation of goethite and kaolinite. The haematite content is calculated from the total soluble iron content in the sample. The method is precise (maximum relative standard deviation of 3.3%) and accurate (within ±10%), provided the crystallinity and particle size of the minerals in the standards are matched to the minerals in the unknown samples. The method was applied to the characterisation of an iron ore tailing and used to assess the performance of a selective flocculation process for upgrading the tailing.

Reflectivity (visible and near IR), Mössbauer, static magnetic, and X ray diffraction properties of aluminum‐substituted hematites

Because aluminum substitution for iron occurs in polymorphs of Fe2O3 and FeOOH in terrestrial (and by inference Martian) environments, it is important for the mineralogical remote sensing of both planets at visible and near‐IR wavelengths to know the effects of Al substitution on their reflectivity spectra. Diffuse reflectivity (350–2200 nm), Mössbauer, static magnetic, and X ray diffraction data are reported for a series of aluminum‐substituted hematites α‐(Fe,Al)2O3 for compositions having values of Als. (mole ratio Al/(Al+Fe)) up to 0.61. Samples were prepared by oxidation of magnetite, dehydroxylation of goethite, and direct precipitation. Unit cell dimensions decrease with Als but at a rate less than that predicted by the Vegard rule. At 293 K, Mössbauer spectra are sextets (negative quadrupole splitting, QS) for Als up to ∼0.5 and doublets for larger Als. At 21 K, all compositions are sextets; however, there is a discontinuity of ∼0.5 T in the magnetic hyperfine field (Bhf) and QS changes sign at Als = 0.06(2) (Morin transition). Al‐poor compositions have positive QS and higher Bhf. Negative and positive quadrupole splittings are indicative of the weakly ferromagnetic and antiferromagnetic states of hematites, respectively. The position of the least energetic crystal‐field transition (6A1→4T1g) of ferric iron shifts to longer wavelengths with increasing Als. The magnitude of the shift is a linear function of (1/a)5, where a is the hexagonal unit cell dimension. For geologically reasonable amounts of Al substitution (Als < 0.33), the magnitude of the shift is small (∼20 nm), so that it is problematical (based on reflectivity data alone) to uniquely ascribe shifts in 4T1g band positions to different degrees of Al substitution. On the basis of Martian spectral data, the range in Als for Martian hematites is 0 < Als < 0.19.

Nonstoichiometric structures during dehydroxylation of goethite

Nonstoichiometric structures during dehydroxylation of goethite

Nonstoichiometric structures during dehydroxylation of goethite

Abstract Thermal dehydration of goethite (α-FeOOH) at low temperature leads to complete transformation into a hematite-like phase before less than one half of the stoichiometric water content in goethite is expelled. The character of the remaining water has not bee explained as yet. The variations of band positions in the infrared spectra and the specific changes in relative integrated intensities of X-ray lines with the increase of dehydration temperature suggested that OH-groups replace oxygen anions in the hematite crystal lattice, while the electroneutrality is preserved by the cation vacancies. Comparison of integrated intensities observed with intensities calculated on this assumption fully confirms that the hematite-like phase is an iron deficient species with a general chemical formula α-Fe2−x/3-(OH) x O3−x . The accurate measurements of lattice constants reveal that while the crystallographic c o-axis decreases steadily with progressing thermal dehydroxylation, the a o-axis reaches a minimum at the temperature at which the nonuniform X-ray line broadening disappears. The largest unit cell of hematite phase after the goethite/hematite transition and the smallest one after the disappearance of selected broadening of X-ray diffraction peaks represent two transitional phases with hematite structure, protohematite and hydrohematite, displaying their own structural and compositional characteristics. The smallest unit cell appearing when the crystalline phase still holds about three percent of structurally bound water suggests that the stoichiometric hematite is thermodynamically less stable than this intermediate state.

Synthesis and Properties of Poorly Crystalline Hydrated Aluminous Goethites

AbstractAl-substituted goethites were prepared by rapid oxidation of mixed FeCl2-AlCl3 solutions at pH 6.8 in the presence of CO2 at 25°C. A combination of Al substitution and adsorption of CO2 reduced crystal size (except for an increase at small additions of Al) and produced unusual thin, porous particles. Product goethites had surface areas up to 283 m2/g and unit-cell expansions induced by hydration. Substitution of Al for Fe reduced the 111 spacing and increased infrared OH-bending vibrational frequencies. Al substitution split the goethite dehydroxylation endotherm during differential thermal analysis into a doublet and increased the temperature of all reactions. Both cold and hot alkali solutions dissolved Al from the goethite structure.After drying the product in vacuo at 110°C. X-ray powder diffraction data indicated minimal deviation from Vegard's law for the goethite-diaspore solid solution up to about 30 mole % Al substitution. Goethite prepared in the presence of 40 mole % Al had a 111 spacing of 2.403 Å corresponding to 36 mole % structural Al if Vegard's law was obeyed. Rapid oxidation of mixed FeCl2-AlCl3 solutions appears to be conducive to a higher degree of Al substitution in goethite than alkaline aging of hydroxy-Fe(III)-Al coprecipitates.

Thermal transformation of goethite into haematite in alkali halide discs

Parallel thermal treatments have been investigated with powder mixtures containing 33 wt % goethite in alkali halides (KCl, KBr, KI and CsI) and with pressed discs having the same compositions. Infrared and X-ray studies have shown that the reaction scheme occurring in the discs includes: (1) dehydroxylation of goethite leading to the formation of protohaematite (at 250°C) and (2) recrystallization of protohaematite into haematite (at 430°C). On the other hand, only the dehydroxylation stage occurs in the powder mixtures and protohaematite persists even after prolonged heating at 430°C. The recrystallization stage, which is accompanied by crystal growth, is very slow in the CsI discs, whereas crystal growth by aggregation (sintering) is essential in the CsI disc but not in the KCl, KBr and KI discs. The results of weight loss during the thermal treatment indicate that in addition to dehydroxylation other reactions are taking place, mainly in the discs. These reactions may include ion exchange and reduction of ferric iron. Water adsorption of the various salt-free products was studied in three different environments, namely from liquid water and from 100 and 40 % relative humidities. These three experimental series show that the protohaematites obtained in potassium halide powder mixtures are hydrophilic, whereas those obtained in discs are hydrophobic. These differences in the surface properties may indicate that crystal perfection during the recrystallization stage begins in the Fe2O3/K-salt interface.