Mineralization of Iron Deposits from Dehbid Area, Fars Province, South Iran: Geochemical and Mineralogical Data
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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.
Ferrihydrite
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Powder Diffraction
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The concentration and acidity levels of iron(III) solutions are found to be interrelated in their effects on the formation of goethite and hematite at 70°C. A higher iron(III) concentration favors the formation of hematite, whereas a lower concentration favors goethite. For a given iron(III) concentration, the addition of acid favors the formation of hematite. A hypothesis is proposed to generalize the effects of these two factors. Hematite is slightly less soluble than goethite at 70°C. Hematite forms only in solutions that are understaurated with respect to goethite but supersaturated with respect to hematite. In solutions that are supersaturated with respect to both goethite and hematite, goethite will crystallize first. The continuing hydrolysis of the excess Fe3+ results in a reduction in solution (Fe3+)(OH-)3 product. When the solution becomes undersaturated with respect to goethite but supersaturated with respect to hematite, the latter begins to form. The goethite already formed changes to hematite at an extremely slow rate or not at all. Thus the relative abundance of goethite and hematite is mainly governed by the initial solution (Fe3+)(OH-)3 relative to the solubility products of goethite and hematite.
Supersaturation
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The morphologically distinct materials in Darling Range lateritic duricrust (i.e. loose and cemented pisoliths, concretions, matrix, pisolith coatings and void coatings) exhibit goethite/(goethite + hematite) ratios ranging from 0.15 for individual pisoliths to 1.0 for void coatings. Mole % Al substitution ranged from 20 to 34% in goethite and from 2 to 15% in hematite. Goethite and hematite in pisoliths and concretions were mostly highly Al substituted. Al substitution in goethite was positively related (P < 0.01) to Al substitution in hematite. Al substitution in maghemite was less than 5%. Goethite crystals ranged in size from 130 to 260 A. Hematite crystals ranged from 140 to 520 A, and were systematically smaller in pisoliths. Crystal size of goethite and hematite decreased with increasing Al substitution. Hematite crystals were usually about 50% larger than goethite crystals in the same sample, and crystal sizes of goethite and hematite were positively correlated (P < 0.01). Goethite and hematite occurred as aggregates of subrounded platy crystals. Differences in the properties of goethite and hematite between morphologically distinct materials in single hand specimens are indicative of the complex history of these duricrusts.
Maghemite
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The iron (oxyhydr)oxides hematite (α-Fe2O3) and goethite (α-FeOOH) are natural and reactive minerals common in soils and sediments, and their adsorption of Fe(II) produces reactive surface sites that facilitate reduction of oxidized environmental pollutants. Single-exposure experiments with 4-chloronitrobenzene showed that hematite is more reactive than goethite, when normalized by surface area loading. Interestingly, the product of Fe(II) oxidation is a mixture of goethite and hematite, and the goethite to hematite ratio depends on the distribution of Fe(II) activated surface sites, which is a function of aqueous Fe(II) concentration, surface area loading, and pH. More goethite is produced under conditions of higher Fe(II), lower surface area loading, and higher pH. Recurrent-exposure experiments showed a substantial decrease in reaction rate after one to three exposures, a trend suggestive of reaction contributions from the increasing goethite surface area over time. Using known atomic surface geometry for goethite and hematite, the hematite {012} facet is proposed as the site of primary mineral growth with goethite {021} at the interface between the two minerals. These results have implications in contaminant fate modeling, where the mineral phases present in the environment, the minerals likely to form, and the surrounding aqueous conditions all have an impact on contaminant reaction rate.
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