Schwertmannite and the chemical modeling of iron in acid sulfate waters.

VII. Synthesis 2569 VIII. Adsorption and Solid Solution 2570 A. General Observations 2570 B. Adsorption of Specific Species 2571 1. Various Cations 2571 2. Various Anions 2572 3. Organic Species 2573 C. Environmental Implications 2573 IX. Transformation of Ferrihydrite 2574 A. Dry Thermal Transformation 2574 B. Aqueous Transformation 2575 C. Effects of Various Ions on the Aqueous Transformation of Ferrihydrite 2576

Occurrence and Constitution of Natural and Synthetic Ferrihydrite, a Widespread Iron Oxyhydroxide

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

https://science.sciencemag.org/content/sci/316/5832/1726.full.pdf

The Structure of Ferrihydrite, a Nanocrystalline Material

Storage of ferrihydrite in aqueous suspensions at 24°C and pHs between 2.5 and 12 for as long as three years resulted in the formation of goethite and hematite. The proportions and crystallinity of these products varied widely with the pH. Maximum hematite was formed between pH 7 and 8, and maximum goethite at pH 4 and at pH 12. The crystallinity of both products, as indicated by X-ray powder diffraction line broadening and magnetic hyperfine field values and distribution widths, was poorer, the lower the proportion of the corresponding product in the mixture. The existence of two competitive formation processes is suggested: goethite is formed via solution, preferably from monovalent Fe(III) ions [Fe(OH)2+ and Fe(OH)4−], and hematite by internal rearrangement and dehydration within the ferrihydrite aggregates. This concept relates the proportions of goethite and hematite to the activity of the Fe(III) ion species in solution, and implies that conditions favorable for the formation of goethite are unfavorable for that of hematite and vice versa.

Effect of pH on the formation of goethite and hematite from ferrihydrite

Physical and chemical properties of superparamagnetic iron oxide MR contrast agents: Ferumoxides, ferumoxtran, ferumoxsil

The Möessbauer spectrum of ferrihydrite and its relations to those of other iron oxides

The first occurrence of schwertmannite in a natural stream environment

Fourier-transform infrared spectra of a synthetic akaganeite, β-FeOOH, were acquired in transmittance, attenuated total reflectance (ATR) and diffuse reflectance. The transmittance spectra showed a distinct dependence on the mode of sample preparation: measurements taken on pellets prepared by pressing the sample with alkali halides (KBr or CsI) displayed bands at 1096, 1050, and 698 cm−1 that were not observed in spectra of the neat material and must therefore be considered artifacts. Variations in the sampling environment (e.g., water and/or organic volatiles) were also observed to exert a noticeable influence on the development of the IR spectra.

Infrared bands due to akaganeite were found at 3480 + 3390 (doublet), 1630, 850 + 820 (doublet), 650, 490, and ~420 cm−1. Diffuse reflectance spectra dominated by volume scattering (loose), diffuse reflectance spectra comprised of both volume and surface scattering (compacted), ATR spectra (surface only), and transmittance spectra (absorbance only, inverse of ATR) showed compatible trends for the akaganeite features both above and below ~1000 cm−1. This indicates that the multiple akaganeite measurements are consistent and confirms the band assignments.

/pdf/the-infrared-spectrum-of-synthetic-akaganeite-b-feooh-3tvhzxzqjn.pdf

The infrared spectrum of synthetic akaganéite, β-FeOOH

Raman spectroscopy is likely to be one of a number of instruments deployed to Mars on future landed missions. The goals of these missions will include characterizing the mineralogy of the rocks and soils, in addition to determining if the planet may be or may have been habitable for life. The astrobiology questions are linked to understanding the aqueous and biogeochemical history of Mars. Clay minerals are studied here because if they are found on Mars they may provide information about the geochemistry of the primary rocks and the aqueous history of the planet. This study compares Raman and IR spectra of the SWy-1, STx-1 and SAz-1 montmorillonites. These samples contain admixtures of organic material, carbonate, amorphous silica and fine-grained quartz, which can all be detected using Raman spectroscopy. Less than 0.1 wt% C as organic material was detected in each of the samples through peaks associated with CC or CH bonds. Carbonate was detected in the Raman spectra for the sample containing 1.65 wt% CO3 but not for 0.15 wt% CO3. Raman spectra exhibit unique bands near 200 and 700 cm−1 that are due to tetrahedral SiO4 in layer silicates and vary depending on the mineral structure. Raman spectra of clay minerals also have unique metal–OH bending vibrations between 840 and 920 cm−1 that provide information about the octahedral cations present. Copyright © 2004 John Wiley & Sons, Ltd.

Characterization of minerals and biogeochemical markers on Mars: A Raman and IR spectroscopic study of montmorillonite

Ochreous precipitates deposited by waters draining a sulfide-rich lignite seam exposed in an abandoned mine in the Czech Republic show marked variations in color and mineralogy as a function of effluent pH. When initially formed under acidic (pH 3.7) conditions, the precipitates are orange in color and their mineralogy is dominated by schwertmannite. Following confluence with alkaline (pH 8.3) waters that have not been affected by mine drainage, the effluent pH rises to 7.3, the color changes abruptly to reddish-brown, and the principal precipitate mineral is a two-line ferrihydrite. This change in mineralogy and the associated variation in color can thus serve as direct indicators of the genetic environments under which the minerals were formed.

Enver Murad

Papers

The Möessbauer spectrum of ferrihydrite and its relations to those of other iron oxides

The first occurrence of schwertmannite in a natural stream environment

The infrared spectrum of synthetic akaganéite, β-FeOOH

Characterization of minerals and biogeochemical markers on Mars: A Raman and IR spectroscopic study of montmorillonite

Iron-rich precipitates in a mine drainage environment: Influence of pH on mineralogy