New Data and a Revised Structural Model for Ferrihydrite

Abstract

Abstract--Synthetic 2-line and 6-line ferrihydrite samples prepared from ferric nitrate solutions have the bulk compositions Fe4(O,OH,H20)12 and Fe4.6(O,OH,H20)lZ, respectively. The composition depends on crystal size, which averages 20/~ for 2-line and 35/k for 6-line ferrihydrite. X-ray absorption edge spectra indicate the presence of tetrahedral Fe 3+, a conclusion supported by heating experiments which show the development of maghemite after heating to 300~ in the presence of N2, followed by the formation of hematite at higher temperatures. These two reactions are recorded on differential thermal analysis traces by exotherms at 350 ~ and 450~ Transmission electron microscopy shows that 2-line ferrihydrite has no Z-axis regularity but does show hexagonal 2.54-/~ lattice fringes. Six-line ferrihydrite forms faceted crystals having 9.4-/~ c-parameter only detectable in dark field. In bright field, 2.54-/~ lattice fringes indicate greater atomic regularity than in 2-line ferrihydrite. Analysis of the X-ray powder diffraction pattern of 6-line ferrihydrite suggests a structure based on double-hexagonal close-packed oxygens, containing 36% Fe in tetrahedral sites. Selective chemical dissolution, surface area measurements, and magnetic susceptibility are consistent with the recorded properties of ferrihydrite.

Silicon-Rich, Iron Oxide Microtubular Sheath Produced by an Iron-Oxidizing Bacterium, Leptothrix sp. Strain OUMS1, in Culture

“…Thus, the crumpling feature of Si-0 in Figure 2f suggests that the lepidocrocite forms rapidly under the present cultivation conditions in Si-0 and -11.5. Similar to a finding by Eggleton et al [26], we observed a very broad peak at 30°-40° (black triangle), which plausibly was derived from 2-line ferrihydrite (2Fh), for Si-11.5 to Si-300. Based on these patterns, the sheath texture in Si-0 and -11.5 was composed of a single phase of poorly crystalline lepidocrocite and a mixed phase of 2Fh and poorly crystalline lepidocrocite, respectively.…”

The influence of silicate on transformation pathways of synthetic 2-line ferrihydrite

“…A total mass loss of FHYD-000 is 21.28 mass% and remains similar for all the samples studied. The losses are comparable with earlier measurements of both synthetic [5,17,35,37,39] and natural ferrihydrites [16,25,49], but are explicitly higher than theoretical water content (*17 mass%), due to a high amounts of physisorbed water. A bend on DTA curves of low-Si samples at ca.…”

“…These ions affect ferrihydrite composition, surface chemistry, reactivity and sorption properties [14,[17][18][19][20][21][22]. Silicate, the most important impurity, lowers crystallinity of this nanomineral and modifies its morphology, magnetic ordering and solubility [15,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39], though the ferrihydrite structure itself is not directly altered [30]. Reducing crystallinity has been thought to result from the formation of soluble Fe-Si complexes during the oxyhydroxide precipitation.…”

“…The transformation of ferrihydrite to hematite is reflected by the sharp exotherm visible on DTA (or DSC) curves at approximately 300-350°C, though the exact position of the peak depends on both the experimental conditions and the method of synthesis. In some cases, two [1,35] or even three exotherms were recorded [36]. Three different thermal patterns of ferrihydrites, obtained by three different procedures, were argued to reflect various proportions of so-called f-phase in the standard multiphase structural ferrihydrite model [37].…”

Influence of Fe(II) on the Formation of the Spinel Iron Oxide in Alkaline Medium

“…It exhibits all the features of the deficient spinel structure of oxidized magnetite; however, because of the process of formation, long range ordering may still be poor, significant hydroxylation is probably retained, and tetrahedral sites may be partly vacant. Pure ferrihydrite is well known to be a phase which shows variable degrees of hydration and ordering, (e.g., Chukhrov et al, 1973;Eggleton and Fitzpatrick, 1988). The "m" species might actually be an additional variety, with prevailing ccp ordering.…”