Structural Model for Ferrihydrite

Abstract

The structure of 6-line and 2-line ferrihydrite (Fh) has been reconsidered. X-ray diffraction (XRD) curves were first simulated for the different structural models so far proposed, and it is shown that neither of these corresponds to the actual structure of ferrihydrite. On the basis of agreement between experimental and simulated XRD curves it is shown that Fh is a mixture of three components: (i) Defect-free Fh consisting of anionic ABACA... close packing in which Fe atoms occupy only octahedral sites with 50% probability; the hexagonal unit-cell parameters are a = 2.96/k and c = 9.40 A, and the space group is P31c. (ii) Defective Fh in which AClBC2A and AblCb2A structural fragments occur with equal probability and alternate completely at random; Fe atoms within each of these fragments have identical ordered distribution within the hexagonal super-cell with a = 5.126 A. (iii) Ultradispersed hematite with mean dimension of coherent scattering domains (CSD) of 10-20 A. The main structural difference between 6-line and 2-line Fh is the size of their CSD which is extremely small for the latter structure. Nearest Fe-Fe distances calculated for this new structural model are very close to those determined by EXAFS spectroscopy on the same samples.

Distribution, lithotypes and mineralogical study of newly formed thermogenic travertines in Northern Euboea and Eastern Central Greece

“…XRD analysis was performed on this amorphous material and the XRD pattern presents two broad peaks at about 35 • and 62 • in 2θ (Fig. 4), which are typical signatures of amorphous ferrihydrite [34,35]. The Ilia hot spring has significantly higher Fe concentrations (up to 11000 µg/L) compared to all other studied hot springs [32].…”

Transformation of 2-line ferrihydrite to 6-line ferrihydrite under oxic and anoxic conditions

“…Several structural models of 6-line ferrihydrite have been proposed, with the defective hematite structural model proposed by Chukrov et al (1973) and Towe and Bradley (1967) being the most widely accepted. In contrast, Drits et al (1993) proposed on the basis of XRD simulations that all natural and synthetic ferrihydrites are multi-component phases comprised of defect-free and defective ferrihydrite mixed with ultradisperse hematite. The main dif-ference between 2-line ferrihydrite and 6-line ferrihydrite is the size of their coherent scattering domains.…”

Nanoparticulate Iron Oxide Minerals in Soils and Sediments: Unique Properties and Contaminant Scavenging Mechanisms

“…It does not occur in crystallites larger than submicron size, and structural characterization has been debated in the literature for some time (Carlson & Schwertmann, 1981;Drits et al, 1993;Janney et al, 2000). The most disordered variety, referred to as ''2-line'' ferrihydrite, hydrous ferric oxide, or HFO, usually has grain sizes of only a few nm.…”