Ferrihydrite electron micrographs

Fig. 1417 Transmission electron micrographs documenting the transformation of ferrihydrite to hematite (Fischer. Schwert-mann, 1975 with permission).

Fig. 16.9 Electron micrographs of soil lepidocro-cite. a) Large multidomainic lath-like crystal viewed perpendicularto [001] with laminar pores from a re-doximorphic soil, Natal, South Africa, b) Poorly crystalline grassy lepidocrocite crystals mixed with tiny ferrihydrite particles and pseudo-hexagonal kaolinite platelets. Origin as before (a. b courtesy P. Self), c) Small lepidocrocite crystal from a hydromorphic soil (with ferrihydrite) viewed perpendicularto [001] and showing (020) lattice fringes (see also Schwert-mann. Taylor, 1989,with permission).

Fig. 16.20 Electron micrographs of synthetic associations between iron oxides and Si-miner-als. Normal (a, b) and shadowed (c, d) kaolinite - 6-line ferrihydrite associations at pH 3 (a, d) and 9 (b, c) (Saleh Jones, 1984 with permis-...

There is no information on the site of formation of the Fe(III)-0-Fe(III) dimers that have been observed in rHF as well as in horse spleen ferritin (57,112). This may be clarified by Mossbauer spectroscopy on ferritin variants. Possible positions are the putative nucleation center or the double Tb site. In the latter case, either the dimers themselves must move or the iron core nucleus must incorporate iron atoms from the ferroxidase centers. Because ferrihydrite can be deposited both in rLF and in the rHF variant lacking ferroxidase activity, nucleation at this center is not possible for these molecules. Indeed, a specific center may not be required. However, electron micrographs of broken ferritin molecules suggest an attachment of ferrihydrite to the protein shell (113). 

Fig. 5-1. Electron micrographs of Al-substituted acicular goethites illustrating the decrease in crystal size with increasing degree of Al-substitution (given as Al/(AI+Fe) moEmoI). The goethites were produced by aging 2-Iine Al-contain-ing-ferrihydrites in 0.35-0.4 M KOH for 14 days at 70 °C (Cornell and Schwert-marm 1996 with permission)..

Fig. 5-15. Electron micrograph replica of acicular goethite outgrowths on a hematite eore. The crystals grew from ferrihydrite at pH 11.7 and 70 °C in the presence of 10 M maltose in 24 h. Bar =100 nm. (see Cornell, 1985).

Fig 8-2. Electron micrograph of a 6-line (a) and a 2-line (b) ferrihydrite. (Cornell and Schwertmann, 1996 with permission.)... 

Fig. 8-3. High-resolution electron micrograph of a 2-line (upper) and a 6-line (lower) ferrihydrite. The micrographs show lattice fringes and possibly hexagonal crystal shapes, both being better expressed in the 6-line than in the 2-line form (Courtesy D.E. Jaimey), (Janney et al. 2000 with permission).

Al-substituted hematites were also produced from Al-containing 2-line ferrihydrite at room temperature and pH 4-7, although at a much lower rate (months to years) (Schwertmann et al. 2000). For the same initial Al/ (Al+Fe) ratios the substitution is lower than at higher temperatures. Electron micrographs show rhombic crystals at low substitution (Fig. 10-5 a), identical to unsubstituted ones, and framboidal or spindle-shaped crystals with a grainy or layered interior at higher substitution (Fig. 10-5b). The latter diffracted X-rays as single crystals. 

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