Goethite and lepidocrocite

Biogenic hpidocrodte was first discovered by Lowenstam (1967) in the radula teeth of a chiton. The crystals are lath-shaped and several tenths of xm long with terminal 101 faces (Webb et al., 1989) (see Fig. 4.14a). The lepidocrocite is often associated with magnetite and ferrihydrite suggesting an Fe precursor (see Chap. 13). 

Ferrihydrite is the iron oxide with the most widespread distribution in living organisms. In the form of ferritin, an iron storage protein, it is found in all organisms from bacteria through to man (in heart, spleen and liver). It occurs in plants as phytoferritin (review by Seckback, 1982). Ferritin plays a key role in iron metabolism it maintains 

There is a number of synthetic substitutes for natural ferritin and the properties of these have been compared with those of ferritin. The synthetic polysaccharide iron complex (PIC), has a magnetic blocking temperature of 48K (Mohie-Eldin et al. 1994). Iron-dextran complexes are used as a substitute for ferritin in the treatment of anaemia. The iron cores of these complexes consist not of ferrihydrite, but of very poorly crystalline akaganeite with magnetic blocking temperatures of between 150 and 290 K (Muller, 1967 Knight et al. 1999) which were lowered from 55K to 35 and 25K, if prepared in the presence of 0.250 and 0.284 Al/(A1 -i- Fe), respectively (Cheng et al.2001). 

Formation of ferritin involves assemblage of the protein subunits to form the apo-ferritin shell which is then filled with the phosphated ferrihydrite core. The mechanism by which ferritin is filled and the iron core built up, has been investigated intensively in vitro. The experiments usually involved incubating apoferritin (from horse spleen) with Fe salts in the presence of an oxidant such as molecular oxygen. They showed that ferritin could be reconstituted from apoferritin and a source of Fe both the iron and the oxygen enter the protein shell, whereupon oxidation of Fe is catalysed by the interior surface of the protein shell (Macara et al., 1972). 

It is thought that oxidation of Fe takes place at specific sites within the protein shell and is followed by inward migration and hydrolysis to form a stable core nu- 

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Hydroxide Fe(OH)3 (Fe + plus OH ) has definite existence and there are many ill-deiined hydrates used as pigments. FeOOH has two forms goethite and lepidocrocite. Colloidal Fe(OH)3 is easily obtained as a deep red sol. Many Fe(III) hydroxy complexes are known. Fe(OH)2 may be formed from Fe and OH" in the absence of O2 but it is very readily oxidized. 

Goethite and lepidocrocite have recently been moved from Pbnm to Pnma and from Cmcm to Bbmm, respectively. As a result of this change, the crystal planes and directions in this book are different from those in the 1 edition. 

Fig. 18.5), overlain by magnetite (Fig. 18.4 B), with goethite and lepidocrocite at the outer surface of the tubercle (Fig. 18.4A). The magnetite probably formed by oxidation of the green rust. In the interior of the older tubercles, magnetite was oxidized to maghemite. The carbonate form of green rust was also identified in the inner layers of rust tubercles in pipes for drinking water (Stampfl, 1969).

Upon adsorption of Fe " at a solid surface, the standard redox potential of the Fe /Fe pair is reduced substantially from 0.77 V to 0.35-0.45 V (Wehrli, 1990) thereby facilitating the electron transfer. Buerge and Hug (1999) have demonstrated that this higher reactivity may be responsible for the fact that solid phases (Fe oxides, Si02, and clay minerals) in natural systems accelerate Cr reduction and that goethite and lepidocrocite are by far more active in this respect than the rest of the solid phases, because these two FeOOH forms adsorb much more Fe ". The authors attribute this to better overlap and charge delocalization at the surface of the Fe oxides. 

Ona-Nguema, G., Morin, G., Juillot, F. et al. (2005) Jr. EXAFS analysis of arsenite adsorption onto two-line ferrihy-drite, hematite, goethite, and lepidocrocite. Environmental Science and Technology, 39(23), 9147-55. 

Mao, H,-K. Bell, P. M. (1974b) Crystal field effects of ferric iron in goethite and lepidocrocite Band assignment and geochemical applications at high pressure. Ann. Rept. Geophys. Lab., Yearb. 73, 502-7. 

The reaction can be carried out over the pH range 6-14. Between pH 6-7 goethite and lepidocrocite result a pure product of either ean be obtained by adjusting the rate of oxidation and the concentration of carbonate in the system (Sehwertmann, 1959 b Carlson and Sehwertmann, 1990). At pH >8 magnetite is obtained and at pH 14, pure goethite is produced. With very rapid oxidation (e.g. by H2O2) feroxyhyte is obtained. 

Galvez, N., Barron, V and Torrent, J. (1999) Effect of phosphate on the crystallization of hematite, goethite, and lepidocrocite from ferrihydrite. Clays Clay Min. 47 304-311. 

Ferrihydrite, [5Fe203 9H2O], once called "amorphous ferric hydroxide", has been reported in bog iron and drainage ditches and in environments where other compounds prevent the formation of goethite and lepidocrocite (Schwertmann, 1988). It occurs as bulky precipitates containing water, adsorbed ions, and organic material. It appears as small (5.0-10.0 nm), spherical particles with a high surface area (200-350 m /g Schwertmann and Taylor, 1977). Ferrihydrite is probably the form of iron present in the B horizon of true Podzols. 

Figure 7. Electron micrograph taken after 302 days9 aging at Run 4, showing compact crystalline intergrowth of goethite and lepidocrocite...

The relative stabilities of two crystalline oxyhydroxides such as goethite and hematite or goethite and lepidocrocite can reverse with time owing to particle size effects. 

These pQ values are consistent with the results of Langmuir and Whittemore (33) for small crystals of goethite and lepidocrocite. The pQ for synthetic- goethite-coated sand is probably indicative of some quite amorphous material on the sand. By itself, agreement of pQ values would hardly be worth mention. The notable result is the complete agreement of electrode-based calculations over a wide pH range for two very different electrode surfaces, independent of solid phase present. 

Lepidocrocite is much less common than hematite and goethite, but it is not rare. Iron coatings around rice roots, formed of goethite and lepidocrocite, have been identified. Maghemite, magnetite, schwertmannite, and akaganeite are other Fe-oxides present in soil environments, which form under specific conditions (Cornell and Schwertmann, 1996). 

The extent of Cr(VI) reduction by the LA River magnetite is minor (Figure 9A), decreasing from 0.060 to 0.045 mM over 24 hours. No measurable Fe(III) is produced in solution suggesting that even this Cr(VI) loss is not attributable to heterogeneous reduction reactions (equation 14). As summarized by (25), chromate is adsorbed at pHs below 4 on ferric oxides such as the secondary goethite and lepidocrocite which were detected by X-ray analysis in the LA sample. 

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