Akaganeite crystallinity

Fig. 15.8 Well ciystalline euhedral platy hematite (a), poorly crystalline spherical Si-containing hematite together with ring-like layer Fe-silicates ( ) (b), and akaganeite (c) from the Atlantis Deep, Red Sea, (Photo H.-Ch. Bartscherer) (Schwertmann etal., unpubl.)...

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). 

Hydrothermal transformation of various Fe oxides. Ferrihydrite (2-line), lepidocrocite, akaganeite and goethite (if poorly crystalline) can be converted to large (1-3 am) hexagonal plates of hematite if kept under water in a teflon bomb at 180 °C for 10 days. 

Dominant constituent of the Fe oxide precipitate. PCL = poorly crystalline lepidocrocite L = lepidrocrocite A = Akaganeite F = feroxyhyte M = magnetite NC = non-crystaUine. The differential infrared data (not shown) are in accord with x-ray data. 

Ferric oxyhy dr oxide minerals or phases which occur naturally are listed in Table I, and except for akaganeite, their occurrences have been described by Palache et al. (3). The oxyhydroxides found as precipitates in natural waters are usually goethite and x-ray amorphous material. Amorphous material, which comprises a relatively large proportion of most fresh precipitates, is formed under conditions of substantial supersaturation with respect to the crystalline oxyhydroxides. An amorphous phase develops by rapid hydrolysis of dissolved ferric species, particularly at pH s below 4-5 where the total concentration of such species can exceed 0.01 ppm. Amorphous material is also produced during the rapid oxidation and hydrolysis of ferrous iron-rich solutions. 

To conclude, it is clear there is a strong dependence of the performance of synthetic hematite on the deposition technique. While methods such as spray pyrolysis and CVD consistently produce electrodes photoactive for water oxidation, solution-based methods such as sol-gel approaches have failed to produce especially photoactive hematite. This is certainly related to the quality of the prepared material in terms of crystallinity and impurity concentrations. Aqueous methods of preparing hematite typically pass through a phase containing iron hydroxide (e.g., akaganeite, lepidocrocite, or goethite) but primarily hematite is detected after at annealing at 500°C. However, it has been shown that at temperatures up to 800°C, a nonstoichiometric composition remains in hematite when prepared in this way... 

In weathering, steel rust formed on atmospheric corrosion in different environments is composed of crystalline compounds like haematite, magnetite and oxyhydroxides of iron like goethite, akaganeite, lepidocrocite and feroxyhite apart from amorphous ferric oxyhydroxide rust. These rust constituents transform to one another during wet-dry cycles of atmospheric exposure [17].Various phases of corrosion products formed in progressive exposure to atmosphere are given in Table 1.1 [6, 18]. 

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