Akaganeite and schwertmannite

Akaganeite formed by hydrolysis of acid FeCls solutions (OH/Fe = 0) at 25-100 °C precipitates as somatoids between 0.2-0-5 p.m in length and 0.02-0.1 Lim in width (Fig. 4.15 a). The crystals are elongated along the c-axis and are bounded by (001) and (200) planes (Mackay, 1962). Crystals grown at room temperature display a square 

Somatoids are often twinned on the (322) plane to give star-shaped or x-shaped twins (Eig. 4.15 a). Incorporation of low levels of Si in the structure promotes twinning with 0.04 mol mol Si, akaganeite was almost 100% twinned (Cornell, 1992). These crystals have a visibly roughened surface. Increasing citrate concentration during forced hydrolysis at 100 °C and pH 1 reduced the length of the somatoids from 

6 xm in the absence of citrate to some tens of nm at a citrate/Ee ratio of 0.02 a similar reduction in size was observed for goethite crystals (Kandori et al. 1991). 

Rod-like crystals (Eig. 4.15 b) are formed from partly neutralized Fe solutions (0 OH/Ee 3) (Mackay, 1962 Atkinson et al., 1977 Paterson Tait, 1977). They are usually monodisperse, around 50 nm long, 6 nm wide and also elongated in the [010] direction. In concentrated suspensions, these rods associate to form tactoids, 

spindle-shaped, anisotropic liquid droplets (0.2 mm long) of spontaneously orientated particles (Zocher, 1925 Mackay, 1962). 

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Akaganeite and schwertmannite form in acidic solutions by forced hydrolysis of FeCl3 or FeFs and Fe2(S04)3 solutions, respectively. For akaganeite a threshold concentration of cr or F ions must be present. 

Schwertmannite, Fei60is(0H)y(S04)z nHyO, has the same basic structure as akaganeite, but contains sulphate instead of chloride ions. This recently recognized mineral frequently occurs in nature as an oxidation product of pyrite and can be... 

Two poorly ordered compounds both having the akaganeite structme but with sulfate or nitrate instead of chloride in the tunnel have been found recently, The sulfate form oecurs frequently in nature as an oxidation product of pyrite and has, therefore, been reeognized as a mineral with the name schwertmannite (Bigham et al. 1994). Chemically it can be considered a Fe oxyhydroxy sulfate with the ideal formula Feg0s(0Ff)6S04. The corresponding nitrate form can be synthesized by forced hydrolysis of an acidic Fe (N03)3 solution at 80°C and is a precursor of ferrihydrite (Schwertmann et al. 1996). 

A complete series between schwertmannite in the pure sulfate system and akaganeite in the pure chloride system has been synthesized from Fe ksolutions with various C17S0 ratios (Bigham et al. 1990). An ion activity product (lAP) for the solubility of schwertmannite, log lAP = 18.0 2.5, was calculated from the chemical composition of a number of mine drainage solutions by Bigham et al. (1996). 

Schwertmannite, is a common nanoparticle-product of neutralization of sulfuric acid-rich solutions (Bigham et al. 1994). The original structural analysis indicated that sulfate was contained within tunnels similar to those found in akaganeite (FeOOH). However, recent work by Waychunas et al. (2001) suggests that this is a defective, nanoporous phase and that sulfate occupies inner and outer sphere positions on the surface, and probably on the internal surfaces of defect regions within the structure. 

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). 

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