Akaganeite somatoids

Akaganeite Somatoids, Rods Stars, crosses (twins), hexagons, prisms ... 

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... 

Fig. 4.15 Synthetic akaganeite a) somatoidal (Murad, 1979, with permission) (b) rod-like, (c) Si-akaganeite (Schwertmann Cornell, 2000, with permission)...

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... 

The surface area of akaganeite also depends upon the crystal shape (see Chap. 4). Samples of somatoidal crystals usually have an area of between 20-60 m g", whereas areas of rod-like crystals range from 100-150 m g . Type II (Paterson and Tait, 1977 Naono et al., 1982) and type IV N2 isotherms (Gonzalez-Calbet et al., 1981) have been reported for the somatoids. Isotherms for the rod-like crystals were type IVand non reversible (Paterson and Tait, 1977). 

Fig. 12.24 Morphological changes of somatoidal akaganeite C7stals after a) 10 b) 25 c) 50, Cr-shadowed and d) 80% dissolution (Cornell, Ciovanoli, 1988a, with permission).

Hold 2 L of 0.1 M FeCl3 solution in a closed vessel at 70 °C for 48 h. During this period the pH of the system drops from ca. 1.7 to 1.2 and a compact yellow precipitate forms. This method gives around 5 g akaganeite consisting of somatoidal crystals. The presence of Cl is essential. 

Akaganeite cannot be prepared at pHs above 5 because the OH ion is far more competitive than the chloride ion for structural sites. Akaganeite displays two morphologies somatoids or cigar shaped crystals and much smaller rod-like crystals. Samples of somatoids frequently contain a proportion of twinned crystals, whereas the rod-like crystals are never twinned. 

The method gives 5 g akaganeite (27% yield). The crystals are well-formed somatoids around 300 nm in length and elongated in the c-direc-tion (Fig. 9-1). The sample has a surface area of around 30 m /g and sharp XRD peaks (Fig. 9-2), indicating a well-crystallized material. An IR spectrum is shown in Fig. 9-2. The Mossbauer spectrum shows three doublets at 293 K and three sextets in the magnetically ordered state at 78 K (Fig. 9-3). 

Fig. 9-3. Mdssbauer spectra of somatoidal akaganeite recorded at room temperature and at 78 K (see Murad, 1979).

The method produces 2.25 g akaganeite with a surface area of 30-40 m /g. The crystals are approximately somatoidal and between 200-400 nm long. 

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