Akaganeite somatoidal crystals

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). 

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 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... 

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). 

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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