Akaganeite surface area

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). 

Dissolution curves for akaganeite in 0.5 M HCI at 76 °C were sigmoidal. TEM showed that acid attack was concentrated along the [001] direction initially the tapered ends of the spindle-shaped crystals (see Fig. 4.15 a) became squared and, as dissolution continued, the crystals became shorter (Fig. 12.24a-c). With further dissolution the crystals were gradually hollowed out (Fig. 12.24d) and it is the resulting increase in surface area that is thought to be responsible for the shape of the dissolution curve (Cornell Giovanoli, 1988 a). 

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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