Akaganeite and schwertmannite to hematite

Decomposition of akaganeite starts at 150 °C and complete conversion to hematite is achieved at ca. 500 °C. This is not a topotactic transformation it involves a complete breakdown of the bcc anion packing of akaganeite followed by reconstruction of the hep anion array of hematite. Initially, the product is in the form of elongated, porous 

The sample used by Naono et al. (1982) was a non-porous one (based on a t-plot) (Fig. 14.8) with a BET surface area of 22 m g . It developed a maximum surface area of 178 m g at 200 °C due to the formation of a system of slit-shaped pores ca. one nm wide (see Fig. 14.2 c). During this process, a contraction of ca. 30% occurred along [100] and [010], but not along [001], i.e. not along the tunnels. With increasing temperature, the pores widened to mesopores and irregular macropores. The surface area of the hematite that finally formed at 500 °C was only 23 m g .  

There is some uncertainty about whether akaganeite transforms directly to hematite. Some authors (Bernal et al., 1959 Dezsi et al., 1967 Morales et al., 1984) con- 

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