8-FeOOH unit cell

FeOOH is isostructural with Fe(OH)2. It has a hexagonal unit cell with a = 0.294 nm and c = 0.449 nm. A related compound exists in nature under the mineral name feroxyhyte (named by F.V. Chukhrov see Chukhrov et al., 1976), also termed 5 -FeOOH. Feroxyhyte has a hexagonal unit cell with a = 0.294nm and c = 0.456 nm. It contains two formula units per unit cell, whereas the unit cell of the synthetic material contains only one formula unit. 

Fig. 2.8 Structure of high pressure FeOOH. a) Octahedral chains, b) Ball-and-stick model with unit cell outlined, (a, b Stanjek, unpubl.)...

Extensive replacement of Fe by transition metal cations and alkaline earth ions has been reported for b-FeOOH (Okamoto, 1968). Muller et al. (1979) found incorporation of up to 0.4 mol moF Ca solid solutions with the formula Fei xKxOi x(OH)i+x could be identified. Jimenez-Mateos et al. (1990) reported that Co and Mn, respectively, could replace up to 0.3 and 0.5 mol mol Fe. The unit cell parameters decreased in both cases with increasing substitution. These Mn- and Co-substituted 5-FeOOHs decomposed at 200 °C to give poorly crystalline, substituted hematites. 

The thermal transformation of feroxyhyte (5 -FeOOH) was studied by Carlson and Schwertmann (1980). Synthetic feroxyhyte transformed to hematite with non-uni-formly broadened XRD lines at 240 °C (DTA). As the temperature increased further, an exothermic peak appeared and the crystallinity of the hematite improved. In an atmosphere of N2 the transformation of natural feroxyhyte was impeded. As the temperature rose, the crystallinity of this feroxyhyte improved and at 460 °C, the a unit cell edge length dropped from 0.5062 to 0.5027 nm. As this sample contained organic impurities, the final transformation product in this case, even at 800 °C, was maghemite (see p. 368). 

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