Lepidocrocite dehydroxylation

During dehydroxylation of lepidocrocite, the water molecules leave the lepidocrocite crystals causing the crystals to break up, so that only tiny crystals of maghemite form (see Chap. 14). Because the crystals are so small, the vacancies are not ordered, hence the XRD pattern shows no superstructure lines. The [001] and [110] directions of the maghemite crystals parallel the [001], and [100] directions, respectively of the lepidocrocite crystals. 

With lepidocrocite the dehydroxylation endotherm due to transformation to maghemite is followed by an exotherm indicating transformation of maghemite to hematite. The temperature of the dehydroxylation endotherm was found to increase from 270 to 300 °C as A1 substitution rose from Al/(Fe-tAl) of 0 to 0.12 (Schwertmann Wolska, 1990) and that of the exotherm rose from 500 to 650 °C (Wolska et al., 1992). Synthetic feroxyhyte shows a weak dehydroxylation endotherm at ca. 260 °C (Carlson Schwertmann, 1980). 

Under otherwise similar conditions, low oxidation rates appear to promote magnetite and goethite, whereas high rates favor lepidocrocite. Magnetite formation probably requires slow oxidation because complete dehydroxylation of the precursor (green rust) prior to complete oxidation is only possible if sufficient time is available if, on the other hand, complete oxidation is fast and precedes dehydroxylation, lepidocrocite forms in preference to magnetite (Schwertmann Taylor, 1977). Dehydroxylation and oxidation appear to be competing reaction steps. 

Lepidocrocite Maghemite, Hematite Thermal dehydroxylation Gas/vacuum... 

The end product of the dehydroxylation of pure phases is, in all cases, hematite, but with lepidocrocite, maghemite occurs as an intermediate phase. The amount of water in stoichiometric FeOOH is 10.4 g kg , but adsorbed water may increase the overall amount released. Thermal dehydroxylation of the different forms of FeOOH (followed by DTA or TG) takes place at widely varying temperatures (140-500 °C) depending on the nature of the compound, its crystallinity, the extent of isomorphous substitution and any chemical impurities (see Fig. 7.18). Sometimes the conversion temperature is taken from thermal analysis data (e. g. DTA), but because of the dynamic nature of the thermoanalysis methods, the temperature of the endothermic peak is usually higher than the equilibrium temperature of conversion. 

Fig. 14.6 Interface between intact lepidocrocite and collapsed layers (after dehydroxylation) forming maghemite (Giovanoli, Brutsch, 1975 with permission).

Thermal methods of analysis are often useful in the characterization of minerals, as described in Section 7.6.5. Aluminum hydroxides such as gibbsite show a mass loss of 34.6% on dehydroxylation thus, they show an important negative peak in DTA and a marked mass loss in TGA, and so these techniques are employed for both qualitative and quantitative characterization of these minerals. The same happens with iron hydroxides and oxohydroxides, such as goethite, lepidocrocite, and so on also, the presence of OH groups in otherwise thermally inert minerals such as hematite can be detected. 

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