Maghemite from lepidocrocite

Manganese oxides, which have different structural and surface properties, vary substantially in their ability to promote the precipitation and crystallization of Fe oxides and oxyhydroxides. The Mn(II) dissolved from Mn oxides in the presence of Fe(II) also influences the crystallization of oxidation products of Fe(II). The Fe oxides formed as influenced by Mn oxides and dissolved Mn(II) range from lepidocrocite, goethite, maghemite, dkaganeite, feroxyhyte, magnetite, honessite-like minerals, to noncrystalline Fe oxides. Therefore, Mn oxides deserve close attention in the genesis of Fe oxides. 

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). 

Magnetite is obtained in aqueous, alkaline systems by precipitation from a mixed Fe /Fe solution, by oxidation of Fe solution via green rust or Fe(OH)2, or by interaction of Fe with ferrihydrite. Another pathway involves high temperature reduction of Fe oxides (e. g. with H2). Maghemite forms topotactically by wet or dry oxidation of magnetite or by heating lepidocrocite and by thermal decomposition of various organic Fe-salts (cf chap. 20). 

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. 

Figure 10.2 Absorption spectra of Fe(III) oxides in the ultraviolet-visible region (left) and visible-near-infrared region (right) (from Sherman Waite, 1985). (a) Goethite (b) lepidocrocite (c) maghemite and (d) hematite. Measured reflectance spectra were converted into absorption spectra by applications of the Kebulka-Munk function. The vertical bars indicate band positions (listed in table 10.2).

Of the mineral forms, only hematite appears to have been mined and used in its massive form these compounds otherwise occur as greater or lesser components in secondary pigments such as the ochres, umbers and siennas qq.v.), either in the natural state or from calcination. Akaganeite, lepidocrocite and maghemite (with pyrolusite) have been identified by Zolensky (1982) in prehistoric pictographs in Seminole Canyon, USA. 

Synthetic maghemite may be produced by several methods. By far the commonest is heating (synthetic) lepidocrocite or magnetite at 250°C for 2-5 hours in oxidising conditions. It may also be produced from the controlled heating of other iron oxides at 500°C and in the presence of organic matter (Cornell and Schwertmaim, 1996). 

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