Al-maghemite

Bowen, L.H. De Grave, E. Bryan, A.M. (1994) Mossbauer studies in external field of well-crystallized Al-maghemites made from hematite. Hyperfme Interactions 94 1977-1982... 

Fig. 11-3. X-ray diffraetograms of (from top to bottom) pure magnetite Al-sub-stituted magnetite with 7 mol% Al maghemite produced by heating pure magnetite at 250 °C. Note the peak shift relative to pure magnetite due to AI substitution (middle) and to oxidation to maghemite (bottom).

In the case of roasting, the pretreatment process destroys the sulfide matrix by driving off sulfur from the structure. This results in the formation of iron oxide particles that are made of concentrically zoned and porous hematite and maghemite (Paktunc et al. 2006). Arsenic is volatilized as As203 and oxidised to... 

Substitution of a large range of cations can be readily induced in magnetite and maghemite because tetrahedral as well as octahedral positions are available. Sidhu et al. 

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). 

A solid state reaction in which the surface layers of magnetite are converted into maghemite appears to be involved as more chromate is adsorbed, further reduction is halted (Peterson et al., 1996). XAS showed that although adsorbed chromate was not reduced on the (112) plane of hematite, small amounts were reduced on (001) it was suggested that some Fe had been produced on the latter plane during annealing under vacuum (Kendlewicz et al., 1999). 

Litter et al. (1991) found that the dissolution of maghemite was also considerably speeded up, once a dissolved Fe "-oxalate or Fe" -EDTA complex was reduced to an Fe" complex by UV irradiation 1 = 254 nm). This system also showed an induction period which could be eliminated by addition of Fe " (see Fig. 12.28). In a study concerned with dissolution of corrosion oxide, electrons from viologen radicals produced by y-radiation ( Co) were used to dissolve hematite and goethite (Mulvaney et al., 1988) it was observed that the Fe " appearing in solution could only account for a fraction of the electrons consumed. The remainder was involved in conversion of the Fe " oxide into magnetite. 

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