Maghemite unit cell

Maghemite has a cubic unit cell the a value of which was first determined by Hagg (1935). The currently accepted value of a is 0.834 nm. Each cell of maghemite contains 32 0 ions, 21 /s Fe " ions and 2 73 vacancies. Eight cations occupy tetrahedral sites and the remaining cations are randomly distributed over the octahedral... 

A related system is that of the titanomaghemites, Fe2Ti05, which are formed by oxidation of the titanomagnetites. These are spinels with vacancies in some of the cation positions. The detailed distribution of cations and vacancies is not fully understood a discussion of the different models and possibilities is given by Lindsley (1976). A non-linear relationship between the unit cell size of titanomaghemite (a = 0.8483 nm) and that of maghemite (a = 0.835 nm) was matched by a non-linear increase in the Curie temperature from 80 to 450 °C (Dunlop Ozdemir, 1997). 

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). 

Two different pathways of formation are possible (Stanjek, 2000). One route involves aerial oxidation of lithogenic magnetite as suggested for Brazilian Oxisols on basic igneous rocks. The mechanism of this topotactic reaction is described in Chapter 14. These maghemites are usually titaniferous as are the magnetites from which they are derived (see Chap. 15) and almost free from or very low in Al (Allan et al., 1989). Their unit cell size is a function of the residual Fe" and the Ti content. 

Da Motta Kampf, 1992), but no correlation has been found in others (Anand Gilkes, 1987 Zeese et ah, 1994). The ratio may depend on whether or not the two oxides were formed simultaneously in the same environment. Maghemites from tropical soils contained A1 up to an Al/(Fe -i- Al) ratio of ca. 0.15 as indicated by chemical analysis and reduction in unit cell size (Schwertmann Fechter, 1984 Fontes Weed, 1991). 

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