Maghemite transformation

Maghemite is ferrimagnetic at room temperature. Measurement of Tc is difficult because maghemite transforms to hematite at temperatures above ca. 7-800 K. The Curie temperature has been estimated as lying between 820 K and 986 K (Murad, 1988). 

Under hydrothermal conditions (150-180 °C) maghemite transforms to hematite via solution probably by a dissolution/reprecipitation mechanism (Swaddle Olt-mann, 1980 Blesa Matijevic, 1989). In water, the small, cubic crystals of maghemite were replaced by much larger hematite rhombohedra (up to 0.3 Lim across). Large hematite plates up to 5 Lim across were produced in KOH. The reaction conditions influenced both the extent of nucleation and crystal morphology. The transformation curve was sigmoidal and the kinetic data in water and in KOH fitted a first order, random nucleation model (Avrami-Erofejev), i.e. 

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). 

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). 

In the dry state magnetite is readily oxidized to maghemite by air. Ultrafme crystals of magnetite change (over years) from black to the brown of maghemite even at room temperature (Murad Schwertmann, 1993). At temperatures >300°C, the transformation proceeds further to hematite (see section 14.2.7). 

A feature of this transformation is the influence of magnetite crystal size on the nature of the reaction products (Feitknecht, 1964 Gallagher et ak, 1968 Gillot et ak, 1978). At 200-250 °C, crystals smaller than 300 run transformed via the mixed phase to maghemite which in turn transformed to hematite at temperatures above 500 °C. 

Magnetite transforms to maghemite (and thence to hematite) in water or alkali under hydrothermal conditions. Conversion to maghemite also involves outward migration of cations via cation vacancies (Swaddle Oltmann, 1980). The hydrothermal transformation is slower than that in air at the same temperature (180 °C) and it has been suggested that this is because the cation vacancies which assist cation diffusion are reduced or eliminated by the large excess of water. 

In acid media (pH 2) magnetite crystals ca. 10 nm across transform topotactically to maghemite via an adsorption reaction which traps mobile electrons from the bulk material and reduces interfacial Fe the Fe ions that form are selectively leached into solution (Jolivet Tronc, 1988). Electron delocalization also induces ferrihydrite in contact with small magnetite particles to transform into a spinel layer (Belleville etal., 1992). 

Randrianantoandro, N. Mercier, A.M. Hervieu, M. Greneche, J.M. (2001) Direct phase transformation from hematite to maghemite during high energy ball milling. Mat. Letters 47 150-158... 

Swaddle, T.W. Oltmann, P. (1980) Kinetics of the magnetite-maghemite-hematite transformation, with special reference to hydrothermal systems. Can. J. Chem. 58 1763-1772 Swallow, K.C. Hume, D.N. Morel, F.M.M. (1980) Sorption of copper and lead by hydrous ferric oxide. Environ. Sci. Tech. 14 1326-1331... 

Yapp. C.J. (2000) Climatic implications of surface domains in arrays of 5D and 5180 from hydroxyl minerals Goethite as an example. Geochim. Cosmochim. Acta 64 2009-2025 Yariv, S. Mendelovid, E. Villalba, R. (1980) Thermal transformation of goethite into hematite in alkali halide discs. J. Chem. Soc. Faraday Trans. I. 76 1442-1454 Yariv, S. Mendelovid, E. Villalba, R. Cohen, M. (1979) Transformation of goethite to maghemite in Csl discs. Nature 279 519-520... 

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