Variable-Temperature Mössbauer Spectroscopy of Aluminum-Substituted Maghemites
EMSL Project ID
30475
Abstract
Maghemite (γ-Fe2O3) is a ferrimagnetic iron oxide that is isostructural with magnetite (FeII FeIII2O4). It differs from magnetite in that most of the Fe is trivalent, and cation vacancies compensate for the charge imbalance created by Fe oxidation. Maghemites are commonly found in surface soils as fine-grained residues of burning and in a broad array of soils, sediments and corrosion deposits as topotactic oxidation products of precursor magnetite. Recently, maghemite has also been identified as an intermediate product of magnetite dissolution in high alkaline media like those occurring in waste plumes at the Hanford site in southeastern Washington. Other guest cations can replace Fe in the maghemite structure, but factors controlling the limits of solid solution as well as the distribution of foreign species between tetrahedral and octahedral sites are unclear. Aluminum, in particular, is abundant in most geologic substrates and is often mobilized with Fe during weathering. The Al(III) cation is 17% smaller than Fe(III), and it has been reported to replace up to 25 mole % of the Fe in natural maghemites. The presence and distribution of substituted Al(III) may have paragenetic significance (i.e., understanding pathways of mineral formation), and it may also influence the geochemical stability and interfacial properties of fine-grained maghemites in the environment. The objectives of the current study are to obtain a better understanding of 1) how Al(III) is spatially distributed among cation sites in the maghemite structure, and 2) the effect of Al(III) substitution on the physical, magnetic, and surface chemical properties of maghemite. To accomplish these goals, a series of synthetic, Al(III)-substituted maghemites with general formula (γ-Fe2-xAlxO3) will be synthesized and analyzed to determine the salient features of Al-for-Fe replacement in the host mineral structure. A critical component of this research will be the utilization of variable-temperature 57Fe Mössbauer spectroscopy to determine how Al(III) perturbs the local electronic and magnetic environments of Fe(III) in the Al-substituted specimens.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2008-09-22
End Date
2010-09-30
Status
Closed
Released Data Link
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Principal Investigator
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