Tuesday, 11 July 2006 - 8:50 AM

Concomitant Formation of Maghemite and Hematite in Aerobic Soils.

Vidal Barrón, Estrella Cabello, and José Torrent. Universidad de Córdoba, Edificio C4, Campus de Rabanales, Córdoba, Spain

Soil formation usually increases magnetic susceptibility, most often by increasing the concentrations of ferrimagnetic minerals. In soils not affected by detrital magnetic inputs, waterlogging, and/or thermal transformations of other iron (hydr)oxides, magnetic enhancement is ascribed to the neoformation of nanosized magnetite and/or maghemite. Both inorganic and bacterially mediated pathways to magnetite have been proposed that require the presence of reductive conditions and the resulting accumulation of the ferrous ion in the soil solution. On the other hand, laboratory experiments show that synthetic 2-line ferrihydrite can partly transform into maghemite at 150°C in aerobic conditions. This occurs upon aging of ferrihydrite precipitated in the presence of phosphate or other ligands (e.g., citrate) capable of ligand exchange with Fe-OH surface groups. The resulting water-rich maghemite consists of particles in the superparamagnetic-single domain size range (15-35 nm) and is a transient phase in the ferrihydrite-to-hematite transformation. Aging ferrihydrite (pure or doped with different ligands) at room temperature for several months results in a significant increase in magnetic susceptibility, which suggests incipient maghemite formation. We contend that the proposed ferrihydrite → maghemite → hematite pathway has been operative during the genesis of many aerobic soils. This contention is supported by two observations: (i) the concentrations of pedogenic hematite and maghemite are correlated in hematite-containing soils in different climatic regions and on different parent materials, as exemplified by soil profiles on Russian and Argentinian loess (Fig. 1a) and on calcarenites of southern Spain (Fig. 1b), and (ii) in areas where both goethitic and hematitic soils have formed, the magnetic susceptibility attributable to pedogenic Fe oxides is much higher in the latter than in the former. The study of soils of different geographic areas also suggests that the maghemite/hematite ratio is influenced by the environment and degree of development of the soil, so it is a potentially effective tool for paleoenvironmental and planetary studies.

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