Magnetism of intermediate hydromaghemite in the transformation of 2-line ferrihydrite into hematite and its paleoenvironmental implications

Liu, Qingsong, Barrón, Vidal, Torrent, José, Eeckhout, Sigrid G. and Deng, Chenglong (2008) Magnetism of intermediate hydromaghemite in the transformation of 2-line ferrihydrite into hematite and its paleoenvironmental implications. Journal of Geophysical Research, 113, B01103. (doi:10.1029/2007JB005207).

Abstract

A maghemite-like phase referred to here as “hydromaghemite” was obtained as an intermediate product in the hydrothermal transformation of phosphated 2-line ferrihydrite into hematite. In this study, we used magnetic and non-magnetic (e.g., X-ray Absorption Near Edge Structure, XANES) techniques in combination to characterize a series of the intermediate products obtained in the aging of phosphated ferrihydrite (P/Fe atomic ratio = 0.03) at 150°C for 120 days. Particle size calculated from both the specific surface area and average unblocking temperature increased with time. XANES spectra revealed the presence of some tetrahedrally coordinated iron, which is consistent with the formation of hydromaghemite (the dominant magnetic phase in the intermediate products). Thus grain size in newly formed hydromaghemite particles increased with time from the initial values in the superparamagnetic region to others in the single-domain region. Further transformation of hydromaghemite into hematite, which was complete by day 120, was probably due to hydromaghemite becoming unstable relative to hematite when the surface to volume ratio fell below a given threshold. The relationships between pedogenically produced maghemite and hematite contents in various soils and paleosols suggest that the ferrihydrite ? hydromaghemite ? hematite transformation may constitute a major pathway accounting for the magnetic enhancement in many soils.

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