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Clays and Clay Minerals; February 2009; v. 57; no. 1; p. 46-53; DOI: 10.1346/CCMN.2009.0570105
© 2009 Clay Minerals Society
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MAGNETIC ENHANCEMENT DURING THE CRYSTALLIZATION OF FERRIHYDRITE AT 25 AND 50°C

E. Cabello1, M. P. Morales2, C. J. Serna2, V. Barrón1,* and J. Torrent1

1 Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Edificio C4, Campus de Rabanales, 14071 Córdoba, Spain
2 Instituto de Ciencia de Materiales de Madrid, CSIC, Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain

* E-mail address of corresponding author: cr1balov{at}uco.es

Soil formation usually results in an increase in magnetic susceptibility. The magnetic properties of the products of transformation of ferrihydrite, a typical precursor of other soil Fe oxides, were examined in the present work. Synthetic 2-line ferrihydrite was aged at two temperatures (25 and 50°C) and two different relative humidities (80 and 100%) in the presence of silicate, phosphate, citrate, and tartrate as adsorbed ligands (molar anion/Fe ratio = 1–3%). The ligands delayed or prevented the transformation of ferrihydrite to hematite. The magnetic susceptibility of the ferrihydrite transformation products increased with aging, the rate of increase depending on the type of ligand added and its concentration. The largest increase in magnetic susceptibility, sixfold, was obtained with ferrihydrite in a citrate/Fe ratio of 1%, after 1500 days. The resulting magnetic products exihibited superparamagnetic behavior at room temperature and high coercivity at 5 K. The formation of an intermediate ferrimagnetic phase in the ferrihydrite-to-hematite transformation might explain the magnetic enhancement observed in many aerobic soils lacking other sources of magnetic minerals.

Key Words: Crystallization • Ferrihydrite • Ferrimagnetism • Hematite • Maghemite






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