HR: 0800h
AN: GP31B-0833    [Abstracts]
TI: Magnetic Evidence for Metastable Ferrihydrite Particles in a Vertisol
AU: * Gehring, A U
EM: gehring@sl.ethz.ch
AF: Institute of Geophysics, ETH Zurich, Zurich, 8093 Switzerland
AU: Schill, E
EM: eva.schill@mag.ig.erdw.ethz.ch
AF: Institute of Geophysics, ETH Zurich, Zurich, 8093 Switzerland
AU: Weidler, P G
EM: peter.weidler@itc-wgt.fzk.de
AF: ITC-WGT, Forschungszentrum Karlsruhe, Karlsruhe, 76021 Germany
AU: Luster, J
EM: joerg.luster@wsl.ch
AF: WSL/ETH, Zurcherstrasse 111, 8903, Birmensdor Switzerland
AB: Poorly crystalline iron oxides in soils are of ecological importance since their chemical properties (e.g. solubility, surface area and reactivity) affect the bioavailability of iron and promote processes such as adsorption and transport of metals, pesticides, and organic nutrients. In natural environments these highly reactive phases are difficult to detect because they form generally a minor component with a nano-sized nature. Ferrihydrite is the best known phase among these materials with ambiguous identity. For the magnetic and soil-chemical investigation a Vertisol in southern Mali was chosen. Southern Mali has a subtropical climate with a wet and a dry season, an annual rainfall of 1300mm, and an average temperature of $27\deg$C. The Vertisol mainly consists of montmorillonite and quartz, and no iron oxides were detected by X-ray diffractometry. It has a relatively high amount of oxalate extractable Fe and reveals a strong increase of IRM acquisition at 77K compared to room temperature. In order to characterize the low temperature magnetic phase, AC susceptibility frequency experiments on bulk samples were conducted between 2 and 300K with 10 and 1000Hz. The in-phase susceptibility exhibited an increase at 120K followed by two peaks at 65 and 35K and a steep increase at 25K. In the temperature range of the peaks a clear frequency dependency was observed. The out-of-phase component revealed a similar behavior with a more pronounced frequency dependency below 65K. The beginning of magnetic ordering in the out-of-phase component at about 120K and the two peaks are assigned to two ferrihydrites with different magnetic properties. After oven-drying the bulk samples at $60\deg$C, the two peaks had vanished. This behavior suggests that ferrihydrites are metastable and convert into a thermodynamically more stable phase.
DE: 1045 Low-temperature geochemistry
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting