HR: 17:45h
AN: GP34A-08    [Abstracts]
TI: Accurate Granulometry Of As-Substituted Nanophase Ferrihydrite Particles Using Static And Dynamic Magnetic Properties
AU: * Berquo, T S
EM: berqu013@umn.edu
AF: Institute for Rock Magnetism University of Minnesota, 291 Shepherd Labs 100 Union Street S.E., Minneapolis, MN 55455 United States
AU: Guyodo, Y
EM: yohan.guyodo@lsce.cnrs-gif.fr
AF: Laboratoire des Sciences du Climat et de l'Environnement (LSCE) Domaine du CNRS, 12, Avenue de la Terrasse, Gif-sur-Yvette Cedex, 91198 France
AU: Banerjee, S K
EM: banerjee@umn.edu
AF: Institute for Rock Magnetism University of Minnesota, 291 Shepherd Labs 100 Union Street S.E., Minneapolis, MN 55455 United States
AU: Lee Penn, R
EM: penn@chem.umn.edu
AF: Department of Chemistry University of Minnesota, B4, 139 Smith Hall 207 Pleasant Street SE, Minneapolis, MN 55455 United States
AU: Aschbrenner, N
EM: aschbren@yahoo.com
AF: Department of Chemistry Lawrence University, 115 S. Drew Street, Appleton, WI 54912 United States
AU: Solheid, P
EM: peat@umn.edu
AF: Institute for Rock Magnetism University of Minnesota, 291 Shepherd Labs 100 Union Street S.E., Minneapolis, MN 55455 United States
AB: Ferrihydrite typically occurs in the nanometer size range and is suspected to be the precursor common to diagenetic formation of goethite, hematite, and magnetite in near surface environments. Identifying and estimating the relative grain-size of ferrihydrite (Fh) can be achieved rapidly, albeit qualitatively, by low temperature magnetic susceptibility (Guyodo et al., 2003). We have now undertaken a program of cross-validated quantitative granulometry of Fh using multiple magnetic techniques and M\"{o}ssbauer spectral analysis between 4.2 and 300K. Fh is antiferromagnetic, but a superimposed magnetic moment can result from uncompensated Fe$^{3+}$ spins. Here we present results from synthetic ferrihydrite with varying amounts of arsenic substitution (0, 1, and 10 wt %). Static and dynamic magnetic techniques were used to elucidate the magnetic behavior of these samples and estimate the grain size. First, ZFC/FC magnetization curves obtained by cooling in zero field (ZFC) and then measuring the induced magnetization at stepwise increasing temperatures, from 2 K to 300 K, in a small applied field (B=5 mT). The sample was again cooled in the same small field (FC), and FC magnetization curves were obtained by measuring induced magnetization at stepwise increasing temperatures. Second, the samples were cooled, with an applied field of 50 mT and susceptibility was measured in stepwise increasing fields (50 mT) with changing frequency (1, 10 and 100 Hz) and in the temperature range 10-300 K. Finally, M\"{o}ssbauer spectra were obtained at temperatures from 4.2 K up to 300 K. Samples measured at 4.2 K displayed only sextets, indicating a complete magnetic order. A progressive size-dependent thermal unblocking was observed by the collapsing hyperfine field on warming until the spectrum was represented only by a doublet. The three techniques indicated very close values for grain size estimates, (5.5 nm for samples Arsenic-free and 5.0 nm for samples with Arsenic) and are also in agreement with our transmission electron microscopy (TEM) observations. Our success demonstrates the feasibility of accurate granulometry of ferrihydrite by low temperature magnetic and M"{o}ssbauer techniques.
DE: 1505 Biomagnetism
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
DE: 1600 GLOBAL CHANGE (New category)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting