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American Mineralogist; August-September; v. 94; no. 8-9; p. 1120-1129; DOI: 10.2138/am.2009.3167
© 2009 Mineralogical Society of America
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Magnetic and microscopic characterization of magnetite nanoparticles adhered to clay surfaces

Cecilia Galindo-Gonzalez1, Joshua M. Feinberg2,*, Takeshi Kasama3, Lionel Cervera Gontard3, Mihály Pósfai4, Ilona Kósa4, Juan D.G. Duran1, Jaime E. Gil1, Richard J. Harrison5 and Rafal E. Dunin-Borkowski3

1 Department of Applied Physics, Faculty of Sciences, University of Granada, C/Fuentenueva s/n, 18071 Granada, Spain
2 Institute for Rock Magnetism, Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.
3 Center for Electron Microscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
4 Department of Earth and Environmental Sciences, University of Pannonia, POB 158, Veszprém H8200, Hungary
5 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.

Correspondence: * E-mail: feinberg{at}umn.edu

When suspended in solution, clay platelets coated with nanometer-scale magnetite particles behave as magnetorheologic fluids that are important to a variety of industrial applications. Such dual-phase assemblages are also similar to natural aggregates that record the direction and intensity of the Earth’s magnetic field in lake and marine depositional environments. This study characterizes the mineralogical structure and magnetic behavior of montmorillonite platelets coated with aggregates of nanometer-scale magnetite crystals. The distribution of magnetite crystal sizes in three different clay-magnetite assemblages was directly measured using conventional transmission electron microscopy and agrees within error with estimates derived from magnetic hysteresis measurements. Magnetic hysteresis and low field susceptibility measurements combined with electron holography experiments indicate that all three samples behave superparamagnetically at room temperature, and show increasing levels of single domain behavior as the samples are cooled to liquid nitrogen temperatures. At such low temperatures, magnetostatic interactions are observed to partially stabilize otherwise superparamagnetic grains in flux closure structures.

Key Words: Magnetite • clay • smectite • TEM • electron holography • electron tomography • granulometry • magnetic properties






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