Earth Planets Space, Vol. 61 (No. 1), pp. 151-160, 2009
M. L. Rivas-Sánchez1, L. M. Alva-Valdivia1,4, J. Arenas-Alatorre2, J. Urrutia-Fucugauchi1, M. Perrin3, A. Goguitchaichvili4, M. Ruiz-Sandoval5, and M. A. Ramos Molina5
1Laboratorio de Paleomagnetismo, Instituto de Geofísica, Universidad Nacional Autónoma de México, 04510 Mexico D. F., Mexico
2Instituto de Física, Universidad Nacional Autónoma de México, 04510 Mexico D. F., Mexico
3Géosciences Montpellier, Université Montpellier II, 34095 Montpellier Cedex 05, France
4Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofisica, Sede Michoacan, Universidad Nacional Autonoma de Mexico, Campus Morelia, Mexico
5Dirección General y Dirección de Tecnología, Consorcio Minero Benito Juárez, Peña Colorada, S. A. de C. V., Av. del trabajo No. 1000, Manzanillo, Colima, Mexico
(Received September 12, 2007; Revised July 20, 2008; Accepted July 21, 2008; Online published January 23, 2009)
We report on the discovery of magnetite nanoparticles ranging in size from 2 to 14 nm in the mineralized zones of the Peña Colorada iron-ore deposit, southern Mexico. Micrometric scale magnetite was magnetically reduced and divided into distinct size ranges: 85-56 μm, 56-30 μm, 30-22 μm, 22-15 μm, 15-10 μm, 10-7 μm and 7-2 μm. Nanometric-scale magnetite in the size range 2-14 nm was identified. The magnetite was characterized by X-ray diffraction, transmitted and reflected light microscope, high-resolution transmission electron microscopy (TEM), high angle annular dark field, Mössbauer spectroscopy and its magnetic properties. Crystallographic identification of nanostructures was performed using high-resolution TEM. Characteristic changes were observed when the particles make the size transition from micro- to nanometric sizes, as follows: (1) frequency-dependent magnetic susceptibility percentage (χFD%) measurements show high values (13%) for the 2-14 nm fractions attributed to dominant fractions of superparamagnetic particles; (2) variations of χFD% < 4.5% in fractions of 56-0.2 μm occur in association with the presence of microparticles formed by magnetite aggregates of nanoparticles (<15 nm) embedded in berthierine; (3) Mössbauer spectroscopy results identified a superparamagnetic fraction; (4) nanometric and 0.2-7 μm grain size magnetite particles require a magnetic field up to 152 mT to reach saturation during the isothermal remanent magnetization experiment; (5) coercivity and remanent magnetization of the magnetite increase when the particle size decreases, probably due to parallel coupling effects; (6) two-magnetic susceptibility versus temperature experiments of the same 2-14 nm sample show that the reversibility during the second heating is due to the formation of new magnetite nanoparticles and growth of those already present during the first heating process.
Key words: Magnetite nanoparticles, berthierine, particle size, magnetic properties, iron-ore deposit, Peña Colorada, Mexico.