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Natural magnetite nanoparticles from an iron-ore deposit: size dependence on magnetic properties


We report on the discovery of magnetite nanoparticles ranging in size from 2 to 14 nm in the mineralized zones of the Pe~na 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, Mossbauer 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.


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Correspondence to L. M. Alva-Valdivia.

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Rivas-Sánchez, M.L., Alva-Valdivia, L.M., Arenas-Alatorre, J. et al. Natural magnetite nanoparticles from an iron-ore deposit: size dependence on magnetic properties. Earth Planet Sp 61, 151–160 (2009).

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Key words

  • Magnetite nanoparticles
  • berthierine
  • particle size
  • magnetic properties
  • iron-ore deposit
  • Pe~na Colorada
  • Mexico