Natural Magnetite Nanoparticles from an Iron-Ore Deposit: Size Dependence on Magnetic Properties
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Earth Planets Space, 61, 151–160, 2009 Natural magnetite nanoparticles from an iron-ore deposit: size dependence on magnetic properties M. L. Rivas-Sanchez´ 1, 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 Geof´ısica, Universidad Nacional Autonoma´ de Mexico,´ 04510 Mexico D. F., Mexico 2Instituto de F´ısica, Universidad Nacional Autonoma´ de Mexico,´ 04510 Mexico D. F., Mexico 3Geosciences´ Montpellier, Universite´ 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 5Direccion´ General y Direccion´ de Tecnolog´ıa, Consorcio Minero Benito Juarez,´ Pena˜ 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 2to14nm in the mineralized zones of the Pena˜ 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 frommicro- 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) Mossbauer¨ 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, Pena˜ Colorada, Mexico. 1. Introduction producer in Mexico, supplying an important volume of iron Magnetite nanoparticles remain in a particular position to the world market. The Pena˜ Colorada deposit is, volu- of magnetic materials due to their unique physico-chemical metrically, the largest in Mexico, with up to 26–35% weight properties. As reported by Berquo´ et al. (2007 and refer- Fe. Rock magnetic properties and microscopic detailed ences therein), magnetite shows many fascinating phenom- studies have been reported by Alva-Valdivia et al. (1996, ena, such as: (1) charge ordering, mixed valence and metal- 2000) and Rivas-Sanchez´ (2002, 2007), respectively. insulator transition (Verwey transition); (2) extraordinary The magnetic properties of nanosize fine particle distri- biocompatibility for biomedical applications; (3) ultrasmall bution are dominated by effects of the particle size distribu- superparamagnetic (SP) iron oxides for perfusion imag- tion and the behavior of the magnetic anisotropy (Blanco- ing. Magnetite nanoparticles can be obtained by several Mantecon´ and O’Grady, 2006). The magnetite grain size methods, including the polyol process, precipitation route, distribution is one of the fundamental factors controlling sonochemical synthesis, microemulsion technique and bio- the magnetic properties, and the occurrence of nanoparti- compatible coprecipitation (Wu et al., 2007, and references cles at temperatures typical of near-surface to deep-crustal therein). conditions could provide a new tool by which to define the The Pena˜ Colorada iron-ore deposit is located in the Pa- thermal history of nanoparticle-bearing geologic and plan- cific continental margin of Mexico. It is the principal pellet- etary materials (Reich et al., 2006). We report one of the first quantitative results in termsof Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sci- ences (SGEPSS); The Seismological Society of Japan; The Volcanological Society well-identified grain-size distribution that support the gen- of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sci- eral theoretical basis of frequency-dependent magnetic sus- ences; TERRAPUB. ceptibility (Dearing et al., 1996). We characterized natu- 151 152 M. L. RIVAS-SANCHEZ´ et al.: NATURAL MAGNETITE NANOPARTICLES: SIZE DEPENDENCE Mg - Magnetite ously, following a spiral movement (like a cyclone) that sep- B - Berthierine a Q - Quartz arates, selects and accumulates magnetite particles of spe- 100 cific range size. We obtained six fractions formed by mag- 80 netite particles of distinct size range (56–30 μm, 30–22 μm, Mg 22–15 μm, 15–10 μm,10–7μm and 7–0.2 μm). These size 60 fractions and the sample containing magnetite nanoparticles te sit 40 In n y were characterized according to their crystalline, physico- Mg Mg Mg Mg chemical and magnetic properties. We used X-ray diffrac- 20 B BBQ Mg Mg Q tion (XRD) using a Rigaku diffractometer model Geiger- 0 Flex, transmitted and reflected light microscopy, an elec- 100 b tron probe X-ray micro-analyzer (EPMA), humid chemical 80 analyses, high-resolution transmission electron microscopy y (TEM) with a JEOL 2010 FEG FASTEM, and Mossbauer¨ 60 B ts 40 spectroscopy at room temperature using a Fe standard and In en it B 57Co/Rh radioactive source. Magnetic susceptibility at 20 Mg BB Mg B varying frequencies was measured with a Bartington In- 0 struments MS2 linked to a MS2B dual frequency sensor. We used low frequency (χlf = 470 Hz) and high-frequency 10 15 20 25 30 35 40 45 50 5555 60 65 (χlf = 4700 Hz) to detect qualitatively the presence of ul- trafine grain size carriers of SP behavior. The magnetic sus- ceptibility as a function of temperature was determined by Fig. 1. Magnetite X-ray diffraction pattern: (a) micrometric magnetite; a susceptibilimeter Bartington MS2, with a sensor MS2W (b) nanometric magnetite. coupled to a furnace MS2WFP. To measure the hysteresis parameters and isothermal remanent magnetization (IRM) acquisition and back-field demagnetization curves, we used ral magnetite when particle size makes the transition from an alternating field-force gradient magnetometer, Micro- micro- to nanometric sizes, observing distinctive changes mag 2900. in the frequency-dependent magnetic susceptibility percent- χ χ age ( FD%) measurements. The values of FD%inthemin- 3. Results eralized sample of 2–14 nmmagnetite nanoparticles and 3.1 XRD and high-resolution TEM Mossbauer¨ spectroscopy identified a significant proportion The first part of our study consisted of a mineralogical of SP particles. The results of a magnetic susceptibility ver- characterization by XRD analyses of the micro- and nano- sus temperature experiment of the 2–14 nm sample that was metric magnetite magnetic concentrates. The XRD identi- repeated in rapid succession suggest the formation of new fied magnetite of a micro- and nanometric scale as the sole magnetite nanoparticles, which is similar to the results ob- Fe-oxide present in the sample (Fig. 1). These magnetite tained by Hirt and Gehring (1991). particles are closely associated with berthierine and quartz. The micrometric magnetite shows seven well-defined peaks 2. Experimental Methods and Sample Descrip- supported by a narrow base in the XRD spectra, indicating tion a greater degree of crystallinity (Berquo´ et al., 2007). The We prepared a magnetic concentrate from the magnetite opposite is observed for the magnetite nanoparticles, where ore that was classified in grain sizes using hydro-cycle pro- the peaks are not well defined and are supported by a wide cesses programmed in specific conditions of operation (den- base. Table 1 shows the chemical composition of the frac- sity, flux source, elutriation time). The samples are “mag- tions classified by grain size. We found magnetite nanopar- netic concentrates” of different grain size obtained using ticles in one sample using high-resolution TEM; these are the “hydro-cycle method”. These processes are initiated probably the first images of this type of mineral formed in with a base mineral sample, following a sequence of grind- natural environments. Figure 2 shows a high angle annular ing treatment and low-intensity magnetic separation (us- dark field (HAADF) image of our semi-spherical magnetite ing a magnetic permanent core). We obtained two sample nanoparticles embedded in berthierine. The texture rela- types: (1) one magnetite concentrate, strongly magnetic, tionship and mineralogical association of magnetite with that was constituted