Resumen

ALVA-VALDIVIA, Luis M. et al. Microscopy and rock magnetism of fine grain-size titanomagnetite from the Jacupiranga Alkaline Complex, Brazil: unearthing Ti-magnesioferrite nanoparticles. Geofís. Intl [online]. 2013, vol.52, n.2, pp.93-110. ISSN 2954-436X.

Very fine samples from the mineralized zones of the Jacupiranga complex at the Cajatí mine were selected for crystallographic identification of Ti-magnesioferrite (TMf) nanostructures embedded in titanomagnetite (TM) using high-resolution transmission electron microscopy (TEM). A magnetic concentrate obtained of pyroxenite samples (sites 4 to 7) was reduced and divided into fractions of distinct range sizes: 26±2 µm, 19±1 µm, 13±1 µm, 9±1 µm, 6±1 µm and 6-0.1 µm. The mineralized samples of carbonatite and pyroxenitewerecharacterized by X-ray diffraction, transmitted and reflected light microscope, and scanning electron microscope with multielemental analysis. The finest magnetic concentrate sample (MC₆) was analyzed under high-resolution transmitted electron microscopy (TEM) and high angleannulardarkfield and Ramanspectroscopy. Magnetic properties were measured for the distinct granulometric fractions, showing drastic changes when grain sizes go beyond the frontier from micro to nanometer sizes. Frequency-dependent magnetic susceptibility percentage (χ_fd%) report higher values (10.2%) for the finer fractions (6±1 µm and 6-0.1 µm) attributed to dominant fractions of superparamagnetic particles. Nanometer and < 6 µm grain size TMf in TM particles require a magnetic field up to 249 mT to reach saturation during the isothermal remanent magnetization experiment. Coercivity and remanent magnetization of these samples increase when the particle size decreases, probably due to parallel coupling effects. Magnetic susceptibility versus temperature experiments were conducted two times on the same (< 35 nm) sample, showing that the repetition during the second heating is probably due to the formation of new TMf nanoparticles and growth of those already present during the first heating process.

Palabras llave : TMf nanoparticles; magnetic properties; mineralogy; grain size effect; Jacupiranga Alkaline Complex; Brazil.

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