Browsing by Author "Gonzalez-Lucena, Fedora"
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Item EPR studies in soil samples from a prospective area at the Andean Range, Venezuela(2003) Aldana, Milagrosa; Dıaz, Marisel; Costanzo-Alvarez, V.; Gonzalez-Lucena, Fedora; Romero, I.Analyses of electron paramagnetic resonance (EPR), in order to determine the organic matter free radical concentration (OMFRC), and magnetic susceptibility (MS) measurements were carried out in soil (surface and depth (≈ 1-2 meters)) samples from a prospective area located at the southern flank of the Andean Range in Venezuela. The results indicate the presence of anomalous zones in this area. These anomalous values could be the result of the reducing environment induced by the hydrocarbon gas leakage. Results for demineralized samples allowed discussing differences between surface and depth ones in terms of weathering effects and change in organic matter type.Item An integrated rock magnetic and EPR study in soil samples from a hydrocarbon prospective area(2002) Gonzalez-Lucena, Fedora; Aldana, Milagrosa; Costanzo-Alvarez, V.; Diaz, M.; Romero, I.; Aubourg, Charles; Elmore, D.; Hirt, A.; Pesonen, L. J.; Peters, C.; Petrovsky, E.; Scholger, R.Magnetic susceptibility (MS) and organic matter free radical concentration (OMFRC) determined by electron paramagnetic resonance, have been measured in soil samples ( nearly equal 1.5 m depth) from an oil prospective area located at the southern flank of the Venezuelan Andean Range. S-ratios close to 1, as well as high temperature susceptibility analyses, reveal magnetite as the chief magnetic phase in most of these samples. Ethane concentrations, MS and OMFRC normalized data have been plotted against the relative position of 22 sampling sites sequentially arranged from north to south. Although there is not a linear correlation between MS and OMFRC data, these two profiles seem to vary in like fashion. A MS and OMFRC southern anomaly coincides with the zone of highest ethane concentration that overlies a "Cretaceous kitchen". OMFRC highs could be linked to the degradation or alteration of organic matter, the possible result of hydrocarbon gas leakage, whose surface expression is the stressed fern observed by remote sensing studies previously performed in the area. Ethane anomalies are associated to this seepage that also produces changes in the magnetic mineralogies detected as MS positive anomalies.Item Magnetic granulometry from equilibrium magnetization measurements; mineral magnetometry of superparamagnetic particles and application to synthetic ferrihydrites(2004) Rancourt, Denis G.; Gonzalez-Lucena, Fedora; Thibault, Pierre-JeanMineral magnetometry is underused in characterizing natural and synthetic nanophase, nanoparticle, and nanocomposite materials, where it has certain advantages over more sophisticated methods, such as high-resolution microscopies and synchrotron-based spectroscopies. We developed the physical theory of direct current (dc) field equilibrium magnetometry of non-exchange coupled superparamagnetic (SP) particles and determined the most information that can be extracted from field and temperature curves, exactly and without making physical assumptions other than those inherent in superparamagnetism and equilibrium measurements. We gave expressions for all field strengths and we did not make any a priori assumptions about the shapes of the distributions of particle size and of supermoment magnitude (μ) or about any relationship between particle size and particle moment. The extracted number average particle mass, {m}, and extracted moments, {μn}, of the number distribution of μ are valid irrespective of particle shapes and sizes, of the nature of the (magnetocrystalline or other) anisotropy barrier, of dipolar inter-particle interactions, and of the mechanism of formation of the supermoment (antiferromagnetic mismatch, surface pinning, etc.). We generalized the approach further to treat composite samples that included: (1) a magnetically inert (diamagnetic) matrix phase, (2) various populations of paramagnetic (PM) cations, and (3) contributions from ferrimagnetic (FI) and ferromagnetic (FM) phases. We applied these methods to synthetic 2-line ferrihydrites coprecipitated with different anion species (nitrate or sulfate) and to a synthetic 6-line ferrihydrite. These are the first accurate evaluations of μ and its relation to particle size in ferrihydrite. The particle masses agree with known sizes for similar synthetic samples. The μ/m ratios (i.e., intrinsic mass magnetization values) are consistent with a significantly higher degree of intra-particle order in 6-line ferrihydrite and gave insight into the supermoment formation mechanism.