Publication: Anisotropía de la susceptibilidad magnética en cristales naturales de hematites
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2016-02-10
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Universidad Complutense de Madrid
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Abstract
El punto de partida de esta tesis es el registro de un comportamiento inusual en cristales de hematites por el que la susceptibilidad magnética a bajo campo se demuestra estar fuera de la región de Rayleigh para los campos más habituales en instrumentos comerciales. A partir de este registro, se plantea difícil proponer medidas de susceptibilidad a bajo campo para realizar separación de subfábricas magnéticas en muestras ricas en hematites. El principal objetivo de la tesis es determinar qué parámetro físico condiciona dicho comportamiento de la susceptibilidad, con la intención de establecer si es posible, un modelo matemático que sirva en la separación de subfábricas magnéticas y en la corrección por somerización de la inclinación magnética. El primer punto tratado es un análisis estadístico orientado específicamente al tratamiento de medidas de anisotropía de la susceptibilidad magnética para descartar que las variaciones sean consecuencia de una imprecisión instrumental. Para ello se ha realizado un estudio sobre la fiabilidad de los métodos estadísticos empleados en la obtención de los parámetros del elipsoide, en función del error instrumental. A continuación se plantea la dificultad adicional, en cristales de hematites, de proporcionar un valor único de forma y grado del elipsoide de anisotropía de la susceptibilidad, ya que también estos dos parámetros varían en función del campo aplicado. Una vez rechazada la hipótesis de una falta de precisión, los parámetros que se proponen como posibles causas de la variación de la susceptibilidad en función del campo magnético aplicado son: el tamaño de grano, el contenido de cationes, la estructura de las muestras, parámetros magnéticos, la anisotropía del plano basal y el movimiento de las paredes de dominio. Para determinar la influencia del tamaño de grano se han medido fracciones de polvo provenientes de una misma muestra que han sido preparadas hasta obtener especímenes de distintos tamaño de grano. Los resultados muestran, que a excepción de un espécimen con comportamiento lineal, el resto tiene un valor constante de la susceptibilidad en función del campo aplicado, por lo que el tamaño de grano no explicaría la falta de linealidad observada en la susceptibilidad en muestras de cristales naturales. Para determinar el contenido de cationes se han realizado medidas de difracción de rayos X, espectrometría de masas y microsonda. Los resultados indican que la variedad de cationes de las muestras varía dependiendo de la procedencia de las mismas, teniendo todas ellas en común el titanio. Además se puede observar, que muestras con el mismo vii viii Resumen contenido y tipo de cationes exhiben comportamientos distintos, y muestras con distinta cantidad de cationes muestran el mismo comportamiento de la susceptibilidad en cristales naturales. Por esta razón, el contenido de cationes también se descarta como posible factor condicionante del comportamiento de la susceptibilidad...
The starting point of this doctoral thesis is the report of an unusual behaviour in hematite natural crystals. The low field susceptibility is shown to be out of the Rayleigh region at the applied fields of most common commercial instruments. This record makes difficult proposing a most general model to separate magnetic subfabrics in hematite bearing rocks. From this observation, it is planed as the main aim of the thesis, determining the controlling physical parameter that rules the behaviour of low field susceptibility. The final goal is proposing a mathematical model that separates magnetic subfabrics and corrections of the magnetic inclination for inclination shallowing behaviour. The first issue focuses the attention of this work is a statistical analysis oriented specifically toward the analysis of measurements of the anisotropy of magnetic susceptibility. In order to discard that the measurements of anisotropy in hematite bearing rocks are within instrumental errors, the reliability of most common statistical methods in anisotropy of magnetic susceptibility studies (applied to shape and degree of anisotropy of the magnetic ellipsoid) are analysed as a function of instrumental error. The next step has reported the difficulty in providing a magnetic anisotropy single crystal property for hematite natural crystals, since shape and degree of anisotropy vary depending of the strength of the applied low-field. At the light of the statistical analysis, a lack of precision of the measurements is rejected. Therefore, the physical parameters to be analysed their influence in the variation of the susceptibility as a function of applied field are the particles grain size, cations content, samples structure, magnetic parameters, anisotropy within the basal plane and movement of the domain walls. The influence of particles grain size has been studies in samples with controlled grain sized. Results show that, with the exception of the larger grain size fraction that increases linearly the susceptibility with applied field, the rest are constant with applied field. Grain size does not explain the behaviour of susceptibility in natural crystals. In order to determine the cations content, X-ray diffraction, mass spectrometry and microprobe measurements have been carried out. Results show that cations content varies depending on the origin of the samples. With the exception of Titanium, which is common, there is not pattern that relates a particular cation with a common susceptibility behaviour. X-ray diffraction allows the determination of the unit cell parameters of the crystals. ix x Abstract Specimens from the same sample and crystallographic structure, have different low field susceptibility as a function of applied field. Therefore, the cell parameter does not have a relevant roll...
The starting point of this doctoral thesis is the report of an unusual behaviour in hematite natural crystals. The low field susceptibility is shown to be out of the Rayleigh region at the applied fields of most common commercial instruments. This record makes difficult proposing a most general model to separate magnetic subfabrics in hematite bearing rocks. From this observation, it is planed as the main aim of the thesis, determining the controlling physical parameter that rules the behaviour of low field susceptibility. The final goal is proposing a mathematical model that separates magnetic subfabrics and corrections of the magnetic inclination for inclination shallowing behaviour. The first issue focuses the attention of this work is a statistical analysis oriented specifically toward the analysis of measurements of the anisotropy of magnetic susceptibility. In order to discard that the measurements of anisotropy in hematite bearing rocks are within instrumental errors, the reliability of most common statistical methods in anisotropy of magnetic susceptibility studies (applied to shape and degree of anisotropy of the magnetic ellipsoid) are analysed as a function of instrumental error. The next step has reported the difficulty in providing a magnetic anisotropy single crystal property for hematite natural crystals, since shape and degree of anisotropy vary depending of the strength of the applied low-field. At the light of the statistical analysis, a lack of precision of the measurements is rejected. Therefore, the physical parameters to be analysed their influence in the variation of the susceptibility as a function of applied field are the particles grain size, cations content, samples structure, magnetic parameters, anisotropy within the basal plane and movement of the domain walls. The influence of particles grain size has been studies in samples with controlled grain sized. Results show that, with the exception of the larger grain size fraction that increases linearly the susceptibility with applied field, the rest are constant with applied field. Grain size does not explain the behaviour of susceptibility in natural crystals. In order to determine the cations content, X-ray diffraction, mass spectrometry and microprobe measurements have been carried out. Results show that cations content varies depending on the origin of the samples. With the exception of Titanium, which is common, there is not pattern that relates a particular cation with a common susceptibility behaviour. X-ray diffraction allows the determination of the unit cell parameters of the crystals. ix x Abstract Specimens from the same sample and crystallographic structure, have different low field susceptibility as a function of applied field. Therefore, the cell parameter does not have a relevant roll...
Description
Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra, Astronomía y Astrofísica I (Geofísica y Meteorología), leída el 11-12-2015