Identification of magnetic minerals by scanning electron microscope and application of ferrofluid (original) (raw)
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Geophysical Journal International, 2007
In paleomagnetic and environmental magnetic studies the magnetomineralogical identification is usually based on a set of rock magnetic parameters, complemented by crystallographic and chemical information retrieved from X-ray diffraction (XRD), (electron) microscopy or energy dispersive spectroscopy (EDS) of selected samples. While very useful, each of these supplementary techniques has its limitations when applied to natural sample material which are related to low particle concentrations (down to the ppm range in marine sediments) and very fine grain sizes (down to the nm scale). Therefore, meaningful application of such techniques depends on sample quality. Electron backscatter diffraction (EBSD) of individual grains in scanning electron microscopy (SEM) enables mineralogical identification of grains down to ~0.2 micrometer and is particularly powerful when combined with EDS. In this study, we show the merits of EBSD for rock magnetic investigations by analyzing titanomagnetites and hemoilmenites of various compositions and submicron lamella of titanomagnetite-hemoilmenite intergrowths. Such particles often occur in natural marine sediments where EDS often has a semi-quantitative character and compositionally similar intergrowths may be difficult to distinguish. With the mineralogical information provided by EBSD unambiguous identification of spinel-type and trigonal oxides is obtained. Optimal EBSD patterns are gathered from smooth, polished surfaces, but here we show that interpretable EBSD patterns can be obtained directly from the surface of unconsolidated, so called `non-embedded' particles from marine sediments. This information enhances the interpretative value of rock magnetic parameters.
Lithos, 2018
Rock samples can have wide range of magnetic properties depending on composition, amount of ferromagnetic minerals, grain sizes and microstructures. Here, we used scanning magnetic microscopy, a highly sensitive and high-resolution magnetometric technique to map remanent magnetic fields over a planar surface of a rock sample. The technique allows for the investigation of discrete magnetic mineral grains, or magnetic textures and structures with submillimeter scale resolution. Here, we present a case-study of magnetic scans of pristine and serpentinized dunite thin sections from the Reinfjord Ultramafic Complex, in northern Norway. The magnetic mineralogy is characterized by electron microprobe, scanning electron-and optical-microscopy, and with rock magnetic methods. In serpentinized samples the magnetic carrier is end-member magnetite occurring as large discrete grains and small grains in micron scale veins. By contrast, the pristine dunite sample contains large Cr-spinel grains with very fine equant exsolutions ranging in composition from ferrichromite to end-member magnetite. Forward and inverse modeling of the magnetic anomalies is used to determine the remanent magnetization directions and intensities of discrete magnetic sources observed in the scanning magnetic microscopy. The finescale magnetization of the rock sample is used to investigate the magnetic carriers and the effect of serpentinization on the magnetic properties of the dunite. Modeling shows that the dipolar magnetic anomalies that are mapped by scanning magnetic microscopy are caused by grains with heterogeneous magnetic sources. The intensity of the magnetization and the amount of magnetic minerals are higher in the serpentinized sample than the pristine dunite sample, consistent with the measured bulk magnetic properties. Furthermore, the serpentinized samples show a larger variability in the direction of the magnetization and a stronger heterogeneity with respect to the pristine sample. The ability to rigorously associate components of the bulk magnetic properties to individual mineral phases creates new possibilities for rock magnetic, paleomagnetic, and exploration applications.
Properties of Rocks and Minerals – Magnetic Properties of Rocks and Minerals
Treatise on Geophysics, 2007
This review describes the current state-of-the-art in the field of computational and experimental mineral physics, as applied to the study of magnetic minerals. The review is divided into four sections, describing new developments in the study of mineral magnetism at the atomic, nanometer, micrometer, and macroscopic length scales. We begin with a description of how atomistic simulation techniques are being used to study the magnetic properties of minerals surfaces and interfaces, and to gain new insight into the coupling between cation and magnetic ordering in Fe-Ti-bearing solid solutions. Next, we review the theory of off-axis electron holography, and its application to the study of magnetotactic bacteria and minerals containing nanoscale transformation-induced microstructures. Then, we review the theory and application of micromagnetic simulations to the study of non-uniform magnetization states and magnetostatic interactions in minerals at the micrometer length scale. Finally, we review recent developments in the use of macroscopic magnetic measurements for characterizing and quantifying the microscopic spectrum of coercivities and interaction fields present in rocks and minerals.
Fundamental relations of mineral specific magnetic carriers for paleointensity determination
Physics of the Earth and Planetary Interiors, 2017
A fundamental linear relationship exists between the efficiency of thermoremanent magnetization measured at room temperature and the magnitude of the ambient field at the time of acquisition. The magnetic efficiency (the ratio of thermoremanent magnetization to saturation remanent magnetization) multiplied by the saturation magnetization is proportional to the magnetizing field, where the proportionality constant is independent of magnetic mineralogy and domain state. The empirical constant for this equation was determined using existing experimental data of single domain through pseudosingle domain to multidomain states of iron, meteoritic iron, magnetite, maghemite, pyrrhotite, and hematite. We show that the acquired magnetic efficiency is closely related to two types of demagnetizing fields that act as barriers against domain wall pinning during magnetic acquisition. The first relates to the saturation magnetization that is derived from the distribution of Bohr magnetons within the crystal lattice, and the second originates from grain shape. Theoretical considerations imply a factor of two difference in the magnetic efficiency acquired during laboratory and geologic timescales. This equation reveals that troilite may be a potentially important magnetic carrier for extraterrestrial magnetism. Using magnetic scanning techniques, our relationship allows for estimating the paleointensity from samples that contain more than one magnetic species.
Journal of Geophysical Research, 2007
Magneto-optical imaging based on the Faraday effect has been used to characterize magnetic minerals embedded in a nonmagnetic matrix. We have studied magnetite grains and magnetite-magnetite grain boundary regions in samples of skarns and serpentinites. Distributions of the remanent magnetic field were measured across at the surface of polished thin sections kept at room temperature. The magneto-optical images resolve directly magnetic structures on length scales ranging from millimeter down to micrometer, thereby revealing the shape and arrangement of the magnetite grains and allow determination of the grain magnetization. We find that (1) for the skarns the intergrain interactions do not affect the magnetic properties of magnetite grains within 0.6-60 mm of each other, while the saturation remanence decreases weakly with increasing grain size from 40 mm to 0.6 mm, and (2) for the serpentinites the magnetic properties of the stripes are size-dependent due to variations in chemical composition.
Micromagnetics of paleomagnetically significant mineral grains with complex morphology
Geochemistry, Geophysics, Geosystems, 2010
1] Micromagnetic calculations relevant to paleomagnetism have generally focused on ideal shapes such as spheres and cubes. However, oxide grains that occur naturally in rocks often have irregular morphologies, highly skeletal dendritic forms being particularly common. To investigate the potential effects of complex grain morphology on magnetic stability, we have carried out two parallel sets of calculations. The first is based on randomly distorted 30-120 nm magnetite spheres. We use a three-dimensional finite element/ boundary integral micromagnetic model able to generate suitable morphologies unrestricted by the regular cell structure required by finite difference models. The second model consists of a 300 nm magnetite octahedron from which most of the material has been removed to leave a small octahedral crystallite at each of the six apices connected by an orthogonal framework of thin rods. The results obtained imply that micromagnetic models currently provide no unambiguous evidence that morphological complexities endow magnetite nanoparticles with enhanced coercivity.