instrumental techniques applied to mineralogy and geochemistry

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TEM in Geology. Basics and applications 27 a completely different system of structural analysis based on the dynamical diffraction theory. Analytical electron microscopy (AEM) The intensity of a given emission line is proportional to the concentration of the corresponding element. In thick samples, the beam penetrates to considerable depth (Fig. 3b). The resulting X rays emitted are subject to absorption and fluorescent effects. Nevertheless, in the thin electron-transparent films generally used in TEM, the paths of emitted X-rays are so short that absorption and fluorescence can be neglected. This is the so-called Cliff-Lorimer (Cliff and Lorimer, 1975) approximation. The ratio of atomic concentrations of two elements (c a /c b ) is directly proportional to the ratio of the intensities of the emission lines of those elements (I a /I b ). Only a proportionality factor K ab is necessary to calculate the relative amount of the elements from the measured intensities. Therefore, it is a relative method in which the concentration ratios of elements can be known, but not their absolute quantities. This is a basic limitation of the technique, but it is not particularly limiting in mineralogy as mineral compositions are usually normalized to their formulae. To present an analysis in the form of oxides in the traditional fashion is artificial and may be misleading. It is the concentration ratios (formulae) that should normally be presented as the basic AEM data. The software of EDX equipment contains theoretical K factors, calculated from first principles (Goldstein et al., 1986) to be used in the so-called standardless analyses. Nevertheless, the K factors need to be experimentally determined for a standard of known composition, as they are valid only for values obtained under specific conditions for a specific instrument. In fact, should the counter window or crystal be replaced, large changes in K-values are possible. Therefore, standardless analyses produce only approximate results very far from being considered quantitative. Electron energy-loss spectroscopy (EELS) The energy lost by beam electrons during inelastic interactions is converted into secondary signals as X-rays, cathodoluminescence, and Auger electrons. In addition to these signals, the changes in energies of the beam electrons also provide information about both the types of atoms in the specimen and about their chemical states (bonding,
28 Fernando Nieto valence, coordination). Electron energy-loss spectroscopy, or EELS for short, is the measurement of the energy distribution of electrons that have passed through a specimen. EELS reflects a primary event of energy change, in contrast with EDX, which reflects a secondary event, the energy of radiation released upon relaxation during the return of the atom to its ground state. Since most electrons that cause ionization can be collected by the EELS spectrometer, the method has high detection efficiency. In comparison to EDX, EELS is better suited for light elements, with the possibility of measuring up to Li. In general, quantification of EELS spectra is more difficult and the results less accurate than for EDX because of the broad shapes of the EELS edges and the high background. Elemental and chemical analysis is probably the major application of EELS for mineralogy, but the spectra contain far more information. Valence states and local environments of atomic species can be determined from EELS spectra by using EXELFS (extended energy-loss fine structure) and ELNES (energy-loss near-edge fine structure). Small changes in the edge energy are caused by differences in valence state. These chemical shifts can be used to determine the valence states of elements in a specimen; this technique has been successfully applied to a variety of minerals. It is difficult to obtain such information at such a fine spatial resolution by any other technique. Nevertheless, while it is easy to see changes in oxidation states, one limitation is that it is difficult to quantify their fractions. Applications The scales for the observation of geological phenomena range from 10 6 m for plate tectonics to 10 -10 m for mineral lattices. Many characteristics may be recognized in such a wide range of dimensions with similar meaning (e.g. folds). The various subtechniques included in TEM, presented above, allow the observation and study of a plethora of phenomena—among them the following can be mentioned as examples. - Electron diffraction Identification, orientation, and cell parameters of minerals.
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instrumental techniques applied to mineralogy and geochemistry

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