Nanoscopic Approaches in Earth and Planetary Sciences
The properties of matter at extreme length scales and the respective processes can differ markedly from the properties and processes at length scales directly accessible to human observation. This scale-dependent behaviour is possible in both directions; towards very large and very small scales. Scientists explore the frontiers of these extreme length scales in an effort to gain insight into yet unknown properties and processes. While the exploration of larger scales has been established since the Renaissance era, a comprehensive investigation of small scales was impeded by the limitations of optical microscopy. These imitations were overcome in the 20th century. Since then, a continuous series of developments in analytical power has taken place. Today these developments allow studies of properties and processes even at the molecular or atomic scale (often referred to as nanoscience). These modern nanoscientific possibilities have triggered new innovative projects in geosciences, providing fascinating insights into small scales. Therefore, nanogeoscience has become a very important geoscientific subdiscipline.
Electron energy-loss spectroscopy and energy-filtered transmission electron microscopy: Nanoscale determination of Fe3+/ΣFe ratios and valence-state mapping
Published:January 01, 2010
Ute Golla-Schindler, Peter A. van Aken, 2010. "Electron energy-loss spectroscopy and energy-filtered transmission electron microscopy: Nanoscale determination of Fe3+/ΣFe ratios and valence-state mapping", Nanoscopic Approaches in Earth and Planetary Sciences, Frank E. Brenker, Guntram Jordan
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Iron, the fourth most abundant element in the Earth, commonly occurs in two valence states, Fe2+ and Fe3+, even within a single mineral. Determination of Fe3+/EFe ratios in minerals at sub-micrometre scale has been a long-standing objective in geosciences. One of the most important characteristics of iron is that the charge on the metal is extremely sensitive to its surrounding reduction-oxidation (redox) conditions, which often results in changes in iron valence state reflecting these conditions. The quantification of ferrous/ferric ratios in minerals can therefore provide great insights into physico-chemical conditions of rock formation such as temperature and oxygen fugacity, and allows the determination of redox states for mineral crystallization and the interpretation of geological and geochemical processes. The high spatial resolution available on a (scanning) transmission electron microscope ((S)TEM) combined with the benefits of electron energy-loss spectroscopy (EElS) allows detailed analysis of multivalent element ratios (e.g. Fe2+ and Fe3+) on the scale of nanometres.
Electron energy-loss spectroscopy is a powerful technique for analyzing the interactions of fast probe electrons with matter, and the energy transferred for a certain excitation process can be measured as an energy loss of the incident electron which reduces its kinetic energy. The probability of inelastic scattering over energy loss is called energy-loss spectrum which results from the excitation of inner-shell, valence or conduction electrons. Excitations are only possible from occupied states below the Fermi level to allowed unoccupied states beyond it. Maxima in the energy loss spectrum correspond to strong electron-specimen interactions. apart from the qualitative and quantitative determination of elements, it is also possible to determine quantitative concentration ratios of