Mössbauer spectroscopy: Basic principles
Among the various spectroscopic methods which today are applied in geochemistry and mineralogy, Mössbauer spectroscopy plays an important role for mainly two reasons: First, the high resolution and accuracy of the method enables quantitative measurements by the detection of very small energy differences. Second, although the applicability of Mössbauer spectroscopy is limited to a relatively small number of isotopes, the most suitable and common Mössbauer active element, iron, belongs to the five most abundant elements of the earth, and is by far the most abundant transition element. Accordingly, many of the important rock-forming or ore minerals contain iron as a main or substitutional ion and much important petrological and geochemical information may be obtained by the study of iron, using the Mössbauer effect. For instance, the oxygen fugacity fO2 is a very important parameter in rocks and ore forming processes. Changing Fe2+/Fe3+ ratios in Fe-bearing minerals document varying oxygen fugacities during their formation and their subsequent geological history. The Mössbauer effect is particularly well suited to study special properties of transition metals (such as Fe), e.g. changing oxidation and spin states, site-dependent electrical fields, magnetic hyperfine interactions etc. Therefore, most of the Mössbauer studies in geochemistry and mineralogy are made on 57Fe. Similarly, this paper deals mainly with Mössbauer spectroscopy on 57Fe, which is the Mössbauer active Fe isotope with 2.17% natural abundance. However, there are a number of other Mössbauer isotopes, such as 119Sn, 121Sb, 197Au etc., which have been investigated successfully with regard to geochemical as well as crystal chemical applications.
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Spectroscopic methods provide information about the local structure of minerals. The methods do not depend on long-range periodicity or crystallinity. The geometric arrangement of atoms in a mineral phase is only one aspect of its constitution. Its vibrational characteristic, electronic structure and magnetic properties are of greatest importance when we consider the behaviour of minerals in dynamic processes. The characterisation of the structural and physico-chemical properties of a mineral requires the application of several complementary spectroscopic techniques. However, it is one of the main aims of this School to demonstrate that different spectroscopic methods work on the same basic principles. Spectroscopic techniques represent an extremely rapidly evolving area of mineralogy and many recent research efforts are similar to those in materials science, solid state physics and chemistry. Applications to different materials of geoscientific relevance have expanded by the development of microspectroscopic techniques and by in situ measurements at low- to high-temperature and high-pressure conditions.