Mössbauer spectroscopy: Applications
The Mössbauer effect is the recoilless absorption and emission of γ rays by specific nuclei in a solid, and provides a means of studying the local atomic environment around the nuclei. It is a short-range probe, and is sensitive to (at most) the first two coordination shells, but has an extremely high energy resolution that enables the detection of small changes in the atomic environment. Mössbauer spectroscopy therefore provides information at the atomic level.
There are numerous textbooks and review chapters written about Mössbauer spectroscopy. A starting point is the previous chapter in this volume (Amthauer et al., 2004), and other reference sources directed at mineralogists include Bancroft (1973), Hawthorne (1988) and McCammon (2000).
The main objective of this chapter is to provide a brief introduction to Mössbauer spectroscopy applications in mineralogy. The approach is intended to be instructional rather than encyclopaedic in nature, so emphasis is placed on worked examples that are chosen to complement the theory presented in the previous chapter. Studies of the 14.4 keV transition in 57Fe comprise more than 95% of mineralogical Mössbauer investigations; hence this chapter is focused exclusively on 57Fe.
The fundamentals of Mössbauer methodology can be found in the previous chapter as well as any textbook devoted to Mössbauer spectroscopy (e.g. Bancroft, 1973; Gütlich et al., 1978), so only those aspects relevant to the present chapter are outlined briefly below.
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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.