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.