Mossbauer spectroscopy of earth and planetary materials

The field of Mossbauer spectroscopy (MS) has recently enjoyed renewed visibility in the diverse geoscience communities as a result of the inclusion of Mossbauer spectrometers on the Mars Exploration Rovers. Furthermore, new improvements in technology have made possible studies involving very small samples (1-5 mg or less) and samples with very low Fe contents (such as feldspars), in addition to samples measured in situ in thin sections. Because of these advances, use of Mossbauer spectroscopy in Earth science applications is expected to continue to grow, providing information on site occupancies; valence states; magnetic properties; and size distributions of (largely) Fe-bearing geological materials, including minerals, glasses, and rocks. Thus, it is timely to review here the underlying physics behind the technique, with a focus on the study of geological samples. With this background, recent advances in the field, including (a) changes in instrumentation that have allowed analysis of very small samples and of surface properties, (b) new models for fitting and interpreting spectra, and (c) new calculations of recoil-free fraction, are discussed. These results have made possible increasingly sophisticated studies of minerals, which are summarized here and organized by major mineral groups. They are also facilitating processing and interpretation of data from Mars.