The field of Mössbauer spectroscopy (MS) has recently enjoyed renewed visibility in the diverse geoscience communities as a result of the inclusion of Mössbauer 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 Mössbauer 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. MS: Mössbauer spectroscopy XPS: X-ray photoelectron spectroscopy EELS: electron-energy loss spectroscopy XANES: X-ray absorption near-edge spectroscopy Mössbauer effect: emission or absorption of a gamma photon without energy loss (or gain) in a transition between the ground state and an excited state of certain nuclei bound in a solid viii Contents

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Fe oxidation processes may have occurred during groundwater‐mediated diagenesis in Meridiani Planum sediments. To address this question, melanterite oxidation experiments were conducted at epsomite saturation as a function of pH. Results show that schwertmannite is initially formed from acidic Fe oxidation and that its formation and aging to mixtures of jarosite and nanocrystalline goethite is strongly controlled by pH over the range ∼2.0–4.0. The pH is controlled in turn by Fe oxidation and Fe3+ hydrolysis. In one 77‐d oxidation experiment, nanocrystalline hematite was tentatively identified by Mössbauer spectroscopy. Accordingly, aging experiments with synthetic nanocrystalline goethite were conducted (1) to further resolve the formation mechanisms of Fe‐phases identified from oxidation experiments and (2) to test whether low water activity (aw) controls the thermodynamically favored goethite to hematite transition at low temperature. Mössbauer spectroscopy and total X‐ray scattering show no observable changes after 4 months of aging, and instead, these results point to a jarosite precursor for the tentatively identified hematite. On the basis of these results, we suggest that the oxidation and maturation of initially formed Fe2+‐bearing saline minerals may account in large part for the distribution of secondary Fe minerals at the Martian surface, contributing to the association of Fe oxides and Mg/Ca sulfates observed from orbital surface analyses. We hypothesize that oxidation of Fe2+ sulfates at low temperature could account for sustained diagenetic acidity in addition to much of the observed Fe mineralogy in Meridiani Planum outcrop rocks. The origin of the gray crystalline hematite at Meridiani, however, is deserving of further experimental work to test this mechanism.

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