ALBERT

Description: Sulfate minerals are important indicators for aqueous geochemical environments. The geology and mineralogy of Mars have been studied through the use of various remote-sensing techniques, including thermal (mid-infrared) emission and visible/near-infrared reflectance spectroscopies. Spectral analyses of spacecraft data (from orbital and landed missions) using these techniques have indicated the presence of sulfate minerals on Mars, including Fe-rich sulfates on the iron-rich planet. Each individual Fe-sulfate mineral can be used to constrain bulk chemistry and lends more information about the specific formational environment [e.g., Fe 2+ sulfates are typically more water soluble than Fe 3+ sulfates and their presence would imply a water-limited (and lower Eh) environment; Fe 3+ sulfates form over a range of hydration levels and indicate further oxidation (biological or abiological) and increased acidification]. To enable better interpretation of past and future terrestrial or planetary data sets, with respect to the Fe-sulfates, we present a comprehensive collection of mid-infrared thermal emission (2000 to 220 cm –1 ; 5–45 μm) and visible/near-infrared (0.35–5 μm) spectra of 21 different ferrous- and ferric-iron sulfate minerals. Mid-infrared vibrational modes (for SO 4 , OH, H 2 O) are assigned to each thermal emissivity spectrum, and the electronic excitation and transfer bands and vibrational OH, H 2 O, and SO 4 overtone and combination bands are assigned to the visible/near-infrared reflectance spectra. Presentation and characterization of these Fe-sulfate thermal emission and visible/near-infrared reflectance spectra will enable the specific chemical environments to be determined when individual Fe-sulfate minerals are identified.

Description: Although Fe-sulfate minerals occur only rarely on Earth as alteration products of sulfidic basalts or in hydrothermal systems, multiple lines of evidence point to the importance of Fe- (and other) sulfate minerals on the surface of Mars. One such martian data set comes from the MIMOS II Mössbauer spectrometers on the Mars Exploration Rovers, which acquired hundreds of spectra from the martian surface at two locations. Interpretation of those spectra has been limited by the lack of a comprehensive set of laboratory analog spectra of the broad range of naturally occurring sulfate minerals. Accordingly, this study reports Mössbauer data of 98 samples representing 47 different sulfate mineral species, all containing six- or higher-coordinated Fe. The resultant Mössbauer parameters are related to the local polyhedral environment around the Fe cation in each mineral to explain variations in spectral characteristics. Results show that the size of the coordination polyhedron is the best predictor of quadrupole splitting, which increases with both octahedral volume and mean bond length. Species within groups of structurally similar minerals are shown to have comparable spectral peaks that generally fall within small ranges. Although coordination polyhedron geometry is not necessarily unique to any particular mineral species or group, Mössbauer data can be used to help constrain mineral identifications from martian spectra. The number of mineral species is large, but the range of crystal structures and hyperfine parameters may be small, so that in many cases, individual minerals cannot be uniquely fingerprinted. Examples would include quenstedtite, coquimbite, kornelite, and lausenite, which have indistinguishable spectra, as do apjohnite, bilinite, dietrichite, and römerite. Overlap of Mössbauer parameters is a particular complication for identification of Fe 3+ -rich phases because the range of Mössbauer parameters for Fe 3+ in any coordination number is so small. Previous analyses of martian Mössbauer spectra reported the presence of jarosite ( Klingelhöfer et al. 2004 ; Morris et al. 2004 ) and an unspecific ferric sulfate ( Morris et al. 2008 ). New data presented here indicate that botryogen, metasideronatrite, and slavikite exhibit Mössbauer spectra similar to those attributed to jarosite at Meridiani Planum. Fibroferrite and rhomboclase have parameters similar to those observed at Arad Samra, and copiapite and parabutlerite could be present at Tyrone Mount Darwin and Berkner Island. Unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a new expansive data set for future interpretation of iron sulfates on Mars.

Description: Phosphate minerals, while relatively rare, show a broad range of crystal structure types with linkages among PO 4 tetrahedra mimicking the hierarchy of polymerization of SiO 4 tetrahedra seen in silicate minerals. To augment previous Mössbauer studies of individual phosphate species and groups of species, this paper presents new Mössbauer data on 63 different phosphate samples, and integrates them with data on more than 37 phosphate species in 62 other studies from the literature. Variations in Mössbauer parameters of different sites in each mineral are then related to both the local polyhedral environment around the Fe cations and the overall structural characteristics of each species. The entire aggregated Mössbauer data set on phosphate minerals is juxtaposed against parameters obtained for spectra from the MIMOS spectrometers on Mars. This comparison demonstrates that signatures from many different phosphate or sulfate mineral species could also be contributing to Mars Mössbauer spectra. Results underscore the conclusion that unique mineral identifications are generally not possible from Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a wealth of new data on Fe-bearing phosphate minerals to bolster future analyses of Mössbauer spectra acquired on Mars.