title: Reflectance spectroscopy of ferric sulfate-bearing montmorillonites as Mars soil analog materials.
abstract: Spectroscopic analyses have shown that smectites enhanced in the laboratory with additional ferric species exhibit important similarities to those of the soils on Mars.  Ferrihydrite in these chemically treated smectites has features in the visible to near-infrared region that resemble the energies and band strengths of features in reflectance spectra observed for several bright regions on Mars.  New samples have been prepared with sulfate as well, because S was found by Viking to be a major component in the surface material on Mars.  A suite of ferrihydrite-bearing and ferric sulfate-bearing montmorillonites, prepared with variable Fe3+ and S concentrations and variable pH conditions, has been analyzed using reflectance spectroscopy in the visible and infrared regions, Mössbauer spectroscopy at room temperature and 4 K, differential thermal analysis, and X-ray diffraction.  These analyses support the formation of ferrihydrite of variable crystallinity in the ferrihydrite-bearing montmorillonites and a combination of schwertmannite and ferrihydrite in the ferric sulfate-bearing montmorillonites.  Small quantities of poorly crystalline or nanophase forms of other ferric materials may also be present in these samples.  The chemical formation conditions of the ferrihydrite-bearing and ferric sulfate-bearing montmorillonites influence the character of the low temperature Mössbauer sextets and the visible reflectance spectra.  An absorption minimum is observed at 0.88-0.89 micrometers in spectra of the ferric sulfate-bearing samples, and at 0.89-0.92 micrometers in spectra of the ferrihydrate-bearing montmorillonites.  Mössbauer spectra of the ferric sulfate-bearing montmorillonites indicate variable concentrations of ferrihydrite and schwertmannite in the interlaminar spaces and along grain surfaces.  Dehydration under reduced atmospheric pressure conditions induces a greater effect on the adsorbed and interlayer water in ferrihydrite-bearing montmorillonite than on the water in ferric sulfate-bearing montmorillonite.  Reflectance spectra of ferric sulfate-bearing montmorillonite include a strong 3-micrometers band that is more resistant to dry atmospheric conditions than the 3-micrometers band in spectra of similarly prepared ferrihydrite-bearing montmorillonites.
authors.affiliation: Department of Chemistry and Geological Sciences, Brown University, Providence, Rhode Island 02912, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Bishop
authors.forename: C M
authors.initials: CM
authors.lastname: Pieters
authors.forename: R G
authors.initials: RG
authors.lastname: Burns
authors.forename: J O
authors.initials: JO
authors.lastname: Edwards
authors.forename: R L
authors.initials: RL
authors.lastname: Mancinelli
authors.forename: H
authors.initials: H
authors.lastname: Fröschl
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry; D001274:Atmospheric Pressure; D001546:Bentonite / Q000737:chemistry*; D000077215:Clay; D003460:Crystallization; D005118:Extraterrestrial Environment; D005290:Ferric Compounds / Q000032:analysis*; D005765:Gastrointestinal Agents / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D018535:Mars; D008962:Models, Theoretical; D017640:Silicates; D012987:Soil / Q000032:analysis*; D015204:Spectroscopy, Mossbauer; D013057:Spectrum Analysis; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D000538:Aluminum Silicates; D005290:Ferric Compounds; D005765:Gastrointestinal Agents; D017640:Silicates; D012987:Soil; D001546:Bentonite; C024823:ferric sulfate; C033214:Smectite; D000077215:Clay