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.