Reflectance Spectroscopy of Ferric Sulfate-Bearing Montmorillonites as Mars Soil Analog Materials

Abstract

Spectroscopic analyses have shown that smecthites enhanced in the laboratory with additionalferric species exhibitimportant 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, becauseS was found by Viking to be a major component in the surface materialon 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, Moessbauer spectroscopy at room temperature and 4 K, differential thermal analysis, and X-ray diffraction. These analyses support the formationof ferrihydrite of variable crystallinity in the ferrihydrite-bearingmontmorillonites and a combination of schwertmannite and ferrihydritein the ferric sulfate-bearing montmorillonites. Small quantities of poorly chrystalline and nanophase forms of other ferric materials mayalso 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 moessbauer sextets and the visible reflectance spectra. An absorptionminimum is observed at 0.88-0.89 muem in spectra of the ferric sulfate-bearing samples, and at 0.89-0.92 muem in spectra of the ferrihydrite-bearing montmorillonites. Moessbauer spectra of the ferric sulfate-bearing montmorillonites indicate variable concentrations of ferrihydrite and schwertmannite in the interlaminarspaces and along grain surfaces.