Distinction of nontronite from palagonite by thermal analysis and evolved-gas analysis: Application to Mars surface exploration

Alternative models for the soils of Mars have invoked either the mineral nontronite (an iron-rich smectite clay) or the mineraliod palagonite fan oxidized, hydrated alteration product of basalt glass) as the major silicate component. Laboratory tests on representative terrestrial minerals demonstrate that nontronite is distinguishable from palagonite by their respective responses to combined thermal and evolved-gas analysis. When subjected to differential scanning calorimetry (DSC) or simultaneous thermogravimetric analysis and differential thermal analysis (SDT), combined with mass spectrometry (MS) of evolved gases, nontronite and palagonite differ in the onset temperatures, peak intensities, and peak profiles for their respective dehydration reactions. Using operating conditions apporpriate for a small planetary surface instrument (40 degrees C min(-1), 50 mi Ar min(-1)), nontronite dehydration peaks are sharply centered at similar to 120-125 degrees C and similar to 460-490 degrees C; equivalent peaks for palagonite occur more broadly at similar to 125-145 degrees C and similar to 620 degrees C. Further distinctions can be made from detailed shapes of the respective SDT profiles, especially at temperatures of 1000-1300 degrees C. The SDT-MS data also reveal minor carbonates, sulfates, and nitrates naturally admired with the major silicates.