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

Abstract

Alternative models for the soils of Mars have invoked either the mineral nontronite (an iron-rich smectite clay) or the mineraliod palagonite (an 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°C min −1, 50 ml Ar min −1), nontronite dehydration peaks are sharply centered at ∼ 120–125°C and ∼ 460–490°C; equivalent peaks for palagonite occur more broadly at ∼ 125–145°C and ∼ 620°C. Further distinctions can be made from detailed shapes of the respective SDT profiles, especially at temperatures of 100–1300°C. The SDT-MS data also reveal minor carbonates, sulfates, and nitrates naturally admixed with the major silicates.