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.

Scanning electron microscopy and energy-dispersive X-ray spectrometry of untreated interior chips from three different specimens of the Chassigny meteorite confirm the presence of discrete grains of Ca-carbonate, Mg-carbonate, and Ca-sulfate. Morphologies of these salt grains suggest that the Ca-carbonate is calcite (CaCO3) and that the Ca-sulfate is gypsum (CaSO4.2H(2)O) or bassanite (CaSO4.1/2H(2)O). The morphologic identification of the Mg-carbonate is equivocal, but rhombohedral and acicular crystal habits suggest magnesite and hydromagnesite, respectively. The salts in Chassigny occur as discontinuous veins in primary igneous minerals and are similar to those previously documented in the nakhlites, Nakhla and Lafayette, and in shergottite EETA79001. Unlike those in nakhlites, however, the Chassigny salts occur alone, without associated ferric oxides or aluminosilicate clays. Traces of Cl and P in Chassigny salts are consistent with precipitation of the salts from short-lived, saline, aqueous solutions that postdated igneous crystallization. In contrast with the clear case for nakhlites, stratigraphic evidence for a preterrestrial origin of the salts in Chassigny is ambiguous; however, a preterrestrial origin of the Chassigny salts best explains all available evidence. The water-precipitated salts provide clear physical evidence for the hypothesis, proposed by other workers, that the igneous amphiboles in Chassigny might have experienced isotope-exchange reactions with near-surface water, thereby compromising the original stable-isotope signature of any magmatic water in melt inclusions.

All subgroups of the shergottite, nakhlite, and chassignite (SNC) meteorites contain traces of water-precipitated minerals that include various combinations of carbonates, sulfates, halides, ferric oxides, and aluminosilicate clays of preterrestrial origin. Traces of sulfur- and chlorine-bearing aluminosilicates, which suggest possible affinities with scapolite minerals, occur in one shergottite but might be products of shock-metamorphic reactions rather than aqueous precipitation; even so, they are subordinate to discrete sulfate and carbonate minerals of clearly aqueous origin. If the SNC parent planet is Mars, as previously inferred from independent evidence, the aqueous precipitates indicate that oxidizing, water-based solutions probably have been chemically active on Mars for at least the time interval represented by the radiometric ages of the meteorites, namely, the past 200–1300 million years (myr). Those solutions included the chemical elements H, C, O, P, and S (N has not yet been found among the salts), although inorganic precipitates apparently predominated over organic products. The 13C/12C ratios in carbonates are satisfactorily explained by inorganic chemical reactions without requiring oxidation of 12C-rich organic matter. Further detailed mineralogical and stable-isotopic studies of the secondary minerals might help establish limits for biological activity in water-based Martian chemistry over the past 1300 myr. A mixture of aqueous precipitates found in the SNCs, comprising smectite, illite, and, gypsum (with minor halite ± calcite and hematite), provides a self-consistent, though not unique, model for the bulk elemental composition of surface sediments at the Viking Lander sites. Among other implications, the smectite-illite model lends support to a previous hypothesis that catalytic action of clay minerals was responsible for the surprising chemical reactivity discovered by the Viking Lander biology experiments. The smectite-ellite-salt model is also compatible with visible and infrared spectrophotometry of Martian dust while neither requiring nor excluding scapolite minerals as minor components.

White efflorescences of weathering origin occur superposed on fusion crusts, or along fractures in the interiors, of approximately 5% of all meteorites in the US Antarctic collection. Efflorescences from equilibrated ordinary chondrites consist of the hydrous Mg-carbonates nesquehonite (+/- hydromagnesite). X-ray diffraction and scanning electron microscope studies of efflorescences from LEW 85320 (H5) show abundant elongate prismatic crystals of nesquehonite (idiomorphic, not pseudomorphous after lansfordite), with minor local encrustations of hydromagnesite.Abundances of Na, K, Ca, and Rb in efflorescences from LEW 85320 suggest that the observed contents of these elements would require only modest fractionation of chondritic composition, whereas extensive fractionation would be required to derive the observed cation ratios from terrestrial sea-salts. Therefore, cations in evaporite minerals on Antarctic meteorites are most likely not products of contamination by terrestrial (marine) salts. The Mg in the efflorescences probably originated from weathering of meteoritic olivine; other cations in the efflorescences are also of meteoritic provenance. Thermodynamic analysis of the reaction forsterite + water + carbon dioxide --> nesquehonite + silica at Antarctic temperatures and pCO2 indicates spontaneity for all water activities greater than 0.65, compatible with the presence of liquid water as brines and/or thin films.

Interior samples of three different Nakhla specimens contain an iron-rich silicate "rust" (which includes a tentatively identified smectite), Ca-carbonate (probably calcite), Ca-sulfate (possibly gypsum or bassanite), Mg-sulfate (possibly epsomite or kieserite), and NaCl (halite); the total abundance of these phases is estimated as < 0.01 weight percent of the bulk meteorite. Rust veins are truncated and decrepitated by fusion crust and are preserved as faulted segments in partially healed olivine crystals, indicating that the rust is pre-terrestrial in origin. Because Ca-carbonate and Ca-sulfate are intergrown with the rust, they are also indicated to be of pre-terrestrial origin. Similar textural evidence regarding origins of the NaCl and Mg-sulfate is lacking. Impure and poorly crystallized sulfates and halides on the fusion crust of the meteorite suggest leaching of interior (pre-terrestrial) salts from the interior after Nakhla arrived on Earth but coincidental addition of these same salts by terrestrial contamination cannot be excluded. At least the clay-like silicate "rust," Ca-carbonate, and Ca-sulfate were formed by precipitation from water-based solutions on the Nakhla parent planet although temperature and pressure conditions of aqueous precipitation are unconstrained by currently available data. It is possible that aqueous alteration on the parent body was responsible for the previously observed disturbance of the Rb-Sr geochronometer in Nakhla at or near 1.3 Ga.