MINERALOGICAL EVIDENCE FOR WATER-ROCK INTERACTIONS IN GALE CRATER, MARS: CHEMIN’S PERSPECTIVE
The smectites in Cumberland and John_Klein are likely to be diagenetic because: (1) the samples are from the same stratum, only a few decimeters apart, implying that their difference reflect local processes; and (2) the mineral proportions of both rocks are similar to that of local windblown sand, except for containing: the smectite, more magnetite, and much less olivine. This relationship suggests that the mudstone, as deposited, was like the windblown sand, and that much of its original olivine reacted with water to form smectite + magnetite. Parallel with this reaction, dissolution of olivine can release Fe2+ ions, which (with a CO2 atmosphere) could provide energy for chemolithoautotrophic microbial life.
The third rock sample, Windjana, is a basaltic sandstone rich in K-feldspar. It contains small proportions of phyllosilicates, seen as a broad diffraction rise at ~1.3 nm, and a low sharper peak at ~1.0 nm. The rise is comparable to the diffraction pattern of Cumberland and can be interpreted as smectite; little more can be discerned because its 02L band is obscured by peaks from other minerals. The latter peak is consistent with illite or mica, which could form by alteration of K-feldspar.
Thanks to the whole CheMin and MSL teams!