CRYSTAL-CHEMISTRY OF MARS MINERALS AT ROCKNEST, JOHN KLEIN AND CUMBERLAND

The CheMin instrument on the Mars Science Laboratory (MSL) rover Curiosity performed X-ray diffraction analyses in Gale crater. The first series of experiments, from sols 94-119, were performed on scooped soil at Rocknest. Later analyses focused on the sheepbed mudstone member at the base of the Yellowknife bay formation. The first drilled mudstone site was John Klein. This sample was analyzed over sols 195-238 and 270-272. The second sample of mudstone fines was drilled from Cumberland and analyzed on sols 282-310. Crystalline phases from each of these samples were identified and subsequently their abundances and unit-cell parameters were estimated with the Rietveld method. The most abundant minerals found in these samples include phases from the olivine, plagioclase and pyroxene groups; magnetite is also abundant in the mudstone whereas olivine abundance is diminished.

Hundreds of unit-cell parameters from minerals with known chemical compositions were obtained from the literature to model the relationships between unit-cell dimensions and composition for each major crystalline phase. These crystal-chemical systematics were applied to each of the major phases observed in Gale crater and their chemical formulae were estimated from refined unit-cell parameters. The compositions of the minor phases were assumed to be ideal and were combined with those of the major phases to estimate the bulk crystalline composition for each sample. All three samples contained amorphous material in addition to the crystalline phases. The amorphous compositions were estimated for each sample by subtracting the chemistry of the crystalline component from the bulk chemical composition measured by APXS.

The mineralogical and amorphous composition of the Rocknest soil indicates a basaltic source with global and regional components, similar to that proposed for Gusev crater and Meridiani Planum. The two drilled mudstone samples had mineral compositions similar to those of the Rocknest soil. However, the mudstone mineral assemblage, which includes phyllosilicates, represents a potentially habitable fluvio-lacustrine environment.