MINERALOGY AT BRADBURY LANDING AND YELLOWKNIFE BAY, GALE CRATER, MARS AS MEASURED BY THE CHEMCAM LIBS
In this study, compositions of individual laser shots were determined using partial least-squares (PLS-1) multivariate analysis and then converted to moles to identify minerals by stoichiometry or through use of elemental ratios. In rare cases, individual minerals were sampled by the laser and the mineral stoichiometries of feldspar and Ca-pyroxene can be identified. In others, multiple LIBS shots on the same material produce constant molar ratios that pass through the origin for elements that occur in only a single phase or two phases. For example, a rock in which the laser samples only varying amounts of feromagnesian silicate and plagioclase (plag) with constant compositions will produce linear arrays of points on a graph of Fe vs. Fe+Mg that pass through the origin and have constant slopes equal to the Fe/(Fe+Mg) content of the ferromag. Olivine (ol) vs. pyroxene (px) may then be distinguished on the basis of partitioning by other elements such as Ni. Results suggest that Fe>Mg compositions predominate for both ol and px in sols through 269. Martian plagioclases were studied using the ratio of Na/(Na+Ca), and have an average composition for these sols of Na0.45Ca0.55Al1.5Si2.5O8 with individual spots spanning the entire range from Ca to Na end-members. A small K-feldspar component, generally <10 mole%, is present either in plagioclase or as a distinct phase. Finally, it is also possible to discern compositions of mineral pairs from linear trends with non-zero intercepts in the plots described above, enabling compositions of, for example, albite + nepheline, plagioclase+pigeonite, and plag + olivine to be determined.