COMPOSITIONAL CONSTRAINTS ON MINERALOGY AT THE MERIDIANI PLANUM LANDING SITE

APXS data supplement thermal-emission, visible-near IR, and Moessbauer (MB) spectroscopic methods of determining mineralogy. Bulk chemical compositions, coupled with likely mineralogy and mineral compositions, also constrain mineral proportions. At the Meridiani site, chemical compositions distinguish evaporite-rich rocks, basaltic soils, concretion/hematite-rich soils, and basaltic rocks with distinctive compositions. Craters expose evaporite-rich sedimentary rocks from beneath a thin, basaltic regolith. In addition to hematite and jarosite, indicated by MB, APXS chemical data indicate sulfates, with Mg as a prominent cation, and possibly chlorides. Correlated variations of major oxides such as CaO and Al2O3 with SO3 constrain possible Ca- and Al-sulfate phases to a few percent at most. The trend for SiO2, however, suggests the presence of a silica phase. The outcrop rocks are not pure evaporite; they retain a significant proportion of Al-Si-bearing detrital material, perhaps as much as 50 wt%. Using terrestrial analog minerals, mixing models suggest that the outcrop rocks could contain as much as 10% H2O+OH. Concretions of ~2-5 mm diameter occur in all observed evaporite rocks at the site. Comparing compositions of rocks and soils with concretion-rich exposures permits estimates of concretion composition and provides constraints on mineralogy, in particular, the proportion of hematite. Microscopic images of concretion-rich targets, coupled with mixing models of observed components, suggest concretion compositions consistent with ~35% hematite and 65% detrital material. Basalts as prominent rocks are rare at the sites investigated to date. A solitary rock near Eagle crater nicknamed "Bounce Rock" has a composition that matches closely martian meteorite EETA79001B. This composition is normatively rich in pyroxene, consistent with MB results. A small basaltic cobble found beyond the rim of Endurance crater has an Mg-rich composition and normatively is an olivine-orthopyroxene-rich basalt, which differs in detail from the olivine-rich Gusev basalts. Neither of these basalts is a good match to the local basaltic soils, suggesting either that the soils have a source rock with distinctly different composition or that the source is varied and the soil represents a well-mixed average.