Soil mineralogy at the Mars Exploration Rover landing sites: An assessment of the competing roles of physical sorting and chemical weathering

Soils in Gusev Crater and Meridiani Planum derive primarily from a surface dominated by basalt. The modal mineralogy of primary (igneous) and secondary (alteration) phases in the soils is estimated using Mossbauer, MiniTES, and APXS spectra. Primary minerals include plagioclase, pyroxene, and olivine with less common apatite, magnetite, and chromite. Secondary phases are dominated by sulfates, and include nanophase oxides, chlorides, hematite, and are assumed to include amorphous silica and phyllosilicates. Most soil chemical compositions overlap with basalts indicating that despite the presence of a secondary component in the soils, they have not been significantly chemically weathered. We modeled the significance of olivine dissolution by acid-S by iteratively removing the molar FeOT + MgO component (olivine proxy) from the mean bulk compositions of the Gusev rock classes Adirondack, Algonquin, and Irvine until none remained. Regardless of modeling conditions, acid-S alteration cannot account for many soils in Gusev Crater that are either depleted or enriched in molar FeOT + MgO, although it is a process capable of explaining some soil compositions. Based on a rock and mineral mixing model, supports our hypothesis for soil formation that consists of surface comminution by impact gardening, followed by compositional modification by hydrodynamic sorting and admixing of secondary components, including phyllosilicates and sulfates. Such a physical process can produce the range of molar FeOT + MgO in soils by concentrating or depleting specific minerals. For example, dust and fine sands are enriched in molar FeOT + MgO relative to coarse sand, which suggests accumulation of more mafic phases in finer grain fractions.