A Miniature Thermal Emission Spectrometer (Mini-TES), based on a Michelson interferometer and Cassegrain telescope, was carried by the Spirit rover in Gusev crater and Opportunity rover at Meridiani Planum to determine the bulk mineralogy of surface materials. Spectra from the plains of Gusev demonstrate the ubiquity of olivine-rich basaltic rocks, with additional examples lofted into the adjacent Columbia Hills by meteoroid impacts. Hundreds of rocks observed with mini-TES in the Columbia Hills display spectral characteristics of variable alteration intensity, but likely with very little water involved. Rare exceptions include a tephra deposit cemented by Mg–Fe carbonates and nodular opaline silica rocks, likely indicative of a hot spring/geyser environment. Opportunity’s mini-TES confirmed orbital identification of crystalline hematite at Meridiani Planum and spectral characteristics indicative of a transition from a precursor goethite phase. The sedimentary bedrock that hosts the hematite has spectral features consistent with Al-rich opaline silica, Mg-, Ca-, and Fe-bearing sulfates, plagioclase feldspar, and nontronite. Rare rocks at both sites are recognizable as iron meteorites from their infrared reflective properties.