Goldenrod pigments and the occurrence of hematite and possibly goethite in the Olympus-Amazonis region of Mars

The spectral properties of the Olympus-Amazonis region of Mars were reproduced using laboratory mixtures of palagonitic tephra and goldenrod pigments, which are blends of well-crystalline hematite and goethite. Laboratory spectra for pure pigments and mixtures were obtained at similar to 140 and similar to 305 K to bracket the temperature range of the martian surface. The best fit to the martian surface spectrum was obtained with a mixture of palagonitic tephra and 2.5% BLS12, a goldenrod pigment with 44% of its iron atoms associated with hematite and 56% with goethite (determined from Mossbauer spectroscopy). Although goethite is the major phase, the positions of reflectivity maximum and minimum near 750 and 860 nm, respectively, are determined by the hematite component at both temperatures. Spectrally, the presence of goethite is recognized by increased reflectivity between similar to 400 and 550 nm and, particularly at low temperatures, lowered reflectivity between similar to 1000 and 1300 nm compared to spectra of mixtures containing only hematite. Although the presence of goethite is consistent with the martian spectrum, the presence of the mineralogy is equivocal because other ferric-bearing phases (e.g., maghemite, lepidocrocite, akaganeite, schwertmannite, jarosite, and nontronite) have similar spectral characteristics, making definitive identification difficult. Thus, our results show that spectral data for Olympus-Amazonis, which are hematitic based on positions of spectral features, permit the presence of comparable proportions of other ferric-bearing phases. Weathering products in addition to hematite, such as goethite from oxidative and hydrolytic weathering or jarosite and/or schwertmannite from acid-sulfate weathering, may be present but spectrally masked by hematite. (C) 1998 Academic Press.