Icarus, Volume 271, June 2016, Pages 387–399
Most sulfate minerals form only in specific pH conditions, making them useful markers of past environmental conditions on Mars. However, interpreting past environments requires a full understanding of the suite of minerals present, a task which is complicated by the fact that some minerals can spectrally mask others in the visible- to near-infrared (VNIR, 0.4–2.5 μm). Here, we report VNIR spectra of two-phase mineral combinations obtained from the Río Tinto acid mine drainage system of southern Spain. Our results show that in VNIR reflectance spectroscopy: (1) copiapite masks rhomboclase and partially masks melanterite; (2) coquimbite masks copiapite, jarosite, and rhomboclase; (3) at wavelengths <1.2 μm, gypsum is consistently masked by copiapite, jarosite, and melanterite, though at wavelengths >1.2 μm, gypsum masks these minerals; (4) unlike copiapite, jarosite, or melanterite, halotrichite masks gypsum completely; (5) in two-phase mixtures of copiapite and jarosite, both phases are evident. No consistent VNIR relationship is observed in two-phase mixtures of melanterite and halotrichite, suggesting that microtextures are likely more important than optical properties in determining VNIR reflectance. We also show that the shorter wavelengths are more sensitive to the presence of both phases: even in mixtures where one phase is masking another, both phases usually impact absorptions in the 0.75–0.95 μm region. This region may therefore be useful in remotely identifying mineral mixtures on Mars. These results have implications for several regions on Mars: most notably, they imply that the jarosite exposures reported at Mawrth Vallis may be jarosite–copiapite mixtures.