Raman , Mir , and Nir Spectroscopic Study of Calcium Sulfates

Introduction: Gypsum (CaSO 4 .2H 2 O) and bassanite (CaSO 4 .0.5H 2 O) were identified on Mars by OMEGA on Mars Express and CRISM on MRO [1, 2, 3]. An-hydrite (CaSO 4) is non-active in the spectral range of OMEGA and CRISM, but may also exist [4]. Ca-sulfate was inferred from compositional correlations revealed by APXS on Mars Exploration Rovers [5, 6, 7], but no definitive identification, as well as its hydra-tion state, was made because of the lack of an in situ mineral identification tool (e.g. Raman or XRD). Compared to Mg-sulfates and Fe-sulfates, Ca-sulfates are more stable under Mars conditions and contain a limited range of H 2 O content, 2w, 0.5w, 0w per SO 4 [8], thus they may not play an important role in the water cycle on Mars. However when Ca-sulfates were mixed with others, e.g. Mg-sulfates, Ca-sulfates can affect the hydration and dehydration pathways of the latters [9]. In addition, although Ca does not form solid solution in sulfates with Mg or Fe, but it does form (Ca, Na)-or (Ca, K)-sulfates as solid solutions. Furthermore, the natural occurrence of three Ca-sulfates does not always follow the predictions of thermadynamics or phase boundaries. Therefore, it is very important to establish a good understanding of the spectroscopic characters of pure Ca-sulfates, in order to make definitive identification of many different types of sulfates, as well as other salts. In this abstract, we report a comprehensive spec-troscopic study of gypsum, bassanite, and anhydrite. Raman and Mid-IR spectra were acquired. Their peaks were assigned to the fundamental vibrational modes of (SO 4) 2-and H 2 O in three structures. These knowledge was then used to assign the overtone and combination-al modes in their NIR spectra. This set of spectral peak assignments will help the interpretation of NIR spectral data from current orbital missions (OMEGA and CRISM), and for the future mission to Mars, i.e. Ra-man on ExoMars. Samples and measurements: Pure chemicals were used for this study, which produce the identical spectra as those from natural minerals. They are gypsum (Sigma Aldrich 255548-100G) with 98% purity, bas-sanite (Sigma_Aldrich 307661_25G) with 98% purity, and gamma-anhydrite (Aldrich 237132-100G) with 99% purity. We used an infrared spectrometer (Ther-mo Nicolet Nexus 670) to record the attenuated total reflectance (ATR) Mid-IR spectra (400 – 4000 cm-1) and diffuse reflectance NIR spectra (1.0 – 5.0 μm) of three samples. Their Raman spectra were …