Abundant Bassanite and Γ-caso4 in Mil 03346,168 Meteorite

MIL03346 meteorite: It is a martian meteorite (nakhlite) collected from Miller Range, Antarctica. It contains variety of hydrous Caand Fe 3+ -sulfates, and other H2O/OH-bearing species, thus has been extensively studied, with a purpose to build the links to aqueous alteration processes on Mars [4-9]. The martian origin of Ca-sulfates and jarosites in MIL 03346 is validated by the finding of these sulfates on Mars through orbital remote sensing and surface explorations by rovers [e.g., 1-3], although there are doubts on the potential terrestrial contaminations [4-7]. We conducted a detailed Raman spectroscopy and Raman imaging investigation on a MIL 03346,168 thin section, with the goals (1) to characterize the molecular forms (including hydration degrees) of alteration species, (2) to find the spatial correlations among them, (3) as well as their spatial distribution. Here, we report the abundant existence of bassanite in this meteorite, which agrees with the finding by CheMin on Curiosity rover at Gale Crater in Sheepbed mudstone samples. Sample and Measurements: A thin section, MIL 03346,168, was investigated using a new Raman imaging facility, a multi-wavelength inVia® Raman System (Renishaw Company), at Washington University in St. Louis. The green laser line (532 nm by Nd:YAG laser) is focused on the sample using 50xL or 100xL objectives, leading to an approximately 1 μm diameter spot. This Raman spectrometer has a spectral resolution about 1 cm -1 . Its wavelength calibration and the laser wavelength calibration support a Raman peak position accuracy and precision of ± 0.2 cm -1 . The typical Raman spectra for individual minerals grains were generally collected between 75 to 1350 cm -1 with a exposure time of 30 s for 2 co-add. In this configuration, there was no fluorescence interference for obtaining high quality Raman spectra from this meteorite sample, except when hitting the epoxy filled cracks in thin section. Ca-sulfates and their abundances in MIL 03346: Ca-sulfates with different hydration degrees each have a finger-print Raman spectral pattern that makes their distinction very straightforward. For example, the characteristic sharp and strong ν1 peak for gypsum (CaSO4.2H2O), bassanite (CaSO4.1/2H2O), anhydrite (β-CaSO4) and soluble anhydrite (γ-CaSO4) occur at 1008, 1015, 1017, and 1025 cm -1 respectively, which can be distinguished even at low spectral resolution of the payloads. Ca-sulfates were found in MIL 03346 [5-7], including a study using Raman line scans that identified the presence of gypsum and bassanite [9]. We found that Ca-sulfates in MIL 03346, 168 occur only as vein filling phase (as shown in Fig. 1.). Three hydration degrees of Ca-sulfate exist in this meteorite, i.e., gypsum, bassanite and γ-CaSO4, no anhydrite (β-CaSO4) was identified. Fig. 1a shows a vein filled with Ca-sulfates in region C of MIL 03346,168, evidenced by a gradual change from a single