Temperature-dependent VNIR spectroscopy of hydrated Mg-sulfates

We investigate two poly-hydrated magnesium sulfates, hexahydrite (MgSO4 · 6H2O) and epsomite (MgSO4 · 7H2O), in the visible and infrared (VNIR) spectral range 0.5/4.0 µm, as particulate for three different grain size ranges: 20-50 µm, 75-100 µm and 125-150 µm. All samples were measured in the 93-298 K temperature range. The spectra of these hydrated salts are characterized by strong OH absorption bands in the 1.0-1.5 µm region, and by H2O absorption bands near 2 and 3 µm. Other weak features show up at low temperatures near 1.75 µm (in both hexahydrite and epsomite) and 2.2 µm (only in hexahydrite). The spectral behavior of the absorption bands of these two minerals has been analyzed as a function of both grain size and temperature, deriving trends related to specific spectral parameters such as band center, band depth, band area, and band width. Hydrated minerals, in particular mono- and poly-hydrated sulfates, are present in planetary objects such as Mars and the icy Galilean satellites. Safe detection of these minerals shall rely on detailed laboratory investigation of these materials in different environmental conditions. Hence an accurate spectral analysis of such minerals as a function of temperature is key to better understand and constrain future observations.