The Spectral Properties of Gypsum from -90 to 400 °C and Implications for Mars

Gypsum (CaSO4.2H2O) is a hydrated sulfate commonly found on Earth and at several locations on Mars. Characterizing the spectral properties of gypsum under different temperature conditions supports remote identification of this mineral. Understanding dehydration of gypsum is also important for interpreting the CheMin data at Gale crater and constraining water in the martian regolith, which has important implications for the geochemical history of Mars, its habitability, and the potential for life. To resolve this, we combined thermal decomposition experiments with cryogenic-FTIR to probe the dehydration of the H-O-H bending (1400-2300 cm-1) region. Our Thermogravimetric Analysis (TGA) and temperature programmed desorption (TPD)-FTIR experiments revealed that H2O in gypsum is released by ~100-150°C. Additionally, cryogenic-FTIR measurements of 1% NaCl and 1% CaCl2 mixtures with gypsum demonstrated that gypsum is more soluble in 1% CaCl2 solution at -90 °C. Introduction: Gypsum is an abundant terrestrial sulfate mineral in evaporate settings on Earth along with its dehydrated phases (bassanite and anhydrite), and has been detected at several locations on Mars [1]. This includes detection of gypsum dunes in Olympia Undae by OMEGA/Mars Express in the north polar region of Mars [2], Meridiani Planum [3] and Gale Crater [4]. Understanding the conditions governing dehydration of gypsum will help constrain the aqueous and geochemical history of Mars. Water loss from the mineral gypsum can occur through (i) heating over 100 °C [5, 6, 7] or (ii) the reaction of gypsum with salts at ~83 °C that both lead to the transformation of gypsum and basanite to anhydrite [5]. This study combines thermal decomposition experiments and cryogenicFTIR spectroscopy of gypsum and Cl-salt mixtures to provide an experimental study to describe the dehydration features of Ca-sulfates and their spectral properties under a range of temperatures to support interpretation of aqueous environments on Mars where Ca-sulfates are present. Methods: We used gypsum sample JB1464 [5] that was crushed and dry sieved prior to the experiments. Spectra were collected either as a powder or mixed with brine solutions for cryogenic-FTIR measurements. TGA. 7.2 mg of gypsum was heated from 25 to 400 °C in the Mettler Toledo TGA instrument with a heating rate of 1 °C / min under N2(g). These experiments were performed in two stages as described in our previous TGA study with the iron-rich sulfate, rozenite [8]. TPD-FTIR. The gypsum powder was coated on a tungsten mesh while applying 5 N/m pressure. Prior to heating, the sample was dried at 25 °C in vacuo (< 0.78 Torr) for 10 min in a transmission IR cell. The temperature was increased from 25 to 400 °C at a rate of 10 °C/min and spectra were collected every 89 sec, based on our previous study [8]. Cryogenic-FTIR. We mixed 50 mg sample with 20 μL 1% NaCl and 1% CaCl2 solutions and equilibrated them for 15 min. 10 μL of the aqueous suspensions were applied onto an attenuated total reflectance (ATR) accessory at 25 °C and flash-frozen to -90 °C in 5 min. The temperature was then increased to 25 °C at a heating rate of 10 °C/min to probe the phase variations from H2O/chloride ice to H2O bound in gypsum, monitoring the H-O-H bending region at 4 cm-1 spectral resolution with 100 scans per spectrum every 89 sec. Results: Our TGA analysis (Fig. 1) revealed that gypsum lost substantial water (~13.8 wt.%) through dehydration up to 80 °C. An additional ~5.4 wt.% water was lost by 110 °C, after which the sample remained