Transmission X-ray Diffraction (xrd) Patterns Relevant to the Msl Chemin

Introduction and Background: The Mars Science Laboratory (MSL) CheMin instrument on the Curiosity rover is a transmission X-ray diffractometer (Co-K radiation source and a ~5 to ~52 2 range) where the analyzed powder samples are constrained to have discrete particle diameters <150 μm by a sieve [1]. To date, diffraction patterns have been obtained for one basaltic soil (Rocknest (RN)) and four drill fines of coherent rock (John Klein (JK), Cumberland (CB), Windjana (WJ), and Confidence Hills (CH)) [2-4]. The CheMin instrument has detected and quantified the abundance of both primary igneous (e.g., feldspar, olivine, and pyroxene) and secondary (e.g., Ca-sulfates, hematite, akaganeite, and Fe-saponite) minerals [2-5]. The diffraction patterns of all CheMin samples are also characterized by a broad diffraction band centered near 30 2 and by increasing diffraction intensity (scattering continuum) from ~15 to ~5, the 2 minimum. Both the broad band and the scattering continuum are attributed to the presence of an XRD amorphous component. Estimates of amorphous component abundance, based on the XRD data itself [2,4] and on mass-balance calculations using APXS data crystalline component chemistry derived from XRD data, martian meteorites, and/or stoichiometry [e.g., 6-9], range from ~20 wt.% to ~50 wt.% of bulk sample. The APXSbased calculations show that the amorphous component is rich in volatile elements (esp. SO3) and is not simply primary basaltic glass, which was used as a surrogate to model the broad band in the RN CheMin pattern [2]. For RN, the entire volatile inventory (except minor anhydrite) is assigned to the amorphous component because no volatile-bearing crystalline phases were reported within detection limits [2]. For JK and CB, Fesaponite, basanite, and akaganeite are volatile-bearing crystalline components. Here we report transmission XRD patterns for sulfate and silicate phases relevant to interpretation of MSL-CheMin XRD amorphous components. Samples and Methods: For sulfate experiments, 10 acid-sulfate solutions were prepared from standard reagents (Table 1). Cryoprecipitation (precipitation by removing water as ice) was induced by freezing solutions using liquid N2. After freeze drying, the precipitates were stored in closed containers in a glove box purged with dry-N2 gas. For silicate experiments, silicate liquids having compositions equivalent to representative MER rocks and soils (SO3and Cl-free) [e.g., 10, 11] were equilibrated at 1350-1450C in a 1atm gas mixing furnace at IW+1 oxygen fugacity and quenched in water to room temperature. Transmission XRD diffraction patterns were obtained on a CheMin-4 diffractometer which is a laboratory version of the MSL CheMin instrument. Dry N2 purge gas was used for the sulfate measurements.