A Comprehensive Spectroscopic Study ( Raman , Mir , Vis - Nir , Libs , Xrd ) Of

SYNTHETIC Fe, Fe, Mg , Al COPIAPITES. W. G. Kong , Alian Wang, J. J. Freeman, P. S. Sobron, Dept Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, MO, 63130 USA ([email protected]), 2 School of Space Science and Physics, Shandong University, Weihai, 264200, China. Introduction: Sulfates were found on Mars by spectrometers on orbiters (OMEGA and CRISM) and on two Mars Exploration Rovers (Spirit, Opportunity). Compared with Mgand Ca-sulfates, there were very few reports on the finding of Fe-sulfates from orbital remote sensing, while Fe-sulfates were found on the ground at two rover exploration sites. Jarosite (K,Na,H3O)Fe3(SO4)2(OH)6) was identified by Mössbauer spectrometer (MB) on Opportunity taking ~10 wt% of Meridiani outcrop [1]. Within the lighttoned salty soils exposed by Spirit rover at several different sites (Paso Robles, Dead Sea, and Tyrone) in Columbia Hills at Gusev Crater [2, 3], the MB spectral analysis indicated the existence of a non-specific ferric sulfate [4]. A Pancam spectral deconvolution suggested the existence of ferricopiapite, hydronium jarosite, fibroferrite, rhomboclase and paracoquimbite in these soils [5]. Another study combining the Pancam and MiniTES data from Spirit with laboratory experiments also suggested that ferricopiapite is the major constituent in Paso Robles salty soils [6]. Both of these studies reinforce the importance of copiapite group minerals for the study of martian salts. Copiapite group minerals are frequently observed in terrestrial acidic drainages. A variety of bivalent and trivalent cations entered the copiapite structure and formed a series of copiapite with different colors. Similar process can happen on Mars. The cations of martian sulfates would normally come from the weathering of martian igneous minerals. In this study, four cations (Fe, Fe, Mg, Al) important for Mars are used for synthesis. The goal of this comprehensive spectroscopic investigation (Raman, MIR, VIS-NIR, LIBS, and XRD) is to build the linkage between the laboratory spectroscopic characters with data from Mars, including the surface explorations and the orbital remote sensing. Synthesis of copiapite minerals: Four copiapites [copiapite(FeFe4(SO4)6(OH)2•20H2O);magnesocopia pite(MgFe4(SO4)6(OH)2•20H2O);ferricopiapte(Fe2/3Fe4 (SO4)6(OH)2•20H2O);aluminocopiapite(Al2/3Fe4(SO4)6( OH)2•20H2O)] were synthesized in our laboratory. Materials used were Fe2(SO4)3·5H2O, FeSO4·7H2O, MgSO4·H2O, and Al2(SO4)3·18H2O providing cations for various copiapite. The chemical reactions involved are as follows. For bivalent cations (where X=Fe, Mg) copiapites: XSO4 + 2Fe2(SO4)3 + 22H2O → XFe4(SO4)6(OH)2 • 20H2O + H2SO4. For trivalent cations (where Y=Fe, Al) copiapites: 1/3Y2(SO4)3 + 2Fe2(SO4)3 + 22H2O → Y2/3Fe4(SO4)6(OH)2 • 20H2O + H2SO4 The synthesis methods were based on the reports made by Friedlander and Majzlan [7, 8]. The saturated aqueous solutions were made by mixing different reagents with distilled water and H2SO4. The copiapites were crystallized from the solution after days in an oven at 60 ̊C. The remained H2SO4 in the powder samples were washed off using ethanol. Their structures were confirmed by XRD (a Rigaku Geigerflex X-ray diffractometer with a CuKα radiation source). Their chemical compositions were verified by laser-induced breakdown spectroscopy (LIBS, with a Continuum MiniLite pulse laser of 1064 nm, an Andor Mechelle 5000 spectrometer and DH734 ICCD).