Spectroscopy of jarosite minerals, and implication for the mineralogy of Mars



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The Mars Exploration Rover Mössbauer spectroscopy team identified jarosite, a sulfate which forms in the presence of water, in spectra of a layered outcrop in Meridiani Planum (Klingelhöfer et al. 2004). They made this assignment based on a doublet with quadrupole splitting of ~1.22 mm/s at T=200-280K that was assigned to either K- or Na-jarosite with the possibility of some Al substituting for Fe in octahedral sites. In order to further characterize this jarosite at Meridiani and search for jarosite elsewhere, careful spectroscopic characterization of the Mössbauer parameters of jarosite over a range of compositions and temperatures is needed, which is the goal of this study. The jarosite group has the formula AM3(SO4)2(OH)6, where A is usually a monovalent cation (K+ and Na+ are most common, although Pb+, Ag+, NH4+ and H3O+ are also observed). M is primarily Fe3+ for jarosite, Al3+ for alunite or another trivalentcation. The structure is composed of SO42- tetrahedral and MO2(OH)4 octahedra, both somewhat distorted and each forming a layer in the a direction. Each SO42- tetrahedron has one O bound to Al or Fe, producing a symmetry of C3v instead of Td. The jarosite samples used in this study were synthesized by Brophy (Brophy, 1965) and were generously provided to us by him. They represent a series of 17 compositions in which the values of potassium (K), sodium (Na), and hydronium (H3O) all vary. Mössbauer spectra of all samples were acquired at room temperature; sample #2 was also measured over a range of 16 temperatures from 12-295K at low He gas pressure. A source of 100-70 mCi 57Co in Rh was used on a WEB Research Co. model W100 spectrometer equipped with a Janus closedcycle He refrigerator. Run times ranged from 1-12 hours; results were calibrated against α-Fe foil. At low K contents, Na and H3O+ were co-varied, so the trends are not clear. These data contrast with the much older work, who suggested that the QS values of jarosite are negatively correlated with K contents. However, those earlier studies used Lorentzian doublets rather than QSD s, employed natural samples that were part of mixtures of minerals, and/or had far fewer compositions represented. Our multi-temperature Mössbauer spectra show that magnetic ordering in jarosite with a composition of K.74Na.02(H3O).24 occurs at 90 K. The NIR spectrum of K-jarosite exhibits an OH stretching band at 1.47 μm, an OH stretch + 2 bend combination doublet at 1.849 and 1.864 μm, plus an OH stretch + bend combination triplet at 2.215, 2.265, and 2.300 μm and additional OH and SO4 combination features near 2.40, 2.46, 2.50, 2.60 and 2.62 μm. H3Oand Na-jarosite spectra exhibit broader features and the doublet is less resolvable. The spectrum of Najarosite contains a band at 1.48 μm, a broad asymmetric band near 1.85 μm and a triplet near 2.235, 2.275, and 2.310 μm, plus additional features near 2.42, 2.47, 2.52, 2.62 and 2.64 μm.