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Estimating the Chemical Composition of Alunite-Jarosite Group Minerals

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Estimating the Chemical Composition of Alunite-Jarosite Group Minerals Estimating the Chemical Composition of Alunite-Jarosite Group Minerals Powered by: at MSL Drill Sites Using XRD Patterns from Synthetic Analogs Chris Donaldson & Tom McCollom General formula for LASP, Boulder, CO ([email protected]) alunite-jarosite group: Introduction Synthesis Experiments (Na,K,H3O)(Al,Fe)3(SO4)2(OH)6 The CheMin instrument on MSL’s Curiosity rover has detected the presence of jarosite at Temperature: 155 ± 5 °C several locations. The occurrence of jarosite provides an opportunity to place constraints on the chemical environment at the time the minerals precipitated, which, in turn, can provide Pressure: ambient Fe # = [Fe/(Fe+Al)] x 100 insight into fluid-rock interactions during diagenesis of the sediments that compose the lower Media: H O or 0.1 M H SO slopes of Mount Sharp. In addition, jarosite and other members of the alunite group can have 2 2 4 + Substrates: Al-Fe-and-NaSO and Fe#=0 Fe#=12 Fe#=36 Fe#=72 Fe#=100 considerable compositional variability, including Al-for-Fe and Na- or H3O -for-K substitutions 4 which, if quantified, would provide additional insight into the processes involved in these KCl mixtures sediment’s formation. Accordingly, our study investigated how XRD patterns and spectral Products: rinsed with ethanol and features change in response to these variations, and our results allow preliminary constraints to be placed on the chemical composition of the jarosite at Mount Sharp. dried at room temp. Potassium Jarosite (Fe#=60) Potassium Alunite-Jarosite XRD XRD peaks display systematic shifts with VNIR VNIR spectral features show systematic shifts with Fe-Al Raman Raman peaks also display systematic shifts + + + Fe-Al and Na-K-H3O substitutions. substitutions, but little variation with Na-K-H3O substitutions. with Fe-Al and Na-K-H3O substitutions. Potassium Alunite-Jarosite Potassium Alunite-Jarosite Potassium Alunite-Jarosite Fe#=0 Fe#=0 Fe#=13 Fe#=13 Fe#=0 Fe#=18 Fe#=18 Fe#=36 Fe#=13 Fe#=18 Fe#=60 Fe#=60 Fe#=36 Fe#=72 Fe#=60 Fe#=72 Fe#=72 Fe#=90 Fe#=100 Fe#=100 Fe#=100 Sodium Alunite-Jarosite Sodium Alunite-Jarosite Sodium Alunite-Jarosite Fe#=0 Fe#=10 Fe#=0 Fe#=0 Fe#=20 Fe#=10 Fe#=10 Fe#=20 Fe#=20 Fe#=25 Fe#=25 Fe#=25 Fe#=49 Fe#=49 Fe#=49 Fe#=61 Fe#=61 Fe#=61 Fe#=65 Fe#=65 Fe#=65 Increasing Fe content Fe Increasing Increasing Fe content Fe Increasing Increasing Fe content Fe Increasing Fe#=84 Fe#=84 Fe#=92 Fe#=84 Fe#=92 Fe#=100 Fe#=92 Fe#=100 Fe#=100 Hydronium Alunite-Jarosite Hydronium Alunite-Jarosite Hydronium Alunite-Jarosite Fe#=83 Fe#=83 Fe#=90 Fe#=90 Fe#=90 Fe#=100 Fe#=100 Fe#=100 Angle (degrees 2Θ Co-Kα) Wavelength (nm) Raman Shift (cm-1) Mount Sharp Jarosite Conclusions Comparison of our synthetic samples with The noticeable shifts in XRD patterns and spectral features with varying J = jarosite CheMin results from three Murray Formation composition suggests that using these indicators to infer chemical Px = pyroxene composition is feasible. With that said, our results indicate that the jarosite Pl = plagioclase drill sites on Mount Sharp. on the lower slopes of Mount Sharp has significant Al for Fe substitution and + is dominated by K as opposed to Na or H3O . Moreover, using our inferred Single diffraction peak at ~34° rather than a composition and an estimated 3.1 wt% jarosite in the Mojave sample [2], we Mojave 2 (sol 882) doublet is indicative of K dominated alunite or estimate that the jarosite can account for ~30 wt% K2O, approximately half jarosite with little to no Na or H O+. of the total amount of K2O measured in the sample (0.72 wt%) [3]. Also, in Confidence Hills (sol 759) 3 many terrestrial settings, jarosite forms by oxidative weathering of Fe-sulfide Sebina (sol 1496) minerals, and sulfides have been identified in small amounts in other samples within Gale crater [4]. However, it is not clear that this process could The position of both the peak at ~34° and the Fe#=100 (K) account for the inferred presence of substantial Al in the jarosite observed in Fe#=90 (K) peaks at 17-21° indicate that Mount Sharp the Murray formation samples, and may instead point to formation of the Fe#=72 (K) jarosite minerals have significant Al-for-Fe jarosite by interaction of acidic, sulfate-rich fluids with K-, Fe-, and Al-bearing Fe#=60 (K) substitution (Fe# between 75 to 90) with a silicate minerals. Fe#=100 (Na) Fe#=84 (Na) possible slight increase in Al contents from Fe#=100 (H3O) Confidence Hills to Sebina to Mojave2. References: [1] McCollom et al., 2014, Am Min 99, 948-964. [2] Rampe et al.,2017, EPSL 471, 172-185. [3] APXS data downloaded from Planetary Data System (PDS). [4] Vaniman D. T. et al. (2014) Science, Angle (degrees 2Θ Cu-Kα) 343, DOI: 10.1126/science.1243480. [5] Curiosity data downloaded from: odr.io/chemin..
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