Halogen partitioning and structural incorporation in K- and Na-jarosite: Experimental insights under Mars-relevant conditions

Jarosite is an important Martian sulfate mineral with the potential to record the physicochemical conditions and fluid evolution of aqueous environments on Mars. Previous studies suggest that jarosite can incorporate trace halogens such as bromide (Br−), offering insights into paleo-fluid composition and halogen cycling on early Mars. However, the mechanisms governing halogen incorporation remain poorly constrained. In this study, we systematically examined K- and Na-jarosite synthesized from Br−/Cl−−bearing solutions prepared via two distinct pathways: Fe2+ oxidation at 25 °C and Fe3+ hydrolysis at 140 °C. Building on previous studies of low-temperature K-jarosite, we broadened the dataset to include both K- and Na-jarosite synthesized via Fe2+ oxidation and Fe3+ hydrolysis, and performed integrated chemical, crystallographic, and spectroscopic analyses to assess halogen uptake and substitution mechanisms. Our results show that Br− is preferentially incorporated into K-jarosite, particularly under low-temperature conditions, with solid-liquid partition ratios (Cs-Br/Caq-Br) exceeding 18. In contrast, Na-jarosite remains halogen-poor and exhibits structurally Fe deficiency across all tested conditions. Collectively, Raman spectroscopy, XRD lattice contraction, and stoichiometric data indicate that Br− and Cl− primarily substitute for structural OH− groups, with maximal substitution in low-temperature K-jarosite. Halogen incorporation is strongly controlled by the A-site cation and synthesis pathways, with K-jarosite accommodating far more halogen than Na-jarosite. Low-temperature conditions may promote Br− uptake via defect trapping and slower crystallization kinetics, whereas hydrothermal synthesis enhances crystallinity but reduces halogen incorporation. The Br− enrichment in jarosite parallels that observed in kainite-type double salts, suggesting that jarosite can act as a selective Br− sink in oxidizing acidic systems. On Mars, Br-bearing jarosite may reflect formation in low-temperature, chemically evolved brines. These findings underscore the dual role of jarosite as both paleoenvironmental proxy and an active participant in halogen cycling. Variations in halogen content, crystallinity, and lattice parameters may provide valuable constraints for reconstructing aqueous histories in future returned Martian samples.