Naturally occurring arsenic (As)-bearing minerals in iron (Fe)-poor, sulfate (SO4)-rich sediments are poorly understood due to the complexity of As speciation and the scarcity of environments where As concentrations exceed mineral saturation. Sediment chemistry, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopic (EDS) data from As-bearing minerals and sediments found in the mudflat of Laguna Chiar Khota, Bolivia, are presented from a 170 m transect. From the salar, five distinct lithofacies were identified in the lake sediment and mudflat sediments. Facies (I) sediment occurs below the sediment-water interface (0–10 cm) of the salar and contains As-copper (Cu)-sulfide precipitates. Within the mudflat sediments are: Facies (II) black, reduced sediments below the water table; Facies (III) As-rich, orange-yellow sediment at the redox boundary above the water table containing realgar, an unidentified As-S compound, and a magnesium (Mg)-arsenate phase; Facies (IV) structureless, As-poor, pale brown sediment composed of diatom frustules; and Facies (V) algal-rich surface sediment containing hörnesite [Mg3(AsO4)·8H2O]. The presence of realgar and hörnesite in the Fe-poor/S-rich environment is unexpected based on geochemical models that predict orpiment should dominate in these conditions. Within the transitional layer of Facies III, temporal and spatial variability within the mudflat affect the As:S ratio across the continuous lithofacies found along the 170 m transect. Realgar is present at percent levels when the As:S ratio is >1 in the solid sediments, while the unidentified As-S phase dominates when the As:S ratio is <1, indicating an important role for the As:S ratio in this system. Co-existing oxidizing and Mg-rich conditions in the redox boundary result in the precipitation of Mg-arsenate from the reaction of As with Mg-rich pore fluids or from the dissolution of realgar. In the mudflat sediments, Mg enrichment towards the surface occurs due to calcite and gypsum precipitation as the solutions evolve during evaporative concentration. Hörnesite present at the surface of the mudflat is associated with green-pink algae, due to the reaction of As with Mg-enriched fluids. Hörnesite represents a possible As-sequestering mineral for remediation of As-contaminated sites, and a potential candidate to study microbial As mineralization.

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