^1,2S. R. Sutton,¹A. Lanzirotti,¹M. Newville,^3,4M. D. Dyar,⁵M. McCanta, ANGSA Team
Journal of Geophysical Research: Planets (in Press) Link to Article [https://doi.org/10.1029/2025JE009416]
¹Center for Advanced Radiation Sources, The University of Chicago, Chicago, IL, USA
²Department of the GeophysicalSciences, The University of Chicago, Chicago, IL, USA
³Planetary Science Institute, Tucson, AZ, USA
⁴Department ofAstronomy, Mount Holyoke College, South Hadley, MA, USA
⁵Department of Earth and Planetary Sciences, University ofTennessee, Knoxville, TN, USA
Published by arrangement with John Wiley & Sons

Chromium and vanadium valence measurements were obtained on 17 lunar glass beads from Apollo 15 and 17 regolith materials using microscale X-ray absorption spectroscopy methods. Interior Cr valences ranged from 1.97 ± 0.02 to 2.88 ± 0.02 (Cr2+ to Cr3+). Interior V valences ranged from 2.82 ± 0.02 to 3.76 ± 0.10 (V2+/V3+ mix to V3+/V4+ mix). The interior valences of most beads cluster near V3+ and Cr2+/Cr3+ ≅ 0.6, that is, close to valences expected at IW-1, but there is significant variability and several outliers exist. For main cluster beads, Cr valence-inferred fO2 ranged from IW-1.5 to IW+1. These beads have V valence-inferred fO2 ranges from IW-2 to IW. These ranges significantly overlap but V tends to be slightly more reduced than Cr, suggesting there could be some decoupling of the Cr and V barometers. Valences for the Apollo 15 glass beads are tightly clustered, as are the Apollo 17 bottom drive tube samples 73001. In contrast, the Apollo 17 upper-drive tube samples 73002 are variable. Cr in the bead rims tended to be oxidized relative to the interiors, whereas V tended to show no redox difference between the rims and interiors. Processes responsible for establishing the redox states of the rims must be complex. Apparent fO2 conditions inferred from Cr valence tended to be slightly more oxidized than those inferred from V. Parental magmas may have possessed variable compositions that in turn experienced varying degrees of assimilation of Cr3+-rich phase(s). Valence-altering secondary processes may also have been significant.

¹Sayantan Guha,¹Shiba Shankar Acharya,²Mruganka Kumar Panigrahi
Journal of Geophysical Research: Planets (in Press) Link to Article [https://doi.org/10.1029/2025JE009601]
¹Department of Geology, Presidency University, Kolkata, India
²Department of Geology and Geophysics, Indian Instituteof Technology, Kharagpur, India
Published by arrangement with John Wiley & Sons

The western Kutch basin, India, provides a unique window into aqueous alteration under extreme acid-sulfate conditions. While previous research focused primarily on the Matanomadh Formation, this study presents a systematic investigation of hydroxy-sulfate minerals-including jarosite, alunite, minamiite, gypsum-across several chronostratigraphic units, spanning the pre-Deccan Ghuneri Member (Late Cretaceous) through post-Deccan formations (Matanomadh, Naredi, Harudi). Using a comprehensive analytical suite (XRD, XRF, FTIR, Raman, SEM-EDS, δ34S) and laboratory-synthesized potassium jarosite dissolution experiments, this work provides a complete solution to the source, formation, and preservation of these minerals. Isotopic data identify the primary source of iron and sulfur as the oxidation of precursor pyrite by meteoric water. A significant finding is the documentation of a natural natrojarosite–natroalunite solid solution, where Al-substitution enhances structural stability, making Na-jarosite more abundant than K-jarosite. Notably, field associations and geochemical data indicate that host-rock composition exerts only a minor influence on the formation of these jarosites. Crucially, our data reveal that the formation of these hydroxy-sulfate phases cannot be attributed to a single geological event or a specific past timeframe. Instead, we demonstrate that these minerals are geologically recent and continue to form as an ongoing process under current environmental conditions. The long-term preservation of these assemblages is primarily governed by the region’s prevailing aridity and localized mineral buffering associated with their mode of occurrence along the fractures of host rock. The discovery of the natrojarosite–natroalunite solid solution provides key insights into acid-sulfate system evolution on both Earth and Mars.