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Uranium (U) is an important global energy resource and a redox sensitive trace element that reflects changing environmental conditions and geochemical cycling. The redox evolution of U mineral chemistry can be interrogated to understand the formation and distribution of U deposits and the redox processes involved in U geochemistry throughout Earth history. In this study, geochemical modeling using thermodynamic data, and mineral chemistry network analysis are used to investigate U geochemistry and deposition through time. The number of U6+ mineral localities surpasses the number of U4+ mineral localities in the Paleoproterozoic. Moreover, the number of sedimentary U6+ mineral localities increases earlier in the Phanerozoic than the number of U4+ sedimentary mineral localities, likely due to the necessity of sufficient sedimentary organic matter to reduce U6+–U4+. Indeed, modeling calculations indicate that increased oxidative weathering due to surface oxygenation limited U4+ uraninite (UO2) formation from weathered granite and basalt. Louvain network community detection shows that U6+ forms minerals with many more shared elements and redox states than U4+. The range of weighted Mineral Element Electronegativity Coefficient of Variation (wMEECV) values of U6+ minerals increases through time, particularly during the Phanerozoic. Conversely, the range of wMEECV values of U4+ minerals is consistent through time due to the relative abundance of uraninite, coffinite, and brannerite. The late oxidation and formation of U6+ minerals compared to S6+ minerals illustrates the importance of the development of land plants, organic matter deposition, and redox‐controlled U deposition from ground water in continental sediments during this time‐period. Plain Language Summary: Uranium (U) is the most widely used fuel in nuclear fission power plants, and nuclear power results in lower greenhouse gas emissions than fossil fuel energy. The different types of U deposits and minerals have evolved throughout the Earth history with changing environmental conditions and formation processes. In this study, we use modeling calculations and network analysis to understand how the transforming Earth system impacted the global U cycle, evolving U mineral chemistry, and deposit formation through time. Modeling calculations show that the abundant U mineral uraninite (UO2) formed in much greater quantities in the anoxic conditions of the Archean eon than present day oxic conditions. We also find that U minerals are increasingly oxidized through time, in agreement with modeling calculations, resulting in new minerals with diversifying chemical element associations and expanding distribution of U in the environment. The earlier increase in the number of oxidized U minerals in sedimentary localities than unoxidized U minerals in sedimentary localities 350–250 million years ago reflects the importance of land plants and organic matter in the formation of unoxidized U minerals in sedimentary settings. Key Points: Model calculations indicate Earth surface oxidation limited U4+ uraninite formation, in agreement with increased observed U6+ mineralsU oxidation increases the diversity of mineral chemical element associations and distribution in the environmentThe later formation of U6+ minerals compared to S6+ minerals represents differential Earth surface oxidation [ABSTRACT FROM AUTHOR]

Clay minerals detected with orbital and in situ instruments in ancient Martian terrains constrain Mars' climate and aqueous alteration history. Early in its history, Mars experienced an atmospheric redox change and iron‐bearing clay minerals may preserve the effects of that transition. Ferrous smectites, the thermodynamically predicted product of chemical weathering of basalts under anoxic conditions, may have undergone oxidation by exposure to chemical oxidants in the atmosphere or regolith, or by direct photooxidation at the surface. To assess these potential oxidation pathways, ferrous trioctahedral smectites of varying initial iron content were synthesized and subjected to oxidation by ultraviolet (UV) irradiation. Experimental UV irradiation under an anoxic atmosphere equivalent to approximately 7 years of flux on the Martian surface caused partial oxidation of smectite‐bound Fe (Fe3+/ΣFe = 16–18%) and octahedral sheet contraction. Metal‐OH vibrational bands in visible/near infrared (VNIR) reflectance spectra of oxidized smectites changed in band depth and asymmetry with higher iron content. X‐ray diffraction patterns of UV irradiated samples indicate the formation of a mixed di‐ and trioctahedral smectite or a secondary nontronite phase, possibly on the surfaces of higher iron content smectites. These experiments suggest that UV irradiation is able to oxidize structurally bound iron in smectites without the presence of other chemical oxidants. Photooxidation may have influenced the mineralogy, both syndepositionally and postdepositionally, of Martian alteration assemblages formed near the surface and this process needs not be limited to one part of their formation history. Plain Language Summary: Martian mineral assemblages observed with orbiters and rover instruments allow us to understand the planet's past climate and aqueous activity. Hydrated minerals that contain Fe(II) are of particular interest as they would have been affected by the early atmosphere becoming more oxidizing overtime, changing the structural iron into Fe(III). In this study, we synthesized Fe(II) smectites with varying iron content and irradiated them with an ultraviolet (UV) light source to test whether they could be photooxidized. Iron in the smectite minerals was incompletely oxidized by UV radiation after being exposed for the equivalent of approximately 7 years of exposure on the early Mars surface. The smectites with higher iron content showed more changes with photooxidation in their visible/near infrared spectra as well as in their X‐ray diffraction patterns, two datasets collected by instruments on currently active Martian missions. These results suggest that UV photooxidation is a plausible contributor to the mineralogy and redox state of clay mineral assemblages observed at the Martian surface. Key Points: Ultraviolet (UV) irradiation of initially ferrous smectites of varying iron content partially oxidized structural iron (Fe3+/ΣFe = 16–18%)Smectites with greater iron had more observable changes in their visible/near infrared spectra regardless of the similar extent of oxidationUV photooxidation is proposed as a viable oxidation pathway for surficial Martian smectites [ABSTRACT FROM AUTHOR]

A comprehensive investigation of the complementary H-bonding-mediated self-assembly between dipyrrolo[2,3- b:3′,2′-e]pyridine (P2P) electron donors and naphthalenediimide/perylenediimide (NDI/PDI) acceptors is reported. The synthesis of parent P2P and several aryl-substituted derivatives is described, along with their optical, redox, and single-crystal packing characteristics. The dual functionality of heteroatoms in the P2P/NDI(PDI) assembly, which act as proton donors/acceptors and also contribute to π-conjugation, leads to H-bonding-induced perturbation of electronic levels. Concentration-dependent NMR and UV/Vis spectroscopic studies revealed a cooperative effect of H-bonding and π-π stacking interactions. This H-bonding-mediated co-assembly of donor (D) and acceptor (A) components leads to a new charge-transfer (CT) absorption that can be controlled throughout the visible range. The electronic interactions between D and A were further investigated by time-dependent DFT, which provided insights into the nature of the CT transition. Electropolymerization of difuryl-P2P afforded the first conjugated polymer incorporating H-bonding recognition units in its main chain. [ABSTRACT FROM AUTHOR]

The aqueous concentration of lead [Pb(II)] in geochemical environments is controlled by the solubility of Pb-bearing minerals and their weathering products. In contaminated soils, a common method for in situ stabilization of Pb(II) is the addition of phosphate to convert more redox sensitive sulfide minerals into sparingly soluble pyromorphite [Pb5(PO4)3X]. In this study, we conducted experimental studies to investigate the fate of reduced sulfur during the conversion of galena [PbS] to chloropyromorphite [Pb5(PO4)3Cl]. Powder X-ray diffraction analysis indicated that the reaction of phosphate with galena under oxic conditions resulted in the oxidation of sulfide and formation of elemental sulfur [S8]. Under oxic abiotic conditions, the S8 was retained in the solid phase, and negligible concentrations of sulfur as sulfide and thiosulfate were detected in the aqueous phase and only a small amount of sulfate. When PbS reacted in the presence of the chemoautotrophic organism Bosea sp. WAO, the S8 in the secondary mineral was oxidized to sulfate. Strain WAO produced significantly more sulfate from the secondary S8 than from the primary galena. Microscopic analysis of mineral-microbe aggregates on mineral-embedded slide cultures showed that the organism was colocalized and increased in biomass over time on the secondary mineral surface supporting a microbial role. The results of this study indicate that stimulation of sulfur-oxidizing activity may be a direct consequence of phosphate amendments to Pb(II)-contaminated soils. [ABSTRACT FROM AUTHOR]