Significance: When oxygenic photosynthesis evolved is debated with an uncertainty of approximately 1 Gy. It is generally assumed that the oxidation of manganese minerals requires biological catalysis or molecular oxygen and therefore is often used as a proxy for the presence of oxygenic photosynthetic organisms. We show that anoxic, abiotic oxidation of the mineral rhodochrosite (MnCO₃) by UV light forms H₂and manganite (γ-MnOOH). Our results reveal an alternative mechanism for producing manganese oxides from rhodochrosite in the absence of molecular oxygen. These results demonstrate the potential impact of photogeochemical processes on the redox state of transition metals and hence question the interpretation of the rise of atmospheric oxygen based on the oxidation of transition metals, such as Cr isotopes.

The precipitation of mixed metal–aluminum-layered double hydroxides (Me(II)–Al(III)-LDH)) may be an important control of the solubility of Mn(II), Fe(II), Ni(II), Zn(II), and Co(II) in soils, but assessment of this process is hindered by a lack of thermodynamic data. Here, we determined the solubility products (Ksp) of the Me(II)–Al(III)-LDHs formed by these metals based on long-term Me(II)–γAl₂O₃ sorption studies complemented with X-ray absorption spectroscopy (XAS) measurements. The LDH phases had the chemical formula Me(II)_2/3Al_1/3(OH)₂Cl_1/3. Solubility products were derived as Ksp = (Me²⁺)_aq^2/3(Al³⁺)_aq^1/3(OH⁻)_aq²(Cl⁻)_aq^1/3, where brackets represent aqueous activity values determined from the equilibrium solution chemistry of the sorption samples. The Ksps are metal-dependent, with values increasing by two orders of magnitude in the order Ni(II) < Zn(II) < Co(II) < Fe(II) < Mn(II). Comparison to the solubility of β-Me(OH)₂ suggests considerable thermodynamic preference of Me(II)–Al(III)-LDH over β-Me(OH)₂ in soil environments and reveals a linear relation between the log-transformed Ksp values of Me(II)–Al(III)-LDH and β-Me(OH)₂. Solubility plots suggest that Ni(II)–, Zn(II)–, and Co(II)–Al(III)-LDH may form in metal-polluted soils. Fe(II)–Al(III)-LDH may occur in riparian soils undergoing reduction, but precipitation of Mn(II)–Al(III)-LDH appears unlikely as it requires [Mn²⁺]_aq much higher than commonly encountered in reducing soils. Additional thermodynamic and field studies are needed to further assess the importance of Me(II)–Al(III)-LDHs in soils and related geochemical systems.