Structural-Redox Domains in Vivianite-Group Minerals: Site-Selective Oxidation and the Prospect of Higher-Valent Fe–O Chemistry

Oxidized vivianite-group minerals pose a persistent crystal-chemical and nomenclatural problem because historical names, structural datasets and redox interpretations do not always coincide. We reassess coordinate models of vivianite, metavivianite, the Dormann phosphate dataset (MvivD), parasymplesite and the Runčevski arsenate dataset (SymR) using reconstructed Fe–O distances, convex-hull polyhedral volumes, transformed lattice metrics and bond-valence sensitivity analysis. The two crystallographically distinct Fe subsystems respond sequentially: edge-sharing Fe(a) octahedra contract mainly in the intermediate domain, whereas the isolated Fe(a′)O6 site undergoes the dominant additional contraction at advanced oxidation, reaching 10.534 Å3 in MvivD and 10.137 Å3 in SymR. These local and lattice responses define reduced, intermediate and strongly oxidized crystalline structural-redox domains. Ordinary Fe(III) phosphates reach similarly small FeO6 volumes; a small volume alone is therefore no evidence of oxidation beyond +3. However, ferristrunzite, the closest hydrated mixed-donor Fe(III) comparator in the present benchmark set, remains larger at 11.023 Å3. The structural data exclude a conventional Fe(II)-like description of MvivD and SymR but do not uniquely distinguish fully ferric, ligand-hole or formal higher-valent states. Site-sensitive spectroscopy and proton-sensitive structural data are therefore required for final electronic-state assignment.