Structural features in the formation of the green rust intermediate and gamma-FeOOH

Electron microscopy data reveal a pattern of changes in the particle shape/morphology during the preparation of gamma-FeOOH. The initial shapeless material develops to form large thin hexagonal crystals which are shown to be Green Rust-II (GR-II). With the initial, sudden pH decrease from about 8 to 6.5 at about 33% conversion of the total amount of Fe2+ that is oxidized, small, dense, hexagonal Fe3O4 particles are formed in addition to, or instead of, the GR-II hexagonal crystals. The large thin hexagonal crystals of GR-II contain holes. The disappearance of Fe3O4 and GR-II particles with further oxidation is accompanied by the development of needle-like gamma-FeOOH particles. The formation of gamma-FeOOH by a pathway involving only GR-II is therefore an oversimplification of the actual process.