Green rusts synthesis by coprecipitation of FeII–FeIII ions and mass-balance diagram

A basic solution is progressively added to various mixed FeII–FeIII solutions. The nature and the relative quantities of the compounds that form can be visualised in a mass-balance diagram. The formation of hydroxysulphate green rust {GR( SO 4 2 − )} is preceded by the precipitation of a sulphated ferric basic salt that transforms in a badly ordered ferric oxyhydroxide. Then octahedrally coordinated FeII species and SO 4 2 − anions are adsorbed on the FeOOH surface and GR( SO 4 2 − ) is formed at the solid/solution interface. By using the same method of preparation, other types of green rust were synthesised, e.g. hydroxycarbonate green rust {GR( CO 3 2 − )}. Like other layered double hydroxides, green rusts obey the general chemical formula [ M II ( 1 − x ) M III x ( OH ) 2 ] x − ⋅ [ ( x / n ) A n − ⋅ m H 2 O ] x + with x ⩽ 1 / 3 . Al-substituted hydroxysulphate green rust consists of small hexagonal crystals with a lateral size ∼50 nm, which is significantly smaller than the size of the GR( SO 4 2 − ) crystals (∼500 nm). To cite this article: C. Ruby et al., C. R. Geoscience 338 (2006).