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A basic solution is progressively added to various mixed Fe II -Fe III 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 Fe II 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 formulax+ 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).

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