Green rust

[1] The crystal structure of green rust can be understood as the result of inserting the foreign anions and water molecules between brucite-like layers of iron(II) hydroxide, Fe(OH)2.

The latter has an hexagonal crystal structure, with layer sequence AcBAcB... , where A and B are planes of hydroxide ions, and c those of Fe2+ (iron(II), ferrous) cations.

It has an hexagonal crystal structure as minerals of the sjogrenite (Mg6Fe2(OH)16CO3·4H2O) group, with layers probably stacked in the sequence AcBiAbCjA...[1][7][13] In oxidizing environment, green rust generally turns into Fe3+ oxyhydroxides, namely α-FeOOH (goethite) and γ-FeOOH (lepidocrocite).

[13] Oxidation of the carbonate variety can be retarded by wetting the material with hydroxyl-containing organic compounds such as glycerol or glucose, even though they do not penetrate the structure.

It was conjectured that green rust may be formed in the reducing alkaline conditions below the surface of marine sediments and may be connected to the disappearance of oxidized species like nitrate in that environment.

[16][17][18] Suspensions of carbonate green rust and orange γ-FeOOH in water react over a few days producing a black precipitate of magnetite Fe3O4.

[4] On the basis of Mössbauer spectroscopy, green rust is suspected to occur as mineral in certain bluish-green soils that are formed in alternating redox conditions, and turn ochre once exposed to air.

[5] Hexagonal crystals of green rust (carbonate and/or sulfate) have also been obtained as byproducts of bioreduction of ferric oxyhydroxides by dissimilatory iron-reducing bacteria, such as Shewanella putrefaciens, that couple the reduction of Fe3+ with the oxidation of organic matter.

[19] In one experiment, a 160 mM suspension of orange lepidocrocite γ-FeOOH in a solution containing formate (HCO−2), incubated for 3 days with a culture of Shewanella putrefaciens, turned dark green due to the conversion of the hydroxide to GR(CO2−3), in the form of hexagonal platelets with diameter ~7 μm.