Green rust

Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the hydroxide (OH⁻
) anion, and another anion such as carbonate (CO²⁻
₃), chloride (Cl⁻
), or sulfate (SO²⁻
₄), in a layered double hydroxide (LDH) structure. The most studied varieties are the following:^[1]

carbonate green rust – GR (CO²⁻
₃):
[Fe²⁺
₄Fe³⁺
₂(OH⁻
)₁₂]²⁺ · [CO²⁻
₃·2H
₂O]²⁻;^[2]^[3]^[4]^[5]
chloride green rust – GR (Cl⁻
):
[Fe²⁺
₃Fe³⁺
(OH⁻
)₈]⁺ · [Cl⁻
·nH
₂O]⁻;^[4]^[5]^[6]
sulfate green rust – GR (SO²⁻
₄):
[Fe²⁺
₄Fe³⁺
₂(OH⁻
)₁₂]²⁺ · [SO²⁻
₄·2H
₂O]²⁻.^[5]^[7]^[8]

Other varieties reported in the literature are bromide Br⁻
,^[7] fluoride F⁻
,^[7] iodide I⁻
,^[9] nitrate NO⁻
₃,^[10] and selenate SeO2−4.^[11]

Green rust was first recognized as a corrosion crust on iron and steel surfaces.^[2] It occurs in nature as the mineral fougerite.^[1]

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