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Iron(II) oxide
Other names ferrous oxide
ferrous iron
CAS number 1345-25-1
Molecular formula FeO
Molar mass 71.85 g/mol
Density 5.7 g/cm³
Melting point

1370 °C (1643.15 K)

Boiling point

3414 °C (3687.15 K)

Solubility in water Insoluble
Main hazards pyrophoric
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox references

Iron(II) oxide, also known as ferrous oxide or ferrous iron, is one of the iron oxides. It is a black-colored powder with the chemical formula FeO. It consists of the chemical element iron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. Iron(II) oxide should not be confused with rust, which usually consists of hydrated iron(III) oxide (ferric oxide). Iron(II) oxide is an example of a non-stoichiometric compound and the ratio of the elements iron and oxygen can vary, samples are typically iron deficient with a compositions ranging from Fe0.84O to Fe0.95O.[1]


FeO can be prepared by heating iron(II) oxalate in vacuo[1]:

FeC2O4 → FeO + CO + CO2

The black powder can be made less reactive by heating. The heated sample is quenched to prevent disproportionation.[2] Stoichiometric FeO can be prepared by heating Fe0.95O with metallic iron at 770°C and 36kbar.[3]


FeO is thermodynamically unstable below 575°C, disproportionating to metal and Fe3O4[1]:

4FeO → Fe + Fe3O4


Iron (II) oxide adopts the cubic, rock salt structure, where iron atoms are octahedrally coordinated by oxygen atoms and the oxygen atoms octahedrally coordinated by iron atoms. The non-stoichiometry occurs because of the ease of oxidation of FeII to FeIII effectively replacing a small portion of FeII with two thirds their number of FeIII, which take up tetrahedral positions in the close packed oxide lattice.[3]

Below 200°K there is a minor change to the structure which changes the symmetry to rhombohedral and samples become antiferromagnetic.[3]


Iron(II) oxide is used as a pigment. It is FDA-approved for use in cosmetics and it is used in some tattoo inks.


  1. 1,0 1,1 1,2 Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd Edition ed.). Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
  2. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.
  3. 3,0 3,1 3,2 Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford University Press ISBN 0198553706

ar:أكسيد حديد ثنائي cs:Oxid železnatý de:Eisen(II)-oxid pl:Tlenek żelaza(II) vi:Ôxít sắt (II) uk:Монооксид заліза zh:氧化亚铁