Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs
in nature as the mineral magnetite. It is one of a number of iron oxides, the
others being iron(II) oxide, which is rare, and iron(III) oxide also known as
hematite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO
∙ Fe2O3. This iron oxide is encountered in the laboratory as a black powder. It
exhibits permanent magnetism and is ferrimagnetic, but is sometimes
incorrectly described as ferromagnetic. Its most extensive use is as a black
pigment which is synthesised rather than being extracted from the naturally
occurring mineral as the particle size and shape can be varied by the method of
production. Preparation
Under anaerobic conditions, ferrous hydroxide2) can be oxidized by water to
form magnetite and molecular hydrogen. This process is described by the
Schikorr reaction: 3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O
ferrous hydroxide → magnetite + hydrogen + water
The well-crystallized magnetite is thermodynamically more stable than the
ferrous hydroxide2 ). Magnetite can be prepared in the
laboratory as a ferrofluid in the Massart method by mixing iron(II)
chloride and iron(III) chloride in the presence of sodium hydroxide. Magnetite
can also be prepared by the chemical co-precipitation in presence of ammonia,
which consist in a mixture of a solution 0.1 M of FeCl3·6H2O and FeCl2·4H2O with
mechanic agitation of about 2000 rpm. The molar ratio of FeCl3:FeCl2 can be
2:1; heating this solution at 70 °C, and immediately the speed is elevated to
7500 rpm and adding quickly a solution of NH4OH, immediately a dark precipitate
will be formed, which consists of nanoparticles of magnetite. In both
cases, the precipitation reaction rely on a quick transformation of acidic
hydrolyzed iron ions into the spinel iron oxide structure, by hydrolysis at
elevated pH values. Considerable efforts has been devoted
towards controlling the particle formation process of magnetite
nanoparticles due to the challenging and complex chemistry reactions involved in
the phase transformations prior to the formation of the magnetite spinel
structure. Magnetite particles are of interests in bioscience applications
such as in magnetic resonance imaging since iron oxide magnetite nanoparticles
represent a non-toxic alternative to currently employed gadolinium-based
contrast agents. However, due to lack of control over the specific
transformations involved in the formation of the particles, truly
superparamagnetic particles have not yet been prepared from magnetite, i.e.
magnetite nanoparticles that completely lose their permanent magnetic
characteristic in the absence of an external magnetic field. The smallest
values currently reported for nanosized magnetite particles is Hc = 8.5 A m−1,
whereas the largest reported magnetization value is 87 Am2 kg−1 for
synthetic magnetite. Pigment quality Fe3O4, so called
synthetic magnetite, can be prepared using processes that utilise industrial
wastes, scrap iron or solutions containing iron salts of steel):
Oxidation of Fe metal in the Laux process where nitrobenzene is treated
with iron metal using FeCl2 as a catalyst to produce aniline:
C6H5NO2 + 3 Fe + 2 H2O → C6H5NH2 + Fe3O4 Oxidation of FeII compounds, e.g. the
precipitation of iron(II) salts as hydroxides followed by oxidation by
aeration where careful control of the pH determines the oxide produced.
Reduction of Fe2O3 with hydrogen: 3Fe2O3 + H2 → 2Fe3O4 +H2O
Reduction of Fe2O3 with CO: 3Fe2O3 + CO → 2Fe3O4 + CO2
Production of nano-particles can be performed chemically by taking for
example mixtures of FeII and FeIII salts and mixing them with alkali to
precipitate colloidal Fe3O4. The reaction conditions are critical to the
process and determine the particle size. Reactions
Reduction of magnetite ore by CO in a blast furnace is used to produce iron as
part of steel production process: Fe3O4 + 4CO → 3Fe + 4CO2
Controlled oxidation of Fe3O4 is used to produce brown pigment quality γ-Fe2O3:
2Fe3O4 + ½ O2 → 3(γ-Fe2O3) More vigorous calcining gives red
pigment quality α-Fe2O3: 2Fe3O4 + ½ O2 → 3(α-Fe2O3)
Structure Fe3O4 has a cubic inverse spinel
structure which consists of a cubic close packed array of oxide ions where
all of the Fe2+ ions occupy half of the octahedral sites and the Fe3+ are split
evenly across the remaining octahedral sites and the tetrahedral sites.
Both FeO and γ-Fe2O3 have a similar cubic close packed array of oxide ions
and this accounts for the ready interchangeability between the three
compounds on oxidation and reduction as these reactions entail a relatively
small change to the overall structure. Fe3O4 samples can be non-stoichiometric.
The ferrimagnetism of Fe3O4 arises because the electron spins of the FeII
and FeIII ions in the octahedral sites are coupled and the spins of the FeIII
ions in the tetrahedral sites are coupled but anti-parallel to the former.
The net effect is that the magnetic contributions of both sets are not
balanced and there is a permanent magnetism.
Properties Fe3O4 is ferrimagnetic with a Curie
temperature of 858 K. There is a phase transition at 120K, the so-called Verwey
transition where there is a discontinuity in the structure,
conductivity and magnetic properties. This effect has been extensively
investigated and whilst various explanations have been proposed, it does
not appear to be fully understood. Fe3O4 is an electrical conductor with a
conductivity significantly higher than Fe2O3, and this is ascribed to electron
exchange between the FeII and FeIII centres.
Uses Fe3O4 is used as a black pigment and is
known as C.I pigment black 11. Fe3O4 is used as a catalyst in the Haber
process and in the water gas shift reaction. The latter uses an HTS of iron
oxide stabilised by chromium oxide. This iron-chrome catalyst is reduced at
reactor start up to generate Fe3O4 from α-Fe2O3 and Cr2O3 to CrO3.
Nano particles of Fe3O4 are used as contrast agents in MRI scanning.
Ferumoxytol, also known as Feraheme and Rienso, is an intravenous Fe3O4
preparation for treatment of anemia resulting from chronic kidney disease.
Ferumoxytol is manufactured and globally distributed by AMAG Pharmaceuticals.
Along with sulfur and aluminium, it is an ingredient in a specific type of
thermite useful for cutting steel. Bluing is a passivation process that
produces a layer of Fe3O4 on the surface of steel to protect it from rust.
Biological Occurrence Magnetite has been found as
nano-crystals in magnetotactic bacteria and in homing pigeon beak tissue
See also Mars Black
References