Heated iron metal interacts with steam to form iron oxide and hydrogen gas.
Under anaerobic conditions, ferrous hydroxide (Fe(OH)) can be oxidized by water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:
This works because crystalline magnetite (FeO) is thermodynamically more stable than amorphous ferrous hydroxide (Fe(OH) ).
The Massart method of preparation of magnetite as a ferrofluid, is convenient in the laboratory: mix iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.
A more efficient method of preparing magnetite without troublesome residues of sodium, is to use ammonia to promote chemical co-precipitation from the iron chlorides: first mix solutions of 0.1 M FeCl·6HO and FeCl·4HO with vigorous stirring at about 2000 rpm. The molar ratio of the FeCl:FeCl should be about 2:1. Heat the mix to 70 °C, then raise the speed of stirring to about 7500 rpm and quickly add a solution of NHOH (10 volume %). A dark precipitate  of nanoparticles of magnetite forms immediately.
In both methods, the precipitation reaction relies on rapid transformation of acidic iron ions into the spinel iron oxide structure at pH 10 or higher.
Controlling the formation of magnetite nanoparticles presents challenges: the reactions and phase transformations necessary for the creation of the magnetite spinel structure are complex. The subject is of practical importance because magnetite particles are of interest in bioscience applications such as magnetic resonance imaging (MRI), in which iron oxide magnetite nanoparticles potentially present a non-toxic alternative to the gadolinium-based contrast agents currently in use. However, difficulties in controlling the formation of the particles, still frustrate the preparation of superparamagnetic magnetite particles, that is to say: magnetite nanoparticles with a coercivity of 0 A/m, meaning that they completely lose their permanent magnetisation in the absence of an external magnetic field. The smallest values currently reported for nanosized magnetite particles is Hc = 8.5 A m, whereas the largest reported magnetization value is 87 Am kg for synthetic magnetite.
Pigment quality FeO, so called synthetic magnetite, can be prepared using processes that use industrial wastes, scrap iron or solutions containing iron salts (e.g. those produced as by-products in industrial processes such as the acid vat treatment (pickling) of steel):
*Oxidation of Fe metal in the Laux process where nitrobenzene is treated with iron metal using FeCl as a catalyst to produce aniline:
:CHNO + 3 Fe + 2 HO   →   CHNH + FeO
*Oxidation of Fe 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 FeO with hydrogen:
:3FeO + H → 2FeO +HO
Reduction of FeO with CO:
:3FeO + CO → 2FeO + CO
Production of nano-particles can be performed chemically by taking for example mixtures of Fe and Fe salts and mixing them with alkali to precipitate colloidal FeO. The reaction conditions are critical to the process and determine the particle size.
Iron(II) carbonate can also be thermally decomposed into Iron(II,III):