Black Fe3O4 is a mixed valence oxide of iron with a melting point of 1597℃ and a density of 5. 18g/cm3. Insoluble in water, soluble in acid solution, it occurs in the form of magnetite in nature. At room temperature, it has strong sub-magnetism and high conductivity.

Ferromagnetic and ferrimagnetic substances undergo second-order phase transition above Curie temperature to become paramagnetic material. The Curie temperature of Fe3O4 is 585℃.

Chemical properties:

Burning iron wire in oxygen will produce Fe3O4. Comparing the Gibbs free energy of standard molar formation of iron oxide, it is concluded that Fe3O4 has the greatest thermodynamic stability, so the product is Fe3O4.

When iron comes into contact with air, it forms oxides on its surface. At this time, the chemical composition of the oxide film itself is not uniform.

For example, a piece of low carbon steel can cover three kinds of oxide films: FeO in contact with metal, Fe2O3 in contact with air, and Fe3O4 in the middle. More precisely, perhaps the saturated solid solution mixture of three oxides constitutes the oxide film on the steel surface.