Iron trioxide, chemical formula Fe3O4. Also known as iron oxide black, magnet, magnetite, or black iron, it is a black crystal with magnetism, hence it is also called magnetic iron oxide. It cannot be pronounced as “ferrous metaferrite” or “ferrous metaferrite” Fe (FeO2) 2, and is not a mixture of iron oxide and ferrous oxide, but can be approximated as a compound of ferrous oxide and ferrous oxide. This substance is soluble in acids, insoluble in water, alkali, and organic solvents such as ethanol and ether. Natural ferric oxide is insoluble in acids and can easily oxidize into ferric oxide in the air in humid conditions. Usually used as a pigment and polishing agent, it can also be used to manufacture audio tapes and telecommunications equipment.