Iron(III) oxide, also known as rust, is a reddish substance, an iron oxide. It has a chemical formula of FeO. It is made by the reaction of iron with oxygen. It is found naturally in the ground. It is a major ore of iron (iron comes from this mineral)haematite. It dissolves in acids. It can be mixed with fine powdered granules of aluminum in a 27:80 ratio to form thermite, a semi-explosive powder.