a dark-red, crystalline, water-insoluble solid, Fe 2 O 3 , occurring naturally, as hematite and rust, or synthesized: used chiefly as a pigment, as a mordant, as a coating for magnetic recording tape, and in the manufacture of polishing compounds.
In order to live and grow they oxidise dissolved iron in the water, producing ferric oxide.
British Dictionary definitions for ferric oxide
a red crystalline insoluble oxide of iron that occurs as haematite and rust and is made by heating ferrous sulphate: used as a pigment and metal polish (jeweller's rouge), and as a sensitive coating on magnetic tape. Formula: Fe2O3Systematic name iron (III) oxide