Itauna covers 6 km2 and comprises a number of incomplete but concentric rings and a central plug. It is emplaced into Precambrian gneisses. Pseudoleucite syenites and microsyenites are intruded by a central core of pseudoleucite phonolite. There is also one extensive outcrop and several smaller ones of breccias containing xenoliths of the other rock types. The pseudoleucite phonolite contains, as well as pseudoleucites reaching several centimetres in diameter, rare crystals of nosean and hauyne. Analyses of pseudoleucites indicate them to have unusually potassic compositions (Valenca and Edgar, 1979). Dykes of pseudoleucite tinguaite, phonolite and trachyte occur within the intrusions.