Longonjo is a carbonatite intrusion that forms a hill rising some 280 m above the surrounding peneplained Precambrian granite gneiss. The principal area of carbonatite is 3 km across but the outer margins are everywhere obscured by an apron of superficial deposits. The carbonatite could have a diameter of 6 km. No contemporaneous igneous silicate rocks have been identified within the intrusion. The central rocks were mapped by Lapido-Loureiro (1973) as 'feldspathic carbonatites and carbonatites' and these grade from carbonate-free feldspathic breccias to carbonatites. The feldspathic rocks are concentrated nearer the margins of the outcrop and are clearly fenites, but whether the feldspar is potassic or sodic is not described, although the former seems the more probable. The carbonatites are described by Lapido-Loureiro (1973) as principally calcitic with ankeritic veins, but three carbonatites analysed by Alberti et al. (2000) are dolomite carbonatites. Iron oxides, apatite, barite and quartz are usual in the carbonatites and strontianite and parisite/synchysite have also been identified, while rare earth-, Nb- and Th-bearing minerals have been reported from the central area. The carbonatite analyses reported by Alberti et al. (2000) include a full suite of trace elements and C, O, Sr and Nd isotope data.