Rio Bonito is a zoned stock of some 28 km2 emplaced in Precambrian gneisses and granulites near to the Tangua and Saorinho intrusions (Fig. 2_206). Slightly undersaturated to saturated pulaskites, umptekites and quartz syenites surround and grade into a central core of nepheline syenites, in which sodalite and analcime occur. Amphiboles of the katophorite-Mg-katophorite series are of greater abundance than aegirine/aegirine-augite and biotite in the outer parts of the intrusion, but pyroxene predominates in the nepheline syenites. The colour index decreases inwards and upwards. Dykes of trachyte, phonolite and minette cut the intrusion and surrounding gneisses. There are rare magmatic breccias.