Catalai I is an approximately circular complex covering 27 km2 and lying 15 km south of Catalao II. It is emplaced in mica schists and quartzites of the Precambrian Araxa Super Group, which it fenitizes. Extensive drilling has indicated that a carbonatitic core is surrounded by pyroxenites, serpentinized peridotites and glimmerites. The pyroxenite-peridotite-glimmerite series comprises various combinations of phlogopite, serpentine, olivine, carbonates, amphibole, pyroxene, feldspar, magnetite, apatite and occasionally zircon, sphene and nepheline. A little nepheline syenite has been found. The carbonatites appear to comprise an earlier series of phlogopite carbonatites containing a broad range of accessory minerals, which are cut by veins of beforsite and sovite with many accessories including monazite, perovskite, baryte and pyrochlore. There are late silexite veins throughout the intrusion. In an aureole of fenites 300-500 m wide Na- and K-feldspars, aegirine and sodic amphibole have developed in schists and quartzites.