A considerable part of the nephelinite lavas of the volcano Mt. Nyiragongo in the eastern Belgian Congo contains kalsilite as one of the main constituents. The mineral never occurs as the only feldspathoid of the rock but is accompanied by nepheline, abundant melilite, and, sometimes, by small to moderate amounts of leucite. Other important constituents of these kalsilite-bearing rocks are clinopyroxene, olivine, perovskite, titanomagnetite, sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrysts in which these phases are strictly co-axial and in the fine-grained groundmass as grains separate from those of nephe-line. The complex nepheline-kalsilite phenocrysts exhibit a continuous series of progressing exsolution schematically presented in Fig. 5. The series begins with a perthite-like nepheline-kalsilite core surrounded by a drop-like development of nepheline in the margin of the crystal and ends up with a homogeneous kalsilite core surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing a typically glomeroporphyritic texture. Evidence is presented indicating that, in the very first stages of crystallization, some of the Nyiragongo lavas are able to precipitate small amounts of phenocrysts of approximate composition K 3 NaAl 4 Si 4 O 16 . Through crystal-rise under turbulent currents in the molten lava mass these phenocrysts have been accumulated into aggregates and thus have been preserved until extrusion. Granted sufficiently slow cooling under static conditions, the phenocrysts would have reacted with the molten lava. The roles of the crystal-rise and of the turbulent currents in lava are illustrated by the occurrence of the ‘giant’ leucite aggregates found in the inner walls of the crater and by observations on the lava lake of the mountain. The occurrence of kalsilite in the groundmass is explained by the existence of a two-phase area in the sub-solidus range in the nepheline-kalsilite system. The Nepheline ...