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.