Kalsilite in the lavas of Mt. Nyiragongo (Belgian Congo)

A considerable part of the nephelinite lavas of the volcanoMt. Nyiragongo in the eastern Belgian Congo contains kalsiliteas one of the main constituents. The mineral never occurs asthe only feldspathoid of the rock but is accompanied by nepheline,abundant melilite, and, sometimes, by small to moderate amountsof leucite. Other important constituents of these kalsilite-bearingrocks are clinopyroxene, olivine, perovskite, titanomagnetite,sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrystsin which these phases are strictly co-axial and in the fine-grainedgroundmass as grains separate from those of nepheline. The complex nepheline-kalsilite phenocrysts exhibit a continuousseries of progressing exsolution schematically presented inFig. 5. The series begins with a perthite-like nepheline-kalsilitecore surrounded by a drop-like development of nepheline in themargin of the crystal and ends up with a homogeneous kalsilitecore surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing atypically glomeroporphyritic texture. Evidence is presentedindicating that, in the very first stages of crystallization,some of the Nyiragongo lavas are able to precipitate small amountsof phenocrysts of approximate composition K₃NaAl₄Si₄O₁₆. Throughcrystal-rise under turbulent currents in the molten lava massthese phenocrysts have been accumulated into aggregates andthus have been preserved until extrusion. Granted sufficientlyslow cooling under static conditions, the phenocrysts wouldhave reacted with the molten lava. The roles of the crystal-riseand of the turbulent currents in lava are illustrated by theoccurrence of the ‘giant’ leucite aggregates foundin the inner walls of the crater and by observations on thelava lake of the mountain. The occurrence of kalsilite in the groundmass is explained bythe existence of a two-phase area in the sub-solidus range inthe nepheline-kalsilite system. The Nepheline Aggregate lavas represent the last extrusionsemitted by the Nyiragongo main crater. The nepheline phenocrystscharacteristic of these lavas range considerably higher in potassiumcontent than the nephelines found in other Nyiragongo flows.The crystals are slightly zoned with a large potassium-richcore coated by a narrow margin with gradually decreasing potassiumcontent. The zoning may be detected only by using special methods.The history of crystallization of the nepheline phenocrystsis considered analogous to that of the complex nepheline-kalsilitephenocrysts with the only difference that the nepheline phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution.