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.     

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 nephe-line. 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 nephe-line phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution.     

Evolution of the Nyiragongo Magma

This paper deals with the rocks, mainly foiditic in composition,of the Nyiragongo volcano and compares them with the mainlytephritic to phonolitic rocks of the neighbouring Nyamuragiravolcano and with the South Kivu basaltic rocks. All these rocksoccur in the Lake Kivu area of the Western Rift Valley of CentralAfrica. Two hundred and sixty-four chemical analyses consideredrepresentative were selected of these rock groups and were subjectedto statistical evaluation. The Nyiragongo magma is considered to be of mantle origin withoutsignificant crustal admixture. The Nyamuragira magma and theNyiragongo magma are both potassic in character and, despitebeing geographically closely related, display pronounced differencesin chemistry. The fairly extreme composition of the Nyiragongomagma is interpreted as a result of a locally strong transferand enrichment of mainly alkalis with a passive depletion ofsilica.     

Order-disorder in Natural Nepheline Solid Solutions

The variation of the birefringence with temperature, from roomtemperature up to c. 900? C, was studied of 34 nephelines fromdifferent geological environments. The measurements were madewith a Berek compensator using a microscope heating stage. Two optical types of nepheline were distinguished on the basisof the absolute value for birefringence at room temperatureand on the basis of the optical -value defined as >=(w-E)₉₀₀?-(w-E)₂₀?.The optical type I is characterized by a high birefringenceat room temperature and by a highly negative optical -value,whereas the optical type II shows a low birefringence at roomtemperature and a highly positive optical -value. The nephelinespecimens studied represent optically a continuous gradationfrom type I to type IL Both optical types are represented amongmediopotassic and perpotassic nephelines. A nepheline of the optical type I is considered to representthe ordered form of the mineral and a nepheline of the opticaltype II the disordered form. All plutonic nephelines studiedbelong to the optical type I whereas the volcanic nephelinesrange from the type I to the type II. Using a high temperature attachment for the Norelco diffractometer,the thermal expansion was measured of a few nepheline specimens.A nepheline belonging to the optical type I differs slightlyin thermal expansion from that of the optical type 11. Thisdifference in thermal expansion may qualitatively explain thedifference between the two optical types. The crystal structural nature of the supposed order-disordertransition in nepheline is not known. The analogy with the feldsparswould suggest a varying degree of ordering in the distributionof the silicon and aluminium cations in the oxygen tetrahedra.