The ranges of chemical composition in nepheline and their petrogenetic significance

Abstract

1. (1) Natural nephelines differ from artificial ones formed in dry melt experiments in their range of chemical composition as well as in their probable stability relations. 2. (2) Rocks containing nephelines may be classified into those of the V facies and of the P facies according to the kinds of alkali feldspars associated with the nepheline. Nephelines in rocks of the V facies have a much wider range of chemical composition than those in rocks of the P facies. This fact suggests that the ranges of chemical composition in natural nephelines depend on the physical conditions under which they crystallized. 3. (3) The physical conditions which affect the composition range of nephelines are probably similar in the crystallization of phenocrysts in volcanic rocks and in the crystallization of the component minerals of plutonic rocks of the V facies, but are different in the crystallization of the component minerals of plutonic rocks of the P facies. 4. (4) Plutonic rocks of the V facies occur usually in association with, or as cognate inclusions in. volcanic rocks, while plutonic rocks of the P facies never occur in association with, or as cognate inclusions in, volcanic rocks.