Nephelines as Crystallization Temperature Indicators

In the system NaAlSiO₄-KAlSiO₄-SiO₂-H₂O the limits of solid solution of nepheline have been determined at 775°, 703°, and 500° C. The crystallization temperatures of a number of natural nephelines have been deduced using the experimental results. The possible chemical changes in the composition of nepheline with falling temperature are discussed. A nepheline in equilibrium with a silicate liquid will change composition in two ways: (a) change in the K/Na ratio and (b) change in the Si/Al ratio, Si being replaced by Al. It is suggested that, in postmagmatic cooling, only the Na and K atoms move, and these are exchanged with the alkali atoms in the accompanying feldspar in such a way that the Na:K ratio of the nepheline approaches 3: 1.