The limit of solid solution of SiO2 in nephelines has been determined at 700° C and a water pressure of 15, 000 lb/sq. in. X-ray data are given for the determination of the composition of nephelines in equilibrium with alkali feldspar at 700° C. The break in the plot of lattice parameters against chemical composition for nephelines in the system NaAISiO4–KAISiO4 (Tuttle & Smith, 1958) is found to persist in the ternary system and occurs when six out of the eight alkali atom sites are filled by Na.
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Hamilton et al. (1960) studied this question.