AbstractThe composition of the feldspar phenocrysts, and the enclosing glassy or finely crystalline ground‐masses, of three phonolites suggests that the addition of calcium or the addition of excess sodium (molecular ratio Na2O + Ka2O>Al2O3) to compositions in the feldspar field of the system NaAlSiO4‐KAlSiO4‐SiO2will change the composition of the minimum on the nepheline‐feldspar boundary when this is projected into this