The nephelinekalsilite system; II, Phase relations

Phase equilibrium studies in the system NaAlSiO ~4~ -KAlSiO ~4~ at subsolidus temperatures have demonstrated that a solvus, or miscibility gap, is present and in a general way it is similar to the solvus between albite and orthoclase. At low temperatures the miscibility gap extends in composition from Na ~3~ KAl ~4~ Si ~4~ O ~16~ to KAlSiO ~4~ . The stable modifications of KAlSiO ~4~ were found to be kalsilite and a high temperature orthorhombic modification while kaliophilite is believed to be a metastable phase at all temperatures at atmospheric pressure. NaAlSiO ~4~ has a high temperature form stable between 850 degrees C and 1250 degrees C. . . . Evidence is presented to show that Na ~3~ KAl ~4~ Si ~4~ O ~16~ is a compound (nepheline) and not merely a single composition in a solid solution series.