In the system with the composition NaAlSiO4, such phases as low- and high-nepheline with the structure related to that of tridymite and low-and high-carnegieite with the structure related to that of cristobalite are present, and they have high-low inversion points at 850° C. and 690° C. respectively. Low-carnegieite is formed as metastable phase. The stability relations among these phases are shown in Fig. 1 (Tuttle and Smith, 1958). When the excess amounts of Na2O are present, alkali-rich carnegieite NaAlSiO4 x Na2O is formed.