The thermal-equilibrium relationships in the system NaAlSiO4 - CaO · AI2O3 have been investigated. Complex relations exist at temperatures above the carnegieite-nepheline inversion range, the system at these temperatures being quaternary in nature. At lower temperatures, in the stability field of nepheline, simplification to a binary system is observed over a considerable portion of the system due to the ability of nepheline to take up somewhat over 60 per cent CaO · AI2O3 in solid solution. The complexity at higher temperatures is due to the inability of carnegieite to include more than a small amount of CaO · AI2O3in its structure, and to the intrusion of fields of β-AI2O3, corundum, and CaO · 2 AI2O3.
Soda volatilization is significant in the system, and apparently the rate is increased as lime is added. As soda is lost,β-AI2O3 or corundum tends to crystallize. The possibility of this effect being related to the corundum present in some alkaline rocks, particularly those associated with limestone, and to soda enrichment (albitization, etc.) of the country rock is discussed.