Liquid immiscibility in the join NaAlSi 3 O 8 -Na 2 CO 3 -H 2 O and its bearing on the genesis of carbonatites

Phase relations in the above join were studied through the system Na ~2~ O-Al ~2~ O ~3~ -SiO ~2~ -CO ~2~ -H ~2~ O at 1 kb. Phases encountered were albite, cancrinite, sodium carbonate, a silica-rich liquid, an Na ~2~ CO ~3~ -rich liquid, and vapor. Albite is the main silicate phase at low water contents while cancrinite is the main silicate phase at high water contents. Three fluid phases coexist: (1) an undersaturated alkaline silicate liquid, (2) an alkaline carbonate liquid with a small amount of dissolved silicate, and (3) a vapor phase with a composition varying between H ~2~ O and CO ~2~ and containing Na ~2~ O and SiO ~2~ in solution. These fluid phases are compared with nepheline or ijolite magmas, carbonatite melts, and fenitizing solutions, respectively.