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

Abstract

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.