The system [Na2Ca(CO3)2 ]100−x − (50 wt.% NaCl + 50 wt.% KCl)x (10 < x < 80 wt.%), a join in the quaternary system Na2CO3 – CaCO3 – NaCl – KCl, has been investigated at temperatures ranging from 450° to 900°C at 0.1 GPa. Liquidus temperatures are on the order of 825°C, with a depression on the liquidus to 750°C at about 23 wt.% halide corresponding to a piercing point. Liquids that are poor in halides relative to the composition of the piercing point have a nyerereite-like (Na,Ca,K) carbonate as the primary liquidus phase; it coexists with a one-phase halide-rich liquid. Liquids with ~ 30–70 wt.% halide initially precipitate calcite as the primary liquidus phase. Below 700°C, calcite reacts with liquid to form nyerereite-like (Na,Ca,K) carbonate. From 600° to 550°C, this carbonate coexists with two immiscible halide liquids. Below the solidus at ~550°C, discrete exsolved inclusions of (Na,K)Clss occur in (K,Na)Clss and of (K,Na)Clss in (Na,K)Clss. These data are relevant to the differentiation and crystallization of the natrocarbonatitic melt at Oldoinyo Lengai under hypabyssal and plutonic conditions; they predict the formation of cumulate rocks composed of nyerereite, gregoryite, halite and sylvite. Disaggregation of such assemblages could lead to formation of sylvite–fluorite macrocrystal natrocarbonatite.