• journal_title：The Canadian Mineralogist
• Contributor：Roger H. Mitchell ; Bruce A. Kjarsgaard
• Publisher：Mineralogical Association of Canada
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.46.4.971
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：46
• issue：4
• firstpage：971
• section：Articles

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)Cl_ss occur in (K,Na)Cl_ss and of (K,Na)Cl_ss in (Na,K)Cl_ss. 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.