Liquid immiscibility in the join NaAlSi_3O_8-CaAl_2Si_2O_8-Na_2CO_3-H_2O

Abstract

The phase relationships in the six component system NaAlSi_3O_8- CaAl_2Si_2O_8-Na_2CO_3-H_2O were determined between 650° and 950°C and 1 kb pressure along the joins Ab_(80)An_(20)-Na_2CO_3 and Ab_(50)An_(50)-Na_2CO_3, both with 10 percent H_2O present. The phase relationships are complex. Crystalline phases encountered were plagioclase, nepheline, noselite, cancrinite, wollastonite, sodium carbonate, and sodium calcium carbonate. The solidus was intersected at minimally 695°C. The most important feature is the presence of a two-liquid field above 750°C, separating a carbonate-poor silicate liquid and a silicate-poor carbonate liquid. Electron microprobe analyses of the quenched silicate liquid and atomic absorption data for the quenched carbonate liquid show that the silicate liquid is peralkaline and strongly undersaturated in silica and that the coexisting carbonate liquid is strongly enriched in calcium. A third fluid phase present with these two liquids is an aqueous vapor phase enriched with sodium silicate and CO_2. It is concluded that in this system three fluid phases coexist that correlate well with the natural occurring undersaturated alkalic urtite-ijolite-melteigite rock series, the carbonatites, and the associated metasomatic fenites of the carbonatite-alkalic rock complexes.

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