Phase Relations in the System NaAlSiO₄-SiO₂-Nacl-H₂O at 400°-800°C and 1 Kilobar, and Petrologic Implications

Mixtures of nepheline, quartz, and halite react in the presence of water at 1 kbar total pressure. Depending on bulk composition, subsolidus assemblages-all coexisting with vapor and/or aqueous liquid-are: quartz-albite-halite, albite-sodalite-halite, nepheline-albite-sodalite, nepheline-sodalite. Below 550°C, analcime appears, permitting the assemblages albite-analcime-sodalite, analcime-sodalite, and nepheline-analcime-sodalite. Below 625°C, a supercritical saline fluid coexists with solid phases; when the mole fraction of NaCl in this fluid is high, analcime cannot form. Above 625°C, an aqueous vapor coexists with a dense NaCl-H₂O liquid; the vapor contains little NaCl and PH2O approximates Pₜₒₜₐₗ. Natrolite and the NaCl end-member of the scapolite series were not encountered. Eutectic melting between quartz and albite occurs at Ab₆₃Qz₃₇ at 765∘ ± 10∘C, and appears to be independent of the presence or absence of NaCl. Above 700°C in the silica-oversaturated portion of the system, sodium chloride reacts with quartz and water to produce HCl and presumably a sodium silicate, although the latter was not found. The presence of NaCl in the nepheline-normative part of the system stabilizes sodalite; in the quartz-normative part, it has no perceptible effect upon silicate phase relations above the solidus, because NaCl-rich liquid is immiscible with silicate melt. The experimental results do not support speculations that alkalic rocks can form by anatexis or assimilation of sedimentary sequences containing halite.