Phase relationships in the system NaAlSiO_4-SiO_2-H_2O to 35 kilobars pressure

Abstract

Phase relationships in the system NaAlSiO_4-SiO_2-H_2O are first presented in P-T projections of the composition joins NaAlSi_3O_8 (albite)-SiO_2-H_2O, NaAlSi_3O_8-H_2O, NaAl_2Si_2O_6 (jadeite)-H_2O, and NaAlSi_2O_6-NaAlSiO_4 (nepheline)-H_2O. Crystalline phases encountered include albite, analcite, coesite, jadeite, nepheline, and quartz. Vapor-saturated melting curves for these compositions, including those for the melting of albite + quartz + vapor, albite + vapor, and albite + analcite + vapor, proceed with negative dP/dT slopes to invariant points at pressures between 10 and 13 kb where jadeite coexists with liquid. At higher pressures, the solidus curves for compositions on these joins extend with positive dP/dT slopes, and the quartz-bearing assemblages terminate at an invariant point marking the appearance of coesite. Albite-H_2O compositions melt incongruently at pressures greater than a singular point at 16 kb, and jadeite-H_2O compositions melt congruently above a singular point at 13 kb and form a stable thermal divide within this system. Together with the chemographic methods of Schreinemakers and experimental data for the system NaAlSiO_4-SiO_2, univariant and invariant equilibria for the ternary system are presented in P-T projection. The approximate solubility of H_2O in liquids of albite, nepheline, and jadeite composition at 15 kb is 27 (wt) percent, 36 percent and 22 percent, respectively. Nepheline-H_2O compositions are not binary at these pressures and break down to a jadeite-bearing assemblage at pressures above 10 kb and to jadeite + an unknown phase at higher pressures. Isobaric-polythermal diagrams, constructed on the basis of the melting relationships and solubility data, illustrate the primary fields for the crystalline phases and disclose that critical phenomena will intervene only for compositions rich in SiO_2.

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