Phase relations in the system NaAlSi3O8 – NaAlSiO4 – H2O up to ~15 Kb have been discussed from a theoretical standpoint. The discussion is based on the equilibrium compositions of invariant phases. Two new invariant points, I5 and I6, have been predicted: I5 at ~13 Kb/~500 °C where five phases, jadeite, nepheline hydrate I, analcite, nepheline, and water vapor coexist, and 16 at ~0.5 Kb/~375 °C where albite, nepheline, analcite, nepheline hydrate I, and water vapor coexist. The phase relations around four previously known invariant points including a singular point S4 have been examined and partly modified.The maximum PT stability field of analcite has been deduced. The stability field of subsolidus analcite is extremely large, whereas that of liquidus analcite is very much limited. The maximum stability field of liquidus analcite is a small triangular area defined by three invariant points, I1 (5.15 Kb/657 °C), I2, (11 Kb/650 °C), and I4 (12.5 Kb/575 °C). It is concluded that most liquidus analcite (primary analcite) is attributed to the crystallization from H2O-undersaturated melt.

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