Phase Equilibria in the System NaAlSi3O8–NaAlSiO4–H2O with Special Emphasis on the Stability of Analcite

Phase equilibria were determined in the P–T range of 0.5–10 Kb and 150–900 °C in the system NaAlSi3O8 – NaAlSiO4 – H2O. Two isobaric (2 Kb and 5.15 Kb) T–X phase diagrams (projected to a dry base) were completely determined and show that the stability field of analcite solid solutions has a large distorted pentagonal shape. The phase relations for the transition: nepheline hydrate I [Formula: see text] nepheline + H2O on the composition join NaAlSiO4 – H2O are not binary. It was found that there exists a narrow zone for the transition. The true P–T curve was found and determined in terms of a ternary univariant reaction: nepheline hydrate I + analcite [Formula: see text] nepheline + H2O. In the system NaAlSi3O8 – SiO2 – H2O, albite contains about 5 wt % silica in solid solution at 5.15 Kb and 670 °C.The equilibrium compositions of various univariant phases were determined essentially on the basis of the T–X phase diagrams. Another univariant reaction (zeolite species P = analcite + nepheline – hydrate I + H2O) was found at 2 Kb/215 °C and 5.15 Kb/235 °C and determined on a P–T projection. Three singular points were determined; two of them are located at 0.8 Kb/390 °C and 9.4 Kb/475 °C respectively on a univariant P–T curve for the reaction nepheline hydrate I + analcite = nepheline + H2O; the other one is located at 6 Kb/655 °C on a univariant P–T curve along which nepheline, analcite, liquid, and vapor coexist. The petrogenetic implication of analcite is discussed fully.