Selected parts of the system nepheline-kalsilite-silica-water were studied at elevated temperatures and, for the most part, at a constant PH2o of 1000 bars. Concomitantly, experimental work was carried out with natural specimens of leucite and pseudoleucite.
The liquidus of the isobaric section through the quaternary system (PH2o = 1000 bars) is substantially lower (200°–400° C) than in the ternary system nepheline-kalsilite-silica, and the primary field of leucite is considerably restricted. Leucite solid solution is extensive along the join KAlSi2O6–NaAlSi2O6, reaching 28 weight per cent NaAlSi2O6 at 1000 bars. The solid solution increases as the water-vapor pressure decreases and reaches a maximum of ~40 weight per cent in the dry system.
Below the solidus, sodium-rich leucite becomes unstable and breaks down to nepheline and feldspar as the temperature falls. This subsolidus reaction continues until either the leucite is completely converted to nepheline and feldspar or the remainder is so poor in sodium that it becomes stable. The subsolidus breakdown of leucite solid solution is responsible for the formation of pseudoleucite. Whether or not pseudoleucite may also result from the breakdown of a potassium-rich analcite was not definitely determined.
The importance of the incongruent melting of K feldspar and reactions at the peritectic has been overestimated in explaining the phase assemblage nepheline-feldspar. Most plutonic alkalic rocks that crystallized leucite during their cooling history, and that now consist of nepheline and feldspar, owe their final assemblage to the subsolidus breakdown of leucite solid solution.