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Ussingite is unstable at 1 kb total pressure in the system $$ussingite-H_{2}O$$. Instead, the natural ussingite has an albite liquidus slightly in excess of 775°C. This liquid can dissolve 5.6 wt% $$H_{2}O$$ before becoming saturated. At subliquidus temperatures, two isobaric invariant assemblages occur: (1) nepheline (ne) + albite (ab) + analcite (anl) + vapor (v) + liquid (1) at 525 ± 5°C and (2) ne + ab + anl + sodium disilicate ($$ns_{2} + 1 at 462 \pm 12°C$$. These are most likely the expressions of the univariant reactions $$anl + v \rightarrow ab + ne + 1, and anl + ns_{2} \rightarrow ab + ne + 1$$. Assemblages involving analcite and a persodic liquid in the system $$Na_{2}O-Al_{2}O_{3}-SiO_{2}-H_{2}O$$ are found to occur at lower temperatures for a constant pressure, and at lower pressures for a constant temperature. The high potential water contents of persodic-rich liquids produces a prolonged negative slope to the vapor-saturation boundary at subliquidus temperatures. This will increase by one ...

The pseudoternary system nepheline-diopside-sanidine represents a plane within the six-component system CaO-MgO-Al₂O₃Na₂O-K₂O-SiO₂ as well as a plane through the tetrahedron nepheline-kalsilite-silica-diopside. Liquidus phase relations show primary phase fields of olivine, nepheliness, leucitess, and diopsidess and the presence of piercing points at $$Ne_{51}Di_{21}San_{28}$$ (1,1158° ± 10° C) and $$Ne_{44,5}Di_{12}San_{43,5}$$ (1,120° ± 5° C). Subliquidus phase relations for silica-poor portions of the system exhibit the crystallization of melilite and the disappearance of both melilite and olivine by reaction. The appearance of melilite is attributed to the desilication of pyroxene while its disappearance is probably caused by the desilication of alkali feldspar. The crystallization of melilites in alkali-rich silica-undersaturated rocks is believed to be controlled by the amounts of alkali feldspar in their parental liquids. In the complex CaO-MgO-Al₂O₃-Na₂O-K₂O-SiO₂ system, two planes are designated t...