The System Ussingite-Water and its Bearing on Crystallization in Persodic Portions of the System$Na_{2}O-Al_{2}O_{3}-SiO_{2}-H_{2}O$at 1 Kb Total Pressure

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 types of subsolidus assemblages originally denned by Robertson and Wyllie (1971) in rock-water systems. This new assemblage is designated Type V, water excess-intermediate vapor absent.