The System Nepheline-Diopside-Sanidine and Its Significance to the Genesis of Melilite- and Olivine-Bearing Alkaline Rocks

Abstract

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 the "melilite stability" and "critical melilite stability" planes, the former representing the limit of melilite stability at solidus temperatures, the latter the limit of melilite formation. A flow diagram, derived from theoretical and experimental considerations, illustrates the effects of fractional crystallization of combinations of olivine and melilite on liquids in the system nepheline-diopside-sanidine. Olivine nephelinite, olivine melilitite, and olivine melilite nephelinite represent initial rock types in sequences which may terminate with rocks akin to phonolites. However, the three initial rock types cannot be produced from the same magma.