The System Diopside-Nepheline-Akermanite-Leucite

Abstract

The relationship of the potassium-rich mafic and ultramafic rocks with monticellite and olivine-bearing alnoites is understood by the study of the system forsterite-diopside-akermanite-leucite, in which nepheline is not included, to avoid complexity. However, the absence of nepheline is a serious loss to the system. Compositions of many other simplified potassium-rich rocks can be represented by the system diopside-nepheline-akermanite-leucite (Fig. 11.1). In the system under consideration, forsterite appears as a phase because of a reaction relationship between diopside and nepheline (Bowen 1922a; Schairer et al. 1962). Study of the system is thus important to understand the genesis of many other mafic and ultramafic volcanic rock types such as olivine-melilite-nepheline leucitite, nepheline-bearing katungite, and melilite-nepheline leucitite. Gupta et al. (1973b) studied this system and found that all phases appearing in this system are solid solutions. They established that nepheline incorporates variable amounts of kalsilite and excess silica; melilite contains soda-melilite and gehlenite; forsterite incorporates monticellite; diopside contains Ca-Tschermak’s molecule and leucite incorporates nepheline in solid solution.