Melilite- and Leucite-Bearing Systems

The four essential minerals of many potassium-rich mafic and ultramafic rocks are leucite, augite, olivine, and melilite. The bulk compositions of these simplified rocks lie within the phase volumes of forsterite–diopside–akermanite–leucite. Phase relation within the system is thus applicable to the genesis of potassium-rich mafic and ultramafic volcanic rocks. A detailed study of the system was made by Gupta (1972), who showed that all phases in this system are solid solutions. Forsterite may contain monticellite; and diopside and akermanite possibly incorporate Ca-Tschermak’s molecule and gehlenite in solid solution, respectively. Likewise, leucite may include K2O·MgO·5SiO2 and forsterite probably contains monticellite. The system is bounded by four ternary joins.