The System Forsterite-Diopside-Akermanite-Leucite

The four essential minerals of many potassium-rich mafic and ultramafic rocks are leucite, augite, olivine, and melilite. The bulk composition of these rocks thus lies within the phase volume of forsterite-diopside-akermanite-leucite (Fig. 2.1). A study of the system is thus applicable to the understanding of the phase relations and origin 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; diopside, and akermanite possibly incorporate Ca-Tschermak’s molecule and gehlenite respectively; leucite may include K2O · MgO · 5 SiO2 and monticellite probably contains forsterite. The four joins which bound the system are described below.