Nepheline–Silica and the Rest of the Basalt Tetrahedron

In this chapter, we return to the basalt tetrahedron as a model for the crystallization of basalt magma and conclude the discussion of phase diagrams related to this system at 1 bar pressure. A brief discussion of the system nepheline–silica (NaAlSiO4-SiO2) serves to introduce the important principle of the thermal barrier (or thermal divide), which tends to prevent silica-undersaturated liquids from producing critically undersaturated residua and vice versa. A review of the rest of the tetrahedron serves as a summary of earlier chapters and as a conclusion regarding the lessons of the fundamental basalt tetrahedron. This review will serve as the background for examination, in succeeding chapters, of the effects of other components and of pressure on the phase relations of basaltic systems.