Diopside-Anorthite-Albite

The system Di-An-Ab allows us to study a basaltic model consisting of diopside and an intermediate plagioclase, a model which is much closer to the real thing than we have studied before. The fact that the two joins Di-An and Di-Ab are not binary means of course that Di-An-Ab is not a three-component, or ternary, system. The reasoning of the last chapter suggests that it must be described as a quinary (c = 5) system, CMASN. The non-ternary nature of the system means that liquids at low pressure will always lie on the SiO2-rich side of this plane, driven there by crystals somewhat richer in Al2O3 than the plane. When the plagioclase composition is near An, it is expected that the DiSS will have a composition approximately towards CaTs from Di, and as the plagioclase approaches Ab, the DiSS will become somewhat more magnesian. These effects mean that, as a part of the generalized critical plane of the basalt tetrahedron, Di-An-Ab has the same properties as Di-Ab, so we should expect that liquids might become oversaturated with silica by fractional crystallization in this system, but not undersaturated.