Pseudobinary Systems

Abstract

The pseudobinary systems differ from the true binary ones by the presence of one or more phases with compositions lying outside those of the binary join, i.e., the compositions of all the phases cannot be expressed as a linear combination of the two end member components. In the system albite-orthoclase, sanidine melts incongruently to leucite plus liquid, but the leucite composition cannot be expressed from those of the two feldspars. Similarly spinel occurs in the binary join anorthite-forsterite, but has a composition outside the join. The pseudobinary systems occur frequently in silicate systems and interpretation of these systems is required for an evaluation of important phase relationships. The pseudobinary systems also bear a relationship to P-T diagrams of rock compositions, since these P-T diagrams may be regarded as distorted pseudobinary systems.