Triangular and pyramidal regions in meltability and solubility diagrams ofmulticomponent systems

A rare type of nonvariant points on phase equilibrium diagrams occurring on ly in systems with a large number of components n greater than or equal to 3 (for meltability diagrams) and n greater than or equal to 4 (for solubili ty diagrams) is considered. It is shown that a melt (solution) does not par ticipate in phase processes occurring at such nonvariant points when heat i s removed (solvent evaporated). The stability of monovariant equilibria in the vicinity of such points is studied. The form of fields (volumes, hyperv olumes) of crystallization of one of the phases is then peculiar: this is a curvilinear triangle (curvilinear pyramid) resting on a side (face) of the concentration simplex. The figurative points of the compositions of the so lid phases coexisting at nonvariant points lie on this side (face) of the c oncentration simplex.