Incongruent melting

Incongruent melting occurs when a solid substance being partially melted does not melt uniformly, so that the chemical composition of neither the resulting liquid nor the resulting solid is the same as that of the original solid. For example, melting of orthoclase (KAlSi₃O₈) produces leucite (KAlSi₂O₆) in addition to a melt. The melt produced is richer in silica (SiO₂). The proportions of leucite and melt formed can be recombined to yield the bulk composition of the starting feldspar. Another mineral that can melt incongruently is enstatite (Mg₂Si₂O₆), which produces forsterite (Mg₂SiO₄) in addition to a melt richer in SiO₂ when melting at low pressure. Enstatite melts congruently at higher pressures between 2.5 and 5.5 kilobars.

See also

• Congruent melting
• Incongruent transition
• Phase diagram