Incongruent melting
Incongruent melting occurs when a solid substance does not melt uniformly. During melting a new solid (of different composition) forms. For example, melting of orthoclase (KAlSi3O8) produces leucite (KAlSi2O6) in addition to a melt. The melt produced is rich in silica (SiO2) so that the proportions of leucite and melt created can be recombined to yield the bulk composition of the starting feldspar. Another mineral that melts incongruently is enstatite (Mg2Si2O6), which produces forsterite (Mg2SiO4) in addition to a melt (if melting at low pressure). Enstatite melts congruently between pressures of 2.5 and 5.5 kilobars.
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