Incongruent melting

Incongruent melting occurs when a solid substance does not melt uniformly, decomposing into another solid and a liquid with different compositions. For example, potassium feldspar (KAlSi₃O₈) decomposes to leucite (KAlSi₂O₆) when it melts. The decomposition is not complete, however. Most of the feldspar does melt, a portion of it decomposes to leucite and some quartz (SiO₂) is left over, since the chemical formulas of potassium feldspar and leucite differ by SiO₂. Another mineral that melts incongruently is enstatite (MgSiO₃), which decomposes to forsterite (Mg₂SiO₄). Enstatite does melt congruently between pressures of 2.5 and 5.5 kilobars.