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.
- Incongruent melting from Eric Weisstein's World of Physics