liquefaction of a solid accompanied by decomposition or by reaction with the melt to produce another solid and a liquid that differs in composition from the original solid. For example, enstatite, a magnesium silicate (MgSiO₃), melts incongruently at low pressures to form forsterite, which is another magnesium silicate (Mg₂SiO₄), and a liquid enriched in silica (SiO₂). At pressures between 2.5 and 5.5 kilobars, the melting of enstatite is congruent.