incongruent melting

Most solids you meet melt cleanly: heat pure ice and it turns straight to water of the very same composition. That is congruent melting — the liquid is a faithful copy of the solid. Some compounds refuse to be so tidy. When you heat them, instead of melting into a liquid of their own composition, they break apart into a different solid plus a liquid whose composition is nothing like the original. That messy, split melting is incongruent melting: the compound does not survive to its own melting point but decomposes first.

On a phase diagram an incongruently melting compound stands as a vertical line (its fixed composition) whose top does not reach up into a simple liquid region. Instead it ends at a special horizontal ledge, the peritectic, where the reaction on heating is: solid compound goes to a different solid plus liquid. A textbook mineral example is enstatite, MgSiO3: heated enough, it does not give a liquid of MgSiO3 composition but breaks into forsterite crystals (Mg2SiO4) plus a silica-rich liquid. Potassium feldspar behaves the same way, decomposing to leucite plus liquid — a fact that shapes how porcelains vitrify.

Why it matters to a ceramist: incongruent melting means you cannot make a pure melt of the compound just by heating it, which complicates growing single crystals or casting glasses of that composition. It also means the first liquid on firing, and the crystals that survive into it, are set by the peritectic, not by any simple melting point. Reading incongruent melting off a diagram tells you which solid phase will persist to high temperature and what the coexisting liquid will be — essential for predicting refractory behaviour and for understanding why mullite and forsterite are the survivors they are.