a peritectic
/ peh-rih-TEK-tik /
A eutectic is where one liquid freezes into two solids. A peritectic is its sibling with the roles rearranged: it is the invariant point where, on cooling, an already-formed solid reacts with the surrounding liquid to produce a second, different solid. Written as a reaction it is liquid + solid-1 goes to solid-2 on cooling (and the reverse, solid-2 goes to liquid + solid-1, on heating). It is the phase-diagram fingerprint of incongruent melting — wherever a compound melts incongruently, a peritectic ledge marks the spot.
On a binary diagram at fixed pressure a peritectic is a horizontal line at one temperature with three phases meeting there (two solids and a liquid), so the phase rule again gives F = C - P + 1 = 2 - 3 + 1 = 0 — an invariant point, fixed in both temperature and composition, just like a eutectic. The difference is the geometry and the reaction sense: at a eutectic the liquid composition sits between the two solids, and cooling makes both solids from liquid; at a peritectic the liquid sits to one side, and cooling consumes liquid-plus-one-solid to build the other solid, often as a rim growing around the first crystals.
That rim causes real trouble in practice. Because the new solid grows as a shell around the old, it can seal the original crystals off from the liquid, so the peritectic reaction stalls before it finishes and the diagram's equilibrium is never reached — a classic reason metastable, unreacted cores survive in a fired or cast ceramic. Recognizing a peritectic on a diagram tells a ceramist that the melting behaviour there is incongruent, that reaction rims are likely, and that the equilibrium phase assemblage may be sluggish to appear.
In the alumina-silica system, mullite is often drawn melting at a peritectic near 1828 degrees C: on heating, mullite reacts to give corundum (solid Al2O3) plus a silica-rich liquid. On cooling the reverse should regrow mullite as a rim on the corundum — but that rim can trap corundum cores, so equilibrium mullite is slow to form.
Peritectic: liquid + solid-1 to solid-2 on cooling — an invariant (F = 0) ledge that marks incongruent melting.
A peritectic reaction often fails to finish: the new solid forms a rim that isolates the old crystals from the liquid, so real castings and firings keep unreacted cores the equilibrium diagram says should be gone.