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Earth and Space Sciences · Glossary

What is Incongruent melting, peritectic point?

Also known as: incongruent melting · peritectic point

Definition 7.3 University Earth and Space Sciences — Year 2 · Chapter 7 — Melting the Mantle

A mineral shows incongruent melting when, on heating, it breaks down into another solid and a liquid of a different composition, instead of melting into a liquid of its own composition. On a binary phase diagram, the point where that liquid coexists with both solids is a peritectic point: unlike a eutectic, it does not lie between the compositions of the two solids.

Part of the system MgO–SiO2 at atmospheric pressure, from Bowen and Andersen (1914). Enstatite (MgSiO3) melts incongruently at the peritectic P, 1557\, C, into forsterite and a liquid of 60.9\,\% SiO2; E is the enstatite–cristobalite eutectic, 1543\, C. The forsterite liquidus is measured only near enstatite: the dashed part is drawn straight to the melting point of forsterite.
Part of the system MgO\ce{MgO}–SiOX2\ce{SiO2} at atmospheric pressure, from Bowen and Andersen (1914). Enstatite (MgSiOX3\ce{MgSiO3}) melts incongruently at the peritectic P, 1557 ∘C1557\,{}^{\circ}\mathrm{C}, into forsterite and a liquid of 60.9 %60.9\,\% SiOX2\ce{SiO2}; E is the enstatite–cristobalite eutectic, 1543 ∘C1543\,{}^{\circ}\mathrm{C}. The forsterite liquidus is measured only near enstatite: the dashed part is drawn straight to the melting point of forsterite.
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