Generalities

Melting (or fusion) is an extremely important phenomenon for solids since it causes them to cease being solids and to transform to the liquid state of matter, thereby losing crystalline long-range order and resistance to shear. Melting is a first-order phase transition, i.e. it exhibits discontinuities in the first derivatives of the free energy: volume and entropy.

Pure metals (and more generally elements) have a fixed melting point Tm at a given pressure. The equilibrium between solid and liquid is univariant and the variation of the melting point with pressure is represented by the melting curve: Tm (P).

The situation is more complicated for multicomponent systems such as alloys and mineral crystals. Several cases may arise and are best understood by referring to phase diagrams (see also Section 7.4.1). A phase diagram at constant pressure is a map of the stability domains of the various possible phases in the composition–temperature space. For the sake of simplicity, let us consider only binary systems, whose composition can be defined by the proportions (in atom or mole percents) of two simple end-members. The end-members that will concern us here can be elements (e.g. Fe, S) or simple binary oxides (e.g. MgO, SiO2) that exhibit congruent melting like elements (i.e. the solid melts at fixed temperature, giving a liquid with the same chemical composition). For various definite proportions of the end-members, compounds may exist, e.g. FeS, FeS2 or MgO,SiO2 (MgSiO3) or 2MgO, SiO2 (Mg2SiO4).