Extension to Ternary Systems

In petrology, the usual representation of phase equilibria in the crystalhzation process of ternary systems is based on a projection from the T axis onto the 

2 Formation of an Intermediate Compound with Fixed Stoichiometry 

Let us examine the crystallization process of an initial composition Cl the first crystals to form are those of the pure component 2 (y , T = 1175 °C). Crystallization of y drives the residual liquid radially away from corner 2 along direction 2-Cl, until peritectic curve PP is reached. Here, crystals y react with the liquid and are transformed into intermediate compound C . Displacement along the peritectic curve terminates when all phase y is completely resorbed. This occurs geometrically when trajectory C-Cl encounters peritectic curve PP. Crystallization of Cj then proceeds, while the residual liquid moves radially away from C . The liquid path then reaches cotectic curve E -Em and crystals y begin to precipitate in conjunction with Cj. Crystallization continues along the cotectic line and, at ternary eutectic Em, crystals y precipitate together with C and y until the residual liquid is exhausted. 

Let us now consider the crystallization process operating on a composition C2, initially poorer in component 1 this composition falls into the compositional triangle C -2-3. Crystallization begins in primary phase field y (T = 1225 °C). Precipitation of y crystals drives the residual liquid radially away from C2 along direction 2-C2. At T = 1100 °C, the composition of the liquid reaches cotectic line Ejii-P, where crystals y begin to form together with y . At peritectic point P, crystals y are partially resorbed by reaction with the liquid to form intermediate compound C . However, the peritectic reaction is not completed, because of ex- 

The compositional paths of liquids for melting processes in closed systems of initial composition Cl or C2 are exactly the opposite of those observed in crystallization events. 

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