Haplodioritic and Related Magmas. 

 Three-Phase Boundaries. 



169 



In the simple system showing a ternary eutectic the location 

 of the boundary curve and isotherms is sufficient to determine 

 the crystallization of any mixture. The crystallization curves 

 (curves indicating the change of composition of the liquid) are, 

 within each field, straight lines radiating from the composition 

 of the pure component (see fig. 7). The composition of the 

 liquid passes along one of these straight lines until it meets a 



Fig. 



Fig. 7. Crystallization curves in a eutectic system. 



boundary curve and then follows the boundary curve to the 

 ternary eutectic where crystallization is completed. On the 

 other hand, in any system in which, on account of the exist- 

 ence of solid solution, there is no ternary eutectic the crystalli- 

 zation curves in the solid solution field are not straight lines, 

 and there is no single temperature common to all mixtures at 

 which crystallization is completed. Evidently, then, further 

 determinations besides the locating of boundary curves and iso- 

 therms are necessary in the present system before the crystalli- 

 zation of individual mixtures can be described. 



Equilibrium in a eutectic system at a certain temperature T° 

 may be represented graphically in fig. 8. DEF is the isotherm 

 for T°. The phases present within the various areas are as 

 follows : 



