66 Rankin and Wright — Ternary System CaO-Alfi^SiO^. 



the other partially or wholly disappears. A tangent drawn at 

 any point on a boundary curve of this class will intersect a 

 prolongation of the line joining the compositions of each of 

 the two phases. This class is illustrated by the boundaries 

 9- 7, B'-±, Z-16, 18-Z, and D-17. 



Knowing, then, the original composition of a ternary 

 solution and the class of boundary curve to which crystallization 

 proceeds on cooling the solution, one can ascertain from a 

 study of the equilibrium diagram precisely which phases 

 separate, their order of crystallization and the final product of 

 crystallization. 



The final product of crystallization of ternary solutions of 

 CaO, A1 2 3 and Si() 2 always consists of three solid phases 

 whose fields of stability are adjacent. The same three solid 

 phases will be the final product of crystallization from any 

 solution whose composition lies within the triangle formed by 

 lines joining the compositions of these three phases. 



A recognition of the above facts is of considerable import- 

 ance in the study of ternary systems. If one is locating the 

 boundary curve and quintuple point temperatures by the dis- 

 continuities in, say, a time curve of cooling, it is quite neces- 

 sary to know the cause of each break (energy change). 

 Ordinarily one starts with an unsaturated solution, so that the 

 first break is at the freezing-point of the first phase ; the second 

 break occurs at the intersection of the boundary curve, and 

 the third break, in such a case, is at the quintuple point. 

 Solutions of silicate mixtures, however, undercool to such an 

 extent that a time curve of cooling is valueless ; hence for 

 experimental work of this nature, the time curve of heating is 

 used, in the course of which the solid phases melt in the 

 reverse order of crystallization. Even with heating curves, 

 however, the breaks obtained for the melting of certain silicates 

 are not sharp ; in such cases the quenching method, with 

 which one is able to determine the optical properties of a phase 

 and its temperature of melting, is more suitable. 



With this quenching method we have made actual observa- 

 tions from which one can deduce what would take place under 

 the ideal conditions for crystallization of cooling solutions of 

 CaO, A1 2 3 and Si0 2 . The results thus obtained, which give 

 the phases present and their temperature range of stability for 

 certain solutions, illustrate all types of crystallization curves 

 described; they are presented in Tables XXI-XXIY. 



The data in Table XXI illustrate that type of crystallization 

 curve which follows a boundary directly to a eutectic. Thus 

 in the solution of the composition CaO 38, A1 2 3 51*5, Si0 2 

 10*5, 2CaO.Al 2 3 .Si0 2 starts to crystallize at a temperature 

 between 1540° and 1520°; at a temperature of 1515° the 



