58 Rankin and Wright — Ternary System OaO-Al 3 3 -Si0 2 . 



tallization has been continuous from the beginning to the end 

 of the curve which has been a eutectic point where three solid 

 phases crystallize together ; on such crystallization curves the 

 composition of the liquid phase at any instant can be made up 

 of the three solid phases which crystallize together at the 

 eutectic. In other words, such a crystallization curve lies 

 entirely within the triangle formed by joining the compositions 

 of the three solid phases present at the quintuple point (eutec- 

 tic), for the reason that the area within this triangle represents 

 all possible concentrations of these three solid phases. Thus 

 the crystallization curve of solution h begins, of course, at b 

 and ends at eutectic 2; its course is entirely within the tri- 

 angle S- OS- OAS 2 , which represents all possible concentrations 

 of these three components. Therefore all points on it repre- 

 sent solutions of S, OS and OAS 2 . 



We now come to the consideration of crystallization curves 

 which lie partially without the triangle formed by the lines 

 joining the compositions of the three solid phases present at 

 the quintuple point (which in such cases is not a eutectic) 

 which is the end of the crystallization curves. For example, 

 consider solution a within the triangle O AS.- AS- A (tig. 17). 

 The crystallization curve starts at a and is a straight line to a v 

 thence follows the boundary P 9 to point 9, where it ends. 

 From a to a 1 A crystallizes and from a x to 9 OAS 2 crystallizes 

 with A. The composition of the solution is given by points 

 on the crystallization curve a—a -9 ; that of the solid crystalliz- 

 ing is obtained by extrapolation of straight lines through a 

 and points on a-§ to intersect the composition line OAS 2 -A. 

 The final solid will, of course, have the gross composition of 

 the original solution a if no solid is removed during the course 

 of crystallization. It will be seen that point 9 lies outside of 

 the triangle OAS 2 -AS-A ; that is, point 9 which represents 

 the composition of the solution when the crystallization curve 

 reaches 9 is a composition such that it can not be repre- 

 sented in terms of OAS 2 , AS and A. But we know that solu- 

 tion a goes solid at the temperature of 9 and that the three 

 solid phases present are OAS 2 ,AS and A. Such being the 

 case, the only way this result may be achieved is for some of 

 the A1 2 3 which has crystallized to unite with the Si0 2 which 

 is in solution to formes' (Al 2 3 .Si0 2 ), and this is what actually 

 happens. As soon as the crystallization curve reaches point 9, 

 A1 2 3 unites with Si0 2 in solution to form Al 2 3 .Si0 2 , the 

 Cab.Al 2 3 .2Si0 2 continuing to crystallize. This process goes 

 on until either the Si0 2 disappears from solution or A1 2 3 from 

 the solid. In the case under discussion (solution a) the Si0 2 

 will disappear first and as the solution solidifies at 9, the com- 

 position of the solid phase changes from a 2 to a, reaching a as 

 the last drop of solution disappears. 



