Rankin and Wright — Ternary System, CaO-AlJJ-SiO r 59 



If, however, we start with a solution a' whose composition 

 lies within the triangle which represents all possible concentra- 

 tions of S, AS, and GJl$„ then, when the crystallization curve 

 </'-<( ^9 reaches point 9 the A1 2 3 will disappear from the solid 

 before all the Si0 2 from solution is used up and the crystalliza- 

 tion curve follows the boundary 9-1, AS and CAS. 2 crystalliz- 

 ing together. At point 1 the crystallization ends, the solution 

 going solid, AS, S, and CAS. 2 crystallizing together, and the 

 gross composition of the solid phase changes again to a'. 



There are several portions of the diagram which represent 

 concentrations such that their solutions have this general type 

 of crystallization curve. Thus all solutions represented by 

 points within the area C\ A S-ll- A- O z A 6 have crystallization 

 curves along which crystallization is continuous until quintuple 

 point 11 is reached. Some of these curves will end at that 

 point, others w T ill follow boundary 11-7 and end at eutectic 7. 

 Following the reasoning applied to the crystallization curve 

 for solutions a and a l9 it is evident that solutions within trian- 

 gle C[AS-A-C 3 A 5 have crystallization curves which end at 

 quintuple point 11, and that those solutions within triangle 

 C 2 AS-ll-A will pass through point 11, where G 3 A 6 completely 

 disappears and continue along boundary 11-7 to point 7, where 

 the crystallization curve ends, A, 2 AS and GAS 2 crystallizing 

 together. 



In the same way it can be shown that crystallization curves 

 for all solutions within the area C 2 S-C[AS-CA-14: will pro- 

 ceed first to quadruple point 13. For those solutions within 

 triangle GJS-G^AS-GA crystallization curves will end at 13 

 and for those within triangle GJS-GA—1A crystallization curves 

 will pass through point 13 and end at eutectic 14. 



In all of the crystallization curves so far studied two phases 

 have crystallized together at all points on the boundary curves ; 

 in other words, the composition of the solid phase present on 

 a boundary curve could be represented in terms of the two solid 

 phases separating. This has been so because the line joining 

 any two points on a boundary curve on extrapolation has 

 intersected the straight line joining the compositions of the 

 two phases separating. There are cases, however, in which 

 such a line on extrapolation will not intersect the composition 

 line of the two phases separating but will intersect an exten- 

 sion of this composition line. 



For example, consider solution r whose composition lies 

 within the triangle CS-CJS^-G^AS. C\S separates until the 

 crystallization curve reaches the boundary curve B'-± at 7\, fig. 

 18 (fig. 18 is an enlarged portion of fig. 17). From the latter 

 point the boundary curve is followed to 4, along which the 

 solid consists of C 9 S and G&. As the solution changes com- 



