56 Rankin and Wright — Ternary System OaO-Alfi^-SiO v 



^h + PA- i 

 PA PA 



Similarly for any other pairs of temperatures (or compositions) 

 within this region. 



" The expression crystallization curve will be used to denote 

 the locus of points which represent the compositions of the 

 solutions formed on cooling any given solution from any given 

 temperature to the quintuple point at which it becomes solid, 

 under the assumption that no phase is removed during the 

 cooling."* The crystallization curve discussed begins with b, 

 is a straight line to b 19 turns sharply at that point and follows 

 the boundary curve to 2. From any point on the crystalliza- 

 tion curve a line drawn through the initial solution will extra- 

 polate to the point which represents the mean composition of 

 the solid phase which has separated up to that time in the 

 process of cooling. Thus the line b x to b extrapolates to S; 

 b 2 to b extrapolates to /3 2 ; etc. When the point 2 is first 

 reached, the solid phase has the composition # 3 . As the 

 solution continues to solidify the gross composition of the solid 

 phase changes from /3 3 to b, reaching b as the last drop of 

 solution disappears. 



From this study of the crystallization of solution b cer- 

 tain generalizations may be drawn. (1) The crystallization 

 curve for any solution whose composition lies within a field 

 will proceed along a definite straight line to a boundary curve. 

 (2) The mean composition of solid crystallized or of liquid 

 remaining at any point on a crystallization curve, or the amount 

 of solid crystallized, may be determined geometrically from 

 the equilibrium diagram. 



We will now consider the nature of the crystallization curve, 

 up to the boundary curve, for any solution of CaO, A1 2 3 , 

 Si0 2 . As we have pointed out, the crystallization curve for 

 any solution follows a straight line until this line intersects a 

 boundary curve, and the origin of this straight line is the point 

 representing the composition of the primary phase of the 

 solution. To show the course of the crystallization curves 

 within each field, straight lines have been drawn which radiate 

 from the point representing the composition of the primary 

 phase for each field ; these lines end at the boundaries of each 

 of these fields. Thus within the field, Si0 2 -J.-2-W, Si0 2 is 

 the primary phase and the lines radiating from Si0 2 to the 

 lines A-2, 2-1 and 1-e/show the direction of the crystallization 

 curves, up to the boundaries, for all solutions whose composi- 

 tions lie within this field. Within each field in which the 

 primary phase is a compound stable at its melting point, these 



*Geer, ibid., p. 263. 



