Rankin and Wright — Ternary System CaO-Al 2 O s -Si0 2 . 51 



(d) The composition line for the compounds C 3 S and 3 A, 

 both of which are unstable at the melting point, lies entirely 

 within the f eld for C. — This line, then, is the melting curve 

 of C in solutions of C 3 S and C\A. 



Crystallization in Ternary Systems. 



The results of this investigation, thus far, have been plotted 

 graphically in diagrams to show the equilibria between the 

 various phases of this system of CaO-Al a 3 -Si0 2 . To the end 

 that these data be utilized in practical work it is desirable to 

 know what is the composition of the solid phases which sepa- 

 rate when a ternary solution is cooled. The systematic 

 elaboration of the theory of phase separation has been carried 

 out in a paper by W. C. Geer, entitled "Crystallization in 

 Three-Component Systems."* Instead of giving merely the 

 reference to this important work, it has seemed desirable to 

 present again in some detail his deductions on crystallization 

 in three-component systems as it is a very concise statement of 

 the theory involved; for we believe that crystallization of 

 ternary solutions is not a matter of general knowledge even 

 amongst those most concerned and the theory is of vital 

 importance to an understanding of the behavior of ternary 

 systems. Where Geer has used hypothetical compounds and 

 conditions we have substituted actual data on which to base 

 the deductions. Otherwise the matter presented is in sub- 

 stance identical with Geer's paper, except for a few minor 

 changes in arrangement and in the wording of certain explana- 

 tions. 



For the discussion of the theory of crystallization as applied 

 to solutions made up only of the components CaO, A1 2 3 and 

 SiO Q , we will use fig. 17. Within this diagram the same num- 

 bers and letters are used for the designation of quintuple and 

 quadruple points as in the previous equilibrium diagrams. 



The experimental data represented by this diagram are such 

 that all the possible theoretical types of ternary crystallization 

 discussed by Geer are found therein. Solid solution we shall 

 leave out of account, since it occurs only in the case of 

 CaO.Si0 2 and then only to the extent of 2 per cent with each 

 of the compounds with which it is associated, namely : Si0 o , 

 3Ca0.2Si0 2 , CaO.Al 2 3 .2Si0 2 , and 2CaO.Al 2 3 .Si0 2 . 



In taking up this discussion we will consider first the sim- 

 plest type of crystallization, which at the same time is that 

 most commonly observed in solutions of CaO, A1 2 3 and Si0 2 . 

 For this purpose it will be convenient to consider that portion 

 of fig. IT which is included within the triangle formed by the 

 *W. C. Geer, J. Phys. Chera., viii, p. 257, 1904. 



