432 Andersen — System Anorthite-Forsterite-Silica. 



of B, C and D only and still had a mean composition a, and 

 that is impossible. During the final process of solidification 

 the composition and temperature of the liquid remains constant 

 (expressed by the properties of the point o) and the mean com- 

 position of the total solid changes from a' to a. During this 

 final solidification at o, while all the liquid disappears, the 

 solid phases C and D crystallize out, whereas some of the phase 

 A is redissolved. 



A liquid the composition of which is expressed by the point 

 c on the conjugation line C D crystallizes as follows : From c 

 to h crystallization of A alone. Between h and o simultaneous 

 crystallization of A and D. At o final solidification with total 

 dissolution of A and simultaneous crystallization of A and C. 



For a melt whose initial composition is expressed by a point 

 on the conjugation line we thus see that the total amount of 

 the phase A separated before o is reached disappears during 

 the final solidification at that point, and the resulting solid con- 

 sists of C and D, as if these two phases had formed a simple 

 binary system with eutectic crystallization. 



Let us now consider a melt whose composition is expressed 

 by a point d within the part of the field of A that extends 

 across the conjugation line. In this case the crystallization 

 curve cannot end at o because this would mean that the final 

 solid consisted of A, C and D and contained no B, although it 

 must have the composition of d which is a mixture of B, C 

 and D. The curve must, therefore, proceed to the ternary 

 eutectic m. Between d and <?, A alone crystallizes out ; between 

 e and o A and C crystallize together. The mean composition 

 of the solid separated between d and o is expressed by the 

 point d' . When crystallization begins to proceed along o m 

 the mean composition of the solid has changed to d" . While 

 crystallization goes on along o m the mean composition of the 

 solid changes from d" to d'" and during the final eutectic 

 crystallization at m from d'" to d. 



We see that in all the three melts considered there will be a 

 dissolution of the solid phase A at the quintuple point o. In 

 the melt a only a part of A is dissolved and the final solidifica- 

 tion takes place at o. In the melt c all A is dissolved at the 

 same time as the liquid disappears and all becomes solid at o. 

 The crystallization of the melt d, finally, proceeds to the ter- 

 nary eutectic m because there is liquid left when all A has 

 been dissolved at o. 



Type Jf, — The points m, o and h,j all lie on the same side of the 

 conjugation line, which is not intersected by the boundary 

 curve j o. 



In the type to be considered below the compound D is 

 unstable at its melting point and also breaks up in mixtures at 



