436 Andersen — System Anorthite-Forsterite-Silica. 



the boundary curve j o which in Type 5 has such a course that 

 one of its tangents goes through the point D as shown in fig. 8. 

 The point of tangency,^?, divides the boundary curve j o into 

 two different parts, p o, the melting curve, and j p, the alter- 

 ation curve. 



Recurrent crystallization of a solid phase. — Some character- 

 istic crystallization curves will be described. The melt a 

 crystallizes as follows : From a to b crystallization of A alone. 

 From b to c crystallization of D and dissolution of A, the last 

 of A disappearing at o. Between c and d crystallization of D 

 alone. At d, A begins to crystallize out for the second time 

 and from d to o there is simultaneously crystallization of A 

 and D. At o all A is completely dissolved once more with 

 simultaneous crystallization of C and D. Between o and m 

 simultaneous crystallization of C and D and at m eutectic 

 crystallization of B, C and D. It is seen that during the pro- 

 cess of solidification of this melt a there is a twice repeated 

 crystallization and subsequent total dissolution of the phase A. 

 A consideration of the diagram shows that the section in which 

 this kind of crystallization curves appear is bounded by the 

 straight lines o D (not drawn in fig. 8) and p D and the part 

 of the boundary curve j p that falls between these two lines. 

 For other melts, namely those within the remaining part of the 

 section^' p D, the crystallization curves also leave the boundary 

 curve j p, but the point d will be located either on the curve 

 o m or on h m, and there will be no second crystallization and 

 dissolution of the phase A. 



A melt e within the section Ap D crystallizes as follows: 

 From e to b separation of A alone. Between b and^> crystal- 

 lization of D and dissolution of A. There is, however, still 

 some A left at p, where A begins to crystallize out for the 

 second time. Between p and o simultaneous crystallization 

 of A and p. At o A is again partly redissolved while C and 

 D crystallize out and the final solid consists of A, C and D. 

 In this case it is seen that while A never disappears from the 

 solid its crystallization is interrupted twice, each time with a 

 partial redissolution of the crystals already separated. 



For other sections of the diagram the crystallization curves 

 show no qualities not already pointed out in -the description of 

 the previous types. 



V. The System Anorthite-Forsteeite-Silica. 

 The spinel field. 



Among the ternary mixtures of anorthite, forsterite and 

 silica there are, as already mentioned, some which crystallize 

 with spinel as primary phase and therefore cannot be properly 



