434 Andersen — System Anorthite-Forsterite-Silica. 



one (iig. 6) lies in the qualities of the boundary curve j o. The 

 relations of this curve are best understood by discussing some 

 of the crystallization curves which intersect it. For a melt of 

 composition e (fig. 7) the crystallization curve has the course 

 eh o. Between e and b the solid phase A crystallizes out. 

 At b the compound D begins to crystallize out and A to redis- 

 golve and this process continues ail along the curve b o. That 

 this must be so is inferred from the fact that all tangents to 

 the boundary curve j o intersect the prolongation of the line 

 A D and not this line itself. If a tangent intersected the line 

 between A and D it would mean that the solid crystallizing at 

 the point of tangency of any such tangent consisted of a mix- 

 ture of A and D (see p. 428) and by a further crystallization 

 the amount of A would increase. If the tangent went through 

 the point D itself, D only would separate and the amount of 

 A that might be present in the solid when the point of tan- 

 gency was reached would neither increase nor decrease. If, 

 therefore, the tangent intersects the prolongation of A D this 

 can only mean that in the solid separated at the point of tan- 

 gency the amount of A present in the solid when this point is 

 reached decreases, and this holds true for all points on the 

 boundary curve j o. We would arrive at the same conclusion 

 if we calculated the amount of different phases present at b and 

 the amount present when crystallization reaches o. We would 

 see that between b and o the amount of A would decrease and 

 that of D increase. At o there is (exactly as in the case of 

 Type 3) a further dissolution of A and a simultaneous crys- 

 tallization of D and C until all becomes solid consisting of a 

 mixture of A, C and D. 



For a melt of composition a the crystallization curve will 

 have the following course : From a to b A separates. Between 

 b and c D crystallizes out and A dissolves, and in the moment 

 the point c is reached, all A has disappeared. This is inferred 

 from the fact that the mean composition of the total solid 

 separated between a and c is indicated by the point D. From 

 <?, therefore, the crystallization breaks away from the boundary 

 curve j o and continues along the line c d through the field of 

 D with D alone separating. From d to m B and D crystallize 

 together and at m the final solidification takes place with eutectic 

 crystallization of B, C and D. For all melts within the section 

 o j Y> the crystallization curves will break away from the 

 boundary curve j o and pass through the field of I). Some of 

 these crystallization curves intersect the boundary curve h m 

 like that of the melt a, others intersect o m. 



Boundary curves likej o (fig. 7) are often called alteration 

 curves. Whenever the crystallization curve passes an altera- 

 tion curve one solid phase will dissolve and the other crystallize 



