448 Andersen — System Anorthite-F'orsterite-Silica. 



In fig. 12 only the heavy lines A L and L E represent points 

 in the plane of the section itself. The other points (lines F M 

 and M L) are projected on this plane with the point D (the 

 forsterite point) as eye point. In the cross section, therefore, 

 vertical lines, like g M, correspond to straight lines through 

 D in the ternary diagram, and the different points are plotted 

 on these lines according to the temperatures determined. 

 With this explanation in mind the diagram is easily under- 

 stood and a further discussion of it is unnecessary. 



Crystallization with removal of forsterite from the liquid. 



In the preceding discussion it has been taken for granted 

 that the solids were always in contact with the liquid from 

 which they separated, that is, there was every opportunity for 

 a complete reaction between the solids and the liquid. We 

 may now consider briefly some cases where the solid is wholly 

 or in part kept away from the liquid during the solidification. 

 This may be caused by a simple mechanical removal of the 

 crystals as soon as they separate, or by a protection of the 

 crystals by their becoming embedded in crystals separating 

 around them. The effect upon the . crystallization of the 

 remaining liquid will in both cases be the same. 



We see that in simple eutectic systems* (or eutectic parts 

 of systems) a removal (or protection) of crystals will not in any 

 way affect the further crystallization from the remaining 

 liquid, because no part of the solid is ever taken back into solu- 

 tion during the solidification, and thus the change of the compo- 

 sition of the liquid is simply caused by the separation of solids. 

 We, therefore, consider only that part of the anorthite-forster- 

 ite-silica system which lies within the section ADFM (fig. 9), 

 the reaction field of forsterite. 



From a melt within the section DE^ (fig. 9) forsterite will 

 separate till a point on the boundary curve F M is reached. 

 If all this forsterite were removed as it separated, the further 

 crystallization would not follow the boundary curve F M but 

 would go through the clinoenstatite field with separation of 

 clinoenstatite alone, and the final solidification would take 

 place at the ternary eutectic !N" with simultaneous separation of 

 anorthite-clinoenstatite and tridymite. We would obtain prac- 

 tically the same course of crystallization if, as a result of the 

 first reaction at the boundary curve, the forsterite crystals 

 were imbedded in a protecting coating (reaction rim) of clino- 

 enstatite. This would prevent a dissolution of forsterite on 

 further cooling and the crystallization would proceed with 

 separation of clinoenstatite and end with eutectic crystalliza- 



* Where all boundary curves are melting curves and all quintuple points 

 ternary eutectics. 



