446 Andersen — System Anorthite- For sterite-S'dica. 



anorthite. In this case there is liquid left when all forsterite 

 is dissolved at M and the crystallization continues along M N 

 to 1ST as repeatedly described. In all mixtures within the 

 field E M g the crystallization proceeds as discussed for m. 



It will appear that in all the mixtures hitherto described, 

 lying within the field D K A M E, where forsterite is formed 

 at sometime during the solidification, the crystallization curves 

 always pass through the alteration point M, or else the crys- 

 tallization is completed at that point. 



For the little part of the forsterite field which now remains 

 to be considered, the field E F M, the crystallization curves 

 leave the boundary curve F M before the quintuple point M 

 is reached. This will be plainly seen by considering the crys- 

 tallization of a mixture a (fig. 11). From a to b forsterite 

 crystallizes out. At b clinoenstatite begins to crystallize, for- 

 sterite to redissolve, and this reaction goes on while the com- 

 position of the liquid changes along the line F M. "When the 

 point c (lying on the continuation of the line E a) is reached, 

 no more forsterite is left, and the further crystallization, there- 

 fore, does not follow the line F M, but proceeds through the 

 clinoenstatite field along the line c d, clinoenstatite crystalliz- 

 ing all the time until the point d is reached. At this point 

 anorthite also begins to separate, and the crystallization goes on 

 along d N to N, where there is the usual eutectic crystalliza- 

 tion of anorthite, clinoenstatite and silica. A mixture of com- 



Table 9. 



Illustrating the crystallization of a mixture An 8, En 89, Si0 2 3 (of type a 



fig. 11). 





Amount present in weight per cent of the initial mixture 





1 



2 



2-1 



3 



4 



5 



5-4 



Phase 



o 



a 



a ° 



o 



H 



N 



'eg 

 1c » 



Showing amount of crys- 

 tallization and dissolu- 

 tion between b and c 



.2 



<S 

 U CO 



si 



.2 



go *-\ 



f-i <D 



I 1 



When solidification at N 

 is completed 



Showing' the amount of 

 eutectic crystallization 



atN 



Liquid 



Forsterite 



Clinoenstatite 



Anorthite 



Tridymite 



97 

 3 

 





 



49 

 



51 

 

 



— 48 



— 3 

 + 51 





 



16 

 



84 

 

 



11 







86 

 3 



0. 





 9 



89 

 8 

 3 



—11 





 + 3 

 + 5 

 + 3 



