Andersen — System Anorthite- For sterite-Sili 



en. 



113 



a system on cooling with equilibrium need no detailed explana- 

 tion. 



A typical example is represented in Table 6, which shows 

 the amounts of the different phases present at the principal 

 stages in the crystallization of a mixture of composition An 15, 

 En 70, Si0 2 15. This mixture lies within the clinoenstatite 

 field and its ciwstallization curve is of the type corresponding 

 to e F N (fig. 11):* between e and f crystallization of clino- 

 enstatite alone, between f and N simultaneous crystallization 

 of clinoenstatite and tridymite. At N eutectic crystallization 

 of clinoenstatite, tridymite and anorthite. 



Table 6.f 



Illustrating the crystallization of a mixture 

 An 15, En 70, Si0 2 15 (of type e fig. 11). 





Amount present in weight per cent of the initial mixture. 





1 



2 



3 



3-2 



Phase 



When crys- 

 tallization 

 reaches /. 



When crys- 

 tallization 

 reaches N. 



34 

 61 



5 







When 

 solidification 



at N is 

 completed 





 70 

 15 

 15 



Showing 

 amount of eu- 

 tectic crystalli- 

 zation at N. 



Liquid 



Clinoenstatite 

 Tridymite 

 Anorthite 



68 



32 











—34 



+ 9 

 + 10 

 -4-15 



f The calculations of the tables 6, 7, 8 and 9 are based on the diagram 

 fig. 9. The figures are rounded off to whole numbers. 



The reaction field. — The mixtures within the field D K 

 A M F, the reaction field (figs. 9 and 11), where forsterite 

 appears during some period of the crystallization, behave in a 

 little more complicated manner, and the course of crystalliza- 

 tion of various mixtures of this kind will be discussed below. 



In mixtures within the field AKI, anorthite is the primary 

 phase and the second step of the crystallization brings out the 

 simultaneous separation of forsterite and anorthite along the 

 curve K M. At the point M there is a reaction between 

 forsterite and liquid in such a way that both decrease in 

 amount, while clinoenstatite crystallizes out. At the same 

 time, anorthite continues to crystallize. If the original mix- 

 ture has a composition within AKL the liquid will disappear 

 before the forsterite and the crystallization will stop at M, the 

 result being a mixture of anorthite, forsterite and clinoenstatite. 



*To avoid crowding of lines no crystallization curves are drawn in fig. 9. 

 The location of any points in fig. 11 is, of course, quite arbitrary and does 

 not correspond to the ratios of Table 6 (or the following Tables 7, 8, 9). 



