424 Andersen — System Anorthite-ForsteriteSilica. 



Under conditions of perfect equilibrium, the crystallization 

 on cooling may be inferred as follows : In mixtures richer in 

 silica than the point P (fig. 2)* cristobalite is the primary 

 phase. On reaching P cristobalite inverts to tridymite and the 

 crystallization proceeds with separation of tridymite along 

 PI. At I there is eutectic crystallization of tridymite and 

 anorthite. Mixtures between P and I have tridymite and 



Fig. 3. 































































































FORSTER 



TE 



\ 



















& 























LIQUIC 



) 













•ST 





















°\ / 















\c^ 







>-^ B Jf 

















1400 









SPINEL 



etc 



a 



LIQUID 



i 



-^- — ib 



20 30 40 50 60 70 



WT PER CENT 



those between I and A anorthite as primary phase, and the 

 eutectic crystallization takes place at I. 



The System For sterite- Anorthite. 



Owing to the reaction between forsterite and anorthite with 

 formation of spinel, the f orsterite-anorthite system is not really 

 binary. The greater part of the system, however, may within 

 certain temperature intervals be treated as if it were binary. 

 The relations in the part where spinel occurs can not be prop- 

 erly explained more simply than in terms of the quaternary 

 system MgO-CaO-Al 2 3 -Si0 2 . The two breaking-up points 

 (B and C, fig. 3) and the liquidus were determined by the 

 quenchings contained in Table 4. The point B was approxi- 

 mately located by determining the temperature at which the 

 mixture Fo 10, An 90 first showed crystals of spinel. The 

 intersection between the isotherm through this point and the 

 anorthite liquidus determined roughly the point. To get an 

 accurate check of the result a mixture Fo 14, An 86 was made 



*The point is not determined, but only plotted on the silica liquidus at 

 1470° according to C. N. Fenner, this Journal (4), xxxvi, 331, 1913. 



