Plagioclase Feldspars. 



587 



plagioclase of a rock, and that of the "first" plagioclase, but 

 the actual determination of this magnitude has had to wait for 

 the development of appropriate methods of measurement. 



Fig. 



10 



ANORTHITE 



5 1.S7 



< 



i 



— , 





















X 







z 























> 







< 























Ill 



(r 





 1566 























1520 



V 



^0 



















1480 



\ 



\ 



















1440 





\ 



s. 



+ 



^*4 













14O0 







\ 





^ 



i\ 















\ 



\ 















1360 









t320 









\ 



\ 





■f-- 









I280 























1240 





















\ 



I200 















\ 







\ 



1160 















A 



\ 





\ 



1120 

















131, 



\ 



\ 























nJ 



40 50 60 



MOL PERCENT 



90 



ALBITE 



Fig. 2. + Total composition of charge and temperature of experiment, 

 x Determined refractive index of glass phase. 



Theoretical Considerations. 



Van Laar, in 1906, endeavored by means of a general equa- 

 tion, derived by consideration of the thermodynamic potential, 

 to estimate the magnitude of the melting intervals of the 

 plagioclase feldspars. He made use of the value of Day and 



