Plagioclase Feldspars. 



583 



The data are shown plotted on a temperature-composition 

 diagram in fig. 1. A clear idea of the extent and accuracy 

 of the data on which the diagram rests may be obtained from 

 Table II. 



Fig. 1. 



o 

 1560 























1520 























1480 























I440 



i >c\ 

 A 



I 















T400 





















1360 



1 



| 



















1320 









\ 



\ 













1280 











\ 



a 











1240 













\ 











1200 























1160 























1120 























J 

















Nj 



10 



20 



30 



40 



50 60 



70 



so 



90 



ANORTHITE 







MOL 



PERCENT 







ALBITE 



Order of Accuracy. 



For the reason that it is somewhat easier to detect a few 

 crystals embedded in glass than to find a small amount of glass 

 in a crystalline aggregate, the accuracy of the liquidus is some- 

 what greater than that of the solidus. No difficulty was found, 

 however, in establishing the solidus within limits of zb 4°. 



