372 



HAROLD L. ALLING 



Indices of refraction : 

 Potash phase 



a 1.528 



Soda phase j/3 i. 533 



It I-53S 

 Extinction angles: 



Potash phase (001) i ■ 5°~2° 



(010) 6.° -8° 



Soda phase (001) 4 . ° 



(010) 18. ° 



We can draw the conclusion that celsian is not isomorphous with 

 any of the plagioclase feldspars, including albite. 



STRONTIUM FELDSPARS 



Eskola says: "It was .... found that the strontium feld- 

 spar is, in its optical properties, exactly like the calcium feldspar. 

 .... It is ... . probable that the strontium feldspar forms a 

 complete series of solid solutions with anorthite."' He does not 

 state its relation to orthoclase but it is reasonable to expect that it 

 does not form solid solutions with it to any extent. As an illustra- 

 tion of barium and strontium bearing feldspars the following analysis 

 of an albite from Seiland, Norway, is introduced. 



TABLE VII 



Albite from Seiland, Norway 

 Analysis calculated to 100 



' p. Eskola, "Silicates of Strontium and Barium," Amer. Jour. Sci. (5), IV (1922), 

 p. 367- 



