566 



JV. L. Bowen — The Binary System. 



Table III. 



Indices of Refraction of Nephelite Mix-crystals. 



CaAl 2 Si 2 8 



0) 



£ 



Birefringence 



Optical character 







1-537 



1-533 



•004 



— 



10 



1-537 



1-535 



•002 



— 



20 



1-537 



1-537 — 



very weak 



— 



23 



1-537 



1-537 



none 



isotropic 



25 



1-537 



1-537 + 



very weak 



+ 



35 



1-537 



1-539 



•002 



+ 



It will be noted that the refractive index of the ordinary ray 

 remains sensibly constant, while that of the extraordinary ray 

 mounts from 1*533 to 1*539. At the composition Ne 77 An 23 



Diagram III. 



10 zo 



(approx.) we have the unique case of hexagonal crystals iso- 

 tropic with respect to sodium light. 



Two different kinds of material were available for these 

 determinations : homogeneous crystalline aggregates prepared 

 by crystallizing the various compositions at about 1200° (area 

 E C F H, Diagram II) and euhedral crystals Ne e2 An 18 -N"e 65 

 An 35 embedded in glass obtained from quenchings within the 

 area CBGF, Diagram II. With the former material the 

 optical character of nephelite (Ne) was determined as negative 

 and that of nephelite (Ne 65 An 35 ) as positive by the ordinary 

 interference figure method. With the latter material the 

 optical character could be determined where the birefringence 

 was extremely small because the relation of indices to crystal- 

 lographic directions was known. Important aid was thus given 



