The Binary System Akermanite-Gehlenite. 137 



Table II 

 Optical characters of artificial crystals of the Akermanite- 



Gehlenite series. 



Composition 



wt. percent. 



Gehlenite. Akermanite. 



100 

 80 

 60 

 50 

 40 

 30 

 25 

 10 

 







20 

 40 

 50 

 60 

 70 

 75 

 90 

 100 



1.669 



1.664 



1.657+ 



1.653 



1.648— 



1.645 



1.643 



1.637 



1.632+ 



1.658 



1.657 



1.654 



1.652 



1.649+ 



1.648 



1.647 



1.643 



1.639+ 



Birefrin- Optical Elonga- 

 gence. character, tion. 



0.011 — 



.007 



.004 



.001 



.001+ 



.003 



.004 



.006 



.007 



+ 

 + 

 + 

 + 

 + 



o /o 

 G£//l£WT£ 



40 JO 60 

 WT P£f?C£/VT 



70 eo <?o /oo 



Z.68 



Fig. 3. — Densities plotted against composition in weight percent, for 

 crystals and glasses. 



The optical character of akermanite is positive and that 

 of gehlenite is negative. In intermediate mixtures the 

 birefringence for sodium light decreases from 0.011 for 

 gehlenite at one extreme and 0.007 for akermanite at 

 the other, to zero at a composition lying between 50 per 

 cent gehlenite-50 per cent akermanite, and 40 per cent 

 gehlenite-60 per cent akermanite. Crystals of a certain 

 definite composition between these limits are isotropic 

 for sodium light. Owing to fluctuations of temperature, 

 zoned crystals were repeatedly obtained from solutions 

 of each of these compositions which exhibited a core of 

 higher index of refraction, negative optical character, 

 and very low birefringence with blue-grav interference • 

 colors surrounded by a zonal growth of material of lower 



