308 F. E. Wright — Optical Properties of Roscoelite. 



Sample No. 2 is finer than No. 1 and is more or less mixed 

 with free quartz, but it is practically identical with No. 1 in 

 its optical properties : y = 1'692 ± 0'005 ; /3 = 1682 db 0*005 ; 

 a = 1-615 d= 0-005. 2E = 55° to 80°. Dispersion of optic axes 

 strong 2E V >2E,.. Pleochroism and absorption as in Sample 

 1. Interference colors often abnormal, especially on cleavage 

 flakes. Gliding planes normal to cleavage after (010) less fre- 

 quent than in No. 1, but not rare. The roscoelite of both 

 samples is remarkably fresh and unaltered. Sharp crystal out- 

 lines were not observed, and are evidently not characteristic of 

 this member of the mica group. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C. 



