250 schaller: refractive indices of strengite 



the values: a = 1.70-1.71, 7 = 1.72-1.73. Lacroix^ deter- 

 mined the mean- index of similar material (called angelardite by 

 him), as approximately 1.70. It therefore seemed necessary to 

 redetermine the refractive indices of typical strengite in order to 

 bring the conflicting determinations in accord. 



Professor Ledroit of Mainz very kindly presented me with 

 several specimens of the mineral from the Eleonore mine on the 

 Diinsberg, near Giessen. The strengite forms small spherulites 

 associated with beraunite (eleonorite), cacoxenite, etc. Crushed 

 fragments of these spherulites were used for determining the 

 minimum and maximum refractive indices. The oil immersion 

 method was"used and the values obtained are: 



a (normal elongation of fibers) =1.71 

 7 (parallel elongation of fibers) = 1.735 

 (7 - a) = 0.025 



The values given by Schroeder van der Kolk are therefore wrong 

 and it seems probable that his figure 1.81 was miscopied for 1.71. 



* Mineral ogie de la France 4: 523, 1910. 



