204 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



found to be tetragonal, isomorphous with and closely resembling 

 cassiterite crystals. The basal plane is very common, and the pre- 

 vailing forms are like Figures 37 and 38. Many of the smaller 

 crystals have merely the steep pyramid v(301) as in Figure 37. The 

 best crystal is one one-hundred-and-fiftieth of an inch long with the 

 form and proportions shown in Figure 39. The forms noted are as 

 follows: 



a .(100) 

 c(001) 



c(101) 

 i>(3Ql) 



x(332) 



37 38 



Figs. 37-38.— Plattnerite crystals. You Like Lode, Shoshone County. After Yeates and 



Ayres 



The form a; (332) is of rare occurrence, beveling the edge of v(301) . 

 The prism faces are rounded and deeply striated. v(301) is un- 

 usually bright, large, and well defined. The angle (301) A (301) was 

 taken as the fundamental angle; the mean of many measurements 

 gave 127° 32'. This angle gives: 



a:c = l :0.G7643 



