500 



KNOPF: PLUMBOJAROSITE FROM NEVADA 



erties of beaverite are nearly identical with those of plumbo- 

 jarosite. It is seen to be crystallized in minute hexagonal 

 plates of yellow color. According to Mr. E. S. Larsen'^ the 

 optical properties of the type material are: Optically negative, 

 probably uniaxial; w = 1.83; € = 1.79. From these data it is 

 obvious that under the microscope the only way of distinguish- 

 ing plumbojarosite from beaverite lies in determining the re- 

 fractive index oj of the hexagonal plates. 



Computation of analysis 



0.027X 694=18.74) 

 0.002X 636= 1.27/ 

 0. 060X1131 = 67. 86~ 

 0.003X1002= 3.00 

 O.OIOX 972= 9.72 



= 20.01 per cent beaverite. 



80.58 per cent plumbojarosite. 



100.59 



On careful re-examination of the material from the Boss mine by 

 immersion in a liquid of index of approximately 1.84 certain of the 

 hexagonal plates were found to have indices that were below this 

 value, and the remainder were found to .exceed considerably 

 this yalue. Those of the lower index were then determined by 

 F. E. Wright to have an index whose value is 1.84 ±0.01, sug- 

 gesting strongly, therefore, the presence of beaverite. The 

 chemical analysis was then computed as follows: The bismuth, 

 which is possibly present in the native state, since no compound 



' Personal communication. 



