434 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



CUSTER COUNTY 



The evansite from Goldburg is massive and amorphous and occurs 

 in seams. It is brittle, with a conchoid al fracture and varies in color 

 through shades of yellow, white, brown and dark red. The brown 

 variety very much resembles common brown opal. An analysis of 

 this brown variety gave the following results : 



Analysis of evansite from Goldburg, Idaho 



(W. T. Schaller, analyst) Per cent 



Water (H 2 0) 36.96 



Phosphoric acid (P 2 5 ) 19. 14 



Ferric iron (Fe 2 3 ) 5. 49 



Alumina (A1 2 3 ) 34.48 



Lime CaO 4. 32 



Magnesia (MgO) Trace. 



Ferrous iron (FeO) None. 



Total 100. 39 



The temperatures at which the water content of the mineral is 

 given off were determined and the following table shows the amount 

 of water expelled at the several temperatures : 



Loss of water oj evansite from Idaho 



Per cent 



At 107° C 20.00 



At 175° C 7.36 



At 255° C 3. 13 



At 290° C .94 



At low red heat 3. 90 



At blasted 1. 61 



Total 36. 94 



A density determination by means of Thoulet solution of the rela- 

 tion of the density to the amount of iron present gave the following 



values : 



Relation of density to iron content 



By plotting these values it was found that the density of the mineral 

 free from iron should be between 1.8S and 1.91, though this value 

 does not necessarily represent that of pure evansite as the material 

 on which these determinations were made contains considerable 

 calcium. 



A determination by F. E. Wright of the index of refraction of the 

 colorless evansite from Idaho gave 1.485, the index for the colored 

 varieties not varying more than 0.01 from this value. 



