190 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



and were combinations of the cube, c(010) predominating, with modi- 

 fying faces of the octahedron p(lll) and the rhombic dodecahedron 

 d(110), as shown in Figure 32. The isometric symmetry is estab- 

 lished by the following measurements: 



Angles of cuprite from Custer County 



Quality, description 



Very good- 



do 



Medium.. 

 do 



Measured 



00 



00 90 00 



41 56 90 00 



45 09 54 42 



Calculated 



00 

 45 00 

 45 00 



00 



90 00 



90 00 



54 44 



In addition to the small symmetrical crystals there occur, on the 

 same specimens, elongated crystals grading toward the capillary 

 form of cuprite called chalcotrichite. These form reticulated meshes 

 in which the angle of intersection is 90 degrees. 



32 33 



Figs. 32-33.-32, Cuprite. Dominantly cubic crystal modified by octahedron and dodecahedron. 

 Mackay, Custer County. 33, Cuprite. Dominantly octahedral crystal modified by dodeca- 

 hedron and cube. Caledonia Mine, Shoshone County 



Specimens from the Skylark mine, Bay Horse district, contain 

 small deep red cuprite crystals coating cracks in rusty iron-stained 

 rock. These are an oxidation product of tetrahedrite. The crystals 



