E. V. Shannon — Naumannite in Idaho. 391 



The specimen of naumannite is an irregularly rec- 

 tangular flat nodule completely coated with grayish clay. 

 The interior is composed of the naumannite which has a 

 dark blue-gray color with metallic luster and a faintly 

 shining gray streak. It is exceedingly sectile and quite 

 malleable. Where broken the fracture is hackly and 

 irregular, the perfect cubic cleavage reported for nau- 

 mannite not being manifest. The color is quite unlike 

 that of argentite and the mineral does not tarnish readily 

 as does the sulphide. The hardness is 2.5 and the specific 

 gravity as determined on a sample not entirely free from 

 clay is 6.527. The pure mineral does not equal 7.0 sp. gr. 

 as compared with 8.0 given for naumannite in textbooks. 

 The descriptions of naumannite from previously reported 

 localities do not emphasize the extremely marked mal- 

 leability and sectility of the mineral. Aside from the 

 clay which coats the outside of the naumannite and is dis- 

 tributed through it, the only other mineral present is mar- 

 casite which occurs as disseminated small grains in the 

 silver mineral. After deducting clay and marcasite the 

 analysis yielded the following results : 



Analysis of Naumannite. 



Per cent. 



Silver 75.98 



Selenium 22.92 



Sulphur 1.10 



100.00 



The presence of sulphur replacing a portion of the 

 selenium is interesting as indicating an isomorphous 

 gradation toward argentite. Lead, which commonly oc- 

 curs in naumannite from European localities, is absent as 

 are copper, gold, zinc, bismuth, antimony, arsenic and 

 tellurium. 



Many tons of such ore as that from which the above 

 described specimen came, were mined and the silver min- 

 eral was passed as argentite. It is not improbable that 

 silver selenide has been similarly overlooked in other 

 western silver ores although the distinctive reactions of 

 selenium are so easily recognized that it is the work of 

 but a moment to prove its presence or absence. 



U. S. National Museum, Washington, D. C. 



