THE MINERALS OF IDAHO 103 



The specimen of naumannite is an irregularly rectangular flat nod- 

 ule about 10 by 7 by 3 centimeters in maximum diameters and 

 weighs about 475 grams (1 pound). Little of the metallic mineral 

 is visible on the exterior of the nodule which is completely coated 

 with a thin tenacious layer of grayish clay (pi. 1, lower). The in- 

 terior of the specimen is composed of the heavy metallic mineral 

 which is so malleable that projecting corners are broken from it 

 with difficulty. In color it is dark blue-gray with metallic luster 

 and faintly shining gray streak. Where broken the fracture is 

 hackly and irregular with no indication of the cubic cleavage which 

 is reported for naumannite. The color is lighter gray than that of 

 argentite and the mineral does not tarnish readily. The hardness 

 is about 2.5 and the specific gravity determined on a sample not 

 entirely free from clay is 6.527. The pure mineral does not reach a 

 specific gravity of 7, suggesting the erroneousness of the value 8 

 given in textbooks. The descriptions of naumannite from previously 

 reported localities do not emphasize the extremely marked sectility 

 and malleability of the mineral. 



Aside from the clay, which coats the exterior of the specimen and 

 is irregularly distributed through the mass, the only impurity is 

 marcasite, which is seen to be distributed in small grains through 

 the naumannite when polished sections are examined with the 

 metallographic microscope in reflected light. The analyzed sample 

 contained 0.90 per cent of iron, all of which was probably present as 

 marcasite. The results of the analysis, recalculated after deducting 

 clay and marcasite gave the following results: 



" Analysis of naumannite 



Per cent 



Silver 75. 98 



Selenium 22.92 



Sulphur 1. 10 



Total 100. 00 



The mineral thus agrees well with the formula given for nauman- 

 nite, the presence of a little sulphur replacing selenium being inter- 

 esting as indicating an isomorphous gradation toward argentite. 

 The ratio of selenium to sulphur is so great (17 : 2) that the mineral 

 must be designated naumannite Ag 2 Se, rather than aguilarite, to 

 which is ascribed the formula Ag 2 S.Ag 2 Se, that is, the sulphur and 

 selenium are supposedly present in molecularly equal amounts. 

 Lead, copper, gold, zinc, arsenic, antimony, bismuth, and tellurium 

 were proven absent. 



The mineral is insoluble in hot concentrated or dilute hydrochloric 

 acid and sulphuric acid but dissolves readily in hot concentrated 

 nitric acid. Heated before the blowpipe on charcoal it yields sele- 



