26 Van Horn — Silver, Copper, and Cobalt Minerals. 



torted crystals of octahedral habit. It is associated in one 

 place with proustite, and in another with silver and barite. 

 One specimen shows many four sided pyramids (evidently 

 octahedrons) each attached to the extremity of wires of silver. 

 It is certain that one mineral originated from the other, but it 

 is impossible to tell which is secondary. An analysis of the 

 argentite was made by Dr. N. A. Dubois, formerly of the 

 Chemical Department of Case School of Applied Science, with 

 the following results : 



Found Theoretical 



Ag 86-18 87*07 



S. 1318 12-93 



Cu 0-70 0-00 



100-06 100-00 



Sp. gr 7-40 7-2-7*4 



The specific gravity given was obtained from the average of 

 six determinations made on two different specimens by means 

 of Jolly balance and pyknometer. The most interesting fact 

 indicated by the analysis is the presence of an appreciable 

 amount of copper. This shows a gradation toward the min- 

 eral jalpaite (Ag,Cu) 2 S, which has been found at but one or 

 two places in the world. It will be shown later that the 

 proustite with which it is associated also contains about the 

 same amount of copper. 



Galena. — This mineral is found in fine granular to some- 

 what fibrous masses. Specimens are occasionally coated with 

 cerussite as a decomposition product, but no anglesite was 

 observed. 



Chalcocite. — This is present in very pure large compact 

 masses showing conchoidal fracture. It is found in some 

 places with chalcopyrite and barite. 



Covellite. — Covellite occurs in very dense fine-grained 

 masses which with the exception of color have the external 

 appearance of chalcocite. On a fresh fracture, the mass is 

 shown to consist of small indigo blue grains which possess the 

 basal cleavage of covellite. Possibly the mineral has resulted 

 from the alteration of chalcocite, on which it is commonly 

 found as a tarnish. 



Chalcopyrite. — This is found in compact masses alone, or 

 mixed with chalcocite. 



Proustite. — The mineral is present in compact masses of 

 brilliant ruby-red color. It shows a distinct rhombohedral 

 cleavage. On one specimen it occurs with argentite, whereas 

 in another place it is associated with calcite in seams of gray 

 magnesian limestone. The mineral was analyzed by Dr. JS". 

 A. Dubois with the appended result : 



