276 H. V. WIXCHELL — SYNTHESIS OF CHALCOCITE AXD ITS GENESIS 



The essential part of these operations may be expressed in a con- 

 densed form as follows : 



1. FeS, + H,0 + 60 = FeSO, + SO, + H,0. 



The SO2 perhaps aided by the ferrous sulphate, reduces a portion of 

 the copper, as follows : 



2. 2CuS0, + SO, + 2H,0 = Cu,SO, + 2H, SO,. 



The acid solution would perhaps form H.^S by attacking the pyrite ; 

 and CuS would be formed, except for the presence of the SO,, which 

 holds the copper in the form of cuprous sulphate and makes the pre- 

 cipitate cuprous sulphide; or, if H,S be not formed by the acid present, 

 and if the CUjS is more insoluble than FeSj, there would be a precipita- 

 tion of CujS and a solution of iron as sulphate. 



3. FeS, + H,SO, -= FeSO, + H,S + S. 



4. Cu.SO, + FeS, = FeSO, + Cu,S -f S. 



In nature, the oxidation of pyrite sometimes leaves residual sulphur, 

 but usually the sulphur is oxidized and aojain taken into solution. 



Chalcocite a secondary Mineral and primary Ore 



It will be observed that the solutions here employed are all acid solu- 

 tions and contain no alkaline carbonates. This corresponds with the 

 conditions existing in the Butte mines now and for a long period since 

 the flows of ascending waters ceased and the work of oxidation and ero- 

 sion began. It seems probable that the waters which have thus deposited 

 chalcocite as a secondary mineral have in most cases been acid waters, 

 and that their movement has been downward. It does not necessarily 

 follow, however, that chalcocite can not be formed in any other way; in 

 fact it is quite possible that ascending alkaline solutions containing cop- 

 per might deposit chalcocite as a primary ore. In that event we should 

 almost certainly find hematite formed as an associated mineral. Indeed 

 this oxide of iron has been reported from some mines under conditions 

 precluding the supposition of origin through the action of descending 

 waters. No hematite, however, is found in the lower levels of Butte 

 mines, nor have alkaline carbonates been found in considerable quantity . 



At many points in the Butte mines are found ores precisely similar in 

 appearance to those thus produced synthetically. Frequently these ores, 

 at first supposed to be pure chalcocite, on analysis prove to be low grade, 

 and on breaking disclose an interior of solid pyrite and a mere surface 

 film of copper glance. So far as I am aware, this is the first account of 

 the synthesis of chalcocite in the wet way, and the first time it has been 

 experimentally demonstrated that important sulphide ore deposits are 

 enriched far below the surface of the ground water by reaction between 

 oxidized solutions from and above the original sulphides. 



