272 H. Y. AVIXCHELL SYNTHESIS OF CHALCOCITE AND ITS GENESIS 



Copper heated to 200° in a closed vessel, in presence of a solution of 

 sulphurous acid, according to the method of Geitner, did not yield satis- 

 factory results. Under similar conditions, a solution of copper sulphite 

 yielded crystalline scales differing from chalcocite in composition, per- 

 haps covellite. 



Margottet^ obtained crystallized copper glance by the slow passage of 

 hydrogen charged with sulphur vapors over copper at red heat. 



Meunierf speaks of the employment of solid precipitants, saying that 

 the reaction in such cases is slow, and favorable for crj'stallization, and 

 moreover reproduces conditions frequently found in nature. He thus 

 obtained crystals of cuprite, alunite, copper, cotunnite, and matlockite. 

 He is convinced, as the result of man}^ experiments, that the natural 

 sulphides, in the presence of suitable metallic solutions, reduce the metal 

 dissolved to the metallic state ; and instances the coating of gold which 

 is deposited on galena when immersed in auric chloride, and of silver 

 when galena is immersed in nitrate of silver. All the sulphides which 

 he has examined produce similar metallic precipitates. 



Chalcocite on Coins 



The usual explanation given for the formation of copper glance on 

 coins in spring waters is that it is due to the reducing action of carbon. 



In his " Mineralogie de la France," J for example, A. Lacroix, in de- 

 scribing the copper sulphide minerals found incrusting old bronze coins 

 in the springs of Bourbonnes-les-Bains,§ says : 



"The chalcocite formed in thermal springs or in fresh water at the expense of 

 Roman coins is explained by the action on the bronze of sohible sulphides, them- 

 selves the product of the reduction of sulphates b}' organic material." 



Organic matter is present in the mine waters of Butte, but is not able 

 to reduce all the ferric sulphate to ferrous sulphate, much less have any 

 appreciable effect on the copper salts. 



Artificial Formation of Chalcocite 



After considering the geological histor}^ and physical structure of these 

 ore deposits, the writer came to the conclusion some three years ago that 

 the copper glance was formed by a chemical reaction between copper 

 sulphate in solution in descending waters and the iron pyrites and other 

 primary sulphide minerals lying below. In order to ascertain the truth 

 or falsit}^ of this theory, laboratory experiments were undertaken by the 



* Margotet : Compt. rend., Ixxxv, 1S77, 114'2. 

 t Meunier, op. cit., 308. 



1 Vol. ii, p. 515. 



2 Daubr^e : Annales des Mines, viii, 439, 1875. 



