158 CARNEGIE INSTITUTION OF WASHINGTON. 



influence is the partial reduction of the copper to the cuprous condition 

 in which it is more readily precipitated. If this be true, more of the 

 copper in the precipitate should be in the cuprous condition when 

 ferrous sulphate is added. This point will be investigated further, for 

 it raises an interesting question. Unless ferrous sulphate is present in 

 the enriching solutions the product always contains cupric sulphide 

 unless repeatedly treated with cupric sulphate. A part of this cupric 

 sulphide, at least, appears to exist in the chalcocite in a dissolved state. 

 Such chalcocites are found in nature and are perhaps the product of 

 enriching solutions which contained little or no iron. 



We have now a method for distinguishing readily between chal- 

 cocites which do and those which do not contain cupric sulphide, and 

 propose to investigate a considerable number of them from various 

 fields where the geological relations have been studied, with the pur- 

 pose of ascertaining whether enriching solutions which contain very 

 little ferrous iron give chalcocites of a different composition. 



The third constituent of the enricliing solutions, sulphuric acid, has 

 an appreciable effect on the process of enrichment. With both pyrite 

 and chalcopyrite, the addition of 1 to 2 per cent of sulphuric acid to 

 the copper sulphate caused retardation of the enrichment. On the 

 other hand, galena, sphalerite, and bornite, which react readily with 

 sulphuric acid to produce hydrogen sulphide, bring down copper more 

 rapidly from enriching solutions which contain free sulphuric acid, 

 because the hydrogen sulphide precipitates copper immediately. 



A number of other reactions have been worked out between the 

 primary sulphides and single constituents of the enriching solutions 

 and with some other substances which may occur locally in the enrich- 

 ment zone. Such products may be economically unimportant in the 

 geology of a region, but economically unimportant minerals sometimes 

 furnish the key to economically important conditions of precipitation. 

 Thus dilute sulphuric acid at 40° decomposes bornite readily, with the 

 formation of covelhte, chalcocite, ferrous sulphate, and hydrogen sul- 

 phide, while at 200°, chalcopyrite and pyrite or marcasite are obtained 

 in addition to the above. 



Dilute sulphuric acid (as low as 40°) with a mixture of pyrrhotite 

 and chalcocite (or covellite) gives ferrous sulphate, hydrogen sulphide, 

 chalcopyrite, and pyrite or marcasite. Bornite is also a product of 

 this reaction if the copper sulphide is in excess. Pyrrhotite and chal- 

 cocite (or covellite) with hydrogen-sulphide water give chalcopyrite 

 at 40°. At higher temperatures bornite is a product of this reaction 

 when the copper sulphide is in excess, but the bornite is formed under 

 a protective covering of chalcopyrite. 



Chalcopyrite has not yet been found as a direct product of enrich- 

 ment on any sulphide except pyrrhotite, nor has bornite been found 

 as an enrichment product of any sulphide. However, both chalco- 



