140 CARNEGIE INSTITUTION OF WASHINGTON. 



found to occur at 100° C, and indications of the same reactions or some 

 of them were also found at 30° C. The conditions of deposition in nature 

 are therefore also accessible and the results comparable with those of 

 the laboratory. Moreover, these reactions have been worked out in 

 a quantitative way, i. e., the quantities of all the products formed have 

 been carefully determined. The ratios of the iron dissolved and of the 

 sulphuric acid which is formed to the copper precipitated have been 

 determined in all the cases. In some of the cases chemical separa- 

 tions of the products can also be made. Thus the following changes 

 have been established: 



Pyrite to chalcocite FeS2 ' CU2S 



Covellite to chalcocite CuS > CU2S 



Chalcopyrite to chalcocite CuFeS2 > CU2S 



Pyrrhotite to chalcopjTite FeS(S)x > CuFeS2 



Bornite to chalcocite and covellite Cu5FeS4 > CuS+Cu2S 



Color changes and incomplete chemical investigations indicate also : 



Pyrite to chalcopjrrite FeS2 » CuFeS2 



Pyrite to bornite FeS2 > Cu6FeS4 



Again, the change at ordinary temperatures from bornite to covellite 

 and chalcocite, FeS2 — >Cu2S and CuS, has been quantitatively worked 

 out and at 300° C. we have found the change CU2S — >Cu. 



All these changes involve an increase of copper in the product, that 

 is, an enrichment. 



Thus far, the conditions which determine any particular one of these 

 changes at 200° C. can only be stated in general terms ; thus, for example, 

 the higher the concentration of copper solutions and the longer the time 

 and the greater the surface of sulphide exposed, the farther the change 

 will go. A large surface of pyrite and a dilute copper solution gives 

 at first considerable chalcopyrite, but eventually all becomes chalcocite. 

 Certainly none of the intermediate copper sulphides between pyrite and 

 native copper can be regarded as stable in the presence of copper- 

 sulphate solutions. Since sulphuric acid is one of the products formed in 

 the oxidation zone whenever pyrite, marcasite, or pyrrhotite is present, 

 we have studied the effect of copper-sulphate solutions on the common 

 sulphides in the presence of acid as well as in neutral solutions. In one 

 instance (the change from pyrite to chalcocite), acid has been found to 

 retard the precipitation of copper, i. e., the enrichment process. Now, 

 mine waters have been proved to be less acid the lower the level. The 

 acid is neutralized by various minerals, like the carbonates, feldspars, 

 and others, with which the solutions come in contact. Therefore 

 we should expect that enrichment would go on more rapidly at some 

 distance below the surface, the distance depending on local conditions. 



In these various ways the problem is leading to a series of reactions 

 bounded by definite conditions which admit of laboratory study and 

 precise definition. This problem will be pursued actively during the 

 coming year. 



