COPPER ORES. 1161 



upward. At greater or less depths below the level of ground water the ores 

 are often dominantly cupriferous pyrites or pyrrhotite, the direct deposit 

 of the ascending waters. At a little higher level oxygen from above may 

 have oxidized a portion of the iron and transported it elsewhere, relatively 

 enriching the deposit in copper; at a still higher level there is a contribution 

 of soluble copper salts from above. Since copper sulphate is certainly the 

 most common salt, for convenience all the soluble salts will be regarded as 

 sulphates. Reactions similar to those given below may easily be written for 

 other salts. 



At the level where salts of copper appear from above, the action of the 

 copper salt on iron sulphide may produce chalcopyrite, the reactions result- 

 ing from the greater affinity of sulphur for copper as compared with iron, 

 and from the law of mass action. With cupric sulphate the reaction may 

 be written, for ferrous sulphide: 



(1) •2FeS-|-CuS0 4 =CuFeS 2 +FeS0 4 



For bisulphide of iron, in the presence of oxygen, the reaction may be : 



(2) 2FeS 2 +CuS0 4 +0 4 =CuFeS 2 +FeS0 4 +2S0 2 



or, if oxygen be absent, 



(3) 7CuS0 4 +8FeS 2 +8H 2 0=7CuFeS 2 +FeS0 4 +8H,S0 4 



Where the iron sulphide is pyrrhotite, intermediate between FeS and 

 FeS 2 , the reactions may be written by combining the equations (1) and (8) 

 in proper proportions. 



The chalcopyrite may also be produced by the reaction of cuprous 

 sulphate on pyrite or marcasite. In the presence of oxygen the reaction 

 is as follows: 



3FeS 2 +Cu 2 S0 4 +40=2CuFeS 2 +FeS0 4 +2S0 2 



and in the absence of oxygen : 



15FeS 2 +70u 2 S0 4 +8H 2 O=14CuFeS 2 +FeSO 4 +8H 2 SO 4 



In passing upward from the lowest level at which the chalcopvrite 

 appears, this mineral may steadily increase in quantity until it becomes an 

 important constituent, and finally the iron sulphide may become subordinate. 

 Under these circumstances bornite is likely to appear also. The produc- 



