72 spencer: chalcocite deposition 



depth, includes oxidation of sulfides (^primary only) by contact 

 with cupric sulfate. Or, if the solution is considered rather than 

 the minerals with which it comes into contact, the three groups 

 of reactions involve successively depletion of free oxygen; reduc- 

 tion of ferric to ferrous sulfate; and finally the decomposition 

 of cupric sulfate derived from previous reactions. This last 

 reaction is, as a whole, one of double decomposition in which 

 soluble ferrous sulfate and insoluble cuprous sulfide are formed 

 at the expense of cupric sulfate and pyrite or chalcopyrite. 



Several of the equations which may be written to represent 

 tentatively the three sets of reactions outlined above are current 

 in the hterature treating of sulfide oxidation and enrichment. 

 Others have been supplied by the writer to complete what may 

 be called natural or obvious sequences. It is believed that no 

 one of the equations presented contemplates the concomitance of 

 bodies that are chemically incompatible. 



Group I. Intermediate equations and final equation repre- 

 senting reactions in the upper zone of complete oxidation : 



FeSo + 60 = FeS04 + SOo (1) 



FeS. + HoO + 70 = FeS04 + H2SO4 (2) 



2FeS04 + H0SO4 + O = Fe2(S04)3 + HoO . (3) 



2FeS2 + H2O + 150 = Fe2rS04)3 + H0SO4 (4)6 



Group II. Intermediate equations and final equation indi- 

 cating oxidation of pyrite by ferric sulfate. This salt is one of 

 the soluble products indicated by equation(4) : 



FeS2 + Fe2 (804)3 = 3FeS04 + 2S (5) 



S + Fe2(S04)3 = 2FeS04 + 2SO2 (6) 



SO2 + 2H2O + Fe2(S04)3 = 2FeS04 + 2H2SO4 (7) 



FeS2 + 8H2O + 7Fe2(S04)3 = 15FeS04 + 8H2SO4 (8)^ 



To this group of equations may be added one representing the 

 decomposition of chalcocite and the simultaneous reduction of 

 ferric sulfate: 



« Derived from equations (2) and (3) by substitution. Where chalcopyrite 

 is present a corresponding equation will indicate the formation of CUSO4 instead 

 of H2SO4. 



7 Derived from the foregoing equations. Note that (8) is strictly comparable 

 with (4). 



