DEPOSITION IN ENRICHMENT ZONE 187 



Though the reduction of copper sulphate to chalcocite does take place 

 to a limited extent in the oxidized zone, where the glance is generally 

 soon changed to oxide, the great bodies of glance and bornite found be- 

 neath the water level are probably not formed in this way ; they result 

 from reactions which do not involve the presence of free oxygen, but 

 depend on a reduction of the copper sulphide by pyrite from the sul- 

 phate solution. The experiments of Brown show that both pyrite and 

 marcasite are decomposed by copper sulphate, and copper sulphide is 

 formed.* According to the modern theory of solutions, cupric sul- 

 phate in solution undergoes partial electrolytic dissociation, forming 



+ = 



Cu (OH) 2 and H 2 S0 4 , the latter being ionized to 2 H and S0 4 , while 



+ = 



part of the CuS0 4 is ionized to Cu and S0 4 . The writer has added H 2 S 



to a solution of pure copper sulphate and obtained a precipitate of copper 

 sulphide, and this is greatly increased in amount if some free H 2 S0 4 be 

 added. It has been asserted that pyrite when powdered and treated 

 with very dilute sulphuric acid will give off sulphuretted hydrogen. 

 Qualitative tests made for me by Dr H. N. Stokes in the Survey labora- 

 tory upon perfectly pure pyrite failed to confirm this statement. If, 

 however, pyrrhotite is added, hydrogen sulphide is at once evolved. It 

 was found, however, that the pyrite from the Butte quartz veins, carry- 

 ing mere traces of copper, when powdered and treated with dilute sul- 

 phuric acid gave off sulphuretted hydrogen, and that the purest chal- 

 copyrite of Ducktown, Tennessee, and cupriferous pyrite of Gold Hill, 

 North Carolina, behaved in the same way. It is therefore certain that 

 the pyrite of these deposits is capable of precipitating copper sulphide 

 from down seeping waters holding copper sulphate and sulphuric acid. 

 This reaction might be expressed as4CuS0 4 4- 3FeS 2 + 4H 2 = 2Cu 2 S + 

 3FeS0 4 + 3H 2 S0 4 4- H 2 S, and for pyrrhotite, 2CuS0 4 + 2FeS = Cu 2 S + 

 2FeS0 4 + S, while for chalcopyrite 2CuS0 4 + Cu 2 SFe 2 S 3 + H 2 S0 4 = 

 2Cu 2 S -j- 3FeS0 4 + H 2 S. The hydrogen sulphide would, of course, at 

 once attack any Cu(OH) 2 present in the water and form copper sul- 

 phide. In these reactions the influence of FeS0 4 has for simplicity been 

 left out. The H 2 S formed by the decomposition of the pyrite will attack 

 the metallic sulphates present and form sulphides, as shown by Doelter.f 



*Proc. Amer. Philos. Soc, vol. xxxiii, May 18, 1894. 



fThis investigator has made sulphides by the method of Senarmont, with the difference that he 

 made them at temperatures below 100 degrees, and the time was extended over several days or 

 weeks. As a starting point he frequently used minerals which contained one constituent of the 

 product which was to be prepared. Thus small cubes of galena were formed from cerussite which 

 was treated several days in a closed glass tube at a temperature of 80 to 90 degrees. In the same 

 manner crystals of pyrite were formed from siderite, magnetite, or hematite ; from cuprite, chal- 

 cocite. From malachite crystals of covellite were formed. (Translated from Chemische Min- 

 eralogie, Reinhard Braun, Leipzig, 189G, p. 26G.) 



XXVII— Bui,t,. Geot,. Soc. Am., Vol. 11, 1899 



