49 



Mr. S. H. Eininens luis tabulated the following steps in the oxidation 

 of pyrite.* Part of the snlphin- of the pyrite is converted by the 

 oxygen and the moisture of the atmosphere into snlphuric acid, leaving 

 a residue of iron monosnlphide. This is then attacked by the snlphuric 

 acid and ferrous sulphate residts, while at the same time sulphuretted 

 hydrogen is evolved. The reactions are as follows :■^ 



(2.) Fe S, + O3 -f H.O = Fe S - HjS Oi- 

 (2.) Fe S - H., S 0, = Fe S O, - H.S. 



If the sulphin-etted hydrogen conies in contact — as it naturally must 

 in passing through the rock — with oxydizing ])yrite, and if, as Emnicns 

 holds, sulphurous anhydrite (S 0.,) is formed, together with the sulphuric 

 acid, the hydrogen sulphide will react with the sulphurous anhydrite 

 and form water and free sulphur. The reactions would be tabulated 

 thus : 



a. Fe S. + O, = Fe S -^ S O.,. 



b. S O. + 2 H,S = 2 H.O - 3 S. 



or, as given by Emmens : 



(5.) Fe S,, -:- O, - 2 H.S = Fe S - 2 H,0 - 3 S. 



This sulphur may be in part deposited, and in part again oxidized to 

 sulphuric acid, thus :i 



(4.) S - O3 + H.O = H.,S 0^ 

 this latter again attacking the monosnlphide (Fe S). 



The third and fourth reactions probably do not take place in these 

 shales, the hydrogen stilphide being directly decomposed by the atmos- 

 lihere, with the formation of sulphur and water, the former being 

 deposited where the oxidation takes place. Thus: 



2 H.S -r- 0, = 2 H,0 - S... 



The ferrous stilphate will absoi'b oxygen, and sulphuric acid, if the 

 latter is in excess, and form ferric sul])hate, according to the following 

 reactions :S 



(5.) 2 Fe vS Oi -^ O ^ H,S O, - Fc, (S OJ 3 ^ H,,0 



which would be the final result of the oxydation. But if the amount of 

 sulphuric acid is insufficient, or if the ferrous sulphate is carried in 

 solution and spread over the surface of the shales, it will oxidize in part 

 to ferric hydrate or limonite, which stains the shales. The reaction, 

 according to Emmens, is : 



Fe S 0^ -r 6 O. + H2O = 4 Fe,,(S O,);, - 2 Fe./);, . H.O. 



'Stephen H. Eninicns: "The Cheinistrv of Gossau," I-;iijL;inceiiiiK ami Mining 

 Jovinial, Dec. 17, 1892, p. 5S2. 

 ttKiiimeiis, U)c. cit. 

 gl-oc. cit. 



