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University of California Publications in Geology [Vol. 10 



iron sulphate and sulphuric acid. These products having been carried 

 down by descending meteoric waters to the ground waterdevel, or the 

 zone of reduction, lost the free acid and the salt was reduced to pyrite. 

 According to Emmons, 14 pyrite oxidizes as follows : 



FeS, + 70 + HX> = FeS0 4 + H 2 S0 4 



6 FeS0 4 + 30 + 3 H 2 = 2 Fe 2 (S0 4 ) :; + 2 Fe(OH), 



Ferric sulphate then breaks down, forming ferric hydroxide and 

 sulphuric acid. However, this last step is not always completed in 

 the zone of oxidation, for the analysis of mine waters commonly 

 reveals the presence of ferric and ferrous sulphates below the zone of 

 oxidation. 



It seems probable that most of the iron sulphate is reduced to 

 sulphide before the ferric hydroxide stage is reached in the process 

 of oxidation, or else limonite would be produced, as pointed out by 

 Emmons. 15 He gives the following equations for this last process: 



Fe.,(SO,):, + 6 H 2 = 2 Fe(OH), + 3 H,S0 4 



4 Fe(OH),= 2 Fe 2 : , + 6 H 2 = 2 Fe 2 3 .3 H 2 + 3 H 2 



It seems, then, that whatever the stage in the process of oxidation 

 sulphuric acid is produced. 



According to the experiments of Allen, Crenshaw, Johnson, and 

 Larsen, 16 pyrite will not form in the presence of a strong acid solution, 

 but weak acid or neutral solutions are necessary for the formation 

 of pyrite at ordinary temperatures. They also conclude from their 

 experiments that neutral solutions and high temperatures favor the 

 formation of pyrite, while acid solutions and low temperatures favor 

 the formation of marcasite. 



In the light of these facts it seems necessary to explain a method 

 of neutralization of the acid formed in the oxidation of the pyrite 

 near the surface. When the rhyolite alters it forms kaolin and silica. 

 Both of these minerals are very abundant in the altered rock. The 

 experiments of Whitman 17 suggest the explanation of at least part 

 of the neutralization of the free acid in the sulphate solution. They 

 show that basic aluminum sulphate is formed and that pyrite is pre- 

 cipitated as a result of the reducing qualities of kaolin. It is a well 



i* Emmons, W. H., The Enrichment of Sulphide Ores, U. S. Geol. Surv., Bull, 

 no. 529, p. 48, 1913. 

 is Op. cit. 



16 Am. Jour. Sci., ser. 4, vol. 33, p. 169, 1912. 



1 7 Whitman, A. R., Vadose synthesis of pyrite, Econ. Geol., vol. 8, p. 455, 1913. 



