1152 A TREATISE ON METAMORPHISM. 



same thing may be seen in Wisconsin, only that the matrix holding the 

 partly dissolved sphalerite crystals or casts is limestone instead of chert. 



The sphalerite oxidized to sulphate is carried downward. Zinc holds 

 sulphur less strongly than lead, but much more strongly than iron. There- 

 fore, the sulphate is reduced to sulphide below the galena, the reactions 

 being similar to those producing the galena. They may be written as 

 follows: 



or, 



ZnSO i +FeS=ZnS+FeS0 4 , 

 ZnCOj+FeS=ZnS+FeC0 3 , 



ZnS0 4 +FeS 2 +0 2 =ZnS+FeS0 4 +S0 2 , 

 ZnC0 3 +FeS 2 +0 2 =ZnS+FeC0 3 +S0 2 



While these, and especially the last two, are regarded as probable 

 reactions, Stokes has shown that at temperatures from 100° to 180° C. 

 precipitation may take place without the presence of oxygen, thus : 



14 ZnC0 3 +8 FeS 2 +Na 2 C0 3 =14 ZnS+4 Fe,0 3 +Na 2 S 2 3 +15 C0 2 



Under the conditions accompanying this reaction, calcium carbonate 

 may be substituted for sodium carbonate, but the reaction is slower. 

 While it has not been shown by experiment, it is probable that, in analogy 

 with the reaction of copper sulphate and lead sulphate, zinc sulphate 

 may at these higher temperatures react upon FeS 2 as follows: 



7 ZnS0 4 +4 FeS 2 4-4 H 2 0=7 ZnS+4 FeS0 4 +4 H 2 S0 4 



As confirmatory of the deposition of secondary sphalerite by the reac- 

 tions above written or by other reactions which produce the same result, 

 Bain has found in the Joplin district a of Missouri at various places ruby- 

 colored crystals of sphalerite superimposed upon the main mass of sphalerite 

 attributed to the first concentration. 



Marcasite and pyrke. — At a certain depth in the opening's below the level of 

 ground water nearly all of the salts of lead and zinc descending from the 

 belt of weathering would be precipitated by reactions between them and 

 the iron sulphide, as above explained. In veins in which the first concen- 

 tration extends to a depth greater than that to which downward-moving 

 waters are effective, only the sulphurets of the first concentration would be 



a Bain, H. F., Van Hise, C. R., and Adams, Geo. I., Preliminary report on the lead and zinc 

 deposits of the Ozark region: Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2, 1901, p. 161. 



