668 ELECTRICAL METHODS [Chap. 10 



with metallic objects, from chemical differences of the materials coming 

 in contact with solutions, and (in exceptional cases) from electrofiltration. 

 These phenomena were discussed in detail in section ii. In eq. (10-la) a 

 relation was given between the potential and the ratio of the solution 

 pressures of two different substances in electrolytic solutions of different 

 concentration; eq. (10-3) stated the conditions responsible for electro- 

 filtration potentials. 



In the spontaneous polarization of ore bodies, the potentials arising 

 from differences in solutions and from differences in materials appear to 

 be related to one another as cause and effect. Differences in the solu- 

 tions contacting different portions of an ore body (which probably con- 

 sisted at first of the same material throughout) have brought about, largely 

 through the medium of oxidation, a condition of unbalance which in turn 

 is responsible for the potentials observed. Near the surface, the atmos- 

 pheric agencies form an aerated zone rich in oxygen, while at the lower 

 portion of the ore body the solutions are either poor in oxygen or are 

 even of a reducing nature. The oxidation of the pyrite at the top proceeds 

 in accordance with the relation 



FeSz + 70 + H2O = FeS04 + H2SO4 . 



The ferrous sulfate is readily transformed to ferric sulfate since 



2FeS04 + H2SO4 + = Fe2(S04)3 + H2O. 



Ferric sulfate in turn changes by hydrolysis to hydrous ferric oxide 

 (limonite) : 



Fe2(S04)3 + 6H2O = Fe203-3H20 + 3H2SO4 . 



The sulfuric acid formed in this process is mostly neutralized by the car- 

 bonates (calcite, limestones, and the like) in the adjacent formations. The 

 last relation explains the formation of the gossan in the zone of oxidation. 

 Minerals contained in it have a lower solution pressure than the unaltered 

 ones in the lower end of the ore body. Hence, in accordance with eq. 

 (10-1 c), a potential difference is set up between the upper end and the 

 lower end of the ore body. A current is then flowing downward in the ore 

 body as well as around it outside; the zones of ingress of this current are 

 above the top of the body and are indicated by a negative potential center 

 (see Fig. 10-21). 



Although pyrite shows the strongest spontaneous polarization, it is not 

 the only mineral exhibiting this property. Activity has been observed 

 also on pyrrhotite, magnetite, cobalt ore, graphite, and anthrazite; on forma- 

 tion boundaries; and in connection with the corrosion of iron pipes. 



