252 MINERALOGY 



In the percolating ground waters many sulphides are soluble, 

 and especially so as these waters are acid in some cases and alkaline 

 in others. These solutions finally, in their descent, reach the area 

 where the pores, fissures and cavities of the rocks are completely 

 filled ; and the solution joins that reservoir of ever flowing water 

 termed the ground water; where through the action of diffusion 

 and flowage, solutions of various substances are mixed as reagents 

 in a beaker, with the resulting precipitations. Here, however, the 

 walls of the containing cavities are active agents and by the inter- 

 change of their elements enter the reactions as important factors in 

 the process of chemical replacement, or metasomasis. 



The products of the oxidation of pyrite are varied, according to 

 the conditions and the amount of oxygen available. 



FeS 2 + 60 = FeSO 4 + SC>2, which yields a solution of ferrous 

 sulphate and an acid solution. 2 FeS 2 + 14 O = Fe 2 (S0 4 ) 3 + SO 2 , 

 yielding a solution of ferric sulphate and an acid solution. This 

 solution may interact with more pyrite, FeS 2 + Fe 2 (S0 4 ) 3 = 

 3 Fe(S0 4 ) + 2 S, yielding ferrous sulphate and free sulphur, also 

 hydrogen sulphide and sulphites may be produced. Ferrous sul- 

 phate in solution is a powerful reducing agent and results in many 

 cases in the precipitation of metals, as silver and copper, and as 

 oxides, as cuprite. Many deposits of iron ore have resulted from 

 the interaction of solutions of iron with carbonates, in which the 

 iron has replaced the calcium of limestones and shells. CaCO 3 

 + FeSO 4 = FeCO s + CaSO 4 , when siderite and gypsum are 

 formed. 



It is by means of a similar replacement or interaction of solutions 

 containing sulphates resulting from the oxidation of sulphides near 

 the surface that such carbonates as smithsonite, rhodochrosite, 

 witherite, cerussite, azurite, and malachite are formed ; these min- 

 erals may also be formed from solutions of carbonates. The more 

 soluble sulphate gypsum may be replaced by barium, strontium, 

 or lead sulphates. The minerals barite, celestite, and anglesite 

 are often precipitated on the walls of fissures and veins as the result 

 of the intermingling of solutions. Minerals formed by chemical 

 replacement occur usually as lens-shaped masses embedded in the 

 rock which has been the cause of their formation. Large areas of 

 limestone have been changed to dolomite by the replacement of 

 calcium by magnesium. Most of the galena and sphalerite 

 deposits of the Mississippi Valley have resulted from replace- 

 ments in limestones. 



