SILVER ORES. 1167 



strongly to sulphur. Ordinarily it is not sufficiently abundant to constitute 

 the main mass of an ore deposit. Since silver holds so strongly to its sul- 

 phur, the silver salts are likely to be found, in the case of lead-zinc-iron 

 compounds, most, abundantly with the lead, less abundantly with the 

 zinc, and least abundantly with the iron ; and in the case of the copper-iron 

 compounds, most abundantly with the copper and less abundantly with the 

 iron. Native silver and the rich silver compounds — cerargvrite (AgCl), 

 argentite (Ag 2 S), proustite (Ag 3 AsS 3 ), pyrargyrite (Ag 3 SbS 3 ), and stephanite 

 (Ag 5 SbS 4 ) — may be found abundantly in the upper parts of mines, but 

 frequently decrease in amount in passing from the surface to the zone of 

 sulphides, and at sufficient depth in this zone may entirely disappear, 

 the products being wholly argentiferous lead, zinc, copper, and iron sul- 

 phides. In many instances independent silver minerals do not occur at all, 

 all of the silver being in the lead, zinc, copper, and iron compounds. In 

 the argentiferous lead, zinc, and iron deposits, the ores are likely to contain 

 less silver as they become poorer in lead and zinc. As the argentiferous 

 copper deposits become poorer in copper with depth, the silver also ordi- 

 narily decreases in amount. Therefore the plumbiferous and zinciferous 

 pyrites and cupriferous pyrites deep in the mines ordinarily contain less 

 silver than the deposits above, which are richer in the base metals. 



In the belt of weathering above the level of ground water native silver 

 and silver chloride, cerargyrite, are likely to be found. The native silver 

 is produced by the direct oxidation of the sulphur, antimony, or arsenic 

 united with the sulphur. For instance, if the silver compound be Ag,S, 

 metallic silver is produced by simple oxidation of the sulphur as follows: 



•Ag 2 S+0 2 =2Ag+S0 2 , 



This reaction is very common, because silver is very slowly oxidized. But 

 argentite may be directly oxidized to sulphate, according to the following 

 reaction : 



Ag 2 S+20 2 =Ag 2 S0 4 . 



This compound is a very readily soluble one, and therefore does not remain 

 in situ, but if chlorides are present they react upon this salt and produce 

 cerargyrite, according to the following reaction: 



Ag 2 S0 4 +2NaCl=2AgCf+Na 2 S0 4 . 



