90 ECONOMIC GEOLOGY 



as threads or plates through the reduction of other ores of silver, 

 or it may occur deposited as a film over other minerals or rocks, 

 as copper or carboniferous shale. The other ores of silver are as 

 follows : 



Argentite, Ag 2 S, containing 87.1 per cent, silver, the only black 

 and sectile sulphide of silver, the most unstable of all the sul- 

 phides of the commoner metals; pyrargyrite, 3Ag2S,Sb2Ss, con- 

 taining 59.9 per cent, silver; stephanite, 5Ag2S,Sb2Ss, containing 

 68.5 per cent, silver; polybasite, 9Ag2S,Sb2Ss, containing 75.6 

 per cent, silver; proustite, 3Ag2S,As 2 S3, containing 65.4 per cent, 

 silver. 



Silver occurs also in association with copper minerals; as tetra- 

 hedrite, 4Cu2S,Sb2S3; although no silver is present in the for- 

 mula, samples of this ore have given 500 lb. of silver to the ton; 

 tennantite, 4Cu2S,As2Ss, sometimes bears silver. 



Silver occurs abundantly in argentiferous galenite. All gale- 

 nite is more or less argentiferous, but the finely crystalline variety 

 contains more silver than the coarse mineral. Silver occurs with 

 tellurium in hessite, Ag 2 Te, containing 63.3 per cent, of silver; in 

 petzite, displacing gold, for petzite is a telluride of gold (Ag,Au)ir 

 Te. It occurs in combination with selenium in naumanite, PbSe,- 

 13Ag 2 Se. and also as the selenide alone, Ag2Se. It occurs with 

 bismuth, copper and mercury, but perhaps more important as the 

 amalgam. This implies varying combinations of silver and mer- 

 cury. Instead of the direct union in the line of atomic weights, 

 seems to unite with mercury in almost all proportions. The 

 amalgam contains 27.5 per cent, to 95.8 per cent, of silver. 



Silver occurs again in combination with the halogens. The 

 most important haloids of silver are: Cerargyrite, AgCl, with 75.3 

 per cent, of silver; embolite, 3AgCl,AgBr, containing 66.9 per 

 cent, silver; bromyrite, AgBr, containing 57.4 per cent, of silver; 

 iodyrite, Agl, containing 45.2 per cent, silver. All of these haloid 

 minerals are soft and sectile. 



Silver is very widely distributed in nature. It is produced by 

 practically all countries of the world, although many of them pro- 

 duce only a small quanity of the metal. It has been observed as 

 a natural constituent of igneous rocks. It has been detected in 

 common salt, in sea weed, in sea water, and in corals. In most 

 cases native silver is of secondary origin, the metal being derived 

 from the reduction of the sulphides and antimonides of silver. 

 Organic matter is a common reducing agent effecting the precipi- 



