1168 A TREATISE ON METAMORPHISM. 



It is well known that in arid regions, native silver and cerargyrite are 

 especially abundant in the belt of weathering. These compounds are 

 produced in the same manner as native copper and its oxidized products, 

 with, however, one additional factor. In arid regions the alkaline salts, 

 such as sodium chloride and similar compounds, are segregated. (See 

 pp. 542-543.) Therefore in such regions the ground waters are particu- 

 larly likely to contain chlorides in solution which react upon the sulphates 

 and thus produce the chloride of silver. Water is not sufficiently abundant 

 to dissolve the chloride and hence cerargyrite accumulates. 



Not all the silver which is oxidized to the soluble form of sulphate is 

 precipitated as chloride, and even the chloride is somewhat readily soluble, 

 so that if the silver be thus transformed a part of it may be held in solution. 

 Therefore silver sulphate, and some silver chloride passes downward from 

 the belt of weathering to the belt of sulphides. Since silver holds to its 

 sulphur more tenaciously than any of the base metals with which it is 

 associated, the first of the sulphides of these metals which is met in mass is 

 reacted on by such silver salts. Where they may come into contact with 

 chalcocite, for instance, argentite is precipitated according to the following 

 reactions : 



Cu,S+Ag,S0 4 =Ag. 2 S+Cu,SO, l . 

 Cu 2 S+2AgCl=Ag 2 S+2CuCl. 



While it would be easy to write equations representing the reactions 

 of the soluble silver salts upon the various sulphides of the base metals, 

 producing silver sulphurets, it hardly seems worth while to do so until 

 experimental work has determined the reactions which actually take 

 place in the different cases. For the present purpose it is necessary only 

 to understand that when soluble silver salts are brought into contact with 

 base sulphides the silver certainly will be precipitated as independent silver 

 sulphides or as rich sulphides in which the silver replaces a part of the base 

 metals. 



As an instance in which silver is concentrated in a base sulphide rather 

 than in the carbonate of the base metal may be mentioned the Leadville 

 ores. Here, according to Emmons, the g-alena is much richer in silver than 

 the associated cerussite. Not only is this so in general, but there are some 

 very interesting special cases. For instance, five assays of galena nodules 

 which had carbonate crusts showed "six times as much silver in the galena 



