188 W. IT. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



Doelter also states that by the reaction of sulphuretted hydrogen on a 

 solution of silver chloride and antimoniate of potash in presence of car- 

 bonate of soda i n closed tubes at 80 to 250 degrees,miargyrite (Ag 2 S, Sb 2 S s ), 

 pyrargyrite (3Ag a S, Sb 2 S 3 ), and stephanite (5Ag 2 S, Sb,S. t ) were formed* 

 Silver sulphide, which is soluble in pure water, would be reduced by 

 H 2 S, with precipitation of the sulphide, and the sulpharsenate and sul- 

 phantitnonates of silver may be formed in a similar way. Zinc sulphate 

 is reduced to blende in the same manner. Doelter says, in writing of 

 the artificial production of these minerals, that dilute solutions are more 

 favorable than concentrated. f Sulphate of zinc is at ordinary tempera- 

 ture almost as soluble as sulphate of copper, even in the presence of 

 carbon dioxide.'! Bischof states that sulphates are precipitated by H 2 S, 

 resulting in dark colored masses, but when precipitated from a dual solu- 

 tion to which H 2 S has been added gradually there forms on the surface 

 of the liquid a thin film with the metallic luster of galena. Putting this 

 in a filter and washing, one finds after partially drying that there are 

 small particles with metallic luster.§ Although the sulphides prepared 

 by precipitation from solutions of metallic salts are mostly amorphous 

 masses without luster, they may be obtained artificially with metallic 

 luster by the slow action of H 2 S upon very weak solutions, or precisely 

 the conditions which prevail in nature. *J[ 



Silver brought into solution by ferric sulphate acting on argentiferous 

 galena, blende, or pyrite probably forms silver sulphate. If there is an 

 excess of ferrous sulphate thus formed and, as no more easily attacked 

 substance is available, the ferrous sulphate and the silver sulphate will 

 then form native silver, viz : Ag 2 S0 4 + 2FeS0 4 = Ag 2 -f Fe 3 (S0 4 ) 3 , and 

 hence in many cases whatever silver is found in the vein remains in the 

 gossan and is not leached out and redeposited at lower levels. Where 

 other conditions prevail redeposition occurs. 



R. C. Hills has suggested that the silver and the gold carried down- 

 ward by waters holding ferrous and ferric sulphates has been precipitated 

 through the decomposition of the sulphates by feldspar.** 



As noted by Vogt and other observers at Konigsberg, native silver is 

 abundant, as an alteration product of silver glance, below the zone of 

 oxidation. He suggests that where native silver occurs in minute cracks 

 in the country rock at such depths it may result from the reducing ac- 



* Braun's Chemischc Mineralogie, p. 2(57. 



|C. Doelter: Allgemeine Chemisehe Mineralogie, Leipzig, 1890, chap, iv, p. 105 et seq. 



% De Launay : L'argent, p. 70. 



g Braun's Chemisehe Mineralogie, p. 2G0. 



fl Bishop : English Trans., voi. iii, p. 451. 



** Proc, Colorado Scientific Society, vol. i, p. 32. 



