PRECIPITATION OF COPPER. 1101 



that such compounds are capable of reducing copper to a metallic condition 

 from its solutions, and this element is more difficult to reduce than 

 silver. That such reduction has actually taken place by the ferrous iron of 

 silicates has been maintained by Vogt for the native silver at Kongsberg, 

 Norway." 



COPPER. 



solution. — Copper in underground solutions is doubtless carried in many 

 forms, probably more largely as copper sulphate, but as a chloride to an 

 important extent. It may be transported also as copper carbonate in 

 carbonate solutions carrying excess of carbonic acid. Copper as a sul- 

 phide is soluble in the alkaline sulphides, especially acid sodium sulphide 

 (NaHS), and is soluble in considerable quantity in alkali sulpharsenates 

 and sulphantimonates. 



precipitation. — Copper as a metallic compound is precipitated by metallic 

 iron and by ferrous compounds. The precipitation by metallic iron is 

 probably unimportant, but that by ferrous compounds is of great conse- 

 quence. Copper occurs most extensively in the metallic form as precipitate 

 of a first concentration in the Lake Superior region. Many years ago 

 JPumpelly 6 noted the fact that in this region there is an intimate connection 

 between the native copper and the iron-bearing- minerals carrying- ferrous 

 iron. On this subject Pumpelly says: 



Throughout its deposits the copper exhibits a decidedly intimate connection 

 with delessite, epidote, and green-earth silicates, containing a considerable percentage 

 of peroxide of iron as a more or less essential constituent; while among the other 

 silicates, viz, analcite, laumontite, datolite, prehnite, only the last named, which 

 alone seems subject to a considerable replacement of its alumina by ferric oxide, is 

 especially favored by copper. This association is so invariable * * * that there 

 exists a close genetic relation between the metallic state of the copper and the ferric 

 condition of the iron oxide in the associated silicates; that the higher oxidation of 

 the iron was effected through the reduction of the oxide of copper and at the expense 

 of the oxygen of the latter. ° 



* * * * -X- * * 



Now, may we not consider the presence of iron in prehnite generally to be due 

 to a beginning change, and the deposition of native copper in the Lake Superior 

 prehnites to be partially or wholly correlated with the higher oxidation of the iron? rf 



"Vogt, J. H. L., Oeber die Bildung des gediegenen Silbers, etc.: Zeitschr. f. prak. Geol., April, 

 1899, p. 118. 



6 Pumpelly, Raphael, The paragenesis and derivation of copper and its associates on Lake Superior: 

 Am. Jour. Sci., 3d series, vol. 2, 1871, pp. 353-354. 



e Pumpelly, cit, p. 353. 



<* Pumpelly, cit., p. 351. 



