138 ECONOMIC GEOLOGY 



L. Michel has reproduced azurite by leaving a solution of copper 

 nitrate in contact with a crystal of calcite for several years. The 

 carbonates have also been observed in the patina of ancient 

 bronzes. 



Nantokite, the cuprous chloride, is rather rare, but atacamite, 

 especially in Chili, is important. According to J. D. Dana, it 

 may be formed by the oxidation ofnantokite. F. W. Clarke 

 states, that it has been observed upon ancient coins and bronzes. 

 The fumes of HC1 acting upon tenorite has produced a hydrous 

 chloride that is not far from atacamite in composition and cor- 

 responds very closely to the hydrous chloride found at Mount 

 Vesuvius as a product of volcanic emanation during the eruption 

 of 1872. 



The two oxides, cuprite and tenorite, are always of secondary 

 origin. They may be formed by the oxidation and reduction 

 of other copper minerals. Cuprite is far the more important 

 species. It has been observed as an incrustation upon ancient 

 objects of copper or bronze. 



Of the two silicates of copper, dioptase and chrysocolla, the 

 first is rare but the second becomes an important copper ore in 

 certain localities. According to F. W. Clarke, chrysocolla is 

 formed by the action of silica-bearing waters upon soluble com- 

 pounds of copper. Also that the mineral may possibly be pro- 

 duced during the processes of secondary enrichment. 



H. Ries gives the following classification of the origin of cop- 

 per ores: 



(1) Magmatic segregations. No workable deposits of mag- 

 niatic origin are known in the United States. 



(2) Contact deposits in crystalline limestone along contact 

 with igneous rocks. The copper has been introduced by vapors 

 from the igneous rocks. 



(3) Deposits formed by ascending circulating hot solutions, 

 depositing ores in fissures, pores, spaces of brecciation, and by 

 replacement of rock. 



(4) Lens-shaped deposits in crystalline schists representing 

 a concentration of material from a disseminated condition in 

 the surrounding rocks. 



The last two are by far the most important, but even here the 

 ores have been enriched by oxidation and the transference of 

 soluble compounds of copper to lower levels to be reprecipitated 

 by limestone and the sulphides of copper and iron. 



