COPPER ORES. 1159 



When the oxidation has gone far enough to produce tenorite, carbona- 

 tion and hydration may take place, and malachite be produced, according to 

 the following reaction: 



2CuO+C0 2 +H 2 0=CuC0 3 . Cu(OH), 



Or if the carbonation goes further as compared with hydration, azurite may 

 be produced, according to the following reaction: 



6CuO+4C0 2 +2H 2 0=2[2CuC0 3 .Cu(OH),] 



The product represented by any stage in this process may be regarded 

 as formed directly by a combination of two or more of the preceding 

 reactions. But in the southwestern portion of the United States it is 

 certain that the reactions frequently if not usually occur step by step, as is 

 shown by numerous specimens In a single specimen I have seen a core 

 of chalcocite surrounded by metallic copper, outside of which is cuprite; 

 beyond this lies tenorite, and beyond the tenorite hydrated carbonate. 

 While it is rather rare to find this full succession, it is easy to get specimens 

 of metallic copper interlaced or surrounded hj cuprite, the latter being 

 surrounded by at least a film of tenorite, beyond which occur the car- 

 bonates. In this connection it may be recalled that cuprite is a rather 

 abundant oxide and that tenorite is rare. The reason which may be sug- 

 gested for this is that as soon as tenorite is formed this oxide may unite 

 with carbon dioxide and water and produce malachite or azurite. This 

 process goes on almost as fast as the tenorite develops, and thus very little 

 of this oxide is found in the mines. The oxidized copper products, instead 

 of being formed from chalcocite, may be produced from any of the leaner 

 sulphides of copper; but in such cases the associated iron forms iron com- 

 pounds which may largely remain in situ as oxides or be carried away as 

 sulphates. 



The oxidized ores of copper occur extensively in arid regions. The 

 precipitation is small, but there is enough moisture underground to carry 

 on the work with the assistance of oxygen and carbon dioxide. In 

 such a region the level of ground water is far below the surface and erosion 

 is exceedingly slow, so that there is ample room and sufficient time for a large 

 amount of material to accumulate above the level of ground water. In 

 humid regions, while water is more abundant and the conditions to that 



