

Till \ \TIYi; U.I.M! 



289 



General description. Copper cry-talli/es in cubes, octahedron-. 

 :ui(l tetrahexahedrons; oilier form- are rare. T\vin> after the spinel 

 law are not uncommon. It occurs more often in distorted form-, 

 or in arborescent, rcticular, dendritic, filiform, and irregular masses. 

 Pseudomorphs after other copper minerals, as cuprite, malachite, 



Fio. 394. Native Copper and Quartz from Lake Superior. 



and azurite, are often formed by double decomposition and reduc- 

 tion. Native copper is usually coated with a coat of oxides or car- 

 bonates ; many masses of cuprite still contain as a central nucleus 

 some of the metallic copper from which they were formed. 



Native copper occurs as a secondary product, either formed by 

 precipitation from solution, or by the chemical action of reducing 

 agents upon minerals containing copper. Percolating ground 

 waters dissolve copper and especially under heat and pressure ; even 

 distilled water will dissolve copper under these conditions, leaching 

 out the original copper content of the igneous rocks and trans- 

 porting it to points, as veins and cavities, where it may be pre- 

 cipitated by contact or by intermingling with other solutions con- 

 taining a reducing agent, as ferrous iron, which is capable ; either 

 as oxide, sulphate, silicate, or carbonate, of precipitating copper 

 from its solutions. 



The large deposits of the Lake Superior region have probably 

 been formed in this way. Here metallic copper, generally in small 

 part ides, is contained in the amygdaloid cavities of a conglomerate. 

 One mass of 400 tons was found in the Minnesota mine. .Origi- 

 nally the copper must have been contained in the adjacent igneous 



