COLLECTIONS IN ECONOMIC GEOLOGY AND METALLURGY. 73 



C OP P E R. 

 CoppEii Minerals. 



COPPER. (MASS OOPPER.) 



Composition: Cu. Copper, frequently containing a little silver, 

 bismnth, and other metals. 



It crystallizes in the isometric system, in cubes, octahedrons, dodeca- 

 hedrous, and the more complex forms; twin crystals a^re common and 

 frequently distorted, producing very complicated crystal masses, often 

 arborescent, with branching at sixty degrees, frequently filiform, and 

 sometimes in sheets of considerable size. Its most common occurrence 

 is massive. It varies in hardness from 2.5 to 3, and its specific gravity 

 is 8.838. It has a full metallic luster and bright red color, can be 

 hio-hly polished, and is malleable and ductile. 



It is a very common mineral, being found over a wide extent of coun- 

 try. At the Lake Superior mines it is almost the only source of copper. 

 The native copper occurs here in tliree distinct forms : 



First, masses of free copper, amounting in some cases to many tons 

 of metal in a single mass, the largest ever found being in the foot wall 

 of the Minnesota Mine and weighing over four hundred tons. 



Second, amygdaloid copper, which is free copper in amygdules, 

 bunches, strings, and sheets, from microscopic size up to a few hundred 

 pounds in weight, with rarely a large mass, disseminated in a soft 

 amygdaloid trap-rock. 



Third, conglomerate copper, which is free copper mostly in strings in 

 a hard conglomerate of ferruginous quartz pebbles. 



At some localities native copper is found forming a central core, sur- 

 rounded by oxydized copper minerals. In such cases, and also at the 

 Lake Superior mines, it is evidently the original copper mineral, but in 

 some localities it has been formed by the reduction of other copper 

 minerals, and is therefore a secondary copper mineral. In the latter case 

 it is never in large amounts. To illustrate the occurrence of free copper 

 six specimens are shown : 



(1) Crystallized. Copper Falls Mine, Keweenaw County, Lake Superior, Michi- 



t'au. (44181.) . ,r- 1 • f-i^as 



(O) Mass Central Mine, Keweenaw County, Lake Superior, Michigan. (o4tSbJ.) 



(3) Aiuygdaloid. The copper occurs iu small amygdules in tlie rock; also in a 

 small vein with calcite and prehnite. Pha-uix Mine, Lake Superior, Mich.oan 



(5367.) , . , 4.- 



(4) Conglomerate. The copper occurs iu small strings, mostly in the cementing, 

 material of the conglomerate, and is quite evenly distril.ute.l through the whole 

 mass The conglomerate is made up of small sized pebbles of a dark red quartz por- 

 phyry CaluraetandlleclaMine, Calumet, Lake Superior. Michigan. (18718.) 



(5) In cupriteand malachite, which have been derived from the oxidation of the 

 metallic copper. Copper Mountain, Arizona. (33386.) 



(6) Crystallized, in decomposed siliceous miterial. Tlie copper has been derived 

 from other minerals, and is secondary. Copper Queen Mine, IMsbee, Cochise 

 County, Arizona. (50041.) 



