298 



MINERALOGY 



General description. Crystals are octahedrons, rhombic 

 dodecahedrons, cubes, or combinations of these forms ; all forms of 

 the type have been observed on argentite, but the other four forms 

 are rare. Fresh surfaces are bright and shiny, but like all silver 

 minerals become dark on exposure. Its occurrence is more often 



dendritic, granular, 

 or disseminated. At 

 the Comstock Lode, 

 Nevada, it alone 

 constitutes a work- 

 able silver ore ; also 

 at Port Arthur, 

 Lake Superior. It 

 occurs in small 

 amounts in most 

 silver mines, and in 

 the cobalt region, 

 Canada, in consid- 

 erable quantities. 

 It is usually as- 



FIG. 397. Argentite from Freiberg, Saxony. SOciated With steph- 



anite, galena, py- 



rite, cobalt, and nickel ores, gold, and silver, the latter being a 

 secondary product reduced from the sulphide. Argentite and 

 galena are isomorphous, and the latter, especially the fine granular 

 varieties, contains small amounts of silver, and the smelting of 

 galena yields each year a considerable amount of the world's pro- 

 duction of silver. 



Silver in solution, either as the sulphate, carbonate, or nitrate, is 

 precipitated by the natural sulphides as pyrite, chalcopyrite, born- 

 ite, or galena, a reaction which without doubt plays an important 

 part in the secondary enrichment of silver ores. 



GALENA 



Galenite. Galena ; Lead glance ; PbS, lead sulphide ; Pb = 

 86.6, S = 13.4 ; Isometric ; Type, Ditesseral Central ; Common 

 forms, a (100), o (111), d (110) ; Twinning plane, 111, both contact 

 and interpenetrating ; Cleavage cubic, perfect ; Brittle ; Fracture, 

 subconchoidal ; H. = 2.5-2.75; G. = 7.4-7.6; Color and streak, 

 lead-gray ; Opaque. 



