300 MINERALOGY 



veins is usually calcite, siderite, barite, or fluorite. Such deposits 

 have been formed by the precipitation of lead, carried in solution 

 as the carbonate, sulphate, or even the sulphide (as galena is depos- 

 ited by some thermal springs), by water, percolating down from the 

 superficial areas, which tends to dissolve the oxidized ores at the 

 surface and again deposit them as sulphides at lower levels; de- 

 posits formed in this way are apt to become poor at depths. Galena 

 may be deposited by ascending solutions, in which case the supply 

 is brought nearer the surface from depths; also lead sulphide is 

 volatile without decomposition when heated in an atmosphere of 

 many gases, and on cooling recrystallizes as cubes ; galena of this 

 nature has been observed in the lavas of Vesuvius. 



Galena is very widely distributed ; of the 24 states commercially 

 producing lead ore in 1909, Missouri, Idaho, Utah, and Colorado 

 produced more than three quarters of the 350 thousand tons of 

 that year. The deposits of Missouri, Southern Illinois, Wisconsin, 

 and the Mississippi valley generally are found in limestone and 

 dolomite. Those of Idaho and Colorado are associated with 

 igneous rocks as well as dolomite ; others, associated in veins with 

 gold, silver, and copper ores, are of a complex nature, and the 

 usual gangue mineral is quartz. 



Artificial galena crystals have been produced by the volatiliza- 

 tion of precipitated sulphide, and by the precipitation of a solution 

 of lead nitrate containing free nitric acid. Octahedral crystals are 

 formed when one part of lead sulphide is fused with six parts each 

 of potash and sulphur. 



CHALCOCITE 



Chalcocite. Copper glance ; Cu 2 S ; Cuprous sulphide; Cu = 

 79.8, S = 20.2 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : 

 c = .5822 : 1 : .9701 ; 100 A 110 = 30 12', 001 A 101 = 59 2', 001 A 

 011=44 8'; Common forms, a (100), b (010), c (001), m(110); 

 Twinning plane, 110 and 032; Cleavage, m distinct; Brittle, frac- 

 ture conchoidal ; H. = 2.5-3 ; G. = 5.5-5.8 ; Color and streak, 

 dark lead-gray, tarnishes to blue on exposure; Luster, metallic; 

 Opaque. 



B.B. On coal fuses easily and boils with spirting. In O. F. 

 yields S0 2 fumes and odor. Powdered and roasted without fusing, 

 then heated in R. F. yields malleable copper, also shows copper 

 with the fluxes. 



