SULPHIDES, ARSENIDES, ANTIMONIDI .> :;n| 



General description. Crystals are tabular parallel to tin- 

 combinations of the base, prism, and pinacoid with a pyramid :ui<l 

 dome ; as the prism angle is 119 35', these combinations are pseudo- 

 hexagonal in symmetry. This is even more striking when twinnec 1 ; 

 such crystals are found at Bristol, Connecticut, and in Cornwall, 

 England. The base is often striated parallel to the edge c/d. 

 Massive, granular, or disseminated chalcocite is more common than 

 tin- crystalline. Specimens are often coated with a black crust of 

 melaconite or of the green and blue carbonates, and less often with 

 the blue sulphide, covellite ; all of which are alteration products of 

 chalcocite. 



Stromeyerite (CuAg) 2 S, as it is variable in composition, is prob- 

 ably a mixture of acanthite, Ag 2 S, the orthorhombic sulphide of 

 silver, and chalcocite, which in many localities contains silver. 



Chalcocite is a mineral of secondary origin associated with other 

 copper ores, with arsenopyrite, tetrahedrite, sphalerite, pyrite, and 

 galena, in veins, joints, lenses, etc., in which the gangue mineral is 

 principally quartz. At Butte, Montana, probably the largest cop- 

 per camp of the world, the sulphide ore is 50 per cent, chalcocite. 

 It is also an important mineral at Bingham Cafion, Utah, asso- 

 ciated with pyrite and chalcopyrite, both of which are enriched with 

 chalcocite. Chalcocite occurs more or less abundantly in all cop- 

 per deposits, in the zone of secondary enrichment, where it has 

 been precipitated from the descending solutions, by contact with 

 pyrite, sphalerite, galena, or sulphides of arsenic, etc. 



Artificial chalcocite may be produced by heating a solution of 

 cuprous sulphide in a sealed tube with a solution of ammonium 

 sulphocyanate. 



SPHALERITE 



Sphalerite. Zinc blende ; black jack, ZnS, zinc sulphide; Zn = 

 67, S = 33 ; Isometric ; Type, Ditesseral Polar ; Common forms, 

 o (111), d (110), a (100); Twinning plane, 111; Cleavage, dodeca- 

 hedral, perfect ; Brittle, fracture conchoidal ; H. = 3.5-4 ; G. = 

 3.9-4.1 ; Color, shades of yellow, brown to black, rarely red, 

 green, or white; Streak, pale to colorless; Luster, adamantine; 

 Transparent to opaque ; n. = 2.369 ; Pyroelectric and polar in the 

 direction of the trigonal .axes. 



B.B. Infusible or nearly so ; powdered and reduced on coal 

 with soda yields a white zinc oxide coat, which may be more or 



