SULPIIO coMPorxns 325 



tetrahedron, or combinations of these two forms, also penetrating 

 supplemental tetrahedral twins, with the perpendicular to 111 as 

 the twinning axis, occur, in veins in a dolomite associated with 

 ehalropyrite crystals and quartz. In crystalline habit tennantite 

 is more apt to be rhombic dodecahedral, with small modifications 

 of the tetrahedrons, as at Redruth in Cornwall, England. Both 

 occur massive, granular, or compact. 



Chemically tetrahedrite may be represented by the formula 

 R":iK'"S ;t , in which R" is mostly copper, but often replaced in part 

 by silver, lead, mercury, zinc, or iron, and R'" represents antimony, 

 arsenic, or bismuth. These substitutions lead to numerous local 

 variations and possibly when not well crystallized to confusion 

 with other minerals of the same nature. It is often an important 

 silver ore, as the variety freibergite, while schwartzite is the mer- 

 curial form. 



Tetrahedrite occurs as a vein mineral associated with sulphides, 

 arsenides, and antimonides in the silver districts of Mexico, in 

 Colorado, and Nevada. It has probably been formed by precipi- 

 tation or replacement at comparatively low temperature. 



Alteration products are bournonite, covellite, malachite, azurite, 

 and oxidized antimony or arsenic minerals. Chalcopyrite often 

 forms a crust enveloping the mother crystal of tetrahedrite, as at 

 Liskeard, Cornwall. Tetrahedrite is an important ore of copper 

 and silver. 



STEPHANITE 



Stephanite. Ag 5 SbS 3 ; Sulphantimonide of silver; Ag = 

 68.5, Sb = 15.2, S = 16.3, Orthorhombic ; Type, Didigonal Polar ; 

 a:b:c = .6291: 1:. 6851, 100 A 110 = 32 10', 001 A 101 = 47 

 26', 001 A Oil = 34 25', 111 A 110 = 37 51'; Common forms, 

 c (001), b (010), m (110), d (Oil), P (111) ; Twinning plane m quite 

 common, others as a and b less so ; Cleavage, b and d imperfect ; 

 Brittle, fracture uneven ; H. = 2-2.5 ; G. = 6.2-6.5 ; Color and 

 streak, iron-black ; Luster, metallic ; Opaque. 



B.B. On coal fuses easily with spirting, yielding a white coat 

 of antimony trioxide, which may become red with oxidized silver. 

 In O. F. yields an odor of sulphur dioxide. Reduced with soda yields 

 a malleable button of silver. 



General Description. Crystals are usually tabular parallel 

 to the base, or short stout prisms, at times elongated parallel to the 

 brachyaxis. Twins with m as the composition face are often 



