pennantite

Pennantite is a copper sulfide mineral that belongs to the tetrahedrite group. It is typically described by a solid-solution formula of Cu12(As,Sb)4S13, reflecting substitution of arsenic and antimony on the same structural site. In practice, pennantite is considered within the tetrahedrite–tennantite series, with end-members that are arsenic-rich (tennantite) and antimony-rich (tetrahedrite).

Crystallography and appearance: pennantite crystallizes in the isometric (c cubic) system and occurs as euhedral to subhedral crystals, though it more commonly appears as coarse-grained, opaque masses or intergrowths with other sulfides. It has a metallic to submetallic luster, a gray to dark gray color, and a gray to gray-black streak. On the Mohs scale, its hardness is about 3.5–4, and its specific gravity ranges roughly from 4.3 to 4.8. Cleavage is indistinct.

Occurrence and associations: pennantite forms in hydrothermal ore veins and in copper- and polymetallic deposits. It is commonly associated with quartz and calcite, as well as other sulfides such as sphalerite, chalcopyrite, bornite, and with related sulfosalt minerals including tetrahedrite and tennantite. It has a wide, but localized, distribution in classic ore districts and in mineralized districts worldwide.

Identification and use: due to its similarity in appearance to related copper sulfides, definitive identification of pennantite typically requires mineralogical analysis such as X-ray diffraction or electron-microprobe techniques. In most deposits it is an ore-associated mineral rather than a primary gemstone material.