But this theory of formation for the sulfosalts is not supported by crystallographic studies, yet the name sulfosalts is still used. Another way to think of sulfosalts is as a double sulfide where the formula for emplectite could be written as CuS-BiS . Emplectite forms in hydrothermal veins with other bismuth and copper minerals. Unfortunately emplectite is a difficult mineral to distinguish from other gray, acicular to massive, dense sulfosalts with a black streak. Chalcostibite has a formula of CuSbS 2 .
Color is iron gray, yellow gray, whitish gray to tin white.
Luster is metallic. Crystals are opaque. Streak is black. Specimens will develop a yellow tarnish. Associated Minerals include quartz , barite , wittichenite , chalcopyrite and skutterudite .