Digenite

Digenite is a rare copper sulfide mineral with the chemical formula Cu₉S₅. It belongs to the sulfide class of minerals and is notable for its distinctive metallic luster and deep blue to iridescent tarnish. The mineral was first described in 1845 by the German mineralogist Friedrich Mohs, who named it after its composition, derived from the Greek words *di-* (meaning "two") and *genos* (meaning "kind" or "race"), though its exact formula was later refined.

Digenite typically forms as a secondary mineral in the oxidation zone of copper deposits, often associated with other copper minerals such as chalcocite, covellite, and malachite. It commonly occurs as massive, earthy, or crust-like aggregates rather than well-formed crystals. The mineral exhibits a metallic to submetallic luster and varies in color from grayish-blue to iridescent purple, green, or blue due to surface oxidation. Under ultraviolet light, digenite may fluoresce a pale blue or greenish hue.

Digenite is found in several copper-rich regions worldwide, including the United States (Arizona, New Mexico), Chile, Peru, and the Democratic Republic of Congo. It is often mined as a minor copper ore, though its economic value is generally low compared to more abundant copper minerals. The mineral’s diagnostic features include its iridescent tarnish and association with other copper sulfides in oxidized environments.

In addition to its geological significance, digenite is occasionally collected by mineralogists and hobbyists for its aesthetic properties. Its relatively soft hardness of 2.5 to 3 on the Mohs scale makes it susceptible to scratching, requiring careful handling. While not a major industrial mineral, digenite serves as an important indicator of copper enrichment in hydrothermal and supergene environments.