Digenite

Chemical formula: Cu<sub>1.8</sub>S

Digenite is a copper sulfide with a characteristic blue-black color, often forming intergrowths with other copper sulfides.

## Characteristics Digenite is an important copper ore mineral belonging to the sulfide group. Its name comes from the Greek word meaning "of two origins," referring to its frequent co-occurrence and intergrowths with chalcocite, to which it is closely related. Typical specimens appear as granular masses or coatings, ranging in color from blue to black. It rarely forms small, pseudocubic (trigonal) crystals, which usually have the shape of pyramids or cubes. ## Physical Properties Digenite is a relatively soft mineral, with a hardness of 2.5-3 on the Mohs scale. It is opaque and has a metallic luster. Its density is significant, approximately 5.6 g/cm³. This mineral exhibits weak electrical conductivity. ## Colors and Varieties It is characterized by colors ranging from lead-gray, through indigo, to blue-black and black. On fresh fracture surfaces, it is lead-gray, but it quickly tarnishes and darkens upon exposure to air. No distinct color varieties or commercial varieties are recognized. ## History and Name The mineral was first described in 1844 by the German mineralogist August Breithaupt. The name "digenite," derived from the Greek word *digenēs* (διγενής) meaning "of two origins" or "of two kinds," was given due to its close relationship and frequent confusion with chalcocite. Initially, it was thought to be a variety of chalcocite. ## Uses As a component of rich copper ores, digenite is a locally important source of this metal. It is primarily of industrial significance. Digenite specimens, especially well-formed crystals or rich aggregates, can also have collector's value.

Properties

Mohs hardness
2.5-3
Luster
Metallic
Streak
Grayish black
Density
5.5-5.7
Cleavage
Poor/Indistinct
Fracture
Conchoidal
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification A key diagnostic feature of digenite is its characteristic blue to blue-black color on fresh surfaces, which quickly darkens. It is a soft mineral that can be scratched with a copper wire. Its metallic luster and relatively high density are also helpful in identification. ## Distinguishing from Similar Minerals Digenite is most commonly confused with chalcocite and covellite. It is distinguished from chalcocite by its more intense blue color (chalcocite is usually lead-gray). Covellite has a similar indigo color but exhibits perfect cleavage and often occurs as thin flakes, whereas digenite forms granular aggregates and lacks such distinct cleavage. Bornite, known as "peacock ore," has similar colors, but these are due to tarnish, and its fresh fracture is reddish-brown. ## Crystal Forms Digenite crystallizes in the trigonal system, but its crystals are rare and usually have a pseudocubic appearance, taking the forms of cubes or octahedra. Most often, it occurs as granular aggregates, massive forms, coatings, and as inclusions and intergrowths in other copper sulfides.

Geological environment

## Genesis Digenite is a hydrothermal mineral, forming in both primary and secondary enrichment zones of sulfide deposits. It forms in medium- and low-temperature ore veins. It can also crystallize directly from hydrothermal solutions or as a product of alteration of other copper sulfides, such as chalcopyrite or bornite. ## Mineral Associations This mineral often co-occurs with other copper sulfides, forming characteristic parageneses with them. It is most commonly associated with chalcocite, covellite, bornite, chalcopyrite, djurleite, and tennantite. It also occurs in association with pyrite and sphalerite. ## Localities Significant deposits and occurrences of digenite are known from many places around the world. The most important include Butte, Montana (USA), where large quantities of this mineral were found, as well as Tsumeb, Namibia, Cornwall, Great Britain, and mines in Chile and Peru. It also occurs in the Mednogorsk region of the Urals (Russia) and in Kazakhstan.

Rarity

Not very common

For collectors

## Quality Criteria Specimens with well-formed, though usually small, pseudocubic crystals are most valued by collectors. Rich, granular aggregates with an intense blue color also enhance their attractiveness. Particularly sought after are specimens where digenite forms distinct associations with other rare sulfides, such as djurleite, or contrasts colorfully with associated minerals. ## Popular Localities Classic and most prized digenite specimens, especially those with distinct crystals, come from mines in Butte, Montana, USA. This locality is historically the most important for this mineral. Other known localities providing collector material include Tsumeb, Namibia, and some mines in Cornwall, England.

Care and storage

## Cleaning Digenite specimens should be cleaned very carefully, preferably dry, using a soft brush to remove dust. The use of water, especially with detergents, is not recommended, as it can accelerate the oxidation process and surface tarnishing. ## What to Avoid Digenite is sensitive to chemicals, especially acids, which can damage it. Exposure to humid air should be avoided, as it causes rapid tarnishing and color change to black. It should not be heated or exposed to sudden temperature changes. ## Storage It is recommended to store digenite specimens in dry conditions, preferably in closed, airtight containers or display cases with a desiccant. Limiting air access will slow down the oxidation process and help preserve the mineral's original luster and color.

External references

Sources

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