Digenite is a copper sulfide mineral with the chemical formula Cu₉S₅ (often expressed as Cu₁.₈S). It belongs to the sulfide class of minerals and is a member of the copper‑sulfide group that also includes chalcocite, covellite, and djurleite.
Crystallography
- Crystal system: Trigonal (hexagonal scalenohedral)
- Space group: R‑3m
- Unit‑cell parameters: a ≈ 3.84 Å, c ≈ 13.5 Å; Z = 3
- Structure: The structure consists of layers of copper atoms alternating with layers containing S₂⁻ units, forming a complex lattice that accommodates a non‑stoichiometric copper‑to‑sulfur ratio.
Physical properties
| Property | Typical value |
|---|---|
| Color | Black to steel‑gray; may acquire a tarnished surface |
| Luster | Metallic |
| Streak | Black |
| Transparency | Opaque |
| Hardness (Mohs) | 3.5–4 |
| Specific gravity | 5.6–5.7 |
| cleavage | None; may display sub‑conchoidal fracture |
| Habit | Massive, granular, or compact aggregates; can occur as tabular or prismatic crystals, though well‑formed crystals are rare |
Occurrence and Formation
Digenite forms in medium‑ to high‑temperature hydrothermal environments and is commonly associated with other copper sulfides such as chalcocite, covellite, and tetrahedrite. It also occurs in supergene enrichment zones where primary copper sulfides are altered by meteoric water. Reported localities include:
- Chile (e.g., Chuquicamata)
- Zambia (Copperbelt)
- United States (e.g., the Morenci district, Arizona; the Copper Hills, Utah)
- Russia, Australia, and various other copper‑rich mining districts worldwide.
The mineral typically precipitates from sulfide‑rich fluids in volcanic‑associated massive sulfide (VMS) deposits, porphyry copper systems, and sedimentary exhalative (SEDEX) deposits.
Economic significance
Digenite is an important ore mineral in many copper deposits. Though it often occurs intergrown with other copper sulfides, it contributes to the overall copper content that can be extracted during ore processing. Its presence is used by geologists as an indicator of the temperature and redox conditions of ore‑forming environments.
Identification
Identification of digenite relies on a combination of physical and analytical techniques:
- Visual inspection: metallic luster, black color, massive habit.
- Hardness testing: 3.5–4 on the Mohs scale, softer than many silicate minerals.
- Streak test: black streak.
- Density measurement: specific gravity ~5.6.
- X‑ray diffraction (XRD): characteristic diffraction pattern consistent with the R‑3m space group.
- Electron microprobe or SEM‑EDS: confirms the Cu:S ratio close to 9:5.
Classification
- Dana system: 02.04.15.01
- Strunz classification: 2.BA.05 (copper sulfides, sulfides with a metal‑to‑sulfur ratio > 1)
History
The name “digenite” derives from the Greek prefix di‑ meaning “two” and genos meaning “type” or “origin,” referencing its intermediate composition between the two better‑known copper sulfides chalcocite (Cu₂S) and covellite (CuS). It was first described as a distinct mineral species in the late 19th century following analyses of samples from the copper deposits of the United States.
All presented data reflect current consensus in mineralogical literature and are drawn from peer‑reviewed mineral databases and standard references such as the Handbook of Mineralogy and the Mineralogical Society of America.