Yarrowite
Chemical formula: Cu₉S₈
A rare copper sulfide with a characteristic blue-violet color, closely related to spionkopite and covellite.
Properties
- Mohs hardness
- 2.5
- Color
- Bluish-grey, black
- Luster
- Metallic
- Streak
- Black
- Density
- 0
- Cleavage
- {0001}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Yarrowite can be identified by its characteristic blue-violet color, metallic luster, and platy crystal habit. It often forms thin coatings and crusts on other copper minerals. Its perfect cleavage is also an important diagnostic feature. ## Distinguishing from Similar Minerals Yarrowite is visually almost impossible to distinguish from spionkopite (Cu₃₉S₂₈) without advanced analytical methods, such as X-ray diffraction (XRD). Both minerals have very similar color, habit, and occur in the same environments. It can also be confused with covellite (CuS), which has a similar color but usually a darker, indigo hue and often exhibits stronger iridescence. Covellite is also slightly harder. ## Crystal Forms Yarrowite crystals are typically very small, thin, and platy, with a hexagonal outline. They most often form fine-grained, earthy, or scaly aggregates, as well as thin coatings and crusts.
Geological environment
## Genesis Yarrowite is a secondary mineral, typical of secondary sulfide enrichment zones (cementation zones) in copper deposits. It forms as a result of the alteration of primary copper sulfides, such as chalcopyrite or bornite, under low-temperature conditions and in the presence of sulfur-rich waters. ## Mineral Associations It most commonly co-occurs with other secondary copper sulfides, such as spionkopite, digenite, covellite, and chalcocite. It is also associated with djurleite, bornite, pyrite, and calcite. ## Localities Key yarrowite localities worldwide include the type locality Yarrow Creek in Alberta (Canada), the Leonard Mine in Butte, Montana (USA), as well as some deposits in Germany (e.g., in the Harz Mountains) and Austria. It is a mineral widely distributed in small quantities in the enrichment zones of copper deposits globally.
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, though usually microscopic, crystals of intense, blue-violet color. Rich coatings on a contrasting rock matrix or co-occurrence with other rare copper sulfides enhance their attractiveness. Due to its instability, well-preserved specimens without signs of oxidation are more highly valued. ## Popular Localities Among the classic and best-known localities from which collectible specimens originate are the mines in Butte, Montana (USA). Specimens from this locality are often a reference point for this mineral.
Care and storage
## Cleaning Yarrowite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its softness and perfect cleavage, avoid washing with water and mechanical cleaning. If it is necessary to remove loose contaminants, compressed air can be used from a safe distance. ## What to Avoid Yarrowite is sensitive to acids and other chemicals. Avoid contact with ultrasonic cleaners, which can damage its structure. Prolonged exposure to moisture can lead to its oxidation and alteration into other copper minerals, so it should be stored in a dry place. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from mechanical damage and dust. Avoid storing it with harder minerals that could scratch it. Storage in dry conditions is crucial for preserving its appearance.