Rollandite
Chemical formula: Cu<sup>2+</sup><sub>3</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·4H<sub>2</sub>O
Rollandite is a rare, hydrated copper arsenate, forming characteristic blue-green, radial aggregates of acicular crystals.
Properties
- Mohs hardness
- 4.5
- Luster
- Vitreous
- Streak
- Pale green
- Density
- 4.23-4.31
- Cleavage
- Good on {100} and {010}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Rollandite can be identified by its characteristic form of occurrence – radial, spherical aggregates – and its intense blue-green color. It forms on host rocks, often in association with other secondary copper and arsenic minerals. ## Differentiation from Similar Minerals It can be confused with other acicular copper arsenates, such as olivenite or cornwallite, but its radial aggregates are quite characteristic. Accurate differentiation from morphologically very similar agardite-(Y) or other minerals from the mixite group often requires advanced chemical analyses (e.g., EDS). ## Crystal Forms Crystals are always acicular or hair-like, very elongated along the [001] axis. They usually form radial, spherical or hemispherical aggregates, as well as bundle-like or fan-like clusters.
Geological environment
## Genesis Rollandite is a secondary mineral, forming in the oxidation (weathering) zones of copper and arsenic ore deposits. It crystallizes in rock cavities and fissures as a result of oxygen-rich waters acting on primary sulfides, such as chalcopyrite and arsenopyrite, or arsenides, like tennantite. ## Mineral Associations It often co-occurs with other secondary arsenate and copper minerals. The most common associated minerals include: tyrolite, as well as minerals from the mixite group (agardite), cornwallite, olivenite, chalcophyllite, azurite, malachite, and tennantite. ## Localities The most important and classic locality (type locality) is the Roua mine in the Gorges de Daluis valley, in the Alpes-Maritimes department of France. This mineral has also been identified in several other locations worldwide, including the Clara mine in the Black Forest (Germany) and the Maria-Josefa mine in Spain.
Rarity
Very rare
For collectors
## Quality Criteria The most prized rollandite specimens are those with well-formed, spherical aggregates of intense, blue-green color, contrasting with a light host rock. The completeness and undamaged form of the delicate, radial clusters are important. Due to the small size of the crystals, specimens are usually "micromount" or miniature type. ## Popular Localities Specimens from the type locality – the Roua mine in France – are by far the most sought after by collectors. Material from the Clara mine in Germany is also known, but French specimens are considered exemplary for this species.
Care and storage
## Cleaning Rollandite specimens are extremely delicate and brittle. Mechanical cleaning is highly inadvisable. If necessary, compressed air can be used from a safe distance to remove loose dust. Contact with water, which can damage the mineral, should be avoided. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. Aggregates are very sensitive to shock and touch – even light pressure can destroy the acicular crystals. It should not be heated or exposed to prolonged strong light. ## Storage Rollandite specimens should be stored in stable, sealed "micromount" boxes that protect them from dust, moisture, and mechanical damage. Nothing should be placed on top of the specimen.