Botallackite is a copper(II) hydroxychloride mineral with the chemical formula Cu₂(OH)₃Cl. It belongs to the atacamite group of minerals, which are composed of copper, chlorine, oxygen, and hydrogen. Botallackite crystallizes in the monoclinic crystal system and typically forms as green, tabular to platy crystals.
Classification
- Mineral class: Halide
- Mineral group: Atacamite group
- Dana system: 01.06.01.01
- Strunz classification: 3.BB.30
Chemical composition
- Formula: Cu₂(OH)₃Cl
- Molar mass: 190.46 g mol⁻¹
- Elemental constituents: copper (Cu), chlorine (Cl), oxygen (O), hydrogen (H)
Crystal structure
- Crystal system: Monoclinic
- Space group: P2₁/c
- Unit‑cell parameters: a ≈ 7.05 Å, b ≈ 7.55 Å, c ≈ 5.23 Å; β ≈ 115.5°
- Botallackite is a dimorph of clinoatacamite; both share the same chemical composition but differ in crystal symmetry.
Physical properties
| Property | Value |
|---|---|
| Color | Light to deep green |
| Habit | Tabular, platy, subhedral aggregates |
| Cleavage | Perfect on {010} |
| Fracture | Uneven to conchoidal |
| Hardness (Mohs) | 2.5–3 |
| Luster | Vitreous to pearly |
| Streak | Light green |
| Specific gravity | 3.6–3.8 |
| Transparency | Transparent to translucent |
| Optical nature | Biaxial (+) |
| Refractive indices | nα ≈ 1.785, nβ ≈ 1.797, nγ ≈ 1.818 |
| Pleochroism | Weak, green shades |
Occurrence and formation
Botallackite forms in the oxidation zones of copper ore deposits, especially in environments where chloride ions are abundant. It is commonly produced as a secondary mineral by the alteration of primary copper sulfides such as chalcopyrite under oxidizing, saline conditions. Typical settings include:
- Marine and coastal zones: weathering of copper-bearing rocks in seawater.
- Anthropogenic corrosion: patina on copper alloys, bronze statues, and ship hulls exposed to salty atmospheres.
- Supergene enrichment zones: within copper mines where groundwater contains chloride.
The mineral is frequently associated with other copper chloride hydroxides such as atacamite, paratacamite, and clinoatacamite, as well as with malachite, azurite, and brochantite.
Historical background
Botallackite was first described in 1865 for specimens collected from the Botallack Mine in Cornwall, United Kingdom, after which it is named. Its identification was confirmed by its distinct crystallography and chemical composition, differentiating it from other members of the atacamite group.
Uses and significance
While botallackite itself has no major industrial applications due to its rarity and limited stability, it is of interest in:
- Mineralogical research: as a natural analogue for studying copper corrosion processes and the behavior of copper chloride hydroxides.
- Cultural heritage conservation: understanding the patination of historic copper artworks and developing strategies to mitigate further deterioration.
- Geochemical indicators: its presence can signal chloride-rich oxidation environments in copper ore districts.
Synthesis
Botallackite can be produced synthetically by reacting copper(II) chloride solutions with alkaline agents under controlled temperature and pH conditions. Laboratory synthesis is employed for comparative studies of its structural and spectroscopic properties.
Safety
As a copper-containing mineral, botallackite may pose health hazards if ingested or inhaled in powdered form. Standard mineral-handling precautions—use of gloves, eye protection, and adequate ventilation—are recommended. Copper compounds can be toxic in large quantities.