Cornwallite
Chemical formula: Cu<sup>2+</sup><sub>5</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>4</sub>
Cornwallite is a rare, green copper arsenate, forming spherical aggregates and crusts, prized by collectors for its intense color.
Properties
- Mohs hardness
- 4.5
- Luster
- Vitreous, Resinous, Waxy, Silky
- Streak
- Green
- Density
- 4.65
- Cleavage
- None
- Fracture
- Conchoidal, Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Cornwallite is most easily recognized by its characteristic, intense green color and typical forms of occurrence – spherical, radial aggregates and crusts. Its streak is light green. It is relatively heavy for a non-metallic mineral. ## Distinguishing from Similar Minerals It can be confused with other secondary copper minerals, such as malachite, pseudomalachite, or tyrolite. - **Malachite** reacts with hydrochloric acid, releasing CO₂ bubbles, while cornwallite does not. - **Pseudomalachite** has a very similar appearance and properties, but it crystallizes in the monoclinic system (cornwallite is amorphous or poorly crystalline) and often has a slightly different shade of green and aggregate forms. Certain distinction requires advanced analyses (XRD). ## Crystal Forms Cornwallite is usually amorphous or occurs as very fine, poorly formed crystals. It forms characteristic radial-fibrous, spherical (spherulitic), botryoidal, and reniform aggregates, as well as dense, compact masses and crusts.
Geological environment
## Genesis Cornwallite is a secondary copper mineral, forming in the oxidation (weathering) zones of copper ore deposits containing arsenic. It forms as a result of the alteration of primary copper sulfides and arsenides under the influence of oxygen-rich waters. ## Mineral Associations It often co-occurs with other secondary copper and arsenic minerals, such as olivenite, scorodite, azurite, malachite, cuprite, tyrolite, conichalcite, as well as with limonite and quartz. ## Localities The most important localities for cornwallite worldwide are the historical mines in Cornwall (United Kingdom), from which its name originates. It also occurs in many other places, including the Miedzianka region and the "Lena" mine in Lower Silesia (Poland), in the Czech Republic (Jáchymov), Germany (Black Forest), Slovakia, France, as well as in the USA (Utah, Arizona, Nevada) and Chile.
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, spherical or radial aggregates of intense, emerald green color. Contrast with the rock matrix on which the cornwallite occurs, and the absence of damage to delicate, fibrous structures, are important. Large, rich crusts are also highly valued. ## Popular Localities Classic and most desirable specimens come from the historical mines in Cornwall, United Kingdom. Specimens from other known localities, such as Majuba Hill in Nevada (USA) or Jáchymov in the Czech Republic, are also valued in the collector's market.
Care and storage
## Cleaning Cornwallite specimens should be cleaned very carefully, preferably using a soft brush to remove dust. Compressed air can be used from a safe distance. Due to its arsenic content, inhalation of dust should be avoided. After contact with the mineral, it is recommended to wash hands. ## What to Avoid Avoid contact with water and chemicals, especially acids, which can damage the mineral. Cornwallite is sensitive to high temperatures and impacts. It should not be cleaned in ultrasonic cleaners. ## Storage Specimens should be stored in a dry place, in closed boxes or display cases, to protect them from dust and mechanical damage. It should be kept away from minerals that could scratch it.