Arthurite

Chemical formula: Cu<sup>2+</sup>Fe<sup>3+</sup><sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>·4H<sub>2</sub>O

Arthurite is a rare, secondary copper and iron arsenate, forming characteristic, acicular crystals of an intense green color.

## Characteristics Arthurite is a hydrated copper and iron arsenate, occurring in the oxidation zones of ore deposits. It forms small, acicular or hair-like crystals, which often aggregate into spherical or radial clusters, as well as crusts and coatings. Its most characteristic feature is its vivid, apple-green to emerald-green color, which makes it easily noticeable despite the small size of its crystals. ## Physical Properties Arthurite crystals are typically very small and brittle. This mineral has a hardness of 3-4 on the Mohs scale. The luster is vitreous to silky, especially in fibrous aggregates. It is a translucent to transparent mineral in thin fragments. Its density is approximately 3.2 g/cm³. ## Colors and Varieties Arthurite occurs in various shades of green, from light green, through apple-green, to dark, emerald-green. No named varieties are distinguished, and the color variability mainly depends on subtle differences in chemical composition and crystallization conditions. ## History and Name The mineral was named in 1964 in honor of two individuals: Arthur E. I. Russell (1881-1964), a British mineral collector, and Arthur W. G. Kingsbury (1906-1968), an English mineralogist who first found this mineral in Cornwall, England. It was described by R.J. Davis and M.H. Hey based on material from the Hingston Down Consols mine in Cornwall, which is its type locality. ## Uses Due to its rarity and small crystal size, arthurite has no industrial applications. However, it is a valued and sought-after collector's mineral, especially well-formed specimens from classic localities.

Properties

Mohs hardness
3-4
Luster
Vitreous
Streak
Pale green
Density
3.2
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Translucent to transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Arthurite is most often identified by its characteristic, vivid, apple-green color and its mode of occurrence – small, acicular crystals forming spherical or radial aggregates. It occurs in the oxidation zones of polymetallic deposits, which is a key field indicator. ## Distinguishing from Similar Minerals Arthurite can be confused with other green secondary copper minerals, such as: - **Olivenite**: Usually has a darker, olive-green hue and forms more prismatic, solid crystals. - **Cornwallite**: Has a similar color but is amorphous or forms radiating-fibrous aggregates, rarely distinct crystals. - **Agardite**: Has a similar acicular habit, but usually a blue-green hue and often occurs in association with other rare earth minerals. - **Tyrolite**: Usually has a blue-green color and a pearly luster on cleavage surfaces. Definitive differentiation often requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms Arthurite crystals are monoclinic, but exhibit an acicular or hair-like habit. They most often occur as radial or spherulitic (spherical) aggregates, as well as thin crusts or coatings on the host rock.

Geological environment

## Genesis Arthurite is a secondary mineral, formed in the oxidation (weathering) zones of hydrothermal polymetallic deposits rich in copper and arsenic. It forms as a result of the reaction of acidic, sulfate-rich waters with primary ore minerals, such as chalcopyrite and arsenopyrite, in the presence of iron minerals. ## Mineral Associations This mineral often co-occurs with other secondary arsenic, copper, and iron minerals. Its most common associations include: scorodite, olivenite, cornwallite, chenevixite, pharmacosiderite, goethite, quartz, and various iron and manganese oxides. ## Localities The most important and classic localities for arthurite are mines in Cornwall (England), especially Hingston Down Consols (type locality) and Calstock. Significant specimens also come from the Clara mine in the Black Forest (Germany) and from Tsumeb (Namibia). Other known occurrences include the Majuba Hill mine in Nevada (USA) and some deposits in Greece (Lavrion) and Spain.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized arthurite specimens are those with well-defined, spherical aggregates of intense, apple-green color, contrasting with the rock matrix. The size of the aggregates is important – the larger and more numerous, the more valuable the specimen. The absence of damage to the delicate, acicular crystals is also crucial. Specimens from classic, historical localities, such as Cornwall, are particularly sought after. ## Popular Localities For collectors, specimens from the type locality in Cornwall (England) are most desirable, possessing historical value. Very aesthetic, rich specimens also come from the Clara mine in Germany, where arthurite forms beautiful, green spheres on a quartz matrix. Specimens from Tsumeb in Namibia are also valued, though often rarer on the market.

Care and storage

## Cleaning Arthurite specimens should be cleaned with the utmost care, using only compressed air to remove dust. Due to its brittleness and potential solubility, contact with water, and especially chemicals, should be avoided. Any mechanical cleaning (e.g., with a brush) is highly risky and can damage the delicate crystals. ## What to Avoid Ultrasonics, steam cleaners, acids, detergents, and solvents should be absolutely avoided. The mineral is sensitive to high temperatures and can be damaged by prolonged exposure to intense sunlight. It should be protected from moisture. ## Storage Arthurite specimens are best stored in closed, padded collector's boxes (so-called "micromount boxes") to protect them from dust, mechanical damage, and sudden changes in humidity. It should not be stored together with harder minerals that could scratch it.

External references

Sources

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