Cuprotungstite

Chemical formula: Cu²⁺₃(W⁶⁺O₄)₂(OH)₂

Cuprotungstite is a rare, secondary copper tungsten mineral, forming characteristic green coatings and crusts.

## Characteristics Cuprotungstite is a copper hydroxide and tungstate. It most often occurs as microcrystalline coatings, crusts, or earthy aggregates. It rarely forms crystals visible to the naked eye, which usually take the form of small, prismatic or tabular columns, often gathered in radial aggregates or rosettes. Its appearance is usually inconspicuous, and identification requires specialized examination. ## Physical Properties This mineral is characterized by a hardness in the range of 4-5 on the Mohs scale. It has a luster described as vitreous to earthy, depending on its form of occurrence. It is a translucent to opaque mineral. Its density is approximately 6.6 g/cm³. ## Colors and Varieties Cuprotungstite ranges in color from pistachio green, through emerald green, to dark green. No distinct color or commercial varieties are recognized. ## History and Name The mineral's name, given in 1868 by Charles F. Chandler, directly refers to its chemical composition – the presence of copper (Latin: *cuprum*) and tungsten (*tungsten*). It was discovered in the Llamuco mine in the Coquimbo region, Chile. ## Uses Due to its rarity and occurrence in small quantities, cuprotungstite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals and for scientists.

Properties

Mohs hardness: 4-5
Color: Emerald green, brown, olive-green, pistachio-green
Luster: Vitreous to earthy
Streak: Greenish gray to greenish yellow
Density: 5.4
Cleavage: None
Fracture: Uneven
Transparency: Translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Identifying cuprotungstite in the field is difficult. Its characteristic features are its pistachio or emerald green color and its occurrence as coatings in the oxidation zones of tungsten deposits. The green color in conjunction with tungsten minerals (e.g., scheelite) is a strong indicator. However, final identification requires chemical analysis (EDS) or X-ray analysis (XRD). ## Distinguishing from Similar Minerals Cuprotungstite can be confused with other secondary copper minerals such as malachite, brochantite, or pseudomalachite. Malachite reacts violently with hydrochloric acid, which cuprotungstite does not. Distinguishing it from other green copper arsenates, phosphates, or sulfates is visually practically impossible without advanced studies. ## Crystal Forms It most often forms earthy, powdery aggregates and crusts. Less commonly, it is found as small, prismatic crystals gathered in radial or fibrous aggregates.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal deposits rich in tungsten and copper. It forms as a result of the alteration of primary tungsten minerals, mainly scheelite (CaWO₄) and wolframite ((Fe,Mn)WO₄), under the influence of copper-rich solutions. ## Mineral Associations It most often co-occurs with the minerals from which it forms, namely scheelite and wolframite. It is also accompanied by other minerals of the oxidation zone, such as cuproscheelite, malachite, chrysocolla, brochantite, as well as quartz, limonite, and hematite. ## Localities The most important and classic localities for this mineral are Cave Creek in Arizona (USA), where well-formed microcrystals were found, and the discovery site in the Coquimbo region, Chile. It also occurs in small quantities in other places around the world, including California (USA), France, Namibia, and Russia.

Rarity

Rare

For collectors

## Quality Criteria The most prized by collectors are specimens with visible, well-formed microcrystals, forming radial aggregates of intense, emerald green color. Rich crusts on a contrasting rock matrix, especially in association with well-defined scheelite crystals, are also highly valued. Due to its rarity, even rich earthy coatings have collector's value. ## Popular Localities The historical finds from Cave Creek in Arizona, USA, are considered the best for this mineral. Specimens from there, although usually microscopic, set the standard of quality for cuprotungstite.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its occurrence as delicate coatings, avoid washing with water, which could damage or dissolve them. If liquid is necessary, isopropyl alcohol applied in minimal amounts is recommended. ## What to Avoid Avoid contact with acids and other chemicals that may react with the mineral. Cuprotungstite is sensitive to abrasion and impact. It should not be heated or exposed to prolonged moisture. ## Storage Cuprotungstite specimens are best stored in closed, padded collector's boxes to protect them from dust, mechanical damage, and sudden changes in humidity. Avoid exposure to direct sunlight.

External references

https://www.mindat.org/min-1194.html