Metazeunerite

Chemical formula: Cu₂+(U₆+O₂)₂(As₅+O₄)₂·₈H₂O

Metazeunerite is a secondary mineral of the autunite group, being a hydrated copper uranyl arsenate, forming characteristic, tabular crystals of an intense green color.

## Characteristics Metazeunerite is a mineral of the autunite group, belonging to the arsenate class. Chemically, it is a hydrated uranyl copper arsenate. It forms as a secondary mineral in the oxidation zones of uranium deposits. Its most characteristic visual feature is well-formed, thin, tabular crystals with a square outline, often with truncated corners. These crystals exhibit an intense, emerald-green or yellowish-green color and often form fan-shaped or almost spherical aggregates. ## Physical Properties This mineral is very soft, with a Mohs hardness of 2 to 2.5, meaning it can be scratched with a fingernail. It is brittle and has perfect cleavage in one direction, parallel to the base of the tabular crystals, which gives it a pearly luster on cleavage planes. On other surfaces, the luster is vitreous. It is a heavy mineral, with a density of approximately 3.64 g/cm³. Metazeunerite is strongly radioactive. ## Colors and Varieties Metazeunerite occurs in shades from emerald-green to grass-green and yellowish-green. No color or commercial varieties are distinguished. Its name is directly related to zeunerite, from which it differs by a lower water content in the crystal structure. Under normal atmospheric humidity, zeunerite spontaneously transforms into metazeunerite, which is why most specimens described as zeunerite are actually metazeunerite. ## History and Name The mineral's name refers to its connection with zeunerite and its lower degree of hydration (the prefix "meta"). Zeunerite was named after Gustav Anton Zeuner (1828-1907), a German engineer and physicist, director of the Mining Academy in Freiberg. Metazeunerite was first described as a distinct mineral in 1937. ## Applications Due to its radioactivity, metazeunerite has scientific significance and is sought after by collectors specializing in uranium minerals. In areas of high concentration, it can be considered a minor uranium ore.

Properties

Mohs hardness: 2-2.5
Luster: Vitreous, Pearly on cleavage
Streak: Pale green
Density: 3.64
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Tetragonal

Diagnostic features

## Identification The most important diagnostic features of metazeunerite are its characteristic habit in the form of square, tabular crystals, intense green color, excellent unidirectional cleavage, and strong radioactivity, which can be measured with a Geiger counter. The streak is light green. ## Distinguishing from Similar Minerals Metazeunerite is visually indistinguishable from metatorbernite. Both form identical green, tabular crystals. Certain differentiation requires chemical analysis – metazeunerite is an arsenate, and metatorbernite is a phosphate. It differs from autunite and meta-autunite in color (autunite is usually bright yellow or yellowish-green) and the absence or very weak fluorescence under ultraviolet light, whereas autunite exhibits strong, green fluorescence. From zeunerite, its fully hydrated form, it is practically visually indistinguishable. ## Crystal Forms Metazeunerite crystallizes in the form of thin to thick tablets with a square cross-section. Crystals often form lamellar, platy, or almost micaceous aggregates. They also occur as rosette-like or fan-shaped aggregates and as crusts.

Geological environment

## Genesis Metazeunerite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal ore deposits rich in uranium, arsenic, and copper. It forms as a result of the alteration of primary uranium minerals, such as uraninite, in the presence of solutions containing copper and arsenate ions. It is a product of the dehydration of zeunerite. ## Mineral Associations This mineral often co-occurs with other secondary uranium minerals. Its most common associates are zeunerite, metatorbernite, torbernite, uranophane, trögerite, walpurgite, and uranospinite. It can also occur in association with relics of primary minerals, such as uraninite, tennantite, or arsenopyrite. ## Localities The most important global localities for metazeunerite include historical deposits in Germany (Schneeberg in Saxony - type locality), the Czech Republic (Jáchymov), and the United Kingdom (Cornwall). Significant specimens also come from the United States, mainly from Utah and Arizona.

Rarity

Not very common

For collectors

## Quality Criteria The most highly prized metazeunerite specimens are characterized by sharply defined, large (over 1 cm) crystals of intense, emerald-green color. Crystal groups forming aesthetic compositions, set on a small, contrasting rock matrix, are particularly sought after. The clarity of the crystals (transparency) and the absence of mechanical damage significantly increase the specimen's collector value. ## Popular Localities The historical mining sites in Europe, such as Schneeberg in Germany and Jáchymov in the Czech Republic, are considered classic and provide the best specimens. Specimens from these localities are valued for their historical significance and excellent crystal quality. Good quality metazeunerite from mines in the southwestern US states is also found on the collector's market.

Care and storage

## Cleaning Metazeunerite specimens are extremely delicate and sensitive. To remove dust, use only a soft brush or a very gentle stream of compressed air. Absolutely avoid contact with water and any chemical agents, as they can cause damage or alteration of the mineral. ## What to Avoid Metazeunerite is strongly radioactive. Direct contact with the mineral should be limited to a minimum, and hands should be thoroughly washed after each touch. It should not be stored in places of permanent human presence (desks, bedrooms). The mineral is very soft and brittle, so it should be protected from impacts, scratches, and vibrations. It is also sensitive to elevated temperatures, which can lead to further dehydration. ## Storage It is recommended to store specimens in closed, airtight containers (e.g., "membrane boxes" or plexiglass boxes), which protect against dust and mechanical damage, and also limit radon emission. Containers should be clearly labeled as containing radioactive mineral. They should be kept in a well-ventilated room, away from bedrooms and workplaces.