Metauranocircite

Chemical formula: Ba(U⁶⁺O₂)₂(PO₄)₂(H₂O)₄·₂H₂O

Meta-uranocircite is a hydrated uranyl barium phosphate, a secondary mineral found in the oxidation zones of uranium deposits, forming characteristic platy crystals of an intense, yellowish-green color.

## Characteristics Meta-uranocircite is a mineral from the meta-autunite group, belonging to the phosphate class. It forms characteristic, thin-platy, square or rectangular crystals, often with truncated corners. These crystals usually occur as radial or fan-shaped aggregates, as well as rosettes, crusts, and coatings on the host rock. It is a highly radioactive mineral and exhibits intense, greenish-yellow fluorescence under ultraviolet light. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2-2.5. It is brittle, and its density is approximately 3.94 g/cm³. The crystals show perfect, unidirectional cleavage, which causes them to easily split into thin lamellae. The luster is vitreous, and pearly on cleavage surfaces. ## Colors and Varieties Meta-uranocircite occurs in shades from greenish-yellow to yellowish-green. Its color is characteristic and relatively constant. No named color or commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition (uranium) and its similarity to uranocircite, with the prefix "meta-" indicating a lower degree of hydration. It was first described in 1904. The type locality (place of first discovery) is given as the uranium deposit in the Krunkelbach valley, in the Menzenschwand area, Germany. ## Uses Due to its radioactivity and composition, meta-uranocircite is primarily of scientific and collector's interest. It has no industrial applications.

Properties

Mohs hardness: 2-2.5
Luster: Vitreous, Pearly
Streak: Yellowish white
Density: 3.94
Cleavage: {001}
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features of meta-uranocircite are its characteristic yellowish-green color, thin-platy crystal form creating fan-shaped aggregates, and very strong, green fluorescence under UV light. Its high radioactivity, measurable with a Geiger counter, is also an important feature. ## Distinguishing from Similar Minerals Meta-uranocircite can be confused with other minerals from the autunite group, such as autunite, meta-autunite, or uranocircite. Autunite is more hydrated and often less stable under room conditions. Uranocircite has a very similar appearance but crystallizes in the tetragonal system. Precise differentiation of these minerals often requires advanced studies, e.g., by X-ray diffraction (XRD). ## Crystal Forms Crystals are typically thin-platy, with a square or rectangular outline. They most often form fan-shaped, radial, or stellate aggregates, as well as rosettes and thin crusts on rock surfaces.

Geological environment

## Genesis Meta-uranocircite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal and sedimentary uranium deposits. It forms as a result of the alteration of primary uranium minerals, such as uraninite, in the presence of waters rich in barium and phosphates. ## Mineral Associations It often co-occurs with other secondary uranium minerals, such as autunite, torbernite, meta-autunite, uranophane, as well as with uraninite, fluorite, quartz, hematite, and goethite. ## Localities The most important occurrences of meta-uranocircite worldwide include the type locality in Menzenschwand (Black Forest, Germany), as well as Wölsendorf in Bavaria. It is also known from numerous occurrences in the Czech Republic, France (e.g., Massif Central), Portugal, Great Britain (Cornwall), USA (South Dakota, Utah, Nevada), and Australia.

Rarity

Not very common

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals forming aesthetic, fan-shaped or stellate aggregates are most valued by collectors. An intense, vivid color and absence of mechanical damage are important. The contrasting placement of yellowish-green crystals on dark host rock significantly enhances the visual appeal of the specimen. ## Popular Localities Classic and most prized specimens come from historical localities in Germany, especially from Menzenschwand in the Black Forest. Good quality collector material was also supplied by some deposits in France and the Czech Republic.

Care and storage

## Cleaning Meta-uranocircite specimens are very delicate and brittle. Cleaning should be kept to an absolute minimum. If necessary, a soft brush can be used to remove dust. Avoid contact with water, which can change the mineral's hydration state and damage the specimen. ## What to Avoid Absolutely avoid contact with water and chemicals. The mineral is sensitive to changes in humidity and temperature, which can lead to its dehydration and disintegration. As a uranium mineral, it is highly radioactive – exposure should be limited, hands should be washed after each contact, and it should not be stored in places of permanent human presence (desk, bedroom). ## Storage Specimens should be stored in dry, stable conditions, preferably in sealed, closed containers (e.g., "perky box" type), to protect them from mechanical damage and humidity changes. Due to radioactivity, it is recommended to store them in separate, labeled boxes, away from other radiation-sensitive minerals and in a location with restricted access.

External references

https://www.mindat.org/min-2665.html