Znucalite
Chemical formula: Zn<sup>2+</sup><sub>10</sub>Ca(U<sup>6+</sup>O<sub>2</sub>)(CO<sub>3</sub>)<sub>4</sub>(OH)<sub>16</sub>(H<sub>2</sub>O)<sub>5</sub>
Znucalite is a rare secondary zinc, calcium, and uranium mineral, forming characteristic bright yellow, spherical aggregates and coatings.
Properties
- Luster
- Dull
- Streak
- Light yellow
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Key diagnostic features of znucalite include its characteristic bright yellow color, occurrence as spherical aggregates and powdery coatings, and strong yellowish-green fluorescence under UV light. It typically forms coatings on other minerals in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals Znucalite can be confused with other yellow-colored secondary uranium minerals, such as studtite, meta-studtite, or some forms of uranophane. Distinguishing it requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS), as visual features are often ambiguous. ## Crystal Forms It forms very fine, acicular or bladed crystals up to several tens of micrometers long. These crystals combine into radial, spherical, or spherulitic aggregates reaching up to 0.5 mm in diameter. It also occurs as thin coatings and powdery crusts.
Geological environment
## Genesis Znucalite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal polymetallic deposits rich in uraninite and zinc. It forms as a result of reactions of carbonate-rich waters with primary minerals containing zinc, calcium, and uranium. It forms under low-temperature conditions, often in rock cavities and fissures. ## Mineral Associations It most often co-occurs with other secondary uranium and zinc minerals. In the type locality (Příbram), it was found in association with gypsum, hydrozincite, aragonite, sphalerite, uraninite, goethite, and cordierite. In other localities, it is accompanied by uranophane, fourmarierite, becquerelite, and boltwoodite, among others. ## Localities This is a very rare mineral. Besides its discovery site in Příbram, Czech Republic, its occurrence has been confirmed in only a few other places worldwide. These include the Shinkolobwe mine in the Democratic Republic of Congo and Jáchymov (also in the Czech Republic).
Rarity
Very rare
For collectors
## Quality Criteria The most valued specimens by collectors are those showing well-formed, spherical aggregates of znucalite, contrasting in color with the dark host rock. The intensity of the yellow color and the richness of the coating are important. Due to the microscopic size of the crystals, specimens are primarily evaluated based on the aesthetics of the entire aggregate. Accurate locality information is also of great importance. ## Popular Localities The most classic and sought-after specimens come from the type locality - the Lill mine in Příbram, Czech Republic. Specimens from this locality are considered exemplary for this mineral.
Care and storage
## Cleaning Znucalite specimens should not be wet cleaned. Due to its powdery, delicate nature, dust is best removed with a soft brush or a photographic air blower. Contact with water can damage or dissolve delicate aggregates. ## What to Avoid Contact with water, acids, and other chemicals should be strictly avoided. The mineral is very brittle and sensitive to shock and abrasion. As a uranium mineral, it is radioactive, so inhalation of dust should be avoided, and it should be stored away from areas of permanent occupancy. Hands should be washed after each contact with a specimen. ## Storage It is recommended to store specimens in airtight, sealed containers (so-called micromount boxes), which protect them from dust, mechanical damage, and moisture. Due to its radioactivity, it should be kept in a designated area for radioactive minerals, away from other radiation-sensitive specimens.