Calcioursilite

Chemical formula: Ca₄(U⁶⁺O₂)₄(Si₂O₅)₅(OH)₆·₁₅H₂O

Calcioursilite is a very rare, pale yellow uranyl mineral, forming microscopic, acicular crystals and coatings.

## Characteristics Calcioursilite is a hydrated uranyl calcium silicate. It occurs as microscopic, acicular or fibrous crystals, which typically form delicate, fluffy aggregates, coatings, and crusts on rock surfaces. Due to the very small size of the crystals, its macroscopic features are difficult to observe, and identification requires advanced analytical methods. ## Physical Properties Calcioursilite crystals are extremely small and brittle. The mineral is characterized by intense radioactivity, typical of uranium minerals. It also exhibits strong, green fluorescence under ultraviolet light. ## Colors and Varieties Calcioursilite has a characteristic, pale yellow to canary yellow color. No varieties are distinguished. ## History and Name The name "calcioursilite" refers to its chemical composition – the presence of calcium (Latin: *calx* – lime) and its structural similarity to another uranium mineral, ursilite. It was described as a new mineral in 1996 based on material from an unspecified locality in Uzbekistan. ## Applications Calcioursilite has no practical or industrial applications. Its significance is purely scientific and as a collector's item, as a very rare secondary uranium mineral.

Properties

Mohs hardness: ~2
Color: Lemon yellow
Luster: Silky
Streak: Light yellow
Density: 0
Cleavage: Perfect
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Identification of calcioursilite in the field is practically impossible. In laboratory conditions, it is recognized based on its characteristic, pale yellow color, the form of microscopic acicular crystals, and strong, green fluorescence under UV light. However, definitive identification requires advanced techniques such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). ## Distinguishing from Similar Minerals Calcioursilite is visually indistinguishable from many other secondary uranyl minerals with similar color and habit, such as ursilite, boltwoodite, or uranophane. Differences between them can only be determined by chemical composition and crystal structure analysis. ## Crystal Forms It forms very fine, acicular or fibrous crystals, which combine into tangled aggregates, fluffy coatings, crusts, and rosettes.

Geological environment

## Genesis Calcioursilite is a secondary mineral, forming in the oxidation zones of uranium deposits. It forms as a result of weathering and alteration of primary uranium minerals, such as uraninite, in the presence of silica- and calcium-rich waters. It is one of the late products of these processes. ## Mineral Associations It co-occurs with other secondary uranium minerals, such as gypsum, schoepite, uranophane, boltwoodite, and with remnants of primary uraninite. ## Localities The only confirmed and thoroughly described occurrence of this mineral is the oxidation zone of the U-Se uranium deposit in the Bota-Burum mine, located on the northern slope of the Altyntau ridge in Uzbekistan.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a calcioursilite specimen depends on the richness and aesthetics of the microcrystalline aggregates. Most valued are samples with clearly formed, fluffy accumulations of intense yellow color, covering as large an area as possible of the host rock. Contrast with a dark substrate enhances visual appeal. Due to its microscopic nature, specimens are primarily evaluated under magnification. ## Popular Localities The only source of specimens available on the collector's market is the Bota-Burum mine in Uzbekistan.

Care and storage

## Cleaning The mineral is extremely delicate and brittle. It should not be cleaned mechanically or chemically. Any physical contact can destroy the microscopic crystals. The safest method is to use compressed air from a distance to remove loose dust. ## What to Avoid Absolutely avoid contact with water and other liquids, which can dissolve or damage the mineral. Avoid vibrations, shocks, and any touching. As a uranium mineral, it is highly radioactive – exposure should be kept to a minimum, avoid inhaling dust, and wash hands after any (even accidental) contact. ## Storage Specimens should be stored in sealed, stable containers, preferably "micromount" type, to protect them from mechanical damage and limit radon emission. Store away from other minerals that could suffer radiation damage. It is essential to clearly label the container as radioactive material.

External references

https://www.mindat.org/min-11360.html