Ježekite

Chemical formula: Na<sub>8</sub>(U<sup>6+</sup>O<sub>2</sub>)(CO<sub>3</sub>)<sub>3</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O

Ježekite is an extremely rare, water-soluble uranyl mineral, forming tiny, acicular crystals of an intense yellow color.

## Characteristics Ježekite is a secondary uranyl-carbonate-sulfate mineral, occurring as very fine, acicular or hair-like crystals. It typically forms small, radial aggregates or fluffy coatings on the surface of the host rock. Due to the extremely small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Ježekite crystals are extremely delicate and brittle. This mineral is water-soluble, which is its key and unusual property. It is highly radioactive due to the presence of uranium. ## Colors and Varieties Ježekite is characterized by an intense, lemon-yellow to greenish-yellow color. No varieties are known. ## History and Name The mineral was named in honor of Bohuslav Ježek (1877–1950), a Czech mineralogist and professor at the Czech Technical University in Prague, author of numerous publications on minerals. It was approved by the International Mineralogical Association (IMA) in 2014. The type locality and only known occurrence is the Svornost mine in Jáchymov, Czech Republic.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light yellow
Density
3.09
Cleavage
Perfect on {010}
Fracture
Splintery
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features of ježekite include its lemon-yellow color, habit of fine, acicular crystals forming fluffy aggregates, and water solubility. Strong fluorescence under UV light and high radioactivity are also characteristic. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals of similar color and form, such as uranophane, zippeite, or schröckingerite. Differentiation requires advanced analytical methods (XRD, EDS). A key distinguishing feature from many of them is its unique solubility in cold water. ## Crystal Forms Ježekite forms very thin, acicular to hair-like crystals that combine into radial or tangled aggregates, as well as fluffy coatings and crusts.

Geological environment

## Genesis It is a secondary mineral, formed in the oxidation zones of uranium deposits as a result of post-magmatic processes. It forms on the walls of old mine workings due to the evaporation of solutions rich in uranium, sodium, sulfates, and carbonates under specific microclimatic conditions. ## Mineral Associations At the type locality, it co-occurs with gypsum, schröckingerite, andersonite, and other secondary uranyl minerals. ## Localities The only confirmed occurrence worldwide is the historic Svornost uranium mine in Jáchymov, Karlovy Vary Region, Czech Republic.

Rarity

Extremely rare

For collectors

## Quality Criteria As an extremely rare "micromineral," ježekite is valued by specialized collectors of rare minerals and those from uranium deposits. The value of a specimen is primarily determined by the abundance and size of aggregates on the host rock, as well as the aesthetics and intensity of its color. Due to the microscopic size of the crystals, specimens are evaluated under magnification. ## Popular Localities The only source of specimens is the type locality – the Svornost mine in Jáchymov, Czech Republic.

Care and storage

## Cleaning The mineral is water-soluble, so wet cleaning is absolutely forbidden. Any contact with water, even a damp cloth, will destroy the specimen. Cleaning is inadvisable and practically impossible due to its delicacy and solubility. ## What to Avoid Avoid water, moisture, all chemicals, high temperatures, and direct sunlight, which can cause fading. This is a highly radioactive mineral – it requires specialized and careful handling. Direct contact, inhalation of dust, and storage in areas of permanent human habitation should be avoided. ## Storage Ježekite specimens must be stored in sealed, dry containers, preferably with a desiccant (e.g., silica gel). Due to radioactivity, they should be kept in special, lead-lined containers, away from other minerals (especially those sensitive to radiation) and in a safe, marked location.

Sources

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