Grimselite
Chemical formula: K₃Na(U⁶⁺O₂)(CO₃)₃·H₂O
Grimselite is a rare, secondary uranyl mineral, forming bright yellow, radial aggregates of acicular crystals.
Properties
- Mohs hardness
- 2-2.5
- Luster
- Vitreous
- Streak
- pale yellow
- Density
- 3.30
- Cleavage
- None
- Fracture
- Conchoidal
- Transparency
- Transparent,Translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Diagnostic features of grimselite include its bright yellow color, characteristic radial or stellate aggregates composed of very fine, acicular crystals, and strong green fluorescence under UV light. It is a secondary mineral, occurring as coatings and clusters on host rocks. ## Distinguishing from Similar Minerals Grimselite can be confused with other secondary uranium minerals of similar color and habit, such as andersonite, liebigite, or schröckingerite. Andersonite crystallizes in the trigonal system and forms more defined, pseudocubic crystals. Liebigite and schröckingerite also form clusters of small crystals and fluoresce strongly, but they differ in chemical composition and subtle differences in crystal habit, requiring advanced studies (e.g., XRD) for certain identification. ## Crystal Forms Grimselite crystallizes as very thin, acicular or hair-like crystals, which almost always occur in radial, stellate, spherulitic, or bundled aggregates. It rarely forms diffuse coatings.
Geological environment
## Genesis It is a secondary mineral, forming in the oxidation zones of uranium deposits. It forms as a result of carbonate-rich waters acting on primary uranium minerals, such as uraninite. It forms under low-temperature conditions, often as post-drying coatings on tunnel walls, in uranium mines, or in rock fractures. ## Mineral Associations It co-occurs with other secondary uranium minerals, such as andersonite, liebigite, schröckingerite, bayleyite, as well as with gypsum, calcite, and uraninite as a primary mineral. ## Localities The most important and classic locality for grimselite is its type locality in the Gerstenegg-Sommerloch tunnel in Switzerland. It is also known from several uranium mines in the Czech Republic (e.g., Jáchymov, Příbram), Germany (Saxony), and the United States (Utah, Wyoming).
Rarity
Rare
For collectors
## Quality Criteria The most prized grimselite specimens are those that exhibit well-defined, radial or stellate aggregates of intense, bright yellow color. Contrast with the dark host rock is important, as it enhances the mineral's aesthetics. The size of the aggregates is crucial – clusters exceeding a few millimeters are already considered significant. The specimen should be free from mechanical damage, which is difficult to achieve due to its fragility. ## Popular Localities Specimens from the type locality in Switzerland are historically most important, although rarely available on the market. High-quality, aesthetic aggregates also come from some mines in the Czech Republic and Germany.
Care and storage
## Cleaning Grimselite specimens are extremely delicate and fragile. Mechanical cleaning, including the use of brushes, should be avoided. If absolutely necessary, dust can be carefully removed with a photographic air blower or compressed air from a distance. The mineral is water-soluble, so contact with water is unacceptable. ## What to Avoid Contact with water, acids, and other chemicals that can dissolve it must be strictly avoided. As a uranium mineral, it is radioactive and requires careful handling – hands should be washed after each contact, and dust inhalation should be avoided. ## Storage Specimens should be stored in closed, airtight containers (e.g., "perky box" type) to protect them from mechanical damage, dust, and moisture. Due to its radioactivity, it should be kept away from areas of permanent human habitation and separated from other minerals that could be damaged by radiation.