Seelite
Chemical formula: Mg(U⁶⁺O₂)₂(As³⁺O₃,As⁵⁺O₄)₂·7H₂O
Seelite is a rare, secondary uranyl and arsenic mineral, forming characteristic yellow, acicular crystals and radial aggregates.
Properties
- Mohs hardness
- 3
- Color
- Canary yellow to bright yellow
- Luster
- Vitreous
- Streak
- White
- Density
- 3.70
- Fracture
- Irregular/Uneven
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Characteristic features of seelite include its canary-yellow color, acicular crystal habit, and tendency to form radial aggregates. An important feature is also its strong, green fluorescence under ultraviolet light (both short-wave and long-wave). ## Distinguishing from Similar Minerals Seelite can be confused with other secondary uranium minerals of similar color and form, such as uranophane, meta-autunite, or zeunerite. Decisive for identification is chemical composition analysis (requiring EDS/WDS analysis to confirm the presence of magnesium and arsenic) and the environment of occurrence. ## Crystal Forms Crystals are elongated, acicular, or hair-like. They typically form radial, stellate, or spherical aggregates, as well as crusts and coatings on the host rock.
Geological environment
## Genesis Seelite is a secondary mineral formed in the oxidation (weathering) zones of uranium ore deposits, in the presence of arsenic minerals. It forms in rock cavities and fissures as a result of surface waters acting on primary uranium ores. ## Mineral Associations This mineral co-occurs with other secondary uranium and arsenic minerals. It is most often associated with uraninite, quartz, hematite, uranyl arsenate, uranophane, schoepite, trögerite, and zeunerite. ## Localities The most important and classic localities for this mineral are the Schmiedestollen mine in Wittichen in the Black Forest (Baden-Württemberg, Germany), which is its type locality. It is also known from several other places in Germany (e.g., Sophia mine near Wittichen) and from Jáchymov in the Czech Republic.
Rarity
Very rare
For collectors
## Quality Criteria Specimens most valued by collectors are those with well-formed, distinct radial aggregates of intense yellow color, contrasting with the dark host rock. Due to the microscopic nature of the crystals, their abundance and aesthetic arrangement on the matrix are crucial. Specimens from the type locality (Wittichen) are particularly sought after. ## Popular Localities By far the most desired specimens come from mines in the Wittichen area of the German Black Forest. These are considered classic for this mineral.
Care and storage
## Cleaning Seelite specimens are extremely delicate and brittle. Mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used from a safe distance to remove dust. Avoid contact with water. ## What to Avoid Seelite is a uranium mineral and is radioactive – direct exposure should be limited, and hands should always be washed after contact. Avoid all chemicals, ultrasound, and high temperatures. As a hydrated mineral, it may undergo changes in overly dry environments. ## Storage Specimens should be stored in closed, stable containers (e.g., "micromount" type) to protect them from mechanical damage and dust. Store away from areas of permanent human presence due to radioactivity.