Metakahlerite
Chemical formula: Fe²⁺(U⁶⁺O₂)₂(As⁵⁺O₄)₂(H₂O)₅·3H₂O
Metakahlerite is a rare, secondary iron uranyl arsenate mineral, distinguished by its sulfur-yellow color and its membership in the meta-autunite group.
Properties
- Mohs hardness
- 2-3
- Luster
- Vitreous
- Streak
- pale yellow
- Density
- 0
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Metakahlerite is primarily identified by its characteristic sulfur-yellow color, small tabular crystals often in rosette-like aggregates, and occurrence in the oxidation zones of uranium deposits. A key diagnostic feature is its strong radioactivity, detectable with a Geiger counter. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals from the autunite group, such as autunite, meta-autunite, or torbernite. Differentiation from kahlerite requires X-ray diffraction (XRD) analysis to determine the degree of hydration. It is distinguished from other similar minerals by its chemical composition (presence of iron and arsenic), which, however, can only be confirmed by advanced analytical methods. ## Crystal Forms It forms very small, thin, tabular crystals with a square or rectangular outline. These crystals often combine into rosette-like or fan-shaped aggregates. It also occurs as earthy coatings and crusts on host rocks.
Geological environment
## Genesis Metakahlerite is a secondary mineral, formed in the oxidation (weathering) zones of hydrothermal uranium deposits rich in arsenic. It forms as a result of dehydration (partial loss of water) of the primary mineral, kahlerite, under conditions of low air humidity. ## Mineral Associations It most commonly co-occurs with kahlerite, from which it directly forms. It is also accompanied by other secondary uranium minerals, such as zeunerite, uraninite (pitchblende), as well as quartz, hematite, and various clay minerals. ## Localities The most important and type locality is the historic Sophia mine in the Böckelsbach valley (Wittichen, Black Forest) in Germany, where it was discovered. It is also known from several other localities worldwide, including uranium mines in the Ústí region of the Czech Republic.
Rarity
Very rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp microcrystals forming aesthetic, rosette-like aggregates. An intense, clean, sulfur-yellow color and contrast with the dark host rock are important. Due to the microscopic size of the crystals, specimens suitable for viewing under magnification are particularly sought after. ## Popular Localities By far the most classic and prized specimens come from the type locality – the Sophia mine in the Black Forest, Germany. Material from this location is considered exemplary for this species.
Care and storage
## Cleaning Metakahlerite specimens should generally not be wet-cleaned. Only gentle dust removal with a soft brush or compressed air from a safe distance is permissible. ## What to Avoid Contact with water and chemicals, which can damage or dissolve the delicate crystals, must be strictly avoided. The mineral is sensitive to changes in humidity and temperature, which can lead to further dehydration or rehydration, destroying the crystal structure. As a uranium mineral, it is radioactive and requires careful handling – avoid inhaling dust and wash hands after each contact. ## Storage Specimens should be stored in sealed, stable containers (e.g., "perky boxes") to protect them from mechanical damage and humidity changes. Due to its radioactivity, it should be kept away from areas of constant human presence, in a designated location, preferably as part of a specialized collection of radioactive minerals.