Metasaléeite

Chemical formula: Mg(U⁶⁺O₂)₂(PO₄)₂·₈H₂O

Metasaléeite is a rare, highly radioactive secondary mineral of the autunite group, prized by collectors for its intense yellow, tabular crystals.

## Characteristics Metasaléeite is a hydrated magnesium uranyl phosphate, belonging to the autunite group. It forms characteristic, thin, square or octagonal crystals with a tabular habit, often with truncated corners. These crystals typically occur as rosette-like or fan-shaped aggregates, as well as thin coatings and crusts on the host rock. Due to its uranium content, this mineral exhibits strong radioactivity and intense, greenish-yellow fluorescence under ultraviolet light (both shortwave and longwave). ## Physical Properties Metasaléeite is a relatively soft mineral, with a hardness of approximately 2.5 on the Mohs scale. Its crystals are brittle and have perfect cleavage in one direction, parallel to the crystal base, which causes them to tend to separate into thin lamellae. The luster is typically vitreous, and on cleavage surfaces, it can be pearly. It is a transparent to translucent mineral. ## Colors and Varieties This mineral occurs in characteristic shades from lemon-yellow to greenish-yellow. Its color is directly related to the presence of the uranyl ion (UO₂)²⁺ in the crystal structure. No named color varieties or commercial varieties are distinguished. ## History and Name Metasaléeite was first described in 1950 by Jacques F. Vaes based on samples from the Shinkolobwe mine in the Democratic Republic of Congo (then Belgian Congo). The name honors the Belgian geologist and crystallographer Achille Salée (1883–1932) from the University of Louvain. The prefix "meta-" refers to a lower hydration state compared to a hypothetical, fully hydrated "saléeite" mineral, analogous to the relationship between metautunite and autunite. ## Uses Due to its rarity and small accumulations, metasaléeite has no industrial application. It is solely an object of interest for collectors specializing in uranium minerals and for scientific institutions.

Properties

Mohs hardness: 2.5
Luster: Vitreous, Pearly
Streak: Light yellow
Density: 0
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Key diagnostic features of metasaléeite are its characteristic lemon-yellow to greenish-yellow color, tabular habit of square crystals, and perfect cleavage in one direction. The most important feature, however, is its strong, greenish-yellow fluorescence under UV light and high radioactivity, easily detectable with a Geiger counter. ## Distinguishing from Similar Minerals Metasaléeite is visually almost impossible to distinguish from other minerals of the autunite group, such as autunite, metautunite, torbernite, metatorbernite, uranocircite, or zeunerite, without advanced chemical (EDS) or X-ray (XRD) analyses. All these minerals form similar tabular crystals and have similar colors. Metasaléeite differs from autunite by a lower degree of hydration, which is an unstable feature and difficult to assess visually. ## Crystal Forms Metasaléeite crystallizes as thin, square or octagonal tablets. These crystals often combine into fan-shaped, radial, or rosette-like aggregates. It also occurs as thin coatings, crusts, and microcrystalline aggregates on rock surfaces.

Geological environment

## Genesis Metasaléeite is a secondary mineral, forming in the oxidation (weathering) zones of uranium ore deposits. It forms as a result of the action of phosphate- and magnesium-rich waters on primary uranium minerals, such as uraninite. It forms under low temperature and pressure conditions, often in voids and fissures of sedimentary rocks (sandstones, shales) or uranium-bearing granites. ## Mineral Associations This mineral often co-occurs with other secondary uranium minerals. Its most common associations include torbernite, autunite, uraninite, uranophane, phosphuranylite, as well as quartz, muscovite, and clay minerals. ## Localities The most important and historical locality, from which classic specimens originate, is the Shinkolobwe mine in Upper Katanga Province, Democratic Republic of Congo. Other significant localities worldwide include the Ranger mine in the Northern Territory, Australia; the areas around Schneeberg and Johanngeorgenstadt in Saxony (Germany); Lodève in Hérault (France); and several locations in the USA, including Utah, Nevada, and Arizona.

Rarity

Rare

For collectors

## Quality Criteria The most prized metasaléeite specimens are characterized by sharply defined, well-formed crystals of intense, lemon-yellow color. Aggregates in the form of aesthetic rosettes or fans, set on a contrasting rock matrix, are highly valued. The transparency and luster of the crystals are also important. Large, undamaged crystals or rich coatings on the matrix significantly increase the collectible value of the specimen. ## Popular Localities Specimens from the type locality – the Shinkolobwe mine in the Democratic Republic of Congo – are considered the best in the world, although they are rare on the market today. High-quality crystals also come from the Ranger mine in Australia. European specimens, e.g., from Germany or France, are also sought after by collectors specializing in systematic or regional minerals.

Care and storage

## Cleaning Metasaléeite specimens are extremely delicate and brittle. Cleaning should be kept to an absolute minimum. If necessary, a soft brush can be used to remove loose dust or compressed air from a safe distance. Contact with water should be avoided, as it can alter the mineral's hydration state and lead to damage. ## What to Avoid Metasaléeite is sensitive to changes in humidity and temperature. Heating or storing in an overly dry environment can lead to further dehydration, causing cracking, crumbling of crystals, and loss of transparency. All chemicals, acids, and detergents should be avoided. Due to its brittleness, ultrasonic and mechanical cleaning should be avoided. ## Storage As a highly radioactive mineral, metasaléeite must be stored with appropriate precautions. It is recommended to keep it in a sealed, lead or thick-glass container, away from other radiation-sensitive minerals. The container should be clearly marked with a radioactivity symbol. It should be stored in a place with stable humidity and temperature, away from direct sunlight and areas of constant human presence.

External references

https://www.mindat.org/min-2664.html