Tengchongite

Chemical formula: Ca(U₆+O₂)₆(Mo₆+O₄OH)₂O₂(OH)₄·₉H₂O

Tengchongite is a very rare, bright yellow uranyl molybdate mineral, forming small, tabular crystals with strong radioactivity.

## Characteristics Tengchongite is a hydrated uranyl calcium molybdate. It occurs as very small, tabular or lath-like crystals, which rarely exceed 1 mm in length. These crystals often form rosette-like or radial aggregates, as well as thin coatings and crusts on rock surfaces. Its most characteristic feature is its intense, bright yellow or canary-yellow color. ## Physical Properties Tengchongite crystals exhibit a vitreous or slightly pearly luster, especially on cleavage surfaces. It is a soft mineral, with a hardness of about 2 on the Mohs scale. Due to the presence of uranium, it is strongly radioactive. ## Colors and Varieties This mineral is monochromatic and occurs exclusively in shades of bright yellow. No color varieties or commercial varieties are distinguished. ## History and Name The name tengchongite comes from its discovery locality – the Rehai geothermal volcanic field in Tengchong County (Baoshan Prefecture, Yunnan Province, China). It was described and approved as a new mineral by the International Mineralogical Association (IMA) in 1984. ## Uses Due to its extreme rarity, small crystal size, and strong radioactivity, tengchongite has no industrial applications. It is of scientific importance only and is an object of interest for specialized collectors of uranium minerals.

Properties

Mohs hardness: 2
Luster: Vitreous, Pearly
Streak: Light yellow
Density: 4.23
Cleavage: Perfect on {100}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification The most important diagnostic features of tengchongite are its bright yellow color, characteristic form of small, tabular crystals forming rosette-like aggregates, and very strong radioactivity, easily detectable with a Geiger counter. It occurs in the specific environment of secondary oxidation zones of uranium deposits. ## Distinguishing from similar minerals It can be confused with other yellow secondary uranium minerals, such as uranophane, carnotite, or tyuyamunite. Differentiation requires specialized tests, most often chemical analysis (EDS) or X-ray diffraction (XRD), as visual features can be very similar. Tengchongite is distinguished by its unique chemical composition, containing calcium, uranium, and molybdenum simultaneously. ## Crystal forms Tengchongite crystallizes in the form of very small, thin, tabular crystals with a rectangular or lath-like outline. These crystals almost always occur as radial or rosette-like clusters and aggregates. Less commonly, they form thin, brittle coatings on the surface of the host rock.

Geological environment

## Genesis Tengchongite is a secondary mineral, formed in the oxidation (weathering) zones of uranium deposits. It forms as a result of molybdenum-rich waters acting on primary uranium minerals. Its formation is associated with hydrothermal activity in volcanic areas, as in the case of its type locality. ## Mineral associations At its type locality (Rehai, China), it co-occurs with uranophane, calcite, and quartz. In other localities, it may be associated with other secondary uranium and molybdenum minerals. ## Localities This is an extremely rare mineral. Apart from its type locality in Tengchong, China, its occurrence has been confirmed in only a few other places worldwide. The most important of these are the Rabejac uranium deposit in the Hérault department (Occitanie, France) and the Krunkelbach uranium mine (Menzenschwand) in the Black Forest (Baden-Württemberg, Germany).

Rarity

Very rare

For collectors

## Quality criteria For collectors, the quality and abundance of well-formed crystals are most important. Specimens with distinct, well-formed, rosette-like crystal aggregates of intense, bright yellow color are most valued. Contrast with the rock matrix and the presence of associated minerals are also important. Due to the microscopic size of the crystals, specimens are usually evaluated under magnification. ## Popular localities By far the most desirable and classic specimens come from the type locality in Tengchong, China. Specimens from the French Rabejac deposit and the German Krunkelbach mine are also valued by specialists, often offering slightly different mineral associations.

Care and storage

## Cleaning Tengchongite specimens are generally not cleaned due to their fragility and radioactivity. Any mechanical (e.g., brush) or chemical intervention risks destroying the brittle crystals. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust particles. ## What to avoid Contact with water and chemicals must be strictly avoided. The mineral is sensitive to changes in humidity and temperature. As a secondary mineral, it can dehydrate in overly dry environments, leading to its disintegration. It should be protected from direct sunlight, which can affect its stability. The greatest threat is its radioactivity – direct skin contact, dust inhalation, and storage in areas of permanent human presence should be avoided. ## Storage Tengchongite specimens must be stored in specialized, sealed, and lead-lined containers designed for radioactive minerals. The container should be clearly marked with the international radioactivity symbol. Store in a dry, cool, and well-ventilated room, away from flammable materials and living spaces.

Sources

IMA Mineral List (CNMNC)