Asselbornite

Chemical formula: Pb₂+(U₆+O₂)₄(Bi₃+O)₃(As₅+O₄)₂(OH)₇·₄H₂O

Asselbornite is a very rare secondary mineral from the arsenate group, distinguished by its intense yellow color and high radioactivity.

## Characteristics Asselbornite is a hydrated lead, uranium, and bismuth arsenate. It forms very small, cubic crystals, rarely exceeding 0.1 mm in size. It usually occurs as thin coatings, earthy or powdery aggregates on the surface of the host rock. Its most characteristic feature is its intense, lemon-yellow to canary-yellow color. Due to its uranium content, it is a highly radioactive mineral. ## Physical Properties Asselbornite crystals exhibit an adamantine to greasy luster, while earthy aggregates are dull. It is a relatively soft mineral, with a Mohs hardness of 3. Due to the presence of heavy elements such as lead, uranium, and bismuth, it has a high density of approximately 6.5 g/cm³. It is brittle and has an uneven fracture. ## Colors and Varieties This mineral is monochromatic – it occurs exclusively in shades of intense yellow, from lemon to canary. No color or commercial varieties are distinguished. ## History and Name Asselbornite was discovered and described in 1982 by Michel Deliens and Paul Piret. The mineral is named in honor of Eric Asselborn (born 1954), a Luxembourgish mineral collector who provided samples for study. The type locality (locus typicus) is the uranium mine in Schneeberg, Saxony, Germany. ## Uses Due to its extreme rarity, small crystal size, and high radioactivity, asselbornite has no industrial applications. It is of scientific interest only and is sought after by specialized collectors of rare and radioactive minerals.

Properties

Mohs hardness: 3
Luster: Adamantine to greasy
Streak: Light yellow
Density: 6.5
Cleavage: None
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Cubic

Diagnostic features

## Identification Key diagnostic features of asselbornite are its intense yellow color, occurrence as tiny, cubic crystals or earthy coatings, and very strong radioactivity. The reaction to UV radiation is variable – some specimens show weak, greenish fluorescence. ## Distinguishing from Similar Minerals Asselbornite can be confused with other yellow secondary uranium minerals, such as uranophane, carnotite, or tyuyamunite. Distinguishing it based on visual characteristics is extremely difficult and most often requires advanced analytical methods, such as X-ray diffraction (XRD) or EDS spectroscopy. The regular (cubic) crystallographic system is an important clue, as most similar uranyl minerals crystallize in other systems. ## Crystal Forms Asselbornite crystallizes in the isometric system, forming very small, well-formed cubes. These crystals rarely occur individually, more often forming dense aggregates, coatings, and crusts on the host rock.

Geological environment

## Genesis Asselbornite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits containing uranium, bismuth, and arsenic. It forms as a result of complex chemical processes occurring under the influence of groundwater, which dissolves primary minerals (such as uraninite, native bismuth, and arsenopyrite) and precipitates new compounds from them under oxygenated conditions. ## Mineral Associations This mineral co-occurs with other secondary minerals of the oxidation zone. It is most commonly associated with uranophane, metauranopilite, walpurgite, zeunerite, torbernite, and quartz. ## Localities It is an extremely rare mineral, known from only a few localities worldwide. The most important and type locality is the historic mining region of Schneeberg in Saxony (Germany). Besides Germany, its presence has also been reported in Jáchymov (Czech Republic) and in the Rabejac mine in the Hérault department (France).

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized asselbornite specimens are those with visible, well-formed, even if microscopic, cubic crystals. The more intense and saturated the yellow color, the more desirable the specimen. The abundance of occurrence on the rock matrix is also important – specimens with dense, extensive coatings are more valuable than those with single specks. Contrast with a dark host rock also enhances aesthetic appeal. ## Popular Localities By far the most classic and valued specimens come from the type locality – Schneeberg, Germany. Specimens from this locality are considered exemplary for this mineral species.

Care and storage

## Cleaning Asselbornite specimens should generally not be wet-cleaned. Due to its softness and often powdery form, any attempt at mechanical cleaning can lead to damage. Only very careful removal of dust with a soft brush or a photographic air blower is permissible. ## What to Avoid Contact with water and chemicals must be strictly avoided. The mineral is soft and brittle, so it should be protected from impacts and vibrations. As a uranium mineral, it is highly radioactive – it requires special precautions. Direct, prolonged contact should be avoided, dust should never be inhaled, and hands should always be washed after handling the specimen. ## Storage Asselbornite must be stored in a sealed, labeled container, preferably made of radiation-blocking material (e.g., lead or in a specialized collector's box). It should be kept away from other minerals that could be damaged by radiation, and away from areas of permanent human habitation.

Sources

IMA Mineral List (CNMNC)