Guilleminite

Chemical formula: Ba(U⁶⁺O₂)₃(Se⁴⁺O₃)₂O₂·₃H₂O

Guilleminite is a rare, bright yellow uranyl and selenium mineral, forming small, platy crystals and characteristic rosette aggregates.

## Characteristics Guilleminite is a hydrated uranyl barium selenite. It occurs as small, bladed or platy crystals, which often form fan-shaped or rosette-like clusters and aggregates. Its most characteristic feature is its intense, canary-yellow to golden-yellow color. It is a secondary mineral, forming in the oxidation zones of uranium deposits containing selenium. ## Physical Properties Guilleminite crystals are very small and brittle. The mineral exhibits strong radioactivity due to its high uranium content. It is characterized by a vitreous or slightly pearly luster on cleavage surfaces. It is a relatively soft and light mineral, although its density is elevated due to the presence of barium and uranium. ## Colors and Varieties Guilleminite occurs in a uniform, characteristic color ranging from canary-yellow, through golden-yellow, to yellow with a greenish tint. No color or commercial varieties are distinguished. ## History and Name The mineral was described in 1964 by R. Pierrot, J. Toussaint, and T. Verbeek. Its name commemorates Jean Claude Guillemin (1923–1994), a French mineralogist and director of research at the Bureau de Recherches Géologiques et Minières (BRGM) in Orléans, France, in recognition of his contributions to the study of uranium minerals. ## Uses Due to its rarity, small crystal size, and high radioactivity, guilleminite has no industrial applications. It is solely an object of interest for collectors specializing in uranium and systematic minerals.

Properties

Mohs hardness: 2
Color: Bright yellow
Luster: Vitreous, Pearly
Streak: Light yellow
Density: 4.88
Cleavage: {100} perfect, {010} good.
Fracture: Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Guilleminite is identified by its intense, canary-yellow color, characteristic rosette or fan-shaped aggregates composed of small, bladed crystals, and strong radioactivity. It also exhibits distinct yellow-green fluorescence under ultraviolet light (both shortwave and longwave). ## Distinguishing from Similar Minerals It can be confused with other secondary yellow uranium minerals, such as autunite, uranophane, or demesmaekerite. It differs from autunite by the lack of distinct, square crystal forms. From uranophane, it differs in the form of aggregates (rosette instead of acicular) and often a more intense hue. Demesmaekerite, also a uranyl selenite, usually has a greenish or brownish color and crystallizes in a different system. Final differentiation requires advanced analyses (XRD, EDS). ## Crystal Forms Crystals are very small, elongated, and flattened (bladed, platy). They most often form radial, fan-shaped, or spherical aggregates and rosettes, as well as coatings and crusts on the host rock.

Geological environment

## Genesis Guilleminite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal uranium deposits that also contain selenium. It forms as a result of the alteration of primary uranium minerals (mainly uraninite) in the presence of selenium-rich solutions. ## Mineral Associations It most often co-occurs with other secondary uranium and selenium minerals. Typical associated minerals include uraninite, demesmaekerite, kasolite, anglesite, barite, chalcedony, and clay minerals. ## Localities The most important and type locality (località tipo) is the Musonoi mine near Kolwezi, in Lualaba Province (formerly Katanga) in the Democratic Republic of Congo. It is also known from the Eureka uranium mine in Spain and from several localities in Germany (Saxony) and the Czech Republic (Jáchymov).

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, distinct rosettes or fans of crystals, of an intense, canary-yellow color. Contrast with the rock matrix and the presence of rare associated minerals are also important. Due to the microscopic size of the crystals, specimens suitable for viewing under magnification (so-called "micromounts") are standard. ## Popular Localities By far the most desirable and classic guilleminite specimens come from its type locality - the Musonoi mine in the Democratic Republic of Congo. Material from this location is considered exemplary for this species.

Care and storage

## Cleaning Guilleminite specimens should not be wet cleaned. Dust can only be removed using a soft brush or a photographic air blower. Due to radioactivity, all operations should be performed with caution, avoiding dust inhalation. ## What to Avoid Contact with water and chemicals must be strictly avoided. The mineral is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight, which can cause dehydration and disintegration. ## Storage Guilleminite must be stored in a specialized, sealed, and appropriately labeled container (e.g., a "perky box") to protect it from mechanical damage and limit radiation emission. It should be kept away from other radiation-sensitive minerals and in a dry place, away from heat and light sources.