Vanmeersscheite

Chemical formula: U⁶⁺(U⁶⁺O₂)₃(PO₄)₂(OH)₆·₄H₂O

A rare, secondary uranyl mineral, forming characteristic canary-yellow aggregates of radial needles.

## Characteristics Vanmeersscheite is a secondary, hydrated uranyl phosphate that visually presents as aggregates of fine, acicular crystals. It forms characteristic, radial aggregates and rosettes of an intense, canary-yellow color. Individual crystals are usually very small and elongated, giving the aggregates a fibrous or silky appearance. ## Physical Properties The crystals are translucent and exhibit a vitreous to silky luster on the aggregate surfaces. The mineral is very soft and brittle. Due to its chemical composition, it is highly radioactive. ## Colors and Varieties This mineral occurs in one characteristic color – canary-yellow. No color or commercial varieties are distinguished. ## History and Name Vanmeersscheite was first described in 1981 by Paul Piret and Michel Deliens. The name honors Maurice Van Meerssche (1923–1988), a Belgian crystallographer from the University of Louvain, who made significant contributions to the structural studies of uranium minerals. ## Uses Due to its rarity and small crystal size, vanmeersscheite has no industrial application. It is solely of interest to collectors specializing in uranium minerals and an object of scientific research.

Properties

Density: 0
Cleavage: {010} good, {100} fair.
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features include its canary-yellow color, the formation of radial or spherulitic aggregates composed of fine, acicular crystals, and strong radioactivity. It occurs in the oxidation zones of granitic pegmatites. ## Distinguishing from similar minerals It can be confused with other secondary uranium minerals of similar color and form, such as meta-autunite, uranophane, or studtite. Definitive differentiation often requires chemical analysis (EDS) or X-ray diffraction (XRD). It is distinguished by its co-occurrence with other uranyl phosphates in its typical locality. ## Crystal forms It forms acicular, elongated crystals, which almost always occur as radial aggregates, rosettes, spherulitic clusters, or crusts with a fibrous texture.

Geological environment

## Genesis Vanmeersscheite is a secondary mineral, forming in the weathering (oxidation) zone of uranium deposits, particularly within granitic pegmatites. It forms as a result of the alteration and decomposition of primary uranium minerals, such as uraninite, in the presence of phosphate-rich waters. ## Mineral associations It most commonly co-occurs with other secondary uranyl phosphates. In its typical locality (Kobokobo), it has been found in association with metatorbernite, meta-autunite, phosphouranylite-Ca, threadgoldite, and mundite. ## Localities The only confirmed and thoroughly described occurrence of vanmeersscheite in the world is its type locality – the Kobokobo pegmatite in South Kivu Province, Democratic Republic of Congo.

Rarity

Very rare

For collectors

## Quality criteria The most desirable specimens are those with well-formed, distinct radial aggregates of intense, canary-yellow color, contrasting with darker host rock. The size and aesthetics of the rosettes, as well as the absence of mechanical damage, significantly increase their collector value. Specimens with well-identified associated minerals are also valuable. ## Popular localities The only source of collector specimens is the Kobokobo pegmatite in the Democratic Republic of Congo. Material from this locality is considered classic and is the standard for this species.

Care and storage

## Cleaning The mineral is extremely delicate and brittle. It should not be cleaned mechanically or with water. The safest method for dust removal is to use a stream of compressed air from a safe distance. ## What to avoid Avoid contact with water, chemicals, ultrasound, and any vibrations or impacts. As a secondary mineral, it is susceptible to changes in humidity conditions. It is highly radioactive, therefore requiring special precautions – avoid direct contact, inhalation of dust, and store it away from areas of permanent human presence. ## Storage Specimens should be stored in sealed, lead-lined or thick plastic containers, appropriately labeled as radioactive material. It is best to keep them in stable conditions, away from light, moisture, and heat sources, in a dedicated place for a collection of radioactive minerals.

External references

https://www.mindat.org/min-4147.html