Oursinite

Chemical formula: Co<sup>2+</sup>(U<sup>6+</sup>O<sub>2</sub>)<sub>2</sub>(SiO<sub>3</sub>OH)<sub>2</sub>·6H<sub>2</sub>O

Oursinite is a very rare, radioactive uranium and cobalt mineral, forming characteristic, acicular aggregates of a pale yellow color.

## Characteristics Oursinite is a uranyl cobalt silicate, occurring in the form of small, acicular crystals. These crystals almost always form spherical or hemispherical, radial aggregates, which can reach several millimeters in diameter. The structure of these clusters resembles small sea urchins, which inspired the mineral's name. The color of oursinite is usually pale yellow, lemon yellow to straw yellow. Due to the fine-grained, fibrous structure of the aggregates, this mineral exhibits a characteristic, silky luster. ## Physical Properties Oursinite is a relatively soft mineral, with a hardness of about 3 on the Mohs scale. Its crystals are brittle. It has a density of approximately 3.5-3.6 g/cm³. It is translucent and exhibits a silky, and on rare crystal surfaces, a vitreous luster. As a uranium mineral, oursinite is strongly radioactive, and this characteristic is crucial for its identification. ## Colors and Varieties This mineral occurs in shades of yellow. No color varieties or commercial varieties are distinguished. ## History and Name Oursinite was first described in 1953 by the Belgian mineralogist Johannes Vaes. The name comes from the French word *oursin*, meaning sea urchin, and refers to the typical form of the mineral's radial aggregates. It also refers to the name of the "Oursin" excavation in the Shinkolobwe mine, where the discovery was made. ## Applications Due to its extreme rarity and strong radioactivity, oursinite has no industrial application. It is solely an object of scientific interest and a prized acquisition in advanced collections of rare and radioactive minerals.

Properties

Mohs hardness
3
Luster
Silky
Streak
Pale yellow
Density
3.5-3.6
Cleavage
Good on {010} and {001}
Fracture
Splintery
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification The most important diagnostic features of oursinite are its unique form (radial aggregates of acicular crystals), pale yellow color, and strong radioactivity, easily detectable with a Geiger counter. The place of origin is also crucial – almost all specimens come from the Shinkolobwe mine. ## Distinguishing from Similar Minerals Oursinite is sometimes confused with other secondary uranium minerals of similar color and acicular form, such as uranophane or sklodowskite. Uranophane (calcium uranyl silicate) often forms similar clusters but usually has a lighter, pale yellow color. Sklodowskite (magnesium uranyl silicate) can also occur in acicular form. Reliable differentiation of these minerals under amateur conditions is very difficult and usually requires advanced chemical analyses (detection of cobalt in oursinite) or X-ray methods (XRD). ## Crystal Forms Oursinite crystals are elongated, acicular (acicular habit). They almost always occur in the form of spherical or hemispherical aggregates with a radial structure, resembling sea urchins or fluffy balls in appearance.

Geological environment

## Genesis Oursinite is a secondary mineral, formed in the oxidation zone of uranium-cobalt deposits. It forms as a result of weathering and alteration of primary uranium ores (mainly uraninite) in the presence of solutions rich in silica and cobalt. ## Mineral Associations This mineral occurs in association with other secondary uranium minerals, such as becquerelite, schoepite, kasolite, curite, sklodowskite, and uranophane. Co-occurrence with these minerals is typical for the oxidation zone of the Shinkolobwe deposit. ## Localities The only significant and confirmed locality of oursinite in the world is its type locality – the Shinkolobwe mine in Katanga Province, Democratic Republic of Congo. This mine has been closed and flooded for years, making new acquisition of this mineral impossible.

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued oursinite specimens are those with well-formed, complete, and undamaged spherical aggregates. The intensity and purity of the lemon-yellow color are important. Specimens on a contrasting rock matrix or in association with other rare uranium minerals (e.g., red curite) achieve significantly higher value. The size of the aggregates is also important, although even millimeter-sized "balls" are considered valuable. ## Popular Localities The only source of valuable collector specimens is historical collections from the Shinkolobwe mine in the DRC. This mineral is a symbol of this classic, now inaccessible locality, which further elevates its status among collectors.

Care and storage

## Cleaning Due to its fragility and potential risk (radioactivity), cleaning oursinite specimens is not recommended. If necessary, a soft brush or carefully applied compressed air can be used to remove dust. Contact with water and chemical agents should be strictly avoided. ## What to Avoid Oursinite is strongly radioactive. Direct, prolonged contact should be avoided, and it should never be stored in residential or office spaces. Inhaling dust is extremely dangerous. The mineral is brittle and susceptible to mechanical damage. It should be protected from high temperatures and humidity. ## Storage Oursinite specimens must be stored in specialized, lead-lined containers that isolate radiation. The container should be clearly marked with the international symbol for ionizing radiation. It should be kept away from other minerals that may be damaged (color degradation) by radiation, such as smoky quartz, fluorite, or topaz.

Sources

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