Uranophane

Chemical formula: Ca(U⁶⁺O₂)₂(SiO₃OH)₂·₅H₂O

Uranophane is a secondary uranium mineral, forming characteristic, radial aggregates of acicular crystals with an intense yellow color.

## Characteristics Uranophane is a hydrated uranyl calcium silicate, belonging to the uranium mineral group. Its most typical forms of occurrence are radial aggregates, crusts, and fibrous or acicular clusters. Well-formed crystals are rare, usually appearing as elongated, flattened needles. This mineral is valued by collectors for its intense, lemon-yellow to greenish-yellow color and characteristic forms. ## Physical Properties Uranophane crystals exhibit a vitreous luster, and a silky or dull luster on fracture surfaces. It is a relatively soft mineral, with a hardness of 2.5 on the Mohs scale. Its density is approximately 3.85 g/cm³. It is transparent to translucent. Uranophane shows strong green fluorescence under ultraviolet light. As a uranium mineral, it is highly radioactive. ## Colors and Varieties Uranophane occurs in various shades of yellow – from pale yellow, through canary and lemon, to greenish-yellow, and even orange-yellow. No named varieties are distinguished, and color differences mainly depend on impurities and crystallization conditions. ## History and Name The name uranophane comes from its chemical composition – its uranium content – and from the Greek word *phaínein* (φαίνειν) meaning "to appear," referring to its frequent occurrence as a weathering product of other uranium minerals. The mineral was first described in 1853 by the German mineralogist Martin Websky. ## Uses Due to its uranium content, uranophane is a minor ore of this element, but it rarely forms deposits of industrial significance. Its main role is as an indicator mineral in the search for richer uraninite deposits. It is primarily an object of interest for collectors specializing in uranium and fluorescent minerals.

Properties

Mohs hardness: 2-3
Color: Light yellow, lemon-yellow, honey-yellow, straw-yellow, green-yellow
Luster: Vitreous
Streak: Pale yellow
Density: 3.8
Cleavage: on {100}
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Uranophane is most easily recognized by its intense, lemon-yellow color, acicular or fibrous aggregate form (often radial "suns" or "hedgehogs"), and strong green fluorescence under UV light. A key diagnostic feature is also its high radioactivity, easily detectable with a Geiger counter. ## Distinguishing from Similar Minerals Uranophane is sometimes confused with other yellow secondary uranium minerals, such as autunite, torbernite (in weathered form), or carnotite. Autunite often forms tabular crystals and has a more intense, greenish-yellow fluorescence. Carnotite is usually more dull, earthy, and forms coatings rather than acicular crystals. Beta-uranophane is its polymorph, and distinguishing it without advanced studies (XRD) is practically impossible. ## Crystal Forms Uranophane crystals are monoclinic, but almost always occur as very thin, elongated needles or blades. They form characteristic radial, fan-shaped, fibrous aggregates, as well as crusts and coatings on other minerals, mainly on uraninite.

Geological environment

## Genesis Uranophane is a typical secondary mineral, forming in the oxidation (weathering) zone of uranium deposits. It forms as a result of the alteration of primary uranium minerals, mainly uraninite, under the influence of waters containing dissolved silica and calcium. It occurs in fractures and fissures of sedimentary rocks (sandstones, limestones) and in granitic pegmatites containing uraninite. ## Mineral Associations It most often co-occurs with the mineral from which it formed – uraninite. It is also accompanied by other secondary uranium minerals, such as beta-uranophane, kasolite, autunite, torbernite, as well as quartz, feldspars, micas, calcite, gypsum, and iron oxides (limonite, hematite). ## Localities Significant uranophane specimens come from many places around the world. In Europe, it is known from Wölsendorf in Bavaria (Germany) and Jáchymov and Příbram (Czech Republic). In the USA, beautiful specimens have been found in mines in Utah and New Mexico, as well as in the Bancroft area in Ontario (Canada). Important localities are also in the Democratic Republic of Congo (Katanga province), where some of the world's best specimens were found in the Shinkolobwe and Musonoi mines.

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued uranophane specimens are those with well-formed, radial aggregates of intense, lemon-yellow color, set on a contrasting substrate (matrix). The size and completeness of the "suns" or "hedgehogs" are important – the larger and undamaged, the more valuable. Specimens with visible, individual acicular crystals are rarer and more sought after. Association with other rare secondary minerals also enhances attractiveness. ## Popular Localities For collectors, the most desirable are classic specimens from Shinkolobwe in the Democratic Republic of Congo, which are considered exemplary. Specimens from Jáchymov in the Czech Republic and from the Madawaska mine near Bancroft in Canada, known for large and well-formed aggregates, are also highly valued.

Care and storage

## Cleaning Uranophane specimens are very delicate and brittle. Cleaning should be kept to an absolute minimum. A soft brush can be used to remove dust or, if necessary, compressed air from a safe distance. Avoid contact with water, which can damage delicate crystals. ## What to Avoid Absolutely avoid washing in water, ultrasonic cleaners, and using any chemical agents. Uranophane is sensitive to changes in temperature and humidity. As a radioactive mineral, it requires careful handling – wash hands after each contact and avoid inhaling dust. ## Storage Specimens should be stored under stable conditions, away from direct sunlight, heat sources, and humidity. It is best to place them in a closed, padded display box or cabinet to protect them from mechanical damage and dust. Due to radioactivity, it should not be stored in bedrooms or places of constant human presence. It is recommended to label the container with a radioactivity symbol.