Françoisite-(Nd)
Chemical formula: Nd(U<sup>6+</sup>O<sub>2</sub>)<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>O(OH)·6H<sub>2</sub>O
Françoisite-(Nd) is a very rare uranyl and neodymium mineral, forming characteristic yellow, acicular crystals in radial aggregates.
Properties
- Mohs hardness
- 3
- Luster
- Silky
- Streak
- Light yellow
- Density
- 4.65
- Cleavage
- Perfect on {100}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Characteristic features include its light yellow color, the form of radial aggregates composed of small, acicular crystals, and strong radioactivity, easily detectable with a Geiger-Müller counter. Under ultraviolet light (both shortwave and longwave), it exhibits strong, green fluorescence. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals of similar appearance, such as studtite, rutherfordine, or schoepite. Definitive differentiation requires advanced analytical methods (e.g., X-ray diffraction XRD or chemical analysis EDS), but the characteristic form of radial aggregates and strong green fluorescence are strong diagnostic indicators. ## Crystal Forms It forms elongated, acicular or bladed crystals, which almost always occur as radial, stellate, or spherulitic aggregates. Single, isolated crystals are extremely rare.
Geological environment
## Genesis This is a secondary uranium mineral, formed in the oxidation zones of uraninite deposits. It crystallizes from aqueous solutions rich in uranium, phosphates, and rare-earth elements, originating from the weathering of primary uranium ores. ## Mineral Associations It most commonly co-occurs with other secondary uranium minerals, such as uranophane, schoepite, becquerelite, as well as with minerals from the monazite group, which are the source of rare-earth elements. It occurs in association with quartz, and also with remnants of primary uraninite. ## Localities The most important and classic occurrence (type locality) is the L’Etoile du Congo mine (formerly Kalongwe) in Katanga Province, Democratic Republic of Congo. This is the only confirmed locality from which well-documented specimens of this mineral originate.
Rarity
Very rare
For collectors
## Quality Criteria The most desirable specimens are those with well-formed, distinct, stellate aggregates on a contrasting rock matrix. The intensity of the yellow color and the size and completeness of the radial "stars" significantly influence collectible value. Specimens with visible associations of other rare uranium minerals are also highly prized. ## Popular Localities The only source of collectible specimens is the L’Etoile du Congo mine in the Democratic Republic of Congo. Material from this locality is considered the type material for this species.
Care and storage
## Cleaning The mineral is very delicate and brittle. Mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust particles. ## What to Avoid Avoid contact with water and all chemicals. The crystals are sensitive to shock and pressure. As a highly radioactive mineral, it requires special precautions. Avoid direct skin contact, inhalation of dust, and storage in areas of permanent human presence. Wash hands thoroughly after each contact. ## Storage Specimens should be stored in a specialized, sealed, lead-lined container to limit radiation emission. The container should be clearly marked with a radioactivity symbol. Store in a dry place, away from other minerals that may be damaged by radiation (e.g., smoky quartz, fluorite).