Françoisite-(Ce)

Chemical formula: Ce₃+(U₆+O₂)₃(PO₄)₂O(OH)·₆H₂O

Françoisite-(Ce) is a rare uranyl and cerium mineral, forming yellow, tabular crystals with strong radioactivity and fluorescence.

## Characteristics Françoisite-(Ce) is a complex, hydrated uranyl and cerium phosphate. It occurs as small, tabular or bladed crystals, often forming radial aggregates, rosettes, or crusts. Crystals are typically very small, rarely exceeding 1-2 mm. The mineral is characterized by an intense yellow color in various shades. ## Physical Properties Crystals exhibit a vitreous or slightly resinous luster. The mineral is translucent. Due to the presence of uranium, it is strongly radioactive. It also displays strong, yellowish-green fluorescence under ultraviolet light (both shortwave and longwave). ## Colors and Varieties Françoisite-(Ce) ranges in color from pale yellow, through golden yellow, to orange-yellow. No distinct color varieties or commercial names are recognized. ## History and Name The mineral was discovered in the Kamoto East uranium mine in the Democratic Republic of Congo (formerly Zaire). It was described in 1988 by Paul Piret and Michel Deliens. The name honors Léon François (1913-1990), a Belgian geologist who made significant contributions to the understanding of uranium deposit geology in the Shaba region. The suffix "-(Ce)" indicates the dominant rare-earth element in its chemical composition, in accordance with IMA nomenclature rules. ## Uses Due to its rarity, small crystal size, and strong radioactivity, françoisite-(Ce) has no industrial applications. It is solely an object of interest for collectors specializing in uranium minerals and for scientists.

Properties

Mohs hardness: 3
Luster: Vitreous to resinous
Streak: Light yellow
Density: 4.69
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features include its characteristic yellow color, tabular or bladed crystal habit forming radial aggregates, strong radioactivity, and intense yellowish-green fluorescence under UV light. ## Distinguishing from Similar Minerals Françoisite-(Ce) can be confused with other yellow secondary uranium minerals such as autunite, meta-autunite, uranophane, or phosphuranylite. Differentiation often requires chemical analysis (EDS) to confirm the presence of cerium. The characteristic radial habit of aggregates is a helpful visual clue, but it is not a unique feature. ## Crystal Forms The mineral forms flattened, tabular crystals with a rhombic outline or blades elongated in one direction. These crystals almost always occur in radial, stellate, or fan-shaped aggregates, as well as in the form of rosettes and thin crusts on the host rock.

Geological environment

## Genesis Françoisite-(Ce) is a secondary mineral of the oxidation zone of hydrothermal uranium deposits. It forms as a result of weathering and alteration of primary uranium minerals (mainly uraninite) in the presence of waters rich in phosphates and rare-earth elements, especially cerium. ## Mineral Associations It most commonly co-occurs with other secondary uranium minerals such as uranophane, curite, soddyite, kasolite, torbernite, as well as with quartz, dolomite, and clay minerals. ## Localities The most important and type locality is the Kamoto East mine, Kolwezi District, Lualaba Province, Democratic Republic of Congo. This is the only confirmed and well-documented locality for this mineral worldwide.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens are those with well-formed, sharp crystals forming aesthetic, radial aggregates of intense, golden-yellow color. Contrast with dark host rock and absence of mechanical damage are important. Due to the microscopic size of the crystals, specimens with larger, eye-visible aggregates are particularly sought after. ## Popular Localities The only source of collector specimens is the historic locality at the Kamoto East mine in the Democratic Republic of Congo. Material from this locality is very limited and rarely appears on the market.

Care and storage

## Cleaning Specimens should not be wet cleaned. A soft brush or compressed air from a safe distance can be used to remove dust. All operations should be performed with extreme caution due to the mineral's fragility and radioactivity. ## 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, although fading is not a primary concern. The greatest danger is its radioactivity – avoid inhaling dust and direct skin contact. ## Storage Françoisite-(Ce) specimens must be stored in specialized, sealed, and appropriately labeled containers (e.g., "perky box" type) away from other radiation-sensitive minerals. They should be kept in a location away from people, especially children, in a well-ventilated room. It is not suitable for display in open showcases.

Sources

IMA Mineral List (CNMNC)