Fontanite
Chemical formula: Ca(U⁶⁺O₂)₃(CO₃)₂O₂·6H₂O
Fontanite is a rare, secondary uranyl mineral, forming transparent, bright yellow crystals and coatings, prized by collectors of rare minerals.
Properties
- Mohs hardness
- 3
- Luster
- Vitreous
- Streak
- White
- Density
- 4.10
- Cleavage
- Good on {100}, {010}, and {001}.
- Fracture
- Uneven
- Transparency
- Transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Fontanite is identified by its bright yellow color, its occurrence as small, tabular crystals forming aggregates, and its characteristic geological environment (oxidation zones of uranium deposits). A key diagnostic feature is its strong radioactivity, easily detectable with a Geiger-Müller counter. ## Distinguishing from Similar Minerals Fontanite can be confused with many other yellow secondary uranium minerals, such as uranophane, boltwoodite, or zippeite. Distinguishing it from these minerals based on visual characteristics is very difficult and often impossible without advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). Good cleavage in three directions can be a helpful clue. ## Crystal Forms Fontanite crystals are small, tabular or bladed, elongated along the crystallographic axis. They typically form radial or tangled aggregates, as well as thin coatings and crusts on rock surfaces.
Geological environment
## Genesis Fontanite is a secondary mineral, formed in the oxidation (weathering) zones of uranium deposits. It forms as a result of the action of carbonate-rich waters on primary uranium minerals, such as uraninite. This process occurs under conditions of low temperature and pressure, close to the Earth's surface. ## Mineral Associations This mineral co-occurs with other secondary uranium minerals. In its type locality (Rabejac Est), it was found in association with uraninite, boltwoodite, uranophane, gypsum, calcite, and manganese oxides. ## Localities The most important and best-documented occurrence of fontanite is its type locality: the Rabejac Est uranium deposit near Lodève in the Hérault department (Occitania, France). It is a very rare mineral, and its occurrence in other places worldwide has not been unequivocally confirmed or is subject to ongoing research.
Rarity
Very rare
For collectors
## Quality Criteria The collector appeal of fontanite primarily depends on the quality and abundance of crystals on the rock matrix. Specimens with well-formed, clearly visible crystals forming aesthetic, radial aggregates are most highly valued. The contrast of bright yellow crystals with a darker host rock also enhances visual appeal. Due to the microscopic size of the crystals, specimens are typically intended for "micromount" collections. ## Popular Localities The only confirmed and recognized source of collectible specimens is the type locality – Rabejac Est in France. Specimens from this location, especially those well-documented, are sought after by collectors specializing in rare and systematic minerals.
Care and storage
## Cleaning Fontanite specimens are extremely delicate and sensitive. Cleaning should be kept to an absolute minimum. If necessary, a soft brush can be used to remove dust or compressed air from a safe distance. Avoid contact with water and any chemicals. ## What to Avoid Fontanite is sensitive to changes in humidity and temperature. It should not be washed in water, especially hot water, nor should ultrasonic cleaners be used. Avoid contact with acids and other chemicals. As a highly radioactive mineral, it requires careful handling – hands should be washed after each contact, and dust inhalation should be avoided. ## Storage Specimens should be stored in stable conditions, away from direct sunlight, heat sources, and moisture. It is best to keep them in closed, padded "micromount" boxes, which protect delicate crystals from mechanical damage. Due to radioactivity, specimens should be stored away from areas of constant human presence, in a well-ventilated room, and appropriately labeled.