Coconinoite

Chemical formula: Fe<sup>3+</sup><sub>2</sub>Al<sub>2</sub>(U<sup>6+</sup>O<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>4</sub>(S<sup>6+</sup>O<sub>4</sub>)(OH)<sub>2</sub>·20H<sub>2</sub>O

Coconinoite is a very rare uranium mineral with a characteristic creamy-yellow color, occurring as small, tabular crystals.

## Characteristics Coconinoite is a hydrated uranyl, iron, and aluminum phosphate-sulfate. It occurs as very small, tabular or bladed crystals, which typically form rosette aggregates, radial clusters, or thin coatings and crusts on the host rock. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Coconinoite crystals are soft and brittle. This mineral exhibits strong, yellowish-green fluorescence under ultraviolet light (both shortwave and longwave), which is typical for uranyl minerals. It is highly radioactive. ## Colors and Varieties Coconinoite has a characteristic, uniform color, ranging from pale cream through creamy-yellow to light yellow. No varieties have been distinguished. ## History and Name The mineral was discovered in the 1960s, and its description was published in 1966. The name comes from Coconino County in Arizona, USA, where its type locality, the "Sun Valley" uranium mine, is located. It was described by E.J. Young, A.D. Weeks, and Robert Meyrowitz. ## Applications Due to its extreme rarity and microscopic size, coconinoite has no industrial applications. It is solely of scientific interest and is sought after by specialized collectors of uranium minerals.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
Pale yellow
Density
3.55
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Diagnostic features of coconinoite include: creamy-yellow color, form of small, tabular crystals forming rosette aggregates, strong yellowish-green fluorescence under UV light, and high radioactivity. It occurs in the oxidation zones of uranium deposits in sandstones. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals of similar color, such as meta-autunite, carnotite, or tyuyamunite. Differentiation requires specialized tests, such as X-ray diffraction (XRD) analysis. However, coconinoite is distinguished by its characteristic chemical composition, containing both phosphate and sulfate groups simultaneously. ## Crystal Forms It forms very small, thin, tabular crystals with a hexagonal or rhombic outline. These crystals often combine into rosettes, fan-shaped clusters, or form thin coatings on the rock surface.

Geological environment

## Genesis Coconinoite is a secondary uranium mineral, formed in the oxidation (weathering) zone of uranium deposits. It forms as a result of groundwater acting on primary uranium minerals, such as uraninite, in sedimentary rocks, mainly sandstones. Its formation is associated with the presence of sulfides (e.g., pyrite) and phosphates (e.g., apatite) in the host rock. ## Mineral Associations It most commonly co-occurs with other secondary uranium minerals, such as uranophane, autunite, torbernite, as well as with gypsum, jarosite, hematite, and quartz. ## Localities The most important and best-documented localities are in the USA, in Arizona (mines in Coconino and Apache counties) and Utah. Outside the USA, small occurrences have been reported in Germany (Saxony) and Italy (Lombardy).

Rarity

Very rare

For collectors

## Quality Criteria Due to the microscopic nature of the mineral, the most important criterion is the richness and aesthetics of the aggregates on the host rock. Specimens with clearly formed, magnification-visible crystal rosettes covering a significant area of the matrix are most valued. Color intensity and lack of mechanical damage also affect value. ## Popular Localities The most classic and prized specimens come from the type locality in Coconino County, Arizona, USA. Specimens from other localities are also sought after by specialists, but those from Arizona are considered exemplary.

Care and storage

## Cleaning Coconinoite specimens are generally not cleaned due to their fragility and rarity. If necessary, compressed air can be used to remove dust. Avoid contact with water. ## What to Avoid The mineral is very brittle and susceptible to mechanical damage. Contact with water and any chemicals should be strictly avoided. As a uranium mineral, it is highly radioactive, so exposure should be limited, and dust inhalation avoided. Hand washing after every contact is mandatory. ## Storage Specimens should be stored in sealed, lead-lined containers or specialized display cases with adequate ventilation, away from other minerals that could be damaged by radiation. It is recommended to store them in a dry place and away from areas of permanent human occupancy.

External references

Sources

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