Bobcookite

Chemical formula: NaAl(U<sup>6+</sup>O<sub>2</sub>)<sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>4</sub>·18H<sub>2</sub>O

Bobcookite is a very rare, water-soluble uranyl mineral, forming bright yellow, acicular crystals.

## Characteristics Bobcookite is a hydrated sulfate of sodium, aluminum, and uranyl. It forms aggregates of fine, acicular or capillary crystals with an intensely yellow to yellowish-green color. Due to its nature, it is extremely delicate and brittle. Crystals are usually very small, rarely exceeding 1 mm in length, and often occur as felted aggregates or radial rosettes on the host rock. ## Physical Properties This mineral is characterized by a silky luster and is transparent. Its hardness on the Mohs scale is very low, estimated at about 1, which means it is extremely soft. It is a mineral with low density. One of its key and unusual features is its solubility in cold water. ## Colors and Varieties Bobcookite occurs in shades from bright yellow to yellowish-green. No varieties have been distinguished. ## History and Name The mineral was named in honor of Robert (Bob) Cook (born 1942), an American mineralogist and retired professor of geology at Auburn University in Alabama, in recognition of his contributions to mineralogy. It was approved as a new mineral species by the International Mineralogical Association (IMA) in 2014. It was described based on samples from the Blue Lizard Mine in San Juan County, Utah, USA. ## Uses Bobcookite has no industrial application. It is solely of interest to collectors specializing in rare minerals, uranium minerals, or minerals from specific localities (so-called "type locality").

Properties

Mohs hardness
1
Luster
Silky
Streak
Light yellow
Density
2.51
Cleavage
Perfect on {010}
Fracture
Splintery
Transparency
Transparent
Crystal system
Triclinic

Diagnostic features

## Identification Key diagnostic features of bobcookite include its habit (fine, acicular crystals), intense yellow color, silky luster, and solubility in water. The mineral exhibits strong, green fluorescence under ultraviolet light (both shortwave and longwave). It is also radioactive. ## Distinguishing from Similar Minerals Bobcookite can be confused with other secondary uranyl minerals of similar appearance, such as zippeite, natrozippeite, or metauranopilite. Distinguishing it from these minerals without advanced analytical methods (such as powder X-ray diffraction - XRD) is practically impossible. A key field indicator may be its solubility in cold water, which is a distinguishing feature among many uranyl sulfates. ## Crystal Forms Bobcookite forms very fine, acicular or capillary crystals, elongated along the [100] axis. They occur as radial or tangled aggregates, as well as coatings and crusts on sedimentary rocks, mainly sandstones.

Geological environment

## Genesis Bobcookite is a secondary uranium mineral that forms as a result of the oxidation of uraninite deposits in the presence of sulfides (e.g., pyrite) in desert or semi-desert conditions. It forms as an evaporation product of solutions rich in uranium, sulfates, sodium, and aluminum on the walls of mine workings or in rock fractures. Its formation is a contemporary process, occurring after deposits have been exposed by mining activity. ## Mineral Associations This mineral co-occurs with other secondary sulfates and uranyl minerals, such as gypsum, zippeite, natrozippeite, johannite, uranopilite, as well as with uraninite (as a primary mineral) and pyrite. ## Localities Bobcookite is an extremely rare mineral, known from only a few locations worldwide. The most important and also the type locality is the Blue Lizard Mine in San Juan County, Utah, USA. It has also been identified in the Freedom Mine in the Marysvale district, also in Utah.

Rarity

Extremely rare

For collectors

## Quality Criteria The most desirable bobcookite specimens are those that exhibit rich, dense aggregates of bright yellow, acicular crystals on a small piece of host rock (matrix). The contrast between the intensely yellow crystals and the darker sandstone background enhances visual appeal. Due to the microscopic size of the crystals, specimens are primarily judged by the richness and coverage of the surface, rather than the size of individual needles. ## Popular Localities The only source of collectible specimens is the type locality – the Blue Lizard Mine in Utah, USA. Specimens from this mine are the standard for this mineral, and practically all available on the market originate from there.

Care and storage

## Cleaning The mineral **must not be cleaned** with water or any liquids, as it is soluble in them. The only safe method for removing dust is to use compressed air from a safe distance or a very soft brush, exercising extreme caution due to the fragility of the crystals. ## What to Avoid - **Water and moisture**: Bobcookite is water-soluble and hygroscopic (absorbs moisture from the air), which leads to its destruction. It should be stored in very low humidity conditions. - **High temperature**: This can cause the loss of water from its crystal structure and disintegration. - **Vibrations and impacts**: Crystals are extremely brittle and delicate. - **Direct sunlight**: As with many uranyl minerals, prolonged exposure can cause chemical changes. ## Storage Bobcookite specimens must be stored in sealed, transparent containers (e.g., "perky boxes") with a desiccant (e.g., silica gel). Due to its uranium content, it is a radioactive mineral and should be stored away from areas of permanent human habitation, in accordance with radioactive material handling guidelines.

Sources

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