Haynesite
Chemical formula: (U<sup>6+</sup>O<sub>2</sub>)<sub>3</sub>(Se<sup>4+</sup>O<sub>3</sub>)<sub>2</sub>(OH)<sub>2</sub>·5H<sub>2</sub>O
Haynesite is a very rare, secondary uranyl and selenium mineral, forming tiny, yellow crystals with a vitreous luster.
Properties
- Mohs hardness
- 1.5-2
- Luster
- Vitreous
- Streak
- Pale yellow
- Density
- 4.88
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Haynesite can be identified by its characteristic light yellow color, acicular crystal habit forming radial aggregates, and occurrence in the oxidation zones of uranium deposits. A key diagnostic feature is its strong radioactivity. Under shortwave UV light, it exhibits weak, green fluorescence. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals of similar color and form, such as uranophane, carnotite, or tyuyamunite. Differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD). However, haynesite is distinguished by its co-occurrence with other uranyl selenites, which is rare for most secondary uranium minerals. ## Crystal Forms It forms elongated, acicular or bladed crystals, terminated by flat faces. These crystals almost always occur as radial or tangled aggregates, as well as coatings and efflorescences on the host rock.
Geological environment
## Genesis Haynesite is a secondary mineral formed in the oxidation (weathering) zone of uranium deposits rich in selenium. It forms as a result of groundwater acting on primary uranium ores (uraninite) and selenium in desert conditions. It forms directly on rock surfaces, in fractures and voids. ## Mineral Associations It most often co-occurs with other secondary uranyl and selenium minerals, such as andersonite, boltwoodite, gypsum, jarosite, and especially with other selenites: guilleminite, demesmaekerite, and marthozite. These minerals often form complex, multicolored coatings. ## Localities The most important and well-documented haynesite localities in the world are in the USA, in Utah (San Juan County), from where its holotype originates (Repete, Freedom #2, Vanadium Queen mines). It has also been found in the Czech Republic, in a uranium deposit near Slavkovice in Moravia.
Rarity
Very rare
For collectors
## Quality Criteria The most prized haynesite specimens are those with well-formed, distinct crystals forming aesthetic, radial aggregates on a contrasting rock matrix. The intensity and purity of the yellow color also increase the specimen's value. Due to the microscopic size of the crystals, specimens are primarily evaluated under magnification. The richness of association with other rare uranyl selenites is also important. ## Popular Localities By far the most sought-after specimens by collectors come from the type locality in San Juan County, Utah, USA. These localities have yielded the best known crystals of this mineral.
Care and storage
## Cleaning Haynesite is extremely delicate, soft, and potentially soluble. Mechanical cleaning is highly inadvisable. If absolutely necessary, a stream of compressed air can be used to remove loose dust. Avoid contact with water and other chemicals. ## What to Avoid Absolutely avoid contact with water, acids, and other liquids that could damage or dissolve it. As a hydrated mineral, it is sensitive to changes in temperature and humidity, which can lead to its dehydration and destruction. Protect it from ultrasound, vibrations, and impacts. Due to its uranium content, it is highly radioactive and requires safe handling. ## Storage Haynesite specimens should be stored in stable conditions, in closed, padded containers (micromount type), to protect them from mechanical damage and dust. Store away from other minerals that could be damaged by radiation. Appropriate precautions for radioactive materials should be observed – store in a well-ventilated area, away from permanent human presence, and wash hands after each contact.