Moreauite
Chemical formula: Al<sub>3</sub>(U<sup>6+</sup>O<sub>2</sub>)(PO<sub>4</sub>)<sub>3</sub>(OH)<sub>2</sub>·13H<sub>2</sub>O
Moreauite is a very rare, secondary uranyl mineral, forming aggregates of small, yellow, tabular crystals.
Properties
- Mohs hardness
- 2
- Luster
- Pearly
- Streak
- Pale yellow
- Density
- 2.5
- Cleavage
- Perfect on {100}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Moreauite can be identified by its characteristic canary-yellow color, pearly luster, and occurrence as small, tabular crystals forming rosetted aggregates. Key diagnostic features include its strong, green fluorescence under shortwave ultraviolet (UV-SW) light and radioactivity, detectable with a Geiger counter. ## Differentiation from Similar Minerals It can be confused with other yellow secondary uranium minerals, such as meta-autunite, uranophane, or phosphuranylite. Differentiation requires advanced analytical methods, such as X-ray diffraction (XRD). In collector's conditions, the characteristic crystal habit and mineral associations from the type locality can be helpful. ## Crystal Forms It forms very thin, tabular crystals with a hexagonal outline. These crystals most often occur as rosetted, radial, or fan-shaped aggregates, as well as thin coatings and crusts on the host rock.
Geological environment
## Genesis Moreauite is a secondary mineral formed in the oxidation (weathering) zones of uraninite deposits. It forms as a result of the interaction of phosphate-rich waters with primary uranium minerals in the presence of aluminum derived from the weathering of clay minerals or feldspars. ## Mineral Associations It most often co-occurs with other secondary uranium minerals. At the type locality (Kobokobo), its presence has been noted in association with meta-autunite, phosphuranylite, threadgoldite, mundite, as well as with quartz, tourmaline, and muscovite. ## Localities The most important and best-documented occurrence of moreauite is its type locality – the Kobokobo pegmatite in South Kivu Province, Democratic Republic of Congo. This is the only locality from which well-formed, identifiable specimens originate.
Rarity
Very rare
For collectors
## Quality Criteria The most prized moreauite specimens are those with rich, well-defined rosetted aggregates of intense, canary-yellow color. The size and aesthetics of these aggregates against the host rock (matrix) are also important. Contrast with darker quartz or other substrate minerals enhances visual appeal. Due to the microscopic size of the crystals, specimens are primarily evaluated based on surface coverage and the quality of the aggregates. ## Popular Localities The only source of collector's specimens is the Kobokobo pegmatite in the Democratic Republic of Congo. Specimens from this locality are considered the global standard for this mineral.
Care and storage
## Cleaning Moreauite specimens should not be cleaned wet. Only gentle dust removal with a soft brush or compressed air from a safe distance is permissible. Contact with water can lead to rehydration and damage to the delicate crystals. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is sensitive to changes in humidity and temperature. As a uranium mineral, it is radioactive, so direct exposure should be limited, and hands should always be washed after handling the specimen. ## Storage Specimens should be stored in closed, airtight containers (e.g., perky boxes) to protect them from mechanical damage, dust, and humidity fluctuations. Due to its radioactivity, it should be kept away from areas of constant human presence and other radiation-sensitive minerals.