Compreignacite

Chemical formula: K₂(U⁶⁺O₂)₆O₄(OH)₆·₇H₂O

Compreignacite is a rare, secondary uranium mineral, forming characteristic yellow to orange-yellow, tabular crystals.

## Characteristics Compreignacite is a hydrated uranyl potassium oxide hydroxide, belonging to the group of rare, secondary uranium minerals. It forms very small, usually microscopic crystals with a tabular or bladed habit, often aggregated into rosettes or radial clusters. Its appearance is characteristic of many minerals from the uranyl hydroxide group, with an intense, bright color. ## Physical Properties This mineral is very soft and brittle. Its luster is described as vitreous to pearly, especially visible on cleavage planes. It is transparent to translucent. The density is approximately 5.03 g/cm³, which is a high value, typical for uranium-bearing minerals. ## Colors and Varieties Compreignacite occurs in shades from light yellow to orange-yellow. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1964 by Fabien Cesbron. Its name comes from the discovery locality – the Margnac mine near the town of Compreignac in the Haute-Vienne department, France. This location is recognized as its type locality. ## Uses Due to its rarity, small crystal size, and radioactivity, compreignacite has no industrial applications. It is solely an object of interest for collectors specializing in uranium minerals and is a subject of scientific research.

Properties

Luster: Vitreous, Pearly
Streak: Pale yellow
Density: 5.03
Cleavage: (001)
Transparency: Transparent,Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Compreignacite is identified by its intense, yellow to orange-yellow color, characteristic tabular crystals forming rosette-like aggregates, and strong fluorescence under ultraviolet light. Its occurrence in the oxidation zones of uranium deposits is also key. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals such as becquerelite, schoepite, or fourmarierite. Becquerelite has a similar color but often forms thicker, prismatic crystals. Schoepite is chemically simpler (does not contain potassium) and can dehydrate. Accurate differentiation of these minerals often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Crystals are very small, hexagonally outlined, in the form of thin tablets or blades. They usually occur as radial or rosette-like aggregates, as well as coatings and crusts on the host rock.

Geological environment

## Genesis Compreignacite is a secondary mineral, forming in the oxidation (weathering) zones of uranium deposits. It forms as a result of the alteration of primary uranium minerals, mainly uraninite, under the influence of oxygen- and potassium-rich waters. ## Mineral Associations It most often co-occurs with other secondary uranium minerals, such as uraninite (as a primary mineral), becquerelite, schoepite, fourmarierite, uranophane, and curite. Iron oxides, such as goethite, may also accompany it. ## Localities The most important and classic locality is the Margnac mine in Compreignac, France. This mineral has also been found in other uranium deposits, for example, in the Shinkolobwe region in the Democratic Republic of Congo, and in several localities in Germany (Saxony) and the Czech Republic (Jáchymov).

Rarity

Very rare

For collectors

## Quality Criteria The most valued compreignacite specimens are characterized by well-formed, sharp crystals forming aesthetic, rosette-like aggregates. The intensity and hue of the color (vibrant, orange-yellow tones are preferred) also influence value. Contrast with the rock matrix and the absence of damage to delicate crystals are important. Due to the microscopic size of the crystals, specimens with larger aggregates visible to the naked eye are particularly sought after. ## Popular Localities Specimens originating from the type locality, i.e., the Margnac mine in France, are historically the most important and most desired by collectors specializing in mineral systematics or uranium minerals.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Any contact with water or other liquids is inadvisable, as it can damage the delicate crystals. Brushes or ultrasonic cleaners should not be used. ## What to Avoid Avoid contact with water and chemicals. The mineral is brittle, so it must be protected from impacts and vibrations. As a uranium mineral, it is radioactive – limit exposure and avoid inhaling dust. Always wash hands after contact with the specimen. ## Storage Compreignacite specimens should be stored in sealed, labeled containers (e.g., "perky box" type), away from other minerals that could be damaged by radiation. Store in a dry place, away from heat sources and direct sunlight, in a location inaccessible to children and pets.

External references

https://www.mindat.org/min-1117.html