Fourmarierite

Chemical formula: Pb<sup>2+</sup><sub>1-x</sub>O<sub>3-2x</sub>(U<sup>6+</sup>O<sub>2</sub>)<sub>4</sub>(OH)<sub>4+2x</sub>·4H<sub>2</sub>O

Fourmarierite is a rare, orange-red uranyl lead oxide mineral, forming characteristic tabular crystals and radial aggregates.

## Characteristics Fourmarierite is a secondary uranium mineral, valued by collectors for its intense, orange-red to golden-red color. It occurs in the oxidation zones of uraninite deposits. It forms small, tabular or lath-like crystals, often grouped into radial aggregates, rosettes, or forming crusts. Its crystals are usually transparent to translucent and exhibit a strong, adamantine or resinous luster. ## Physical Properties This mineral is characterized by a hardness of 3-4 on the Mohs scale, making it relatively soft. It has a high density, approximately 6.05 g/cm³, which is typical for minerals containing lead and uranium. It is strongly radioactive. It exhibits perfect cleavage in one direction, parallel to the basal plane of the crystals. ## Colors and Varieties The dominant and virtually only color of fourmarierite is orange-red, which can transition into golden-red, reddish-brown, or yellowish-orange hues. No distinct color or commercial varieties are recognized. ## History and Name Fourmarierite was first described in 1924 by Henri Buttgenbach. The mineral's name honors Paul Fourmarier (1877–1970), a Belgian geologist and professor at the University of Liège, for his contributions to the study of the geology of Belgium and Congo. ## Uses Due to its rarity and small crystal size, fourmarierite has no industrial applications. However, it is a highly valued and sought-after collector's mineral.

Properties

Mohs hardness
3-4
Luster
Adamantine to resinous
Streak
Orange-red
Density
6.046
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Fourmarierite is relatively easy to identify based on its intense, orange-red color, adamantine luster, and characteristic tabular crystals often forming radial aggregates. Its high density and strong radioactivity are key diagnostic features. It often occurs in association with other secondary uranium minerals. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals of similar color, such as curite, becquerelite, or wölsendorfite. Curite is usually more orange and forms acicular crystals. Becquerelite has a more yellow to yellowish-orange color. Precise differentiation of these minerals often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Fourmarierite crystals are most often thin-tabular, with a rhombic or hexagonal outline. They usually occur as radial or stellate aggregates, rosettes, and also as massive or granular crusts on the host rock.

Geological environment

## Genesis Fourmarierite is a secondary mineral, formed in the oxidation (weathering) zone of hydrothermal and pegmatitic uraninite (pitchblende) deposits. It forms as a result of the alteration of primary uranium minerals under the influence of oxygen- and lead-bearing waters. ## Mineral Associations It most commonly co-occurs with uraninite, which is its parent mineral. It is also accompanied by other secondary uranium minerals, such as curite, torbernite, becquerelite, kasolite, rutherfordine, schoepite, and wölsendorfite. ## Localities The most important and type locality, from which the best specimens originate, is the Shinkolobwe mine in Katanga Province, Democratic Republic of Congo. It is also known from several other localities worldwide, including the Ruggles mine in Grafton, New Hampshire (USA); Great Bear Lake in the Northwest Territories (Canada); the Wölsendorf mine in Bavaria (Germany); and Jáchymov (Czech Republic).

Rarity

Rare

For collectors

## Quality Criteria The most highly prized fourmarierite specimens are characterized by sharply terminated, well-formed crystals with an intense, orange-red color and strong luster. Radial aggregates and rosettes formed from distinct, undamaged tabular crystals are particularly sought after. Contrast with light host rock and co-occurrence with other rare uranium minerals, such as schoepite or becquerelite, significantly increases the specimen's value. ## Popular Localities By far the most classic and valued specimens come from the type locality – the Shinkolobwe mine in the Democratic Republic of Congo. Specimens from this location are considered the global standard for this mineral.

Care and storage

## Cleaning Fourmarierite specimens are very delicate and brittle. Cleaning should be kept to an absolute minimum. If necessary, a soft brush can be used to remove dust. Avoid water and any chemical agents. ## What to Avoid The mineral is susceptible to mechanical damage due to its low hardness and perfect cleavage. Avoid contact with water, acids, and other chemicals. As a secondary mineral, it is unstable and can undergo alterations in contact with moisture. Due to its uranium content, it is strongly radioactive – limit exposure, wash hands after each contact, and do not store it in areas of permanent human presence (desk, bedroom). ## Storage Specimens should be stored in closed, padded "perky boxes" to protect them from mechanical damage and dust. Store in a dry place, away from heat sources and direct sunlight. Due to radioactivity, specimens should be stored in specially marked containers, in a restricted access area.

External references

Sources

Read more