Favreauite

Chemical formula: Pb<sup>2+</sup>Bi<sup>3+</sup>Cu<sup>2+</sup><sub>6</sub>O<sub>4</sub>(Se<sup>4+</sup>O<sub>3</sub>)<sub>4</sub>(OH)·H<sub>2</sub>O

Favreauite is a very rare copper, lead, and bismuth selenite, forming microscopic, tabular crystals of an intense green color.

## Characteristics Favreauite is a hydrated selenite containing copper, lead, and bismuth. It occurs as very small, tabular or bladed crystals, rarely exceeding 0.2 mm in length. These crystals form rosette-like or radial aggregates, as well as thin coatings and crusts on the host rock. Its most characteristic feature is its intense, emerald-green to green color. ## Physical Properties Favreauite crystals exhibit a vitreous luster. They are transparent to translucent. Due to the microscopic size of the crystals, hardness and density have not been precisely measured. The density calculated based on the chemical formula and unit cell parameters is 5.75 g/cm³. ## Colors and Varieties This mineral occurs in a uniform, characteristic color ranging from emerald-green to green. No varieties have been distinguished. ## History and Name Favreauite was discovered in the Musonoi copper and uranium mine, near Kolwezi in Lualaba Province (Democratic Republic of Congo). It was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 2011 (IMA2011-066). The mineral is named in honor of the French collector and micromineralogist, Georges Favreau (born 1948), in recognition of his contributions to the study of secondary minerals from the Musonoi deposit.

Properties

Luster
Vitreous
Streak
Light green
Density
5.75
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of favreauite is based on its unique characteristics: intense green color, characteristic form of thin, tabular crystals forming rosette aggregates, and occurrence in a specific geological environment. Due to its microscopic size, final confirmation requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Favreauite can be confused with other green secondary copper minerals, such as malachite, brochantite, or other selenites (e.g., schmiederite, demesmaekerite). However, it is distinguished by its characteristic habit of tabular crystals in rosette aggregates. Demesmaekerite forms crystals with a different habit, and malachite most often occurs in botryoidal, fibrous forms, or as massive aggregates. ## Crystal Forms Crystals are very small, thin, and tabular, flattened parallel to the {010} plane. They often combine into radial or rosette aggregates, and also form thin coatings on rock surfaces.

Geological environment

## Genesis Favreauite is a secondary mineral, formed in the oxidation (weathering) zone of hydrothermal deposits containing selenium, copper, lead, and bismuth. In its type locality (Musonoi mine), it formed as a result of the weathering of primary uranium, copper, and lead sulfides and selenides. ## Mineral Associations This mineral co-occurs with other rare selenites, such as demesmaekerite, marthozite, schmiederite, as well as with malachite, quartz, and uranium oxides. ## Localities The only confirmed and described occurrence of favreauite in the world is its type locality – the Musonoi mine, located near Kolwezi, in Lualaba Province in the Democratic Republic of Congo.

Rarity

Very rare

For collectors

## Quality Criteria The quality of favreauite specimens is assessed based on the richness and aesthetics of the microcrystal aggregates. Most valued are specimens with well-formed, sharp crystals forming dense, rosette-like clusters of an intense green color. Contrast with the rock matrix and the presence of rare associated minerals are also important. Due to their microscopic nature, specimens are evaluated under magnification. ## Popular Localities The only source of favreauite specimens is the Musonoi mine in the Democratic Republic of Congo. This is a classic locality for many rare secondary minerals, and specimens from there are highly prized by collectors specializing in microminerals.

Care and storage

## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to extreme fragility and reactivity, any contact with water, cleaning solutions, or even soft brushes should be avoided. ## What to Avoid Contact with water and chemicals that could dissolve or damage the microcrystals must be strictly avoided. The mineral is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight. ## Storage Favreauite specimens must be stored under stable conditions, preferably in sealed, airtight "micromount" boxes, away from dust, humidity, and temperature fluctuations. Display should only be in enclosed display cases.

Sources

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