Demesmaekerite

Chemical formula: Pb²⁺₂Cu²⁺₅(U⁶⁺O₂)₂(Se⁴⁺O₃)₆(OH)₆·₂H₂O

Demesmaekerite is a rare lead, copper, and uranyl selenite, forming tiny, tabular, bottle-green crystals.

## Characteristics Demesmaekerite is a complex, hydrated selenite of lead, copper, and uranyl. It occurs as very small, tabular or bladed crystals, rarely exceeding 1 mm in length. It also forms rosette aggregates and earthy masses. Fresh specimens have a characteristic bottle-green color, which over time, due to dehydration (loss of water from the crystal lattice), can change to brownish hues. ## Physical Properties This mineral is characterized by a hardness in the range of 3-4 on the Mohs scale, making it relatively soft. Its density is significant, approximately 5.28 g/cm³, which is typical for minerals containing lead and uranium. It is translucent to opaque. The luster is described as vitreous. ## Colors and Varieties The predominant color is bottle-green. No named varieties of this mineral are distinguished. ## History and Name The mineral was described in 1965. Its name honors Gaston Demesmaeker (1911–?), a Belgian geologist and general manager of the geological department of Union Minière du Haut Katanga, the company operating in the region where the discovery was made. ## Uses Due to its extreme rarity, small crystal size, and content of toxic elements, demesmaekerite has no industrial applications. It is solely an object of scientific interest and a collector's item for advanced collectors specializing in uranium minerals or rare species.

Properties

Mohs hardness: 3-4
Luster: Vitreous
Density: 5.28
Transparency: Translucent,Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Key diagnostic features include the bottle-green color (on fresh specimens), co-occurrence with other secondary selenite and uranium minerals in the oxidation zone of copper-cobalt deposits, and small, tabular crystals. The reaction to UV radiation is not characteristic. Final identification requires advanced analytical methods such as XRD or EDS. ## Distinguishing from Similar Minerals Demesmaekerite can be confused with other green, secondary uranium minerals such as torbernite, zeunerite, or cuprosklodowskite. It is distinguished from them by its crystal habit (small tabular crystals), specific geochemical environment (presence of selenium), and mineral associations. Many of these minerals also have lower specific gravity and different hardness. ## Crystal Forms Crystals are very small, usually tabular or bladed, often flattened. They form fan-shaped or rosette aggregates, as well as coatings and earthy masses on the host rock.

Geological environment

## Genesis Demesmaekerite is a secondary mineral formed in the oxidation zones of hydrothermal uranium-nickel-cobalt-selenium deposits. It crystallizes as a result of the weathering of primary ores containing uraninite, selenides, and sulfides in the presence of solutions rich in copper and lead. ## Mineral Associations This mineral co-occurs with other rare uranyl selenites, such as guilleminite, derriksite, marthozite, and haynesite. It is also often accompanied by malachite, cuprosklodowskite, kasolite, and iron and manganese oxides. ## Localities The most important and classic occurrence (type locality) is the Musonoi mine near Kolwezi, in Lualaba Province, Democratic Republic of Congo. This is the only confirmed locality from which well-formed, identifiable specimens originate.

Rarity

Very rare

For collectors

## Quality Criteria The most valued specimens are those with sharp, well-formed crystals of intense, bottle-green color, set on a contrasting rock matrix. Due to the microscopic size of the crystals, their concentration and aesthetic arrangement on the specimen are crucial. Associations with other rare selenites are also highly prized. ## Popular Localities The only source of collector specimens is the Musonoi mine in DR Congo. Material from this locality is extremely difficult to obtain, and most specimens are found in institutional collections and among specialized collectors of systematic minerals or uranium minerals.

Care and storage

## Cleaning Demesmaekerite specimens are extremely delicate and small. Mechanical cleaning is not recommended. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust. Avoid contact with water, which can accelerate the dehydration process and color change. ## What to Avoid The mineral is sensitive to changes in humidity and temperature, which cause water loss and a color change from green to brown. Ultrasonic cleaners, all chemicals, acids, and solvents should be avoided. As a uranium mineral, it is radioactive and toxic (contains lead and selenium) – avoid inhaling dust and wash hands after any contact. ## Storage Specimens should be stored under stable conditions, preferably in sealed, airtight containers (so-called micromount boxes), to minimize humidity fluctuations and slow down dehydration. Store away from other minerals that could be damaged by its radiation. It is necessary to clearly label the specimen as radioactive and toxic material.