Marthozite

Chemical formula: Cu²⁺(U⁶⁺O₂)₃(Se⁴⁺O₃)₂O₂·₈H₂O

Marthozite is a rare, secondary uranyl and selenium mineral, forming aggregates of tiny, acicular crystals of an intensely yellow color.

## Characteristics Marthozite is a hydrated uranyl copper selenite. It occurs as tiny, acicular or bladed crystals, which often form radial aggregates, rosettes, or crusts. Individual crystals are usually microscopic in size, rarely exceeding 1 mm in length. Due to its habit and small size, its visual features are best observed under magnification. ## Physical Properties Marthozite crystals are flexible and can be bent. Hardness is low, estimated at about 2 on the Mohs scale. Density is significantly higher than average, approximately 4.52 g/cm³. The mineral exhibits strong pleochroism in shades from colorless to pale yellow. It is strongly radioactive. ## Colors and Varieties This mineral has a characteristic bright yellow to golden-yellow color. No color or commercial varieties are distinguished. ## History and Name The mineral was named in honor of Aimé Marthoz (1894-1962), a Belgian mining engineer and general director of Union Minière du Haut Katanga, the company managing the Musonoi mine in the Democratic Republic of Congo, where marthozite was discovered. The mineral was described and approved by the IMA in 1969. ## Uses Marthozite, due to its extreme rarity and microscopic crystal size, has no industrial applications. It is of purely scientific interest and is sought after by specialized collectors of rare and radioactive minerals.

Properties

Mohs hardness: 2
Color: Yellowish-green to greenish-brown
Luster: Vitreous
Streak: Pale yellow
Density: 4.4
Cleavage: Perfect {100}, imperfect {010}
Fracture: Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Characteristic features of marthozite include its bright yellow color, acicular crystal habit forming radial aggregates, and occurrence in the oxidation zones of uranium-selenium deposits. Strong radioactivity is a key diagnostic indicator. ## Distinguishing from Similar Minerals It can be confused with other secondary uranyl minerals of similar color and habit, such as guilleminite, demesmaekerite, or some forms of uranophane. Definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS), due to its specific chemical composition (presence of selenium and copper). ## Crystal Forms Marthozite forms elongated, acicular or bladed crystals, elongated along the [001] axis. These crystals combine into characteristic radial or stellate aggregates, and also form crusts and coatings on the host rock.

Geological environment

## Genesis Marthozite is a secondary mineral, formed in the oxidation zones of hydrothermal uranium-nickel-cobalt-selenium deposits. It forms as a result of weathering and alteration of primary uranium ores (uraninite) and selenides in the presence of copper-rich solutions. ## Mineral Associations It most commonly co-occurs with other rare uranyl selenites. At its type locality (Musonoi mine), it was found in association with demesmaekerite, guilleminite, kasolite, cuprite, quartz, and hydrated iron oxides. ## Localities This is an extremely rare mineral. The most important and type locality for its occurrence is the Musonoi mine, near Kolwezi, in Lualaba Province (formerly Katanga) in the Democratic Republic of Congo. This is the only confirmed and well-documented locality in the world from which all known specimens originate.

Rarity

Extremely rare

For collectors

## Quality Criteria The most highly valued specimens are those with well-formed, rich aggregates of radial crystals of an intense, golden-yellow color, contrasting with the host rock. Due to the microscopic nature of the mineral, the size and richness of the crust are crucial. Association with other rare selenites is also important. ## Popular Localities The only source of collector specimens is the historical discovery at the Musonoi mine in the Democratic Republic of Congo. New specimens do not appear on the market, and those available come from old collections.

Care and storage

## Cleaning Marthozite specimens should generally not be wet cleaned. The tiny, acicular crystals are very fragile and easily damaged. If necessary, compressed air (from a safe distance) can be used to remove dust. ## What to Avoid Contact with water and chemicals should be strictly avoided. The mineral is sensitive to changes in humidity and temperature. As a secondary mineral, it is unstable under unsuitable conditions. Due to the presence of uranium, it is strongly radioactive – direct, prolonged contact should be avoided, dust should not be inhaled, and hands should be washed after any contact. ## Storage Specimens should be stored in sealed, dry containers (e.g., "perky boxes") to protect them from moisture and mechanical damage. Appropriate labeling warning of radioactivity is necessary. Store away from other radiation-sensitive minerals and away from areas of permanent human habitation.