Dumontite

Chemical formula: Pb<sup>2+</sup><sub>2</sub>(U<sup>6+</sup>O<sub>2</sub>)<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>O<sub>2</sub>·5H<sub>2</sub>O

Dumontite is a rare uranium and lead mineral, forming characteristic yellow, acicular crystals with strong radioactivity.

## Characteristics Dumontite is a hydrated uranyl lead oxyphosphate, belonging to the group of secondary uranium minerals. It occurs as small, acicular or hair-like crystals, which often form radial aggregates, crusts, or rosettes. Its most characteristic feature is its intense, lemon-yellow to golden-yellow color. Due to its chemical composition, dumontite is a strongly radioactive mineral. ## Physical Properties Dumontite crystals are very small and brittle. The mineral exhibits a silky to vitreous luster. It is translucent to opaque. Its density is high, approximately 5.5 g/cm³, which is typical for minerals containing lead and uranium. ## Colors and Varieties Dumontite occurs in various shades of yellow, from lemon-yellow, through golden-yellow, to orange-yellow. No distinct color varieties or commercial names are recognized. ## History and Name The mineral was first described in 1924 by the Belgian mineralogist Alfred Schoep. He named it in honor of André-Hubert Dumont (1809-1857), a Belgian geologist who first created a geological map of Belgium. ## Uses Dumontite, like other secondary uranium minerals, has no direct industrial application due to its rarity and occurrence in small quantities. However, it is of interest to collectors specializing in uranium minerals and has scientific significance as an indicator of uranium deposit weathering processes.

Properties

Mohs hardness
2-3
Luster
Silky
Streak
Yellow
Density
5.5
Cleavage
Perfect on {100}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features of dumontite include its intensely yellow color, characteristic form of small, acicular crystals forming radial aggregates, and very strong radioactivity, easily detectable with a Geiger counter. This mineral often occurs on or near uraninite. ## Distinguishing from Similar Minerals Dumontite can be confused with other yellow secondary uranium minerals, such as autunite, torbernite (in meta- form), or kasolite. Differentiation often requires chemical analysis (EDS) or X-ray diffraction (XRD). Kasolite also contains lead but has a different structure and often forms more compact aggregates. Autunite and torbernite typically form platy crystals and exhibit strong fluorescence under UV light, which dumontite does not possess. ## Crystal Forms Dumontite crystallizes in the monoclinic system. It forms elongated, acicular or hair-like crystals, often flattened. These crystals almost always occur as radial or stellate aggregates, as well as in the form of crusts and coatings on other minerals.

Geological environment

## Genesis Dumontite is a secondary mineral, formed in the oxidation (weathering) zone of hydrothermal uranium deposits. It forms as a result of the alteration of primary uranium minerals, mainly uraninite, in the presence of phosphate- and lead-rich waters. ## Mineral Associations This mineral most commonly co-occurs with uraninite (as the parent rock), as well as with other secondary uranium minerals such as kasolite, torbernite, meta-torbernite, dewindtite, phosphuranylite, and saleeite. ## Localities The most important and classic locality for dumontite is the Shinkolobwe mine in Haut-Katanga Province, Democratic Republic of Congo, where it was discovered. It is also known from several other localities worldwide, including the uranium mines in the Écarpière region of France and some pegmatites in North Carolina, USA.

Rarity

Very rare

For collectors

## Quality Criteria Specimens most valued by collectors are those with well-formed, distinct radial aggregates of intense, golden-yellow color. Contrast with the matrix mineral on which dumontite grows is important. The size of the aggregates and the abundance of occurrence on the specimen significantly increase its value. Due to the microscopic size of the crystals, clarity is not a key criterion. ## Popular Localities By far the most sought-after and classic specimens come from the type locality – the Shinkolobwe mine in the Democratic Republic of Congo. Material from this mine is considered exemplary for this species.

Care and storage

## Cleaning Dumontite specimens should generally not be wet cleaned. The fine, acicular crystals are extremely fragile and easily damaged. Compressed air from a safe distance or a very soft brush can be used to remove dust, proceeding with utmost caution. ## What to Avoid Contact with water and chemicals must be strictly avoided. As a strongly radioactive mineral, it requires special handling. Direct skin contact should be avoided, and hands should be thoroughly washed after any contact (even with the container). It should not be stored in places where a lot of time is spent (bedrooms, desks). ## Storage Dumontite must be stored in a sealed, lead-lined or thick-glass container, clearly marked as radioactive material. The container should be placed in a well-ventilated area, away from other radiation-sensitive minerals (e.g., smoky quartz, some fluorites) and photographic materials.

Sources

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