Nielsbohrite

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

Nielsbohrite is a very rare, radioactive uranyl arsenate mineral, forming microscopic, yellow, tabular crystals.

## Characteristics Nielsbohrite is a hydrated potassium uranyl arsenate. It occurs as extremely small, tabular or platy crystals, rarely exceeding 0.2 mm in length. These crystals are usually transparent to translucent and are characterized by a bright yellow color and vitreous luster. Due to its chemical composition, this mineral is strongly radioactive. ## Physical Properties Nielsbohrite crystals are too small to precisely determine their hardness, but it is estimated to be around 2 on the Mohs scale. The density calculated from the formula and unit cell parameters is 4.63 g/cm³. The mineral exhibits perfect cleavage in one direction. ## Colors and Varieties Nielsbohrite occurs in a uniform, bright yellow color. No color or commercial varieties are known. ## History and Name The mineral was described in 2002 by P. C. Burns, K. A. Hughes, J. B. Parise, and Y. Lee. Its name honors Niels Bohr (1885–1962), a Danish physicist and Nobel laureate, for his fundamental contributions to the understanding of atomic structure and quantum mechanics. The type locality (place of first discovery) is the Krunkelbach mine in Menzenschwand, in the German Black Forest. ## Uses Nielsbohrite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare uranium minerals.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light yellow
Density
4.63
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Nielsbohrite is recognized by its characteristic appearance: microscopic, bright yellow, tabular crystals occurring in paragenesis with other secondary uranium minerals. A key diagnostic feature is its strong radioactivity. Final identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical spectroscopy (EDS). ## Distinguishing from Similar Minerals It can be confused with many other yellow secondary uranyl minerals, such as meta-autunite, uranophane, or studtite. Differentiation by the naked eye is practically impossible. Nielsbohrite is distinguished by its specific crystal habit (thin tablets) and unique chemical composition (presence of potassium and arsenic), which requires laboratory confirmation. ## Crystal Forms Nielsbohrite crystals have a habit of thin, rectangular tablets or plates, often flattened parallel to the cleavage plane. They usually form small, loose aggregates or grow on the surface of the host rock.

Geological environment

## Genesis Nielsbohrite is a secondary mineral formed in the oxidation zones of uranium deposits. It forms as a result of weathering and alteration of primary uranium minerals, such as uraninite, in the presence of solutions rich in arsenic and potassium. It forms under low temperature and pressure conditions, often in voids and fractures in rocks. ## Mineral Associations This mineral co-occurs with other secondary uranium minerals. At its type locality (Krunkelbach mine), it was found in association with uraninite, hematite, quartz, and other rare uranyl arsenates such as metakahlerite, uranospinite, and heinrichite. ## Localities The only confirmed and detailed described occurrence of nielsbohrite in the world is its type locality – the Krunkelbach mine (Grube Krunkelbach) near Menzenschwand in the Black Forest (Baden-Württemberg, Germany). This is a historic uranium mine known for the occurrence of many rare and unique secondary minerals.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of nielsbohrite specimens is primarily assessed based on the size and development of the crystals, which is rare. The most prized specimens are those with clearly visible, sharp, undamaged crystals of intense yellow color. Contrast with the rock matrix and the presence of interesting associated minerals are also important. Due to the microscopic size of the crystals, most specimens are intended for "micromount" collectors. ## Popular Localities The only source of collector specimens is the Krunkelbach mine in Germany. Material from this locality is extremely rare and sought after by specialized collectors of systematic and uranium minerals.

Care and storage

## Cleaning Nielsbohrite specimens should generally not be cleaned. The crystals are extremely small, fragile, and sensitive to any mechanical manipulation. Any contact with water or other chemicals can damage them. If necessary, dust can be removed very carefully with a soft brush or a stream of compressed air from a distance. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is brittle and sensitive to shocks and ultrasound. As a secondary mineral, it is unstable under fluctuating humidity conditions. Due to its uranium content, it is strongly radioactive – exposure should be limited, and dust inhalation avoided. ## Storage Nielsbohrite must be stored under specialized conditions. A sealed, closable "micromount" box is recommended to protect against dust and mechanical damage. It should be kept away from other minerals that could be damaged by radiation. Store in a dry place, away from direct sunlight and heat sources. Due to radioactivity, it should be stored in a place inaccessible to children and pets, preferably in a ventilated room.

Sources

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