Clarkeite

Chemical formula: Na(U<sup>6+</sup>O<sub>2</sub>)O(OH)·nH<sub>2</sub>O

A rare, highly radioactive uranium mineral, formed by hydrothermal alteration of uraninite in granitic pegmatites.

## Characteristics Clarkite is a secondary mineral from the uranium oxide group, formed as a result of the alteration of uraninite. It usually occurs as dense, compact masses or as dark rims around uraninite grains. It rarely forms discernible crystals, and its aggregates are most often massive or earthy. Due to its strong radioactivity, it is a dangerous material and requires specialized handling. ## Physical Properties This mineral is characterized by high density, approximately 6.39 g/cm³. Its Mohs hardness is estimated at 4-4.5. The luster is usually dull, earthy, or waxy. It is opaque. ## Colors and Varieties Clarkite ranges in color from dark brown to reddish-brown, and in powdered or weathered form, it can be yellowish-brown. No commercial or colored varieties are distinguished. ## History and Name The mineral is named in honor of Frank Wigglesworth Clarke (1847–1931), an American geochemist and former Chief Chemist of the United States Geological Survey (USGS). The mineral was first described in 1931 by Charles Palache based on material collected from the Spruce Pine mine in North Carolina, USA. ## Uses Clarkite, like other secondary uranium minerals, is primarily of scientific importance as an indicator of geochemical processes occurring in uranium deposits. Due to its rarity and strong radioactivity, it has no industrial or commercial applications. It is sought only by specialized institutions and advanced collectors of radioactive minerals.

Properties

Mohs hardness
4-4.5
Luster
Waxy
Streak
Brownish-yellow
Density
6.39
Cleavage
None
Fracture
Conchoidal
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Clarkite is primarily identified by its strong radioactivity, detectable with a Geiger counter. Its dark brown color, massive form, and co-occurrence with uraninite in pegmatites are key visual characteristics. It often forms transitional zones with other uranium oxides. ## Distinguishing from similar minerals It can be confused with other massive, dark uranium minerals, such as uraninite or gummite (mixtures of uranium oxides). Definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD), due to visual similarities and common co-occurrence. ## Crystal forms Clarkite usually occurs in compact, massive, or earthy aggregates. Microscopic, poorly formed tabular or platy crystals are very rarely observed.

Geological environment

## Genesis Clarkite is a secondary mineral that forms as a result of hydrothermal alteration of primary uraninite. This process occurs in the oxidation zones of uranium deposits, most often in granitic pegmatites, under conditions of elevated temperature and pressure, with the involvement of sodium-rich solutions. ## Mineral associations This mineral co-occurs with uraninite (as a product of its alteration), as well as with other secondary uranium minerals such as curite, becquerelite, schoepite, and fourmarierite. In pegmatites, it is also accompanied by feldspars (especially albite), quartz, and micas. ## Localities The most important and classic occurrence of clarkite is its type locality - the Spruce Pine district in Mitchell County, North Carolina, USA. It has also been reported in several other uranium pegmatites worldwide, including in India.

Rarity

Very rare

For collectors

## Quality criteria In the case of clarkite, collector value is not assessed according to typical aesthetic criteria, such as luster or crystal form, which are usually poorly developed. The most important factors are the purity of the material, confirmed by analyses, and accurate documentation of the specimen's origin. Due to its extreme rarity, every confirmed specimen has high scientific and collector value for specialized collectors. ## Popular localities The only source of historically significant and reference specimens is the Spruce Pine area in North Carolina, USA. Specimens from other localities are extremely rare and are subjects of scientific research.

Care and storage

## Cleaning Cleaning clarkite specimens is highly inadvisable and dangerous without specialized equipment and knowledge. Mineral dust is highly radioactive and toxic. All activities should only be performed in a professional laboratory. ## What to avoid Avoid any skin contact, inhalation of dust, and ingestion. The mineral must be protected from water and moisture, which can accelerate its weathering and the release of radon. It should not be heated or exposed to chemicals. ## Storage Clarkite specimens must be stored in sealed, lead-lined containers, clearly marked with the international radioactivity symbol. These containers should be kept in a well-ventilated area, away from places of permanent human habitation and other minerals that could be damaged by radiation.

External references

Sources

Read more