Jáchymovite

Chemical formula: (U<sup>6+</sup>O<sub>2</sub>)<sub>8</sub>(S<sup>6+</sup>O<sub>4</sub>)(OH)<sub>14</sub>·13H<sub>2</sub>O

Jáchymovite is a very rare, bright yellow uranyl mineral, occurring as microscopic acicular crystals and efflorescences.

## Characteristics Jáchymovite is a secondary uranyl sulfate, forming characteristic, bright yellow to yellowish-green efflorescences and crusts. It occurs as aggregates of very fine, acicular or bladed crystals, which rarely reach sizes visible to the naked eye. Due to its habit, jáchymovite aggregates often have a delicate, silky luster. It is a strongly radioactive mineral. ## Physical Properties Crystals are extremely small and brittle. Hardness is very low, estimated at about 2 on the Mohs scale. The calculated density is 3.99 g/cm³, but due to the nature of the aggregates, it is difficult to measure. The mineral exhibits strong, yellowish-green fluorescence under ultraviolet light (both shortwave and longwave). ## Colors and Varieties Jáchymovite occurs in shades from bright yellow to canary yellow and greenish-yellow. No color or commercial varieties are distinguished. ## History and Name The mineral was described in 1996 by a team of mineralogists (P. Ondruš, J. Čejka, R. Skála, J. Hloušek, I. Čejková, T. Bábek). Its name comes from the discovery locality – the historic mining town of Jáchymov (formerly St. Joachimsthal) in the Czech Republic, known for its abundance of uranium minerals. Type specimens come from the Geschieber vein in the Svornost mine. ## Uses Jáchymovite has no industrial application. It is of purely scientific importance and is sought after by specialized collectors of rare uranium minerals.

Properties

Mohs hardness
2
Luster
Silky
Streak
Light yellow
Density
3.99
Cleavage
None
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Jáchymovite can be identified by its bright yellow color, form of fine, acicular crystals forming efflorescences and crusts, and silky luster. A key diagnostic feature is its strong yellowish-green fluorescence under UV light. It occurs in paragenesis with other secondary uranium minerals. ## Distinguishing from Similar Minerals It can be confused with other yellow, secondary uranium minerals such as uranophane, zippeite, or johannite. Uranophane forms radial aggregates, and its crystals are often larger. Zippeite has a similar appearance but often forms more earthy or scaly aggregates. Definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms It forms very fine, elongated crystals with an acicular or bladed habit, rarely exceeding 0.1 mm in length. These crystals group into tangled aggregates, efflorescences, crusts, and coatings on host rocks.

Geological environment

## Genesis Jáchymovite is a secondary mineral, formed in the oxidation zones of hydrothermal deposits rich in uraninite. It forms as a result of weathering and alteration of primary uranium ores under the influence of sulfate-rich waters. It is one of the alteration products found in old mine workings. ## Mineral Associations It co-occurs with other secondary uranium minerals, such as adolfpateraite, zippeite, uranopilite, johannite, as well as gypsum and uraninite as a primary mineral. ## Localities The most important and classic locality, from which the type material originates, is the Svornost mine in Jáchymov (Krušné Hory, Czech Republic). It has also been reported in uranium mines in the Schlema-Hartenstein area in Saxony (Germany).

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the intensity and purity of color and the richness of the efflorescence on the rock matrix are most important. Specimens where well-formed, individual acicular crystals are visible (even under high magnification) are much more valued than dense, earthy efflorescences. Association with other rare uranium minerals is also important. ## Popular Localities By far the most valued and sought-after specimens come from the type locality – the Svornost mine in Jáchymov, Czech Republic. This is the classic and historical source of this mineral.

Care and storage

## Cleaning Jáchymovite specimens are generally not cleaned. Any attempts at mechanical or chemical cleaning will almost certainly destroy the delicate crystals. Only very careful removal of dust with a soft brush or a photographic air blower is permissible. ## What to Avoid Avoid contact with water and chemicals, which can dissolve or damage the mineral. It is sensitive to changes in temperature and humidity. Due to its uranium content, inhalation of dust and direct skin contact must be strictly avoided. Always wash your hands after handling a specimen. ## Storage Specimens should be stored in airtight, transparent containers (micromount type) that protect them from mechanical damage, dust, and humidity changes. Due to radioactivity, it should be stored away from areas of permanent human presence, in a marked location, preferably in a radiation-shielding container (e.g., lead).

Sources

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