Mathesiusite

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

Mathesiusite is a rare, radioactive secondary uranium mineral, forming characteristic, acicular crystals of an intense yellow color.

## Characteristics Mathesiusite is a hydrated uranyl, potassium, and vanadium sulfate. It occurs as very fine, acicular or fibrous crystals, most often forming radial aggregates, rosettes, and crusts on the host rock. Its most characteristic feature is its intense, canary-yellow or greenish-yellow color, which makes it easily noticeable despite the small size of the crystals. ## Physical Properties This mineral is very brittle and soft, with a hardness of about 2 on the Mohs scale. The luster is vitreous on crystal faces and silky in fibrous aggregates. It is transparent to translucent. Due to its high uranium content, it has a significant density of about 4.03 g/cm³. ## Colors and Varieties Mathesiusite occurs in uniform, bright shades of yellow, from canary yellow to greenish-yellow. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was officially described in 2013 by a team of mineralogists led by Jakub Plášil. Its name honors Johannes Mathesius (1504–1565), a German clergyman, preacher, and chronicler, who was one of the first to describe minerals from Jáchymov, Czech Republic – the discovery locality of mathesiusite (Rovnost I mine). ## Uses Due to its rarity, mathesiusite has no industrial applications. However, it is an object of scientific interest and a valued collector's mineral, sought after for its attractive form and intense color. It should be noted that as a uranium mineral, it is radioactive.

Properties

Mohs hardness
2
Luster
Vitreous, Silky
Streak
Light yellow
Density
4.03
Transparency
Transparent to Translucent
Crystal system
Tetragonal

Diagnostic features

## Identification The most important diagnostic features of mathesiusite are its form – radial aggregates composed of fine, acicular crystals – and its intense, canary-yellow color. A key diagnostic feature is also strong, green fluorescence under ultraviolet light (both shortwave and longwave). ## Distinguishing from Similar Minerals Mathesiusite is sometimes confused with other secondary uranium minerals of yellow color and similar form, such as zippeite or uranopilite. Zippeite often forms more compact, earthy crusts and exhibits slightly different fluorescence. Uranopilite may have a more greenish hue. Certain differentiation of these minerals without advanced studies (chemical analysis, XRD) is very difficult and often relies on the analysis of associated minerals at a given locality. ## Crystal Forms Mathesiusite crystals are always very small, acicular or fibrous. They almost never occur as single crystals but form characteristic, radial aggregates resembling small suns, fans, or fluffy spheres. They can also form dense, matted coatings on rock surfaces.

Geological environment

## Genesis Mathesiusite is a secondary mineral, formed in the oxidation zones of uranium deposits. It forms as a result of the weathering of primary uranium ores (mainly uraninite) in an environment rich in sulfates, potassium, and vanadium. It is a typical mineral of the weathering zone, often found on the walls of old mine workings, where it crystallizes from seeping, acidic mine waters. ## Mineral Associations This mineral co-occurs with other secondary uranium sulfates and vanadates. Its most common associations include adolfpateraite (at the type locality), zippeite, uranopilite, johannite, and gypsum. ## Localities The most important and best-known worldwide occurrence of mathesiusite, from which the best specimens originate, is the Rovnost mine complex in Jáchymov (Czech Republic). It has also been identified in several other locations worldwide, including the Blue Lizard mine in San Juan County (Utah, USA).

Rarity

Rare

For collectors

## Quality Criteria The most highly prized mathesiusite specimens are characterized by rich crusts with numerous, well-formed, radial aggregates of an intense, canary-yellow color. The contrast with the dark host rock (matrix) and the size of individual "suns" are important. The presence of other rare secondary minerals on the same specimen adds additional value. ## Popular Localities Jáchymov in the Czech Republic is the classic and practically only commercially significant locality for this mineral. Specimens from this location are considered exemplary for the species and fetch the highest prices on the collector's market.

Care and storage

## Cleaning Mathesiusite specimens are extremely sensitive to water, in which they can dissolve. Cleaning should be limited to very gently removing dust with a soft, dry brush or carefully using a stream of compressed air from a safe distance. ## What to Avoid Absolutely avoid contact with water and other liquids. Specimens should be protected from moisture, high temperatures, and direct, prolonged exposure to sunlight. Due to its significant brittleness, the mineral is susceptible to mechanical damage. As a uranium mineral, it is radioactive – avoid inhaling dust and wash hands after each contact with the specimen. ## Storage It is recommended to store specimens in a dry place, preferably in tightly sealed containers (e.g., "micromount" type), which protect against dust, moisture, and damage. Due to radioactivity, specimens should be kept away from areas of permanent human habitation, and collections of radioactive minerals should be stored separately.

External references

Sources

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