Plášilite

Chemical formula: Na(U⁶⁺O₂)(S⁶⁺O₄)(OH)·₂H₂O

Plášilite is a rare, secondary uranyl mineral, distinguished by its intense yellow color and occurrence as small, tabular crystals.

## Characteristics Plášilite is a hydrated sodium-uranyl sulfate. It forms characteristic, thin crystals with a tabular habit, which often group into small aggregates, rosettes, or crusts. Individual crystals are usually very small, rarely exceeding 1 mm, and are characterized by an intense yellow, canary-yellow color. Due to its uranium content, this mineral is radioactive. ## Physical Properties Plášilite crystals are brittle. They exhibit strong, greenish-yellow fluorescence under ultraviolet light (both longwave and shortwave). The mineral is transparent to translucent, with a luster described as vitreous to pearly, especially on cleavage surfaces. ## Colors and Varieties Plášilite occurs in a uniform, intense yellow color. No color or commercial varieties are distinguished. ## History and Name The mineral was discovered in the dumps of the Blue Lizard uranium mine in San Juan County, Utah (USA). It was described in 2012 by a team of mineralogists led by Anthony R. Kampf. The name "plášilite" honors the Czech crystallographer and mineralogist, Dr. Jakub Plášil, for his contributions to the study of uranium and selenium minerals. ## Uses Plášilite has no industrial application. Due to its rarity, attractive color, and well-formed microcrystals, it is a sought-after mineral by specialized collectors of uranium minerals.

Properties

Mohs hardness: 2-3
Color: Greenish yellow
Luster: Vitreous to Pearly
Streak: White
Density: 3.726
Cleavage: Two perfect cleavages, {010} and {001}.
Fracture: Irregular/Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Plášilite can be identified by its intense yellow color, characteristic tabular crystal habit, and strong greenish-yellow fluorescence under UV light. Its occurrence in association with other secondary uranium minerals on sedimentary rocks is also a key indicator. ## Distinguishing from Similar Minerals It can be confused with other yellow, secondary uranyl minerals, such as johannite, uranophane, or boltwoodite. Differentiation is often impossible without advanced analytical methods (e.g., XRD, EDS). However, plášilite is distinguished by its specific chemical composition (it is a sodium-uranyl sulfate) and crystallizes in the monoclinic system, which differentiates it from orthorhombic johannite. ## Crystal Forms Crystals are thin-tabular, often with rectangular or hexagonal outlines. They usually form small, rosette-like or radial aggregates, as well as thin crusts on the host rock.

Geological environment

## Genesis Plášilite is a secondary mineral, forming in the oxidation zones of uranium deposits. It is formed by the weathering and alteration of primary uranium minerals (mainly uraninite) in the presence of sulfate-rich waters. It crystallizes under low temperature and pressure conditions, forming in voids and fissures of sedimentary rocks, such as sandstones. ## Mineral Associations This mineral often co-occurs with other secondary sulfates and uranyl minerals. At the type locality (Blue Lizard mine), it was found in association with gypsum, johannite, natrozippeite, watevilleite, as well as newly described minerals such as bluelizardite and redcanyonite. ## Localities The most important and typical occurrence of plášilite is the Blue Lizard mine in Red Canyon, San Juan County, Utah, USA. Beyond this locality, its presence has been confirmed in several other uranium mines in the same region (e.g., Giveway-Simplot mine). It is a characteristic mineral of this specific geological area.

Rarity

Very rare

For collectors

## Quality Criteria The collector's value of a specimen is primarily determined by the quality and size of the crystals. Samples with sharp, well-formed, transparent crystals forming aesthetic, rosette-like aggregates are most prized. Contrast with the rock matrix and the richness of association with other rare uranyl minerals are also important. Due to the microscopic nature of the crystals, specimens are evaluated under magnification. ## Popular Localities The only source of valuable collector specimens is the Red Canyon area in Utah, USA, particularly the Blue Lizard mine, which is the type locality for this mineral.

Care and storage

## Cleaning Plášilite specimens should generally not be wet-cleaned. Only gentle dust removal with a soft brush or a photographic air blower is permissible. Contact with water can lead to the dissolution or damage of delicate crystals. ## What to Avoid Contact with water, any chemical agents, ultrasonic cleaners, and sudden temperature changes must be strictly avoided. As a uranyl mineral, it is sensitive to direct, prolonged sunlight, which can cause its slow degradation. Due to radioactivity, direct contact should be limited, and hands should always be washed after handling the specimen. ## Storage Specimens should be stored in a dry place, preferably in a tightly sealed, transparent box (a so-called micromount box), which protects them from dust, moisture, and mechanical damage. Storage in sealed containers also limits radon emission. It should be kept away from other minerals that could be damaged by its radiation.