Sejkoraite-(Y)

Chemical formula: Y₂[(U₆+O₂)₈O₆(S₆+O₄)₄(OH)₂]·₂₆H₂O

Sejkorite-(Y) is a very rare, secondary uranyl and yttrium sulfate mineral, forming microscopic, yellow, platy crystals.

## Characteristics Sejkorite-(Y) is a hydrated uranyl and yttrium sulfate. It occurs as very small, platy or bladed crystals, rarely exceeding 0.2 mm in length. These crystals often form radial or stellate aggregates, as well as thin coatings and crusts on rock surfaces. Due to the microscopic size of the crystals, its visual features are difficult to observe without specialized equipment. ## Physical Properties Sejkorite-(Y) crystals exhibit a vitreous luster. The Mohs hardness is estimated to be around 2. The mineral is flexible, meaning its thin lamellae can be bent. It is a strongly radioactive mineral due to its high uranium content. ## Colors and Varieties Sejkorite-(Y) has a characteristic, light yellow to yellowish-green color. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was discovered in the uranium mine in Jáchymov, Czech Republic, and described in 2001. Its name honors the Czech mineralogist and crystallographer, Jiří Sejkora (born 1970), for his contribution to the study of secondary uranium minerals. The suffix "-(Y)" in the name indicates the dominance of yttrium in its chemical composition. ## Applications Sejkorite-(Y) has no industrial applications. Due to its rarity, attractive aggregate forms, and association with a historical locality, it is a prized collector's mineral, sought after by specialists collecting uranium minerals or type locality minerals.

Properties

Mohs hardness: 2
Luster: Vitreous
Streak: Light yellow
Density: 4.68
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Sejkorite-(Y) can be recognized by its characteristic yellow color, platy or bladed crystal habit, and tendency to form radial aggregates. A key diagnostic feature is its occurrence in paragenesis with other secondary uranium minerals. It exhibits strong, green fluorescence under ultraviolet light (both shortwave and longwave). However, definitive identification requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other yellow, secondary uranium minerals, such as uranophane, zippeite, or johannite. It differs from uranophane by its crystal habit (uranophane forms needles), and from zippeite and johannite by subtle differences in hue and aggregate form. Certain distinction is possible almost exclusively based on chemical or crystallographic analysis. ## Crystal Forms It forms flattened, platy or bladed crystals, elongated along the [010] axis and flattened parallel to {001}. These crystals combine into characteristic, radial or stellate aggregates, and also form thin coatings on the host rock.

Geological environment

## Genesis Sejkorite-(Y) is a secondary mineral, formed in the oxidation zones of uraninite (pitchblende) deposits. It forms as a result of weathering of primary uranium ores in the presence of sulfate-rich waters, which originate from the oxidation of sulfides, such as pyrite. It forms under low temperature and pressure conditions, crystallizing in voids and fissures in rocks. ## Mineral Associations It most commonly co-occurs with other secondary uranium minerals, such as gypsum, zippeite, uranopilite, johannite, and natrozippeite. The primary mineral from which it forms is uraninite. ## Localities The most important and type locality for sejkorite-(Y) is Jáchymov in the Czech Republic, a historical uranium mining region. It has also been identified in other localities, such as the Rabejac mine in the Hérault department, France, and in several locations in Utah, USA, including the Delta and Giveway-Simplot mines.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, radial crystal aggregates, contrasting with a dark host rock. The intensity and purity of the yellow color and the absence of damage to delicate structures are important. Due to the microscopic nature of the mineral, specimens are evaluated under magnification. Those originating from the type locality – Jáchymov – have particular value. ## Popular Localities The most sought-after specimens come from Jáchymov in the Czech Republic. Material from the Rabejac mine in France and from mines in Utah (USA) is also present on the collector's market and valued by specialists.

Care and storage

## Cleaning Sejkorite-(Y) specimens are generally not cleaned due to their extreme fragility and microscopic size. Any attempt at mechanical (e.g., brushing) or chemical cleaning will almost certainly destroy the crystals. If dust removal is necessary, a gentle stream of compressed air from a distance can be used. ## What to Avoid Contact with water and chemicals, which can dissolve or damage the mineral, should be strictly avoided. The crystals are very brittle and sensitive to shocks and temperature changes. As a uranium mineral, it is strongly radioactive – exposure should be limited, it should be stored away from frequently occupied areas, and hands should be washed after each contact. ## Storage Specimens should be stored under stable conditions, in closed, padded containers (micromount type), to protect them from mechanical damage, dust, and moisture. Due to radioactivity, it is recommended to store them in special lead containers or in a separate, well-ventilated display case, away from other radiation-sensitive minerals.

Sources

IMA Mineral List (CNMNC)