Písekite-(Y)

Chemical formula: (Y,As₃+,Ca,Fe₂+,U₄+)(Nb₅+,Ti₄+,Ta₅+)O₄

A historical, discredited name for a tantalum-rich variety of samarskite-(Y), found in pegmatites near the town of Písek in the Czech Republic.

## Characteristics Písekit-(Y) is a currently unrecognized mineralogical name, historically given to specimens later identified as a tantalum-rich variety of samarskite-(Y). It is a black, less commonly brownish-black, massive mineral with high density. It usually occurs as irregular lumps or grains embedded in pegmatite rock. Its crystal structure is destroyed due to radiation emitted by its constituent elements, such as uranium (metamict state), hence it does not form well-developed crystals. The surface can be dull or covered with yellowish-brown weathering. ## Physical Properties This mineral is relatively hard and brittle, and its fracture is conchoidal, typical of amorphous materials. The luster is described as vitreous to resinous, and on fresh fracture surfaces, it can be almost submetallic. It is opaque; only in very thin splinters can it be slightly translucent brown. It is characterized by high density, resulting from the presence of heavy elements such as niobium, tantalum, yttrium, and uranium. It exhibits distinct radioactivity. ## Colors and Varieties Its color is uniform, from velvety black to dark brownish. The name "písekit" itself refers to a specific variety of samarskite-(Y) from a particular locality, distinguished by an elevated tantalum content. ## History and Name The name comes from the place of discovery – the vicinity of the town of Písek in southern Bohemia, Czech Republic. The mineral was first described by F. Ulrich in 1929. Initially considered a distinct species, after more thorough research, its status was discredited, and the name was attributed to a local variety of samarskite-(Y). ## Uses Due to its rarity and historical significance, písekit-(Y) is of interest exclusively to collectors and scientists studying rare earth minerals and the phenomenon of metamictization.

Properties

Mohs hardness: 5-6
Luster: Vitreous to resinous
Streak: Dark reddish-brown to black
Density: 5.6-5.8
Cleavage: Indistinct on {010}
Fracture: Conchoidal to subconchoidal
Transparency: Opaque, translucent in thin splinters
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features include black color, high density (the specimen is surprisingly heavy for its size), resinous or submetallic luster, conchoidal fracture, and radioactivity, which can be detected with a Geiger counter. Occurrence in granitic pegmatite is also an important clue. ## Distinguishing from Similar Minerals It can be confused with other black, heavy pegmatitic minerals such as euxenite, columbite, tantalite, or fergusonite. Definitive differentiation is usually impossible without advanced analytical methods, such as X-ray microanalysis (EDS/WDS). For a collector, the most important clue is the confirmed locality – Písek in the Czech Republic. ## Crystal Forms Due to its metamict state, písekit-(Y) practically does not form crystals. It occurs as irregular, rounded masses and grains. It is rarely found as pseudomorphs after primary, poorly formed, prismatic samarskite crystals.

Geological environment

## Genesis Písekit-(Y) is an accessory mineral in granitic pegmatites, especially in their zones enriched in rare earth elements (REE), niobium, tantalum, uranium, and thorium. ## Mineral Associations It co-occurs with typical pegmatitic minerals such as quartz, albite, microcline, muscovite, as well as other rare minerals, e.g., beryl, monazite, xenotime, and minerals from the columbite group. ## Localities The only and historical locality for which the name "písekit" was used are the pegmatites in the Písek region in the South Bohemian Region of the Czech Republic.

Rarity

Rare

For collectors

## Quality Criteria The collecting appeal of specimens is primarily associated with their origin from the historical, type locality. Larger, solid fragments of the mineral are valued, as are specimens embedded in the rock matrix, showing associations with other pegmatitic minerals. Due to its metamict nature, well-formed crystalline forms practically do not exist and would be extremely rare. ## Popular Localities The most valuable and "classic" specimens are considered to be those from old collections, obtained from pegmatites in the vicinity of Písek in the Czech Republic.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they can cause brittle lumps to crack. ## What to Avoid The mineral is brittle and sensitive to impact. Contact with strong chemicals should be avoided. As a radioactive mineral, it requires limiting direct, prolonged contact. It should not be stored in a bedroom or in places of permanent residence. ## Storage It is recommended to store it in separate, labeled boxes, away from minerals that may be damaged (discolored) by radiation, such as fluorite, smoky quartz, or topaz. Display should be in a closed display case.

Sources

IMA Mineral List (CNMNC)