Petříčekite

Chemical formula: Cu²⁺(Se₂)²⁻

Petříčekite is a very rare copper selenide with a metallic luster, occurring as microscopic, black crystals.

## Characteristics Petříčekite is an extremely rare mineral from the marcasite group, chemically classified as a copper selenide. It forms very small, tabular or lath-like crystals, rarely exceeding 50 micrometers in length, which often occur in radial or tangled aggregates. Its black color and metallic luster are characteristic of this mineral. ## Physical Properties This mineral is opaque and relatively soft, with a Mohs hardness of 2-2.5. It has a very high density, calculated at 6.673 g/cm³, which is typical for minerals containing heavy elements like selenium. The fracture is uneven. ## Colors and Varieties Petříčekite occurs exclusively in black. No colored or commercial varieties are known. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2017. Its name honors František Petříček (1950-2016), a Czech mineral collector who specialized in the mineralogy of hydrothermal uranium veins and was the first to find material containing this new mineral. The discovery was made at the Předbořice deposit in the Czech Republic. ## Uses Due to its extreme rarity and microscopic crystal sizes, petříčekite has no industrial applications. It is solely an object of scientific and collecting interest for specialized collectors (so-called micromounters).

Properties

Mohs hardness
2-2.5
Luster
Metallic
Streak
Black
Density
6.673
Fracture
Irregular/Uneven
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of petříčekite is impossible without advanced analytical methods such as energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Visually, under high magnification, black, metallic-lustered, lath-like crystals can be observed in rock cavities. A black streak is an auxiliary feature. ## Distinguishing from Similar Minerals Petříčekite can be confused with other rare, black selenides such as umangite, berzelianite, klockmannite, or tyrrellite, with which it often co-occurs. Definitive differentiation is only possible through chemical and structural analysis. ## Crystal Forms Petříčekite crystals are very small, usually lath-like or tabular in habit, flattened parallel to {010}. They often form radial or haphazardly arranged aggregates and clusters in rock fissures.

Geological environment

## Genesis Petříčekite is a hydrothermal mineral. It forms under low-temperature conditions in calcite-selenide veins cutting through volcanic rocks. At the type locality (Předbořice), it occurs in calcite veins cutting dacites. ## Mineral Associations This mineral co-occurs with other selenides. At the type locality, it was found in association with umangite, berzelianite, klockmannite, tyrrellite, and chalcomenite, although not all of these minerals are in direct contact. The main associated mineral is calcite. ## Localities The only confirmed occurrence of petříčekite in the world is its type locality: the Předbořice deposit, near the town of Kovářov in the South Bohemian Region, Czech Republic.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a petříčekite specimen is primarily assessed based on the abundance and development of its microscopic crystals. Samples with numerous, well-formed, lustrous crystalline aggregates, clearly visible against a light calcite background, are most prized. Due to the microscopic nature of the mineral, the quality of the specimen under high magnification is crucial. ## Popular Localities The only source of specimens is the type locality in the Czech Republic (Předbořice). Material from this location is extremely difficult to acquire and appears on the collector's market very sporadically.

Care and storage

## Cleaning Specimens of petříčekite should generally not be cleaned mechanically or chemically due to the microscopic size and fragility of the crystals. Any attempts at cleaning can easily damage or remove the mineral from the host rock. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust. ## What to Avoid Avoid contact with chemicals, acids, and ultrasound. The mineral is potentially sensitive to oxidation, so it should be protected from moisture and sudden temperature changes. ## Storage Specimens should be stored under stable conditions, in closed, dry containers (e.g., "micromount boxes"), to protect them from dust, moisture, and mechanical damage. Storage in darkness will help prevent potential chemical changes.

External references

Sources

Read more