Felbertalite

Chemical formula: Cu₁+₂Pb₂+₆Bi₃+₈S₂-₁₉

Felbertalite is a rare copper, lead, and bismuth sulfide, forming acicular crystals with a metallic luster.

## Characteristics Felbertalite is a complex lead, copper, and bismuth sulfosalt. It occurs as very fine, acicular or hair-like crystals, which often form tangled aggregates or radial clusters. Individual crystals are elongated and striated along the elongation axis. The mineral is characterized by a metallic luster and a steel-gray to black color. ## Physical Properties Felbertalite crystals are opaque and exhibit a strong, metallic luster. Due to the small size and acicular form of the crystals, precise determination of hardness and density is difficult. The density calculated based on the chemical formula and unit cell parameters is approximately 7.08 g/cm³. The mineral is brittle. ## History and Name The mineral's name comes from its discovery locality – the Felbertal valley in the Hohe Tauern, Salzburg state, Austria. It was first described in 1999 by Werner H. Paar, Andrew C. Roberts, Dan Topa, Chris J. Stanley, and Emil Makovicky. Approved by the International Mineralogical Association (IMA) in 1998 (IMA1998-052). ## Uses Felbertalite has no industrial significance. It is solely an object of interest for collectors specializing in rare or systematic minerals, as well as a subject of scientific research.

Properties

Luster: Metallic
Streak: Black
Density: 7.08
Cleavage: Perfect on {001}
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Felbertalite can be recognized by its characteristic form – very thin, acicular or hair-like crystals of steel-gray color and metallic luster. It often forms tangled or radial aggregates on the surface of the host rock. However, identification requires advanced analytical methods, such as chemical analysis (EDS) and X-ray diffraction (XRD). ## Distinguishing from similar minerals It can be confused with other acicular sulfosalts, such as jamesonite, boulangerite, or aikinite. Distinguishing it from these "by eye" is practically impossible and requires specialized research equipment. Geological context and associated minerals (e.g., other bismuth sulfides) can be indicative but are not decisive. ## Crystal forms Crystals are strongly elongated, acicular, reaching up to 2 mm in length but very thin (about 5-10 micrometers). They occur as single needles, tangled wool-like aggregates (felted aggregates), or in the form of radial rosettes.

Geological environment

## Genesis Felbertalite is a hydrothermal mineral. It forms in Alpine-type veins cutting metamorphic rocks, such as gneisses and amphibolites. Its crystallization occurs in the late stage of hydrothermal processes, at relatively low temperatures. ## Mineral associations This mineral coexists with other sulfides and sulfosalts. At the type locality (Felbertal), it was found in association with native bismuth, bismuthinite, chalcopyrite, pyrite, quartz, calcite, albite, and chlorite. ## Localities The most important and well-documented occurrence of felbertalite is its type locality in the Felbertal valley in Austria. This is the only confirmed locality of this mineral in the world, making it extremely rare and endemic to this area.

Rarity

Very rare

For collectors

## Quality criteria The most valued felbertalite specimens are those with rich, dense clusters of well-formed, undamaged acicular crystals, forming visually attractive aggregates. Contrast with a light host rock (e.g., quartz or albite) is also important. Due to the microscopic nature of the crystals, specimens are often evaluated under magnification. ## Popular localities The only source of felbertalite specimens is its discovery locality – the Felbertal valley in Austria. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Felbertalite specimens are extremely delicate. Mechanical cleaning should be avoided. If absolutely necessary, compressed air (from a safe distance) can be used to remove dust. The use of water or any chemical agents is not recommended. ## What to avoid Avoid contact with chemicals, especially acids, which can react violently with sulfides. The crystals are very brittle and sensitive to shocks and ultrasound. Prolonged exposure to humid air can lead to slow oxidation and dulling of the surface. ## Storage Specimens should be stored in stable conditions, in closed boxes or display cases, to protect them from dust, mechanical damage, and humidity changes. Due to their brittleness, they should not be placed loosely with other, harder minerals.

Sources

IMA Mineral List (CNMNC)