Grundmannite

Chemical formula: Cu₁+Bi₃+Se₂-₂

Grundmannite is a very rare copper bismuth selenide, forming tiny, metallic inclusions in other minerals.

## Characteristics Grundmannite is an extremely rare mineral belonging to the sulfosalt group, specifically selenides. Chemically, it is a copper bismuth selenide. It occurs as very fine, anhedral (irregularly shaped) grains, usually not exceeding 100 micrometers in size. Most often, it forms inclusions in other, more common minerals such as berzelianite, umangite, or clausthalite. Due to its size and mode of occurrence, it is impossible to observe with the naked eye. ## Physical Properties As a metallic mineral, grundmannite is opaque and exhibits a strong, metallic luster. Its hardness and density have not been precisely measured due to the microscopic size of the grains, but they are estimated based on its chemical composition and comparisons with similar minerals. It does not exhibit cleavage, and its fracture is uneven. ## Colors and Varieties On a fresh surface in reflected light (under a microscope), grundmannite is grayish-white with a slight creamy tint. No varieties are known. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2015. Its name honors Toni

Properties

Mohs hardness: 2.5-3
Luster: Metallic
Streak: Black
Density: 6.46
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of grundmannite is possible only through advanced laboratory methods, such as chemical composition analysis (e.g., electron microprobe - EDS/WDS) and X-ray diffraction (XRD). In reflected light under a polarizing microscope, it is characterized by a grayish-white color with a creamy tint, distinct pleochroism (from gray to grayish-white), and strong anisotropy. It does not exhibit internal reflections. ## Differentiation from Similar Minerals It is distinguished from other bismuth selenides and sulfosalts (e.g., wittichenite, emplectite) based on its precise chemical composition, especially the ratio of copper to bismuth and selenium. Visually, without specialized equipment, it is impossible to distinguish from many similar, microscopic minerals with a metallic luster. ## Crystal Forms To date, it has only been observed in the form of anhedral, or irregularly shaped, grains ranging from a few to about 100 micrometers in size. These grains form inclusions or aggregates with other minerals.

Geological environment

## Genesis Grundmannite is a hydrothermal mineral. It forms under low-temperature conditions in calcite-uranium veins rich in selenium. Its crystallization is associated with mineralization processes in which hydrothermal solutions transported and deposited metals, including copper, bismuth, and selenium. ## Mineral Associations This mineral occurs in close association with other selenides. It is most commonly accompanied by berzelianite (Cu₂Se), umangite (Cu₃Se₂), clausthalite (PbSe), eucairite (AgCuSe), eskebornite (CuFeSe₂), as well as uraninite and calcite. It often forms complex, microscopic intergrowths with them. ## Localities The only confirmed occurrence of grundmannite in the world (type locality) is the historic mine dump of the Schlema-Alberoda mine in the Erzgebirgskreis district, Saxony (Germany). This is a classic locality for many rare selenides.

Rarity

Extremely rare

For collectors

## Quality Criteria Grundmannite does not occur in macroscopic specimens that could be traded by collectors in the traditional sense. Its value is purely scientific. For specialized collections (so-called "micromounts"), the only criterion is the confirmation of its presence on a sample, usually in association with other rare selenides from the same locality. Such a specimen consists of a fragment of the host rock on which microscopic grains of grundmannite have been identified.

Care and storage

Grundmann, a German mineral collector who provided the material for research. The discovery was made on material from the historic mine dump of the Schlema-Alberoda mine in the Ore Mountains (Saxony, Germany).

Sources

IMA Mineral List (CNMNC)