Stilleite

Chemical formula: Zn<sup>2+</sup>Se<sup>2-</sup>

Stilleite is a very rare zinc selenide of the sphalerite group, forming small, dark brown to black grains in hydrothermal veins.

## Characteristics Stilleite is a mineral from the sulfide class (though it is a selenide), belonging to the sphalerite group. It occurs as very small, anhedral (irregular) grains, rarely exceeding 0.5 mm. Its appearance is inconspicuous – it has a dark brown to black color and a submetallic luster. It is usually intergrown with other minerals, making its identification difficult without specialized equipment. ## Physical Properties The Mohs hardness of stilleite is approximately 5. This mineral is characterized by a submetallic to resinous luster and is opaque in larger fragments, although thin splinters may be translucent. It has a relatively high density, around 5.63 g/cm³. ## Colors and Varieties This mineral is uniform in color and occurs in shades from dark brown to black. No color or commercial varieties are distinguished. ## History and Name Stilleite was first described in 1956. Its name comes from Hans Stille (1876–1966), a German geologist and professor at the universities of Göttingen and Berlin, who made significant contributions to the study of European tectonics. ## Applications Due to its extreme rarity, stilleite has no industrial application. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness
5
Luster
Sub-metallic to Resinous
Streak
Brownish-black
Density
5.63
Cleavage
Perfect on {110}
Fracture
Conchoidal to uneven
Transparency
Opaque, Translucent in thin fragments
Crystal system
Cubic

Diagnostic features

## Identification Identification of stilleite is very difficult and usually requires advanced analytical methods, such as chemical composition analysis (EDS/WDS) or X-ray diffraction (XRD). Preliminary identification is based on its dark color, submetallic luster, and co-occurrence with other selenides in a specific geological environment. ## Distinguishing from Similar Minerals It can be confused with other dark, opaque minerals, such as sphalerite (with which it is isostructural), clausthalite, or tiemannite. Differentiation requires chemical analysis – stilleite is the only one of these that is a pure zinc selenide. Sphalerite is a zinc sulfide, while clausthalite and tiemannite are lead and mercury selenides. ## Crystal Forms It usually forms irregular, rounded grains and granular aggregates. Well-formed crystals are extremely rare and, if present, take forms typical of the isometric system, such as tetrahedra.

Geological environment

## Genesis Stilleite forms under low-temperature hydrothermal conditions. Its formation is associated with solutions rich in selenium and zinc. ## Mineral Associations It most often co-occurs with other selenides, such as clausthalite, a selenium variety of digenite, as well as with uraninite, pyrite, and linnaeite. ## Localities The type locality and main occurrence is the Shinkolobwe mine in Katanga Province (Democratic Republic of Congo). It is also known from Tilkerode in the Harz Mountains (Germany) and from the uranium mine in the Athabasca Lake region in Saskatchewan (Canada).

Rarity

Very rare

For collectors

## Quality Criteria For such a rare mineral, the main criterion is its very presence and the possibility of identification. The most highly valued specimens are those where stilleite grains are relatively large (over 1 mm), clearly visible, and occur in a rich association with other rare selenides. ## Popular Localities The most sought-after specimens come from the type locality – the Shinkolobwe mine in the DRC, which is now inactive and inaccessible, further increasing the value of historical samples.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. The use of water or any liquids is not recommended to avoid damaging delicate mineral associations. ## What to Avoid Avoid contact with chemicals, acids, and ultrasonic cleaners. As a selenide, it can be toxic, so avoid inhaling dust during any preparation and wash hands after contact with the mineral. Protect from impacts due to its perfect cleavage. ## Storage It is best stored in a closed, stable display box, protected from dust, moisture, and direct sunlight. Due to the small size of the grains, storage in "micromount" type boxes is recommended.

External references

Sources

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