Schöllhornite

Chemical formula: Na₀.₃CrS₂·H₂O

Schöllhornite is a rare, hydrated sodium chromium sulfide, occurring as hexagonal platelets with a brown color and metallic luster.

## Characteristics Schöllhornite is a rare mineral from the sulfide group, chemically classified as a hydrated sodium chromium sulfide. It forms very small, thin, hexagonal crystals with a platy habit, usually not exceeding 0.1 mm. It most often occurs as inclusions in other minerals, mainly godlevskite. Its characteristic features are its brown color and metallic luster. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1.5. It has perfect cleavage in one direction, parallel to the basal plane of the crystals {0001}. It is opaque and possesses a metallic luster. Its density, calculated based on the chemical formula and unit cell parameters, is approximately 3.13 g/cm³. ## Colors and Varieties Schöllhornite occurs in a uniform brown color. No color varieties or commercial varieties are known. ## History and Name The mineral was first described in 1985 by M.H. Kuch, W. Piestrzyński, and M. Raith. The name "schöllhornite" was given in honor of Professor Robert Schöllhorn (born 1935), a German chemist from the University of Münster, for his contributions to solid-state chemistry research, including intercalated compounds of transition metal chalcogenides. The International Mineralogical Association (IMA) approved it as a new mineral in 1983 (IMA1983-069). ## Uses Due to its extreme rarity and microscopic crystal sizes, schöllhornite has no industrial applications. It is solely an object of scientific interest and a collector's item for specialized collectors of rare minerals.

Properties

Mohs hardness
2
Color
Grey
Luster
Metallic
Streak
Brown
Density
0
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Identifying schöllhornite under amateur conditions is practically impossible due to the microscopic size of its crystals. It requires advanced laboratory techniques, such as chemical analysis (EDS) and X-ray structural analysis (XRD). A preliminary indication might be finding brown, metallic, hexagonal platelets in association with nickel and copper sulfides in serpentinites. ## Distinguishing from Similar Minerals It can be confused with other fine, platy sulfides or oxides. Definitive differentiation is possible only based on chemical composition – the presence of sodium, chromium, and sulfur in specific proportions. ## Crystal Forms Schöllhornite forms thin, hexagonal platelets and lamellae. These crystals occur as single inclusions or small aggregates within other minerals.

Geological environment

## Genesis Schöllhornite is a mineral of hydrothermal origin. It forms as a result of the serpentinization processes of ultramafic igneous rocks. It occurs in calcite-serpentine veins cutting through nickel and copper sulfide deposits. ## Mineral Associations This mineral coexists with other sulfides, mainly godlevskite (in which it often forms inclusions), as well as chalcopyrite, pentlandite, millerite, heazlewoodite, and minerals from the serpentine group and calcite. ## Localities The only confirmed occurrence of schöllhornite in the world (type locality) is the "Lubin" copper mine in Lubin, Lower Silesia, Poland. It was found there in black Zechstein shales (Kupferschiefer).

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a schöllhornite specimen is determined solely by its existence and identifiability. Since the crystals are microscopic, any specimen on which the mineral is visible (under high magnification) and has been analytically confirmed is considered exceptional. The most highly valued specimens would be those with possibly "large" (on a micro scale), well-formed crystals, clearly visible against the matrix. ## Popular Localities The only source of specimens is the "Lubin" mine in Poland. Material from this locality is extremely difficult to acquire and is practically unavailable on the collector's market.

Care and storage

## Cleaning Specimens of schöllhornite, due to their microscopic nature and fragility, should generally not be cleaned mechanically or chemically. Any attempts at cleaning can easily damage or completely remove the tiny crystals from the matrix. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust. ## What to Avoid Avoid contact with water, acids, bases, and any solvents. The mineral is very soft and susceptible to scratching. It should be protected from ultrasound, high temperatures, and humidity, which can lead to its chemical degradation. ## Storage Specimens should be stored under stable conditions, in closed, padded "micromount" boxes, to protect them from mechanical damage, dust, and humidity changes. It is best to keep them away from direct sunlight and heat sources.

External references

Sources

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