Linnaeite

Chemical formula: Co<sup>2+</sup>Co<sup>3+</sup><sub>2</sub>S<sup>2-</sup><sub>4</sub>

Linnaeite is a cobalt and nickel sulfide from the thiospinel group, valued by collectors for its well-formed, metallic crystals with an isometric habit.

## Characteristics Linnaeite is a relatively rare sulfide mineral belonging to the thiospinel group. It typically forms small but well-formed, octahedral crystals with a metallic luster. Less commonly, it occurs as granular aggregates or massive forms. Its surface is often covered with a pinkish or purplish patina, which is the result of weathering and oxidation to erythrite. ## Physical Properties This mineral is characterized by a hardness ranging from 4.5-5.5 on the Mohs scale and a significant density of approximately 4.8-5.0 g/cm³. It is brittle and exhibits an uneven fracture. It is non-magnetic. The luster is distinctly metallic, and the mineral itself is opaque. ## Colors and Varieties Fresh surfaces of linnaeite are light gray to steel-gray, often with a slight pinkish tint. When exposed to air, it quickly tarnishes and becomes covered with the aforementioned pinkish-purple patina. There is a nickel-rich variety called polydymite, and a selenium-bearing variety called cadmoindite. Siegenite, in turn, is a nickel-cobalt variety, often difficult to distinguish without chemical analysis. ## History and Name The mineral's name, given in 1845 by Wilhelm von Haidinger, honors Carl Linnaeus, the famous Swedish naturalist and creator of the classification system for organisms. The mineral was first described based on specimens from the Bastnäs mine in Sweden. ## Uses Linnaeite, due to its cobalt content, is a minor local source of this metal. However, its main significance is scientific and as a collector's item. Well-formed crystals are sought after by collectors of rare minerals.

Properties

Mohs hardness
4.5-5.5
Luster
Metallic
Streak
Grayish-black
Density
4.8-5.0
Cleavage
Imperfect on {001}
Fracture
Uneven to conchoidal
Transparency
Opaque
Crystal system
Cubic

Diagnostic features

## Identification Key diagnostic features of linnaeite include its isometric, octahedral crystal habit, metallic luster, steel-gray color, and tendency to form a pinkish patina. A hardness around 5 on the Mohs scale and relatively high density are also helpful in identification. ## Distinguishing from Similar Minerals Linnaeite is sometimes confused with other sulfides of similar appearance. It differs from pyrite by its slightly darker, grayer color (pyrite is brass-yellow) and usually octahedral, rather than cubic, habit. It differs from chalcopyrite by the absence of a distinct golden hue. The most difficult distinction is from other minerals within its own group, such as siegenite or polydymite – this often requires advanced chemical analysis (EDS). ## Crystal Forms Linnaeite crystallizes in the isometric system, forming almost exclusively octahedral crystals. These crystals can be single, embedded in the host rock, or form small, intergrown groups. Granular aggregates and massive forms are less common.

Geological environment

## Genesis Linnaeite is a hydrothermal mineral. It forms in ore veins at medium to high temperatures, most often in association with other nickel and cobalt sulfides. It can also crystallize in metamorphic rocks and in sedimentary deposits where it undergoes redeposition. ## Mineral Associations This mineral often co-occurs with other sulfides, such as chalcopyrite, pyrite, pyrrhotite, millerite, siegenite, gersdorffite, as well as with calcite, dolomite, quartz, and siderite. Its association with pink erythrite, which is a product of its weathering, is characteristic. ## Localities The historical type locality is the mines in Bastnäs, Västmanland, Sweden. Other significant occurrences worldwide include the Siegen area in North Rhine-Westphalia (Germany), where it occurs in association with siegenite; Katanga Province in the Democratic Republic of Congo; Kotalahti in Finland; and also the state of Missouri in the USA (La Motte and Mine-La-Motte mines). In Poland, its occurrence has been reported in the vicinity of Kletno and Miedzianka.

Rarity

Rare

For collectors

## Quality Criteria Specimens with sharp, well-formed, undamaged crystals and a strong metallic luster are most highly valued by collectors. Ideal specimens are single, large octahedra or aesthetic groups of crystals embedded in a contrasting rock matrix. Surface cleanliness, without excessive secondary coatings, also increases value, although a delicate, pinkish patina is considered a characteristic and often desirable feature. ## Popular Localities Classic and most sought-after specimens come from historical localities in Germany (Siegen area) and Sweden (Bastnäs). Currently, specimens from the Democratic Republic of Congo also appear on the market. Specimens from the USA, especially from Missouri, are also valued in specialized collections.

Care and storage

## Cleaning Linnaeite specimens should be cleaned very carefully, preferably dry, using a soft brush to remove dust. Contact with water, especially for extended periods, can accelerate oxidation processes and lead to the formation of undesirable coatings (e.g., erythrite). If wet cleaning is necessary, use distilled water and immediately dry the specimen thoroughly. ## What to Avoid Contact with acids and other chemicals that can react violently with sulfides should be strictly avoided. The mineral is sensitive to humid air, which promotes its weathering. It should not be heated or exposed to prolonged sunlight. ## Storage It is recommended to store linnaeite in a dry place, preferably in closed, airtight containers or display cases with a desiccant. This will help limit the oxidation process and preserve the metallic luster and original color of the specimen.

External references

Sources

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