Sederholmite

Chemical formula: NiSe

Sederholmite is a rare nickel selenide, forming hexagonal crystals with a metallic luster and a brownish hue.

## Characteristics Sederholmite is a rare mineral from the sulfide group, chemically classified as a nickel selenide. It typically occurs as fine, hexagonal grains or plates, rarely exceeding 1 mm. Its appearance is typically metallic, with a characteristic brownish or brownish-pink hue on fresh surfaces. It often forms intergrowths with other selenides, such as clausthalite. ## Physical Properties This mineral is characterized by a metallic luster. It is opaque. Its hardness on the Mohs scale is approximately 2.5, making it relatively soft. The density is high, calculated at 7.98 g/cm³. ## Colors and Varieties Sederholmite has a brownish color, often with a pink or brownish-yellow hue. No color varieties or commercial varieties have been distinguished. ## History and Name The mineral's name honors Jakob Johannes Sederholm (1863–1934), a Finnish geologist and petrologist, former director of the Geological Survey of Finland. Sederholmite was first described in 1964 based on material found in Proterozoic shales in the Kuusamo mine in Finland.

Properties

Mohs hardness
2.5
Luster
Metallic
Streak
Brownish black
Density
7.98
Cleavage
Good on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Sederholmite is difficult to identify visually without specialized equipment. Its key features include a brownish color with a pinkish hue, metallic luster, and occurrence in association with other selenides. Positive identification requires advanced methods such as reflectivity analysis in reflected light or chemical analysis (EDS/WDS) to confirm its composition (selenium and nickel). ## Distinguishing from Similar Minerals It can be confused with other metallic minerals of similar color, such as breithauptite (nickel antimonide), which is harder, or some copper and nickel sulfides. Chemical composition analysis and the environment of occurrence are crucial – sederholmite is characteristic of selenide parageneses. ## Crystal Forms It most commonly occurs as anhedral (irregular) grains or small, hexagonal plates and flakes, usually less than a millimeter in size. It rarely forms well-developed crystals.

Geological environment

## Genesis Sederholmite forms under hydrothermal conditions, at low to medium temperatures. It is a typical mineral of calcite-uraninite veins cutting metamorphic rocks, such as shales. Its crystallization is associated with the presence of selenium-rich solutions. ## Mineral Associations This mineral co-occurs with other selenides and sulfides. It is most commonly accompanied by: clausthalite, penroseite, willyamite, klockmannite, umangite, berzelianite, as well as uraninite, calcite, and hematite. ## Localities The most important and classic locality (type locality) is the Kuusamo area in Finland. Other confirmed localities include the uranium deposit at Beaverlodge Lake in Saskatchewan (Canada) and the Třebíč region in the Czech Republic.

Rarity

Very rare

For collectors

## Quality Criteria As a microscopic mineral, sederholmite is not evaluated according to typical criteria such as color or clarity. The greatest collector value is held by specimens where sederholmite grains are relatively large (visible under a magnifying glass), well-defined, and occur in a rich association with other rare selenides. Accurate documentation and confirmation of the mineral's identification from a given locality are crucial.

Care and storage

## Cleaning Sederholmite specimens are usually very small and occur within the host rock. Cleaning should be kept to an absolute minimum. If necessary, compressed air can be used to remove dust. Avoid contact with water and chemical agents. ## What to Avoid As a selenide, the mineral may be sensitive to acids and oxidizing agents. Avoid moisture, high temperatures, and exposure to chemicals that could cause its decomposition or discoloration. Due to its softness, it is susceptible to scratching. ## Storage Specimens should be stored under stable conditions, in a dry environment, preferably in closed display boxes, to protect them from dust, moisture, and mechanical damage. Avoid direct contact with other minerals to prevent scratching.

Sources

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