Kullerudite

Chemical formula: NiSe<sub>2</sub>

Kullerudite is a rare nickel selenide with a metallic luster, found in low-temperature hydrothermal formations.

## Characteristics Kullerudite is a mineral from the sulfide group, chemically classified as a nickel selenide. It typically forms small, irregular grains or massive aggregates embedded in the host rock. It is rarely observed in the form of poorly developed crystals. It has a characteristic metallic luster and is completely opaque. Its color on a fresh fracture is tin-white to steel-gray, often with a yellowish or reddish tint. On weathered surfaces, it may be covered with a darker tarnish. ## Physical Properties This mineral is characterized by a relatively high hardness for a selenide, ranging from 5.5-6.5 on the Mohs scale, which is comparable to the hardness of pyrite. It has a high density, approximately 6.86 g/cm³. The luster is strongly metallic, and the streak is grayish-black. ## Colors and Varieties Kullerudite occurs in uniform colors: tin-white, steel-gray, sometimes with a delicate yellowish or reddish tint. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1964 based on material from the Flaad nickel mine in Evje (Norway). Its name commemorates Gunnar Kullerud (1921–1989), a Norwegian-American geochemist and experimental petrologist who made significant contributions to the study of sulfide and selenide systems at the Carnegie Institution for Science in Washington. ## Applications Kullerudite has no industrial applications. It is solely an object of scientific interest and is valued by specialized collectors of rare minerals.

Properties

Mohs hardness
5.5-6.5
Luster
Metallic
Streak
Grayish-black
Density
6.86
Cleavage
Good on {100}, {010} and {001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Kullerudite is identified by its metallic luster, tin-white or steel-gray color, high density, and relatively high hardness for a selenide. A key indicator is its paragenesis—co-occurrence with other, often rare selenides in a specific hydrothermal environment. ## Distinguishing from Similar Minerals This mineral can be confused with other metallic sulfides and selenides, such as penroseite ((Ni,Co,Cu)Se₂), which has a very similar appearance but is rarer and often contains cobalt. It differs from pyrite by a slightly different color (pyrite is brass-yellow) and occurrence environment. Positive identification is difficult without advanced analytical methods, such as X-ray diffraction (XRD) or electron microprobe analysis (EDS/WDS). ## Crystal Forms Kullerudite crystallizes in the orthorhombic system. Well-formed crystals are extremely rare. It most often occurs as anhedral (irregular) grains, granular aggregates, or as massive infillings, strongly intergrown with other minerals.

Geological environment

## Genesis Kullerudite is a mineral of hydrothermal origin, forming under low-temperature conditions. It forms in ore veins, where it precipitates from solutions rich in selenium and nickel. ## Mineral Associations This mineral often occurs in association with other selenides, such as clausthalite, tiemannite, umangite, klockmannite, berzelianite, eucairite, and penroseite. It also co-occurs with uraninite, hematite, calcite, and native selenium. ## Localities The most important kullerudite localities worldwide are few. Besides the type locality in the Flaad mine in Evje (Norway), this mineral has been identified in selenide deposits in Pacajake (Bolivia), in the Sierra de Cacho region (Argentina), in the Shinkolobwe mine (Democratic Republic of Congo), and in the Beaverlodge Lake area in Saskatchewan (Canada).

Rarity

Very rare

For collectors

## Quality Criteria The collector's value of kullerudite specimens depends primarily on its abundance in the sample and its association with other rare selenide minerals. The most prized are rich, massive aggregates, as well as specimens with clearly visible, even if small, crystalline aggregates. The origin of the specimen is also of great importance, with samples from the type locality (Norway) and classic localities in Bolivia and Argentina being particularly sought after. ## Popular Localities The most well-known and valued localities by collectors include the historical sites in Norway (Evje), Bolivia (Pacajake), and Argentina (Sierra de Cacho), from which rich selenide parageneses originate.

Care and storage

## Cleaning Kullerudite specimens should be cleaned very carefully, preferably dry, using a soft brush or compressed air to remove dust. Contact with water should be kept to a minimum—if necessary, use distilled water and immediately dry the specimen thoroughly. ## What to Avoid Avoid all chemical agents, especially acids, which can react with the mineral. As a selenide, kullerudite is susceptible to oxidation, so it must be protected from prolonged exposure to moisture and air, which can cause tarnishing and the formation of coatings. ## Storage It is recommended to store specimens in a dry place, preferably in closed, airtight containers or display cases with controlled humidity. This will help preserve its natural luster and prevent oxidation processes.

Sources

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