Trüstedtite

Chemical formula: Ni₂+Ni₃+₂Se₂-₄

Trüstedtite is an extremely rare nickel selenide of the linnaeite group, occurring as tiny, metallic grains in sulfide deposits.

## Characteristics Trüstedtite is a member of the linnaeite group, belonging to the selenides. It forms small, irregular grains with a metallic luster, usually intergrown with other ore minerals. It is rarely observed as poorly formed, small octahedral crystals. It is an opaque mineral with a black color. ## Physical properties Its Mohs hardness is approximately 3. It is characterized by a strong, metallic luster. It is a brittle mineral with an uneven fracture. The calculated density is 7.03 g/cm³, making it a very heavy mineral. ## Colors and varieties It occurs exclusively in black. No color or commercial varieties are known. ## History and name The mineral is named in honor of Otto Trüstedt (1866-1929), a Finnish geologist who studied the Outokumpu copper deposit in Finland. The mineral was finally described and approved by the IMA in 1962 by a team consisting of Olavi Kouvo, Maja Huhma, and Yrjö Vuorelainen. ## Applications Trüstedtite has no industrial applications. It is solely an object of scientific interest and a valued acquisition for advanced systematic mineral and micromineral collections.

Properties

Mohs hardness: 3
Luster: Metallic
Streak: Black
Density: 7.03
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Visual identification is practically impossible. Indicative features may include: black color, metallic luster, high density, and specific mineral parageneses. Certain identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS), to confirm the chemical composition (presence of nickel and selenium). ## Distinguishing from similar minerals In grain form, it is indistinguishable from other black minerals with a metallic luster from the linnaeite group (e.g., linnaeite, polydymite, siegenite) and other sulfides and selenides. The key difference is the chemical composition – the dominance of selenium over sulfur. ## Crystal forms It most commonly occurs as anhedral (unformed) grains less than a millimeter in size. Very rarely, it forms tiny, imperfectly formed octahedra.

Geological environment

## Genesis It forms under metamorphic conditions in selenium-rich, massive metal sulfide deposits. It is a product of hydrothermal or metamorphic processes occurring at high temperatures. ## Mineral associations It often co-occurs with other rare selenides, such as wilkmanite, penroseite, tyrrellite, and blockite. It is also accompanied by more common ore minerals, including chalcopyrite, pyrrhotite, sphalerite, and bornite. ## Localities The type locality and most important occurrence is the Outokumpu deposit in the North Karelia region of Finland. It has also been reported in small quantities on the Taimyr Peninsula in Russia and in the Key West mine in Clark County, Nevada, USA.

Rarity

Extremely rare

For collectors

## Quality criteria For a micromineral like trüstedtite, the value of a specimen is determined by its richness (quantity of visible grains), analytically confirmed identity, and association with other rare selenides. Any specimen with visible, even poorly formed crystals, is considered exceptional. ## Popular localities The most prized and classic specimens come from the type locality – the Outokumpu deposit in Finland.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush. Distilled water can be used, followed by thorough drying of the specimen. ## What to avoid Avoid contact with acids and other strong chemicals. Ultrasonic cleaners are not recommended, as they can damage delicate specimens. ## Storage Store in stable conditions, away from moisture and chemically aggressive substances. It is best to store in a sealed "micromount" box to protect it from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)