Tetrataenite

Chemical formula: FeNi

Tetrataenite is a rare mineral, a natural iron-nickel alloy, found mainly in meteorites, characterized by a tetragonal structure.

## Characteristics Tetrataenite is a mineral from the native elements group, consisting of an ordered iron-nickel alloy. Its name refers to its tetragonal crystal structure and chemical composition (taenite is another Fe-Ni phase). It occurs as microscopic grains and lamellae, most commonly within larger masses of kamacite and taenite in iron meteorites. Due to the size of its crystals, its macroscopic features are difficult to observe; it is usually identified using advanced analytical techniques. ## Physical Properties As a metallic alloy, tetrataenite is characterized by a high density, approximately 8 g/cm³. It has a metallic luster and is opaque. It is strongly magnetic, which is one of its key physical properties. ## History and Name The name "tetrataenite" was first used by mineralogists Clarke and Scott in 1980 to refer to the tetragonal Fe-Ni phase identified in meteorites. It refers to the tetragonal crystallographic system and to taenite, another nickel-iron mineral. Its existence as a distinct mineral was formally approved by the International Mineralogical Association (IMA). ## Applications Currently, tetrataenite has no industrial applications due to its rarity and occurrence primarily in meteorites. However, it is the subject of intensive research in materials science. Its unique magnetic properties, similar to rare-earth alloys, make it a potential candidate for the production of permanent magnets without the use of rare-earth elements. Work is underway on the synthetic production of this material on an industrial scale.

Properties

Mohs hardness: 3.5
Luster: Metallic
Streak: Greyish metallic
Density: 0
Cleavage: None
Fracture: Hackly
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of tetrataenite is impossible without specialized equipment. Identification requires advanced methods such as X-ray diffraction (XRD) to confirm the tetragonal crystal structure and X-ray microanalysis (EDS/WDS) to determine the precise chemical composition (Fe:Ni ratio close to 1:1). In meteorites, it occurs as microscopic lamellae or rims around taenite grains. ## Distinguishing from Similar Minerals It can be confused with other iron-nickel minerals found in meteorites: - **Taenite**: Has a very similar composition but crystallizes in the isometric system. Differentiation requires crystallographic analysis. - **Kamacite**: Has a significantly lower nickel content (typically 4-7.5%) and crystallizes in the isometric system. - **Antitaenite**: Is an unstable phase of similar composition, rarely encountered. ## Crystal Forms It forms microscopic, lamellar or granular aggregates. It often occurs as fine lamellae or as marginal zones around taenite crystals, forming intergrowths with it.

Geological environment

## Genesis Tetrataenite forms in iron meteorites and chondrites as a result of extremely slow cooling (on the order of a few degrees Celsius per million years) in asteroid parent bodies. Below 320°C, the existing Fe-Ni phase (taenite) undergoes atomic ordering, where iron and nickel atoms arrange themselves in alternating layers, leading to a change in crystal symmetry from isometric to tetragonal and the formation of tetrataenite. ## Mineral Associations It co-occurs with other minerals typical of meteorites, such as kamacite, taenite, troilite, schreibersite, and various silicates (olivine, pyroxene) in the case of chondrites. ## Localities As a meteoritic mineral, its localities are meteorite fall sites. It has been identified in many iron meteorites (e.g., Estherville, Carlton, Toluca), as well as in chondrites (e.g., St. Séverin). It is not known to occur in terrestrial rocks.

Rarity

Very rare

For collectors

## Quality Criteria Tetrataenite as such is not an object of collector trade, as it occurs in the form of microscopic inclusions in other minerals. The collector value applies to the entire meteorite specimen in which it has been identified. For meteorite collectors, the type of meteorite, its history (whether it was an observed fall), size, degree of preservation (absence of rust), and aesthetics, such as a well-visible Widmanstätten structure on an etched surface, are important.

Care and storage

## Cleaning Meteorite specimens containing tetrataenite are stable but sensitive to moisture, which can cause rusting. Cleaning should be kept to an absolute minimum. If necessary, a soft brush can be used to remove dust. Water and any chemical agents should be avoided. ## What to Avoid The greatest threat is moisture and atmospheric oxygen, leading to oxidation (rusting). Contact with water, acids, bases, and other chemicals must be strictly avoided. Storage in a humid environment will cause irreversible damage to the specimen. ## Storage Meteorite specimens with tetrataenite should be stored in a dry environment, preferably in sealed containers (e.g., acrylic plastic) with a desiccant (e.g., silica gel). The condition of the specimen should be checked regularly.