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Tetrataenite
Tetrataenite
	Silvery-bright tetrataenite crystals
General
Category	Native element minerals
Formula; (repeating unit)	FeNi
IMA symbol	Ttae
Strunz classification	1.AE.10
Crystal system	Tetragonal
Crystal class	Domatic (m) ; (same H-M symbol)
Space group	Pm
Unit cell	22.92 Å³
Identification
Formula mass	57.27 gm
Color	gray white, silver white
Crystal habit	Granular – Common texture observed in granite and other igneous rock
Cleavage	none
Fracture	malleable
Mohs scale hardness	3.5
Luster	metallic
Streak	gray
Diaphaneity	opaque
Density	8.275
Common impurities	Co, Cu, P
References	[1] [2]

Tetrataenite is a native metal alloy composed of chemically-ordered L1₀-type FeNi, recognized as a mineral in 1980.^[5] The mineral is named after its tetragonal crystal structure and its relation to the iron-nickel alloy, taenite, which is chemically disordered (A1) phase with an underlying fcc lattice.^[6] Tetrataenite is one of the mineral phases found in meteoric iron.^[7]^[3]^[8] Before its discovery in meteoritic samples, experimental synthesis of the L1₀ phase was first reported in 1962 by Louis Néel and co-workers, following neutron irradiation of a chemically disordered FeNi sample under an applied magnetic field.^[9]^[10]^[11] Compared to the magnetically soft, chemically disordered A1 phase (taenite), the tetragonal L1₀ structure of tetrataenite leads to good hard magnetic properties, including a large uniaxial magnetocrystalline anisotropy energy.^[10]^[12] Consequently, it is under consideration for applications as a rare-earth-free permanent magnet.^[13]

Formation

Tetrataenite forms naturally in iron meteorites that contain taenite that are slow-cooled at a rate of a few degrees per million years, which allows for ordering of the Fe and Ni atoms.^[5]^[14] It is found most abundantly in slow-cooled chondrite meteorites,^[15] as well as in mesosiderites.^[5] At high (as much as 52%) Ni content and temperatures below 320 °C (the order-disorder transition temperature^[9]), tetrataenite is broken down from taenite and distorts its face centered cubic crystal structure to form the tetragonal L1₀ structure.^[16]^[14]

It is reported that the L1₀ phase can be synthetically produced by neutron- or electron-irradiation of chemically disordered (A1) FeNi below 593 K,^[9]^[10]^[11] by hydrogen-reduction of nanometric NiFe₂O₄,^[14] by combined application of mechanical stress and magnetic field during annealing of the chemically disordered A1 phase,^[17] or by crystallization of Fe−Ni alloys in the presence of traces of phosphorus.^[18]

In 2015, it was reported that tetrataenite was found in a terrestrial rock – a magnetite body from the Indo-Myanmar ranges of northeast India.^[14]

Potential laboratory protocols for bulk synthesis

Applied Stress and Magnetic Field

It has been reported that the combined application of mechanical stress and a modest magnetic field during the annealing process can accelerate the formation of the atomically ordered L1₀ phase in bulk samples.^[17]

Addition of Phosphorus

In 2022, it was reported that mixing iron and nickel together in specific quantities, with a phosphorus catalyst, and smelting the mixture, forms tetrataenite in bulk quantities, in seconds.^[19]^[20] These claims raised hopes that some of the technologies which currently require the use of magnetic alloys containing rare earths metals may be achievable using magnets made of tetrataenite as an alternative, which would reduce dependence on toxic, environmentally harmful rare earth mines.^[21] However, at present, the reported findings are yet to be independently replicated by other experimental groups.

Crystal structure

Tetrataenite has a highly ordered crystal structure,^[16] appearing creamy in color and displaying optical anisotropy.^[5] Its appearance is distinguishable from taenite, which is dark gray with low reflectivity.^[14] FeNi easily forms into a cubic crystal structure, but does not have magnetic anisotropy in this form. Three variants of the L1₀ tetragonal crystal structure have been found, as chemical ordering can occur along any of the three axes.^[13]

Magnetic properties

Tetrataenite displays permanent magnetization, in particular, high coercivity.^[22] It has a large uniaxial magnetocrystalline anisotropy^[12] and theoretical magnetic energy product, the maximum amount of magnetic energy stored, over 335 kJ m⁻³.^[22] The L1₀ phase has a theoretical Curie temperature of over 1000 K,^[12] resulting in a magnetic anisotropy which is predicted to remain large up to and beyond room temperature.

Applications

Tetrataenite is a candidate for replacing rare-earth permanent magnets such as samarium and neodymium since both iron and nickel are earth-abundant and inexpensive.^[23]