Tetrataenite

Tetrataenite is a native metal composed of chemically-ordered L10-type FeNi, recognized as a mineral in 1980. The mineral is named after its tetragonal crystal structure and its relation to the iron-nickel alloy, taenite. It is one of the mineral phases found in meteoric iron.
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. It is found most abundantly in slow-cooled chondrite meteorites, as well as in mesosiderites. At high (as much as 52%) Ni content and temperatures below 320 °C (the order-disorder transition temperature), tetrataenite is broken down from taenite and distorts its face centered cubic crystal structure to form the tetragonal L10 structure.
The L10 phase can be synthetically produced by neutron- or electron-irradiation of FeNi below 593 K or by hydrogen-reduction of nanometric NiFe2O4, but only on a small scale.
In 2015, it was reported that tetrataenite was found in a terrestrial rock - a magnetite body from the Indo-Myanmar ranges of northeast India.
Crystal Structure.
Tetrataenite has a highly ordered crystal structure, appearing creamy in color and displaying optical anisotropy. Its appearance is distinguishable from taenite, which is dark gray with low reflectivity. 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.
Magnetic Properties.
Tetrataenite displays permanent magnetization, in particular, high coercivity. It has a theoretical magnetic energy product, the maximum amount of magnetic energy stored, over 335 kJ m−3.
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.