Tetrataenite

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.[9][10] It is found most abundantly in slow-cooled chondrite meteorites,[11] as well as in mesosiderites.[9] 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.[12][10]

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.[10]

In 2015, it was reported that tetrataenite was found in a terrestrial rock - a magnetite body from the Indo-Myanmar ranges of northeast India.[10]

Tetrataenite has a highly ordered crystal structure,[12] appearing creamy in color and displaying optical anisotropy.[13] Its appearance is distinguishable from taenite, which is dark gray with low reflectivity.[10] 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.[4]

Tetrataenite displays permanent magnetization, in particular, high coercivity.[5] It has a theoretical magnetic energy product, the maximum amount of magnetic energy stored, over 335 kJ m⁻³.[5]

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.[14]

• Glossary of meteoritics