Haxonite revisited, NiFe22C6: new crystal-chemical formulae and Fe-Ni ordering as a structure-stabilizing factor

Abstract Haxonite is the second most common carbide in meteorites following cohenite. However, unlike the latter, haxonite has no synthetic counterpart, hence its structure type and chemical formula were suggested based on the similarity of the X-ray powder diffraction pattern with that of a tau carbide (synthetic analogue of isovite). During the study of the mineralogy of the Barbianello meteorite (iron ungrouped, Ni-rich ataxite), we identified both cohenite and haxonite within the 2-cm single-crystal host rod of taenite matrix. Both carbides form irregular-angular crystals associated with nickelphosphide, but have never been found in one inclusion together. Cohenite and haxonite vary considerably in Ni content (<1 and 5 wt. %, respectively), while the associated nickelphosphide and taenite have a constant Ni composition (∼27 and 52 wt.%, respectively). The Ni content in haxonite from the Barbianello meteorite is close to ∼1.1 atom per formula unit (apfu), which is a typical value for all previously studied haxonites (∼1.1 - 1.3 apfu). Notably, no Ni-free haxonite crystals are known. Based on single crystal X-ray diffraction data, we demonstrated that Ni orderly occupies single crystallographic site (M1, 4a). The dominance of Ni in a single crystallographic site also leads to the ideal formula of haxonite being M1NiM2-M4Fe22C6. We hypothesize that the presence of Ni stabilizes haxonite crystal structure and makes haxonite common in Ni-rich irons. We propose that other factors promoting haxonite formation include the presence of a precursor Ni-rich phase and relatively low crystallization temperatures.