Fe-Ni ordering in meteoritic nickelphosphide, Ni 2 FeP

Nickelphosphide, a Ni-dominant member of the join Ni 3 P-Fe 3 P, is the second most abundant phosphide in nature, after its Fe-counterpart, schreibersite Fe 3 P. Surprisingly, the Fe-Ni distribution in the crystal structure of natural nickelphosphide has never been studied.In this paper, we present the results of X-ray single-crystal study of 13 nickelphosphide samples from different meteorite groups: irons, pallasites (both the main group and the Eagle Station), mesosiderites, and the recently established achondrite grouplet, tissemouminites.The Ni content varies from 1.52 apfu (atoms per formula unit), or 50.6 mol.%Ni 3 P, in the mineral from the tissemouminite Northwest Africa 1054 (NWA 1054), to 1.87 apfu 62.3 mol.%Ni 3 P in phosphide from the Hainholz mesosiderite.The Fe/Ni distribution over the M1, M2, and M3 sites of the crystal structure exhibits a sharp Fe-Ni ordering, with Ni always predominating at the M3 and M2, leaving the M1 site Fe-dominant even in the Ni-richest varieties.This ordering implies that the ideal end-member formula of meteoritic nickelphosphide is Ni 2 FeP, rather than the currently adopted Ni 3 P. Consequently, the minerals of the considered series can be represented by three different species, with the end members Fe 3 P, Ni 2 FeP and Ni 3 P.