Crystal structure of iron nickel phosphide, Fe1.8Ni1.2P, a Schreibersite extracted from Orange River meteorite

Abstract

Fe1.8Ni1.2P8, tetragonal, I4 (No. 82), a = 9.085(5) A, c = 4.481(3) A, V = 369.8 A 3 , Z =8 ,Rgt(F) = 0.037, wRref(F 2 ) = 0.086, T = 293 K. Source of material Single crystals of the phosphides from “Orange River” were obtained by using a solution with HNO3 (3%) and methanol. After dissolving the meteorite samples only sulphides, carbides, oxides, phosphides and other crystals remained. The solution with the crystals was filtered and the phosphides were picked up from thefiltermaterial.Theywerepreparedandfixedonglasfibresfor X-ray experiments. The chemical composition was determined by electron microprobe analysis (Cameca SX 100). The crystals are homogeneous and don’t show zonations, twinnings or intergrowth sections. Discussion The (Fe,Ni)3P single crystals (xenomorph variant named Schreibersite) from the “Orange River” meteorite with a Fe:Ni-ratioof1.5:1(Fe1.8Ni1.2P)showabodycenteredtetragonal crystal structure with the well known space group I4 typical for meteoritic phosphides. The distribution of Fe and Ni on the three symmetryindependentmetalpositionswasdeterminedearlieron Rhabdites from “North Chile” meteorite using a Co-tube with a wavelength near by the Fe absorption edge. This result showed thatNipreferstheM2andM3sitesavoidingM1[1](cf.also[2]). Additional measurements with synchrotron radiation (HASYLAB at DESY, Hamburg) near the Fe absorption edge in order to separate Fe and Ni using the anomalous dispersion and thedeltasynthesis[3,4]showedthesameordering:Feprefersthe M1 and M2 sites whereas Ni prefers the M2 and M3 sites. Considering this fact, the site occupancy factors of the Fe and Ni atomswerefixedduringrefinement[5].ThebestR-valueisevoked by a distribution with Fe on M1, Fe/Ni on M2 and Ni on M3.