No abstract

A survey is presented on some characteristic features of meteoritic (Fe,Ni) 3 P which is an abundant and important minor phase of most iron meteorites. This mineral (named schreibersite/rhabdite) plays a decisive role during the formation of the so-called Widmanstätten pattern. Different transmission as well as scanning electron microscopic techniques have been applied to get more precise information about the real structure of the phosphide crystals, their chemical composition and the metal distribution across the phoshide/ kamacite interface. X-ray crystal structure determinations have been performed for selected (Fe,Ni) 3 P -cystals from various iron meteorites (Toluca, North Chile, Watson, Orange River, Morasko, Agpalilik, Odessa, Canyon Diablo). These experiments revealed a metal ordering, i.e. for the three non-equivalent metal positions a different substitution of Fe by Ni has been found. The perfection of the brittle (Fe,Ni) 3 P samples differs appreciably and seems to be dependent on the thermal history of each individual meteorite. Moreover, inside Ni-rich rhabdite crystals small monocrystalline inclusions of CrN (carlsbergite) have been detected.

Synchrotron radiation diffraction studies of meteoritic (Fe,Ni)3P crystals have been performed to reveal the ordering of the elements Fe and Ni on the three metal sites M1, M2 and M3 of the unit cell. The delta synthesis technique, which is a two-wavelength method using anomalous dispersion effects, was applied. For (Fe,Ni) phosphide crystals with different Fe:Ni ratios extracted from different meteorites, it was found that Ni occupies the M3 site and also partially the M2 site, avoiding the M1 position, whereas the M1 site is preferentially occupied by Fe. In connection with earlier results known from the literature, this metal distribution seems to be characteristic of this compound, and is independent of thermodynamic formation conditions.

Discussion The (Fe,Ni)3P single crystals (xenomorph variant named Schreibersite) from the "Canyon Diablo" meteorite with a Fe:Ni-ratio of 1.3:1 (Fe1.7Ni1.3P) show a body centered tetragonal crystal structure with the well known space group 74 typical for meteoritic phosphides. The distribution of Fe and Ni on the three symmetry independent metal positions was determined earlier on Rhabdites from "North Chile" meteorite using a Co-tube with a wavelength near by the Fe absorption edge. This result showed that Ni prefers the M2 and M3 sites avoiding Ml [1] (cf. also [2]). Additional measurements with synchrotron radiation (HAS YLAB at DES Y, Hamburg) near the Fe absorption edge in order to separate Fe and Ni using the anomalous dispersion and the delta synthesis [3,4] showed the same ordering: Fe prefers the Ml and M2 sites whereas Ni prefers the M2 and M3 sites. Considering this fact, the site occupancy factors of the Fe and Ni atoms were fixed during refinement [5]. The best R-value is evoked by a distribution with Fe on Ml, Fe/Ni on M2 and Ni on M3. Table 1. Data collection and handling. Crystal: bronze, wedge-shaped fragment, size 0.04x0.07x0.10 mm Wavelength: Mo K a radiation (0.71073 Â) μ: 268.83 cm" 1 Diffractometer, scan mode: Stoe IPDS. φ 20 max : 47.8° N(hkl)measured, N(hkl) unique : 2173, 282 Criterion for / obs , N(hkl) gt : I 0 bs > 2 σ(Ι αbs), 274 N(param) refined: 39 Program: SHELXL-97 [5] Source of material Single crystals of the phosphides from "Canyon Diablo" 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 the filter material. They were prepared and fixed on glas fibres for 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 inter-growth sections. Abstract Fe1.7Ni1.3P8, tetragonal, 74 (No. 82), a = 9.049(1) Á, c = 4.4646(9) À, V = 365.6 À 3 , Ζ = 8, R gt (F) = 0.042, u'ÄreffF 2) = 0.093, T= 293 Κ.

Fe1.65Ni1.35P8, tetragonal, I4̅ (No. 82), a = 9.059(2) Å, c = 4.479(1) Å, V = 367.6 Å3, Z = 8, Rgt(F) = 0.022, wRref(F2) = 0.049, T = 293 K.