EM characterisation of a potential meteorite sample

D. Schryvers1 and B. Raeymaekers2

1. EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium

2. InfraServ Gendorf, D-84508 Burgkirchen, Germany

 

nick.schryvers@ua.ac.be

Keywords: gupeiite, xifengite, meteorite

In the district of Altötting (Upper Bavaria, Germany) four particular and apparently correlated spatial configurations were recently discovered:

–         craterlike structures,

–         anomaly of Ni-traces in honey samples, established during a biomonitoring project with honey bees,

–         severe magnetic anomalies in the stray field of craterlike structures,

–         metallic particles (diameter mostly > 1 cm) in the upper 30 cm of the soil.

Two of the metallic samples were investigated with a variety of conventional EM techniques. The first specimen has a rough surface while the second one looks like a solidified globular “droplet”, both having a metallic shining. In Figure 1 SEM images of three distinctly different surface morphologies are shown, together with the respective EDX spectra. These reveal a primarily Fe-Si content with some micron sized cubic Ti rich, C-containing inclusions. By SEM/WDX the following elements were further detected: C, Al, Ti, V, Cr, Mn, Co, Ni, Ta, Mo, Nb, W. For most of these elements the formation of silicides has been described before [1].

In Figure 2 SAED patterns from internal grains of these samples are shown from which three different phases can be recognised. Figure 2.a reveals the [110] zone of cubic Fe3Si, also known as Gupeiite, while Figures 2b and 2c show the [101] and [201] zones of the hexagonal Fe5Si3 structure, also known as Xifengite. These SAED patterns originate from micron sized grains, whereas the ring pattern in Figure 3b relates to the nanoparticles shown in Figure 3a and can be explained by the bcc a-Fe structure, the poor quality of the image being due to the magnetic distortions of the beam.

It should be noted that the FexSiy, species are lacking in the geology of Bavaria. Nevertheless, over a large surface (30 x 5 km2) these FexSiphases can be found in particles from the upper soil. Given the particular characteristics listed above and since industrial sources are highly improbable, the particles were assigned to the impact of a meteorite, a suggestion which recently has been confirmed by the geophysical characterisation of a few crater structures [2].

Similar effects were reported from the Yanshan area in China [3], where Fe3Si and Fe5Si3 were found in globular particles, with a cosmogenic origin. Moreover, the Ni concentration of 0.1 % in the present samples is identical as in the Yanshan-particles. Worldwide further reports of these species were not found.

The presence of Xifengite and Gupeite in FexSiy-particles over a wide area in undisturbed soil profiles, accompanied by further refractory species, supports the hypotheses of a cosmic origin, in analogy to the results from the Yanshan area.

1.        N. Wiberg, Lehrbuch der Anorganischen Chemie, 1985, Walter de Gruyter, New York, Berlin

2.        V. Hoffmann, W. Rößler, B. Raeymaekers, to be published

3.        Y. Zuxiang, Acta Petr. Miner. et Anal. 3, 231-238

Figure 1.  (a, b, c) Different surface morphologies observed in both samples, (d, e) EDX-spectra from matrix and cubic inclusion, respectively.

Figure 2. SAED patterns of (a) [110] Fe3Si, (b) [101] Fe5Siand c) [201] Fe5Si3.

Figure 3. a-Fe nano particles and the corresponding ring pattern.