Mineralogy

On the origin of the xifengite and gupeiite ferrosilicides

In the eastern Bavarian region between the towns of Traunstein and Straubing, small amounts of ferrosilicides are regularly found in the subsurface. The minerals xifengite and gupeiite predominate, but stoichiometrically different Fe-Si compounds are also encountered. Originally, a relation between the occurrence of the ferrosilicides and the proposed Chiemgau impact was supposed (e.g., Rappenglück et al. 2005) substantiated by

— the proximity to the crater strewn field

— the peculiar find circumstance

— the exotic chemical compound (that can form in purely oxygen-free ambience only)

— the extremely rare detection of the gupeiite and xifengite minerals in a terrestrial   context (natural and technical) and

— the occurrence of these minerals in various meteorites and in the outer space.

Therefore, a detailed mapping of the extension of the ferrosilicides was performed which resulted in a sampling mass of about 2.5 kg over an area of nearly 10,000 km².

In order to clarify the possible cosmic origin of the eastern Bavarian ferrosilicides, material-scientific investigations were performed by Raeymaekers & Schryvers (e.g., 2004) who were the first to detect the xifengite and gupeiite minerals in the Chiemgau material, and later by Fehr et al. (2004). The studies by Fehr et al. comprised also a Pb isotope analysis of one sample. The result was not unambiguous because the analysis pointed to the terrestrial field of lead isotopics and at the same time corresponded to the meteorite isochrone. Thus, the statement by Fehr et al. (2004) the material to be technically produced jumped to conclusions. In fact and as noted in the article, ferrosilicides are produced technically in large quantities, but for the xifengite and gupeiite minerals exactly the converse holds true: an industrial production was hitherto unknown.

However, Fe isotopics analyses of three samples initiated by our working group and conducted by Dr. Stefan Weyer (Institut für Mineralogie der Universität Frankfurt) substantiated a terrestrial origin of the Chiemgau ferrosilicides (pers. comm. S. Weyer). Therefore, we focused on the question of a possible technical source for the exotic material, and we are now able to give an answer.

Ferrosilicides, among them considerable amounts of xifengite and gupeiite, resulted as a byproduct from the production of mineral fertilizer that was predominantly used in agriculture immediately after World War II. (Since the mineralogical composition of the fertilizer byproducts was of no interest, the xifengite and gupeiite remained undetected.) It is true the ferrosilicides were for the most part removed from the fertilizer and worked up elsewhere (the xifengite and gupeiite structures having been destroyed), but minimum quantities may have found their way to the fields at that time. Various processes during the past 50 – 60 years like farming activities, precipitation, vegetation, and animal burrowing could have enabled the particles to attain to their present position in the lower soil mostly at 20 – 40 cm depth. However, it is inexplicit now as before why the particles under discussion are found also in many hundred years old forests, in turf moors and in alp regions at more than 1,000 m altitude, unless also these areas were fertilized in former times. Although rather unlikely, a so-called effect of convergence, that is the inserting of the same kind of ferrosilicide particles by fertilizing as well as by impact processes, cannot completely be excluded.

Here we want to state explicitly that the ferrosilicides were never regarded as in proof of an impact, although in the beginning, they actually were related with the Chiemgau event. The numerous other evidences for an impact in the Chiemgau region, as can be studied on our website, are completely untouched by the new insight into the ferrosilicide complex.

 

References

Fehr, K.T., Hochleitner, R., Hölzl, S., Geiss, E., Pohl, J., Faßbinder, J. (2004): Ferrosilizium-Pseudometeorite aus dem Raum Burghausen, Bayern. Der Aufschluß 55, S. 297-303.

Raeymaekers, B., Schryvers, D. (2004): Iron silicides and other metallic species in the SE Bavarian strewn field). Paneth-Kolloquium Nördlingen.

Rappenglück, M., Schüssler, U., Mayer, W., Ernstson, K. (2005): Sind die Eisensilizide aus dem Impakt-Kraterstreufeld im Chiemgau kosmisch? – Eur. J. Mineral. 17, Beih. 1: 108. Siehe auch

http://www.uni-wuerzburg.de/mineralogie/schuessler/Poster_Eisensilizide_2005.pdf

 

2

Nickel: The Chiemgau gupeiite, the  North Haig ureilite meteorite, and the NWA 1241 ureilite meteorite.

In connection with the finds of the peculiar matter in the Chiemgau strewnfield and its possible cosmic origin, critics always point to the lack of nickel. About that, we present a compilation (Tab. 1) of compositions of a gupeiite sampled in the Chiemgau strewnfield, of a low-nickel suessite from the North Haig ureilite meteorite, and of a low-nickel suessite from the NWA 1241 ureilite meteorite.

The Table reveals

— a very similar composition of all three minerals

— low and comparable nickel contents for all three minerals

— a chromium content for the gupeiite within the ranges of  the chromium contents for the two suessites.

 

	Chiemgau							North Haig	NWA 1241
wt.-%	gupeiite							suessite	suessite
Si	14.83	14.40	11.78	14.85	12.93	13.68		14.6 – 16.4	13
Fe	83.61	83.32	87.59	83.28	86.06	86.00		83.0 – 88.8
Ni	0.53	0.53	0.40	1.02	0.48	0.45		0.50 – 2.40	1 – 4
Cr	0.57	0.59	0.45	0.45	0.46	0.37		0.02 – 0.25	1.1
Total	99.54	98.84	100.22	99.60	99.93	100.50

 

Tab. 1. Composition of one sample of gupeiite from the Chiemgau crater field, measured at several locations with an electron microprobe (Institute of Mineralogy, University of Würzburg), in comparison to the compositional ranges of low-Ni suessites of the North Haig ureilitic meteorite and of  the NWA 1241 ureilitic meteorite. (Analysis of the North Haig ureilite in K. Keil, J.L. Berkley & L.H. Fuchs, 1982, American Mineralogist, 67, p. 126-131; classification and mineralogy of NWA 1241 by F. Wlotzka and M. Kurz, in: S.S. Russell, J. Zipfel, J.N. Grossman and M.M. Grady (2002) The Meteoritical Bulletin, No. 86, 2002 July. – Meteoritics Planet. Sci., 37, (Supplement), A157-A184, 2002).

 

A short description of ureilites (in German) can be found here: http://de.wikipedia.org/wiki/Achondrit .

 

1

Reference

The following important publication on the Chiemgau impact event has escaped our attention:

Schryvers, D. and Raeymakers, B. (2005): EM characterisation of a potential meteorite sample, proceeding of EMC 2004, Vol. II, p. 859-860 (ed. D. Schryvers, J.P. Timmermans, G. Van Tendeloo). See Abstract article and Poster (2 MB) .