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TRIGONAL AND CUBIC FE2SI POLYMORPHS (HAPKEITE) IN THE EIGHT KILOGRAMS FIND OF NATURAL IRON SILICIDE FROM GRABENSTÄTT (CHIEMGAU, SOUTHEAST GERMANY

Authors:
  • vhs Gilching and Observatory Gilching
TRIGONAL AND CUBIC FE2SI POLYMORPHS (HAPKEITE) IN THE EIGHT KILOGRAMS FIND OF
NATURAL IRON SILICIDE FROM GRABENSTÄTT (CHIEMGAU, SOUTHEAST GERMANY): F. Bauer1, M.
Hiltl2, M. A. Rappenglück3, K. Ernstson4. 1Oxford Instruments GmbH NanoScience, Wiesbaden, Germany
(frank.bauer @oxinst.com), 2Carl Zeiss Microscopy GmbH, D-73447 Oberkochen, Germany (mhiltl@online.de),
3Institute for Interdisciplinary Studies, D-82205 Gilching, Germany (mr@infis.org), 4Faculty of Philosophy I, Uni-
versity of Würzburg, D-97074 Würzburg, Germany (kernstson@ernstson.de).
Introduction: Some 30 years ago a metallic, sil-
very gleaming boulder weighting 8 kg (Fig. 1) was
excavated near the town of Grabenstätt on Lake
Chiemsee in Bavaria. As an enigmatic object of com-
pletely unknown origin the private finder bequeathed it
to the family where it fell into oblivion.
Fig. 1. The eight kilograms iron silicide boulder from
Grabenstätt. Centimeter scale. Fig.2. Location map for
the Chiemgau impact crater strewn field and iron sili-
cide occurrences.
The find was brought to mind again when similarly
looking metallic matter became common currency in
the Chiemgau district as basically important for the
meanwhile established Chiemgau meteorite impact
event. Here we report on detailed analyses of the boul-
der that rapidly proved to be iron silicide matter thus
remarkably adding to the iron silicides family from the
Chiemgau crater strewn field so far established and
published [1-5].
Iron silicides and the Chiemgau meteorite im-
pact event: The discovery of the Chiemgau meteorite
crater strewn field was directly paralleled by the abun-
dant finds of iron silicides comprising gupeiite, xi-
fengite, hapkeite, naquite and linzhite, and containing
various carbides like, e.g., moissanite SiC, titanium
carbide TiC and khamrabaevite (Ti,V,Fe)C, and calci-
um-aluminum-rich inclusions (CAI), minerals krotite
and dicalcium dialuminate (Fig. 3). With regard to this
exotic mineral assemblage and the extreme purity of
the carbide crystals that obviously was not achieved
under terrestrial conditions, an industrial or a geogenic
origin was discarded, in particular with regard to the
very specific sampling situations. Hence a cosmic
origin got increasing evidence. So far the total mass of
the iron silicides has amounted to about two kilograms
sampled from the whole strewn field with metal detec-
tors, and the largest specimen was a few centimeters
tall and weighed 160 g. Against this background the
recovery of an iron silicide "monster" from the crater
strewn field weighting eight kilograms proved to be-
come a scientific stroke of luck.
Fig. 3. Various aspects of iron silicides from the
Chiemgau meteorite impact crater strewn field.
Methods: From the strongly magnetic boulder
splinters of thumbnail size were removed and provided
with mirror polish for SEM-EDX and EBSD analyses,
and Raman spectroscopy.
Fig. 4. EBSD reveals nearly 100 % Fe2Si with con-
spicuous grain boundaries (see text).
Results: From the EBSD analyses of one boulder
splinter (Fig. 4) the general texture proved to be Fe2Si
making the main mass (98,3 % phase fraction). The
phase description for the Fe2Si is the trigonal crystal
system [7, 8]. The reniform contact of the four differ-
ently oriented grains far from any crystallographic
direction is enigmatic, because they are not growth-
related and not related to recrystallization either. They
rather point to interpenetration from a rapidly
quenched formation or to a similar process of metaso-
matic overprint.
Within the Fe2Si matrix tiny inclusions with sizes
between a few micrometers and about 1 mm are scat-
tered forming two different clusters as seen in the EDS
Layered Image (Fig. 5). It established one cluster to be
formed of pure carbon particles, and Raman spectros-
copy identified graphite (Fig. 6 A, 7) with typical D, D'
bands of disordered graphene. The second cluster re-
vealed a larger variability, and EBSD suggest gupeiite,
khamrabaevite and possibly zirconium carbide con-
tributing to the inclusions (Fig. 4, Fig. 6 B). A certain
1520.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
amount of uranium could not be ascribed to a reasona-
ble compound. At a second position of the 8 kg block,
titanium carbide/khamrabaevite and carbon inclusions
were established in a matrix of cubic hapkeite and cu-
bic gupeiite.
Fig. 5. EDS reveals two clusters of inclusions within
the iron silicide matrix: irregular orientation of carbon
particles and aligned particles near to khamrabaevite,
(Ti,V,Fe)C.
Fig. 6. Inclusions within the iron silicide matrix.
Graphite has been established by Raman spectroscopy
(Fig. 7). EDS data suggest gupeiite, khamrabaevite and
possibly zirconium carbide.
Fig. 7. Raman spectra taken at the cross positions: typ-
ical graphite spectrum with D, D' bands of disordered
graphene.
Discussion: In the literature a trigonal (P3-m1),
and a tetragonal (Pm3-m) Fe2Si polymorph are de-
scribed [9-12] after a first artificial preparation of
Fe2Si (unclear polymorph) in 1939 [13]. A cubic poly-
morph, especially named hapkeite-1C for its first de-
tection in the lunar Dhofar 280 meteorite [14, 15], was
later also discovered in a sample from the Luna 24
Mare Crisium landing site [16] and in an Apollo 16
regolith sample [17]. Recently hapkeite (12 µm) was
found in a meteorite from Koshava, Bulgaria, [18] and
also discovered in the meteorite DAG 1066 [19] and
occurs in a grain from the FRO 90228 ureilite [20].
Fe2Si reported for magnetic spherules in Hungary
could be related to cosmic dust or a meteorite impact
[21]. Hapkeite was found also in a 7μm Supernova
graphite (OR1d3m-18) from the Orgueil meteorite
[22]. The trigonal polymorph the most stable among
the silicides (up to 255 GPa) in conjunction with
xifengite (Fe5Si3), gupeiite (Fe3Si), and inclusions of
cubic SiC (moissanite) and (Ti,V,Fe)C (khambraevite),
was discovered in a sample coming from the
Chiemgau Impact area [3]. These findings show that
Fe2Si is produced by (1) extreme reduction and shock
heating in an impact melt, (2) condensation of a sili-
cate vapor caused by a massive impact event, and (3)
space weathering. The reported Fe2Si in a fulgurite
[23] is mentioned for reasons of completeness. Only
now Fe2Si (all polymorphs) come into focus for indus-
trial application.
Conclusions: From these analyses and within the
specific context an 8 kg chunk of massive iron sili-
cide containing Fe2Si in close proximity to the Lake
Tüttensee meteorite crater in the Chiemgau impact
strewn field, no artificial production and no fulgurite
it is very probable that the boulder is of extraterrestrial
origin. Hitherto it is the biggest sample known to con-
tain natural Fe2Si in its cubic phase (hapkeite-1C), and
together with the earlier reported polymorph [3] the
first natural occurrence of trigonal Fe2Si. For reasons
of definiteness we suggest to name the trigonal Fe2Si
polymorph hapkeite - 2T possibly rating a new mineral
name.
References: [1] Ernstson, K. et al. (2010) J. Sibe-
rian Federal Univ., Engin. & Techn., 1, 72-103. [2]
Hiltl, M. et al. (2011) 42th LPSC, Abstract #1391. [3]
Bauer, F. et al. (2013) Meteoritics & Planet. Sci., 48,
s1, Abstract #5056. [4] Rappenglück, M.A. et al.
(2014) Proc. Yushkin Memorial, Syktyvkar, Russia,
106-107. [5] Rappenglück, M.A. et al (2013) Meteo-
ritics & Planet. Sci.., 48, s1, Abstract #5055. [6]
Ernstson, K. et al. (2014) LPSC 45th, abstract #1200.
[7] ICSD Database Fe2Si [100094 ] 2931 [8] NIST
Database Fe2Si (1627) [9] Rix, W. (2001) Ph.D. thesis
2001. [10] Khalaff, K. et al. (1974) Less-Common
Metals 35, 341-345. [11] Kudielka, H. (1977) Z. Kris-
tall.-Cryst. Mat. 145, 177189. [12] Chi Pui Tang et al.
(2016) AIP Advances, 6, 065317. [13] Dodero, M.
(1939) C. R. Acad. Sci. Paris, 799. [14] Anand, M. et
al. (2004) PNAS, 101, 6847-51. [15] Anand, M. et al
(2003) LPS XXXIII, Abstract #1818. [16]
https://www.mindat.org/loc-7798.html. [17] Spicuzza
M.J. et al. (2011) LPS XLII, Abstract #2231. [18] Ya-
nev, Y. et al. (2015) Geosciences, 20, 81-82. [19]
Moggi Cecchi, V. (2015) 78th Ann. Meeting Meteoriti-
cal Soc., Abstract #1856. [20] Smith, C.L. et al.
(2008) 39th LPSC, Abstract #1669. [21] Szöör, Gy.
(2001) Nucl. Instrum. Methods in Phys. Res. Section B,
181, 557-562. [22] Croat, T. K. et al. (2011) 42nd
LPSC, Abstract # 1533. [23] Sheffer, A. A. (2007)
Ph.D. thesis, The University of Arizona, 75-79.
1520.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
... Big sparkling crystals (moissanite) protruding from the metallic matrix are visible to the naked eye. Fersilicite/naquite (FeSi), ferdisilicite/linzhiite (FeSi 2 ), hapkeite (Fe 2 Si) as cubic (hapkeite-1C) and trigonal (hapkeite-1T), gupeiite (Fe 3 Si), suessite (Fe,Ni) 3 Si, xifengite (Fe 5 Si 3 ), and in traces suessite (Fe,Ni) 3 Si were detected [404,[424][425][426][427]. Fe x Si y appeared as irregular, round blebs (5-40 µm) and pyramid-shaped formations (≈600 µm) in the microstructure. ...
... Finally, an iron silicide lump (c. 16 cm × 11 cm × 5 cm, 8 kg), found approximately 30 years ago near Grabenstätt at Lake Chiemsee, is reported to contain cubic hapkeite (Fe 2 Si, cubic and trigonal polymorph), gupeiite (Fe 3 Si), xifengite (Fe 5 Si 3 ), titanium carbide (TiC)/khamrabaevite ((Ti,V,Fe)C), moissanite (cubic SiC), zirconium carbide (ZrC), graphite and graphene [424,426]. When writing this review, the block is the largest known example containing natural cubic and trigonal Fe 2 Si. ...
Article
Full-text available
This review systematically presents all finds of geogenic, impact-induced, and extraterrestrial iron silicide minerals known at the end of 2021. The respective morphological characteristics, composition, proven or reasonably suspected genesis, and possible correlations of different geneses are listed and supported by the available literature (2021). Artificially produced iron silicides are only dealt with insofar as the question of differentiation from natural minerals is concerned, especially regarding dating to pre-industrial and pretechnogenic times.
Conference Paper
Full-text available
The finds of iron silicides composed, among others, of xifengite, gupeiite, hapkeite with inclusions of titanium carbide, khamrabaevite and moissanite, and CAIs, together with about 30 elements including uranium and REE, which have been regarded as extraterrestrial for about 15 years in the crater strewn field of the Chiemgau impact, have been enriched by an 8 kg find with analogous composition.
  • K Ernstson
References: [1] Ernstson, K. et al. (2010) J. Siberian Federal Univ., Engin. & Techn., 1, 72-103. [2]
  • V Moggi Cecchi
  • C L Smith
Moggi Cecchi, V. (2015) 78th Ann. Meeting Meteoritical Soc., Abstract #1856. [20] Smith, C.L. et al. (2008) 39th LPSC, Abstract #1669. [21] Szöör, Gy. (2001) Nucl. Instrum. Methods in Phys. Res. Section B, 181, 557-562. [22] Croat, T. K. et al. (2011) 42nd
  • W Rix
  • K Khalaff
ICSD Database Fe2Si [100094 ] 2931 [8] NIST Database Fe2Si (1627) [9] Rix, W. (2001) Ph.D. thesis 2001. [10] Khalaff, K. et al. (1974) Less-Common Metals 35, 341-345. [11] Kudielka, H. (1977) Z. Kristall.-Cryst. Mat. 145, 177-189. [12] Chi Pui Tang et al. (2016) AIP Advances, 6, 065317. [13] Dodero, M. (1939) C. R. Acad. Sci. Paris, 799. [14] Anand, M. et al. (2004) PNAS, 101, 6847-51. [15] Anand, M. et al (2003) LPS XXXIII, Abstract #1818. [16]
LPS XLII, Abstract #2231
M.J. et al. (2011) LPS XLII, Abstract #2231. [18] Yanev, Y. et al. (2015) Geosciences, 20, 81-82. [19]