Die Eisensilizide im Chiemgau-Impakt Kraterstreufeld

Ein neuer, frei zugänglicher (Open Access) Artikel:

In diesem sehr umfangreichen Review-Artikel schreibt der Autor Rappenglück einen sehr ausführlichen Abschnitt über das weltweit einzigartige Streufeld von den Eisensiliziden im Krater-Streufeld des Chiemgau-Impaktes. Dieser Abschnitt wird hier nachfolgend eingefügt.

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Auschnitt über die Eisensilizide im Streufeld des Chiemgau-Impaktes

11. Iron Silicides Associated with Craters

In a few cases, iron silicides may be associated with individual craters or crater fields (Figure 9). The Haughton impact crater (Devon Island, Territory of Nunavut, Canadian Arctic, 75◦220 N, 89◦410 W), 23–24 km in diameter, contains iron silicides together with moissanite (SiC, native Si, and other silicides of Al, Ni, Ba, Ti, and V (here VSi2). The hexagonal crystals comprise vanadium silicide (VSi2) with minor Ti and Ba substitutions for V within silicate glass produced by the impact event [402,403]. The impact is dated to the Eocene, 39 my ago.

Figure 9. Iron silicides associated with craters: (1) Haughton impact crater (2) Chiemgau impact crater strewn field. Source: Michael A. Rappenglück, based on Google My Maps.

A comprehensive relation of iron silicides in craters in an extensive strewn field may be undertaken using material from the Chiemgau Impact site. The crater strewn field of the “Chiemgau impact” is evidence of a large meteorite impact that occurred in prehistoric times in the foothills of the Bavarian Alps [404–406]. The area extends roughly elliptically over an area of about 60 km × 30 km (c. 1800 km2 , 47.8◦–48.4◦ N, 12.3◦–13.0◦ E) between Altötting, Lake Chiemsee and the Alps, Bavaria, Germany. Nearly 80 craters have been documented. The impactor that caused the event is likely to have been a relatively porous object consisting of various components that broke apart in the atmosphere. The analysis of the composition of an impact rock showing the shock metamorphoses typical for an impact and, at the same time, fusing with the metallic components (high lead bronze and iron) of artefacts from the archaeological layer, makes it possible to date the Chiemgau impact to ca. 900–600 BC [407,408]. The published research results evidence an impact event based on the relevant criteria and methodology required in the scientific community. However, the relationship of the geological and archaeological structures and material findings to an impact event has been questioned [409–413] and debated [404,408,414–422].

In the crater strewn field, a total of 2–3 kg of particles, hardly corroded or not corroded at all and showing a metallic sheen, were found distributed over hundreds of square kilometres. They were often are shaped in aerodynamic forms such as ellipsoids, spheres, buttons and drops, but also as splinters and pieces (from 1 mm up to 6 cm and 167 g), or even an 8 kg lump in the subsoil down to the substratum (≈ 30–40 cm) in a glacially formed layer [404,405]. A smoothed convex face and a flat irregularly shaped reverse were frequently observed. The material is tough and magnetic [414,423]. Some specimens show a remaglyptic surface. There is also accretionary lapilli with magnetic xifengite cores. Iron silicide splinters also occurred in foamy-porous carbonate matrices, presumably recrystallised carbonate melt. Big sparkling crystals (moissanite) protruding from the metallic matrix are visible to the naked eye. Fersilicite/naquite (FeSi), ferdisilicite/linzhiite (FeSi2), hapkeite (Fe2Si) as cubic (hapkeite-1C) and trigonal (hapkeite-1T), gupeiite (Fe3Si), suessite (Fe,Ni)3Si, xifengite (Fe5Si3), and in traces suessite (Fe,Ni)3Si were detected [404,424–427]. FexSiy appeared as irregular, round blebs (5–40 µm) and pyramid-shaped formations (≈600 µm) in the microstructure. The intergrown iron silicides formed a matrix for various mineral inclusions. Among them were cubic moissanite ([β]3C-SiC) and titanium carbide (TiC) crystals (≈ 40 µm × 80 µm) of extreme purity, as well as TiC0.63. Khamrabaevite ((Ti,V,Fe)C) was frequently present. There was zirconium carbide (ZrC), possibly baddeleyite (ZrO2) and uranium carbide (UC). Zircon Zr[SiO4] crystals (3–10 µm) and uranium (U) as caps were recognisable. Sometimes, SiC appeared peppered with U blobs. Moreover, calcium-aluminium-rich matter, like the calcium aluminate/krotite (CaAl2O4) and dicalcium dialuminate (Ca2Al2O5) [426], was identified in the material. There were also graphite and nanodiamonds (C). Ni (≈ 0.8 wt%) was present in the suessite (Fe,N)3Si. The amount of Cr was ≈ 0.5 wt%. In addition to the main component, i.e., FexSiy, more than 40 other chemical elements, including uranium and REE (e.g., Y, Ce, La, Pr, Nd, Gd, Yb) have been detected so far. In one sample Th was marginally detectable, and in another, a trace of Po was found. Lead was completely absent. Previous individual findings of a different nature could not be confirmed [409,410]. Although uranium was present in spectra in clear quantities, there was no evidence of daughter nuclides, grandchild nuclides, etc. The microstructure of the material showed clear signs of very intense mechanical overload, which, in principle, could have been caused by high shock effects (pressure, dynamic spallation, and thermal). This caused deformation lamellae and various crack features, e.g., tensile open fractures and groups of subparallel open fissures in FexSiy, TiC crystal, and multiple sets of planar features (PF), kink bands, planar deformation features (PDF) in SiC crystal. The FexSiy matrix was littered with rimmed microcraters (10–20 µm), sometimes showing “ring walls”, probably from the impacts of microparticles. The fersilicites regularly occurred near rimmed nanometre craters. Detailed images showed that zircon crystals struck the plastically deformed or even liquefied matrix of iron silicides. Minerals 2022, 12, 188 27 of 49 It is assumed that disturbance waves ran through the material and suddenly stopped, so that the matrix froze.

The mixture of minerals in the iron silicide matrix was unusual; they were distributed in it with low/high pressure and/or low/high temperature. There was monoclinic high temperature (>1773 K), low-pressure dimorph of CaAl2O4 [419,426], known as krotite. As a natural mineral, it has been identified in meteorites NWA 1934 [428] and in the basic/ultrabasic basaltic volcano complex of Mt. Carmel (Rakefet magmatic complex, Mount Carmel, Haifa District, Israel, 32◦4305900 N, 35◦2 05900 E; see above), dated to the Late Cretaceous (96.7 ± 0.5 Ma) and assigned to kimberlites [429]. At the latter site, orthorhombic dicalcium dialuminate (Ca2Al2O5), was found, i.e., unnamed UM1977-08-O:AlCaH [430], a high-pressure phase (>2.5 GPa) [431] with the brownmillerite-type structure. This was also identified in the iron silicide matrix of the Chiemgau impact [419,426]. That phase can also be produced at ambient pressure but under quite high temperatures [431]. Moreover, in the large area of the Hatrurim Formation (Israel, 31◦ N, 35◦ E), where the rocks, consisting of chalk, limestones, marl, enriched with bituminous compounds, have been intensely heated and metamorphosed, Ca2Al2O5 was also detected [432,433]. The chronology there is Late Cretaceous/Early Eocene (66.0–47.8 mya). Ca2Al2O5 was also detected in the xenoliths of the Ettringer Bellerberg volcanic system (Ettringen, Mayen-Koblenz, Rhineland-Palatinate, Germany, 50◦2100.8800 N, 7◦13041.6500 E), dated c. 0.215 ± 0.004 to 0.190 ± 0.004 mya [434]. In addition, the iron silicide suessite (Fe,Ni)3Si formed from the matrix at more than 2000 K, and cubic moissanite ([β]3C-SiC) as well as nanodiamonds indicated high shock pressure [243]. Xifengite (Fe5Si3) and carbon spherules within amorphous carbon were found in the glazed enamel skin of a pebble from crater #004 in the field. High temperatures (thermal shock), >1773 K and pressures, as well as a magnetic anomaly, have been documented for the rocks in that crater [417,435]. 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 (Fe2Si, cubic and trigonal polymorph), gupeiite (Fe3Si), xifengite (Fe5Si3), 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 Fe2Si.

Collectively, the iron silicides hapkeite (Fe2Si), suessite (Fe,Ni)3Si) and xifengite (Fe5Si3) in the matrix, the mixture of mineral inclusions, which prove the effects of high but also low temperatures and pressures, the large-scale distribution, the association with craters in a strewn field, the finds in proven old layers of the Middle Ages from below a medieval hoard of coins and a castle, in peat mires and on the heights (>1000 m) of the neighbouring Alps exclude an anthropogenic-industrial origin (including bombing) [410] of these materials [404,405,414,435]. A geogenic source is also not plausible [414,435]. A primary extraterrestrial, including perhaps already a mixture in space or a secondary terrestrial (ejecta) source, is suggested [404–406]. The high degree of similarity among the finds from the Chiemgau impact with those from the Alatau and Kalu ranges (Southern Urals, Ishimbayskiy rayon, Republic of Bashkortostan, Russia Ural, Russia) and Laurel Hills, Holmdel (New Jersey, USA) is striking (see above). The findings on the association of uranium and fersilicites, moissanite, titanium carbide, graphite, and the special khamrabaevite are particularly significant. Thus, the iron silicides of the Chiemgau impact can, in principle, also be classified as (distal) impact ejecta. However, in contrast to, and as an extension of, the Alatau and Kalu as well as the Laurel Hills findings, there is a vast crater-strewn field which is genetically associated with the iron silicides, and within the iron silicide matrix are rare krotite (CaAl2O4) and dicalcium dialuminate (Ca2Al2O5). Although FexSiy can be anthropogenic in origin, it is usually not comparable to the iron silicides and associated material found in the Chiemgau strewn field. Given that the known occurrences of FexSiy include several examples of extraterrestrial origin, such an origin is plausible unless a separate, nonimpact origin for FexSiy can be clearly demonstrated.

An additional, still unknown process or a mixture with the extraterrestrial material of the impactor is assumed here.