A meteoric iron arrowhead from the late Bronze Age of Switzerland.

The Iron Age is considered to have begun when people started smelting iron from iron oxide ores. However, some iron artefacts predate this, having been produced from a source which did not require smelting: meteoric iron. In the Old World, Bronze Age meteoric iron artefacts are known from Turkey, Greece, Syria, Iraq, Lebanon, Egypt, Iran, Russia, China, and Poland. To date, the entire complement of Bronze Age meteoric iron artefacts from Europe comprises two rings and an amulet from Greece, a pair of bracelets from Czestochowa-Rakowa in Poland, and an iron axe from Wietrzno, also in Poland. Attempts have recently been made to locate other meteoric iron objects in archaeological collections, using X-ray fluorescence analysis, concentrating on areas where meteoric iron is thought likely to have been available. One such potential source is the Twannberg iron meteorite strewn field in the Jura Mountains of Switzerland, which has led archaeologists to re-examine many Bronze Age artefacrs in Swiss museum collections.

In a paper published in the Journal of Archaeological Science on 25 July 2023, Beda Hofmann of the Naturhistorisches Museum Bern and the Institute of Geological Sciences at the University of Bern, Sabine Bolliger Schreyer of the Bernisches Historisches Museum, Sayani Biswas and Lars Gerchow of the Paul Scherrer Institute, Daniel Wiebe, Marc Schumann, Sebastian Lindemann, and Diego Ramírez Garíca of the Physics Insitute at the University of Freiburg, Pierre Lanari, also of the Naturhistorisches Museum Bern, Frank Gfeller, also of the Naturhistorisches Museum Bern and the Institute of Geological Sciences at the University of Bern, Carlos Vigo, Darbachan Das, and Fabian Hotz, also of the Paul Scherrer Institute, Katharina von Schoeler of the Institute for Particle Physics and Astrophysics at ETH Zürich, Kuzihiko Ninomiya of the Institute of Radiation Sciences at Osaka University, Megumi Niikura of the RIKEN Nishina Center for Accelerator Based Science, Narongrit Ritjoho of the School of Physics at Suranaree University of Technology, and Alex Amato, again of the Paul Scherrer Institute, describe the discovery of a Bronze Age meteoric iron arrowhead in the collection of the Bernisches Historisches Museum.

The arrowhead (specimen number A/7396), was recovered from the Mörigen Pile Dwelling, a Bronze Age stilt house settlement attributed to the Urnfield Culture, on Lake Biel in Bern Canton, which is about 4-8 km south of the Twannberg iron meteorite strewn field. The Mörigen site was discovered by local fishermen in 1843, and was the subject of various amateur excavations until 1873, when the Bern government banned such activities, and arranged for a formal exploration of the site under the leadership of archaeologist Eduard von Jenner and geologist Edmund von Fellenburg. Exactly when arrowhead A/7396 was found is unclear, but it is thought to have been recovered during Jenner and Fellenburg's excavations in 1873 and 1874. It was first observed that the arrowhead was iron rather than bronze by Monika Bernatzky-Goetze in 1987, during a wider examination of arrowheads from Mörigen, though she made no further investigation of it at that time. It has a mass of 2.904 g, and measures 39.3 mm long, 25 mm wide, and 2.6 mm wide, and has a triangular blade with a 13 mm tang. 

(a) Overview of the Mörigen arrowhead (A/7396). Note adhering bright sediment material. Remnants of an older label on the left of the sample number. Total length is 39.3 mm. ( b) Side view of the Mörigen arrowhead. Layered texture is well visible. Point is to the right. Thomas Schüpbach in Hofmann et al. (2023).

Hofmann et al. carried out a metallurgical comparison of arrowhead A/7396, comparing it to two fragments of the Twannberg Meteorite, TW1 (NMBE 36467) and TW934 (NMBE 43747), but found that it was metallurgically quite distinct from these, and therefore derived from a different meteorite. The arrowhead was also examined by light microscopy, X-ray micro-computer tomography, muon induced X-ray emission spectography, scanning electron microscopy, gamma spectroscopy, and Ramen spectroscopy.

The arrowhead is comprised of rust covered iron with a very laminated texture, in places patches of sediment can still be seen attached to the surface, and very small amounts of unrusted iron are visible within a crack on the surface. The surface of the arrowhead has grinding or scratch marks in several places, which are beneath the attached organic material and sediment particles where these are found on the same part of the arrowhead.

X-ray tomography revealed that the rust layer, although covering most of the sirface, is very thin (less than 0.1 mm). The crack observed visually can be seen to extend across almost the whole width of the arrowhead in X-ray tomography, and is largely filled with fine-grained silt sediment. -ray tomography also showed the arrowhead to be of uneven thickness, being 1.2 mm thick on one side, while the other is only 0.6 mm thick. The metal has a pronounced layering parallel to the frontal plane of the arrowhead, something which would not be expected in an iron meteorite, and which is therefore presumed to be an artefact of the way in which the arrowhead was made.

X-ray tomographic sections of the Mörigen arrowhead. (a) Shows four sagittal sections, (b) shows 10 transversal sections. Brightest (densest) areas correspond to metallic iron, brightness of iron metal is variable due to flatness of the object. The layered structure and fractures filled with iron (hydr)oxides/sediment material resulting from oxidative volume expansion are well visible. Hofmann et al. (2023).

The Mörigen arrowhead is very flat, and has probably had its thickness increased somewhat by oxidation. This is not a natural shape for meteors or meteor fragments, suggesting that the metal has been flattened as well as being sharpened. Such working of the metal is a plausible origin for the laminations visible in the X-ray tomograph images of the arrowhead, which is probably a deformed Widmanstätten pattern (Widmanstätten patterns are interleaving of kamacite and taenite bands found in nickel-iron meteorites, where they are believed to be formed by very slow cooling of the metal, probably over millions of years). Similar patterns have been observed in artefacts from Greenland, which are known to have been made by cold working of material from the Cape York meteorite. These Greenland artefacts also have a very flat form, and a layered microstructure made from flattening of large kamacite and taenite grains. Hot working is also a possibility, though heating to above about 700° would probably result in the loss of the banding due to recrystallization. The grinding marks seen on the surface of the arrowhead in places may be a result of this working process. Thus, although the arrowhead is of a similar shape to the bronze arrowheads also found at Mörigen, it appears to have reached this shape via quite a different working process. 

An undeformed Widmanstätten pattern in a section of a meteorite from the Gibeon Cluster in Namibia. Kevin Walsh/Wikimedia Commons.

The oxidised surface of the arrowhead is a less than ideal target for X-ray fluorescence spectroscopy, and is likely to be responsible for the variation in nickel concentrations across the surface of the object; up to 22%, which is improbable on an unoxidized surface, and probably results from element partition during the corrosion process. Muon induced X-ray emission spectography, which can penetrate the surface of objects, found that the nickel content increased and stabilised with depth in both the arrowhead and meteorite fragment TW934 (which also has an oxidised surface) but not meteorite fragment TW1, which does not. Iron, nickel, cobalt, gallium, and germanium, all typical components of iron-nickel meteorites, were all detected by X-ray fluorescence spectroscopy, as were arsenic and copper, which are much more unusual. High levels of lead were found on the parts of the arrowhead with white numbering, implying that a lead-oxide based paint was used.

Scanning electron microscopy revealed the presence of bith taenite and kamacite, which are nickel-rich and nickel-poor phases found in iron-nickel meteorites. Some organic material was present on the surface, and were sediment particles, showing calcium, carbon, oxygen, and silicon, which would fit with a mixture of calcium carbonate and quartz. The pigment of the label was found to contain bith leand and tin.

Scanning electron microscopy images of typical surface areas of the arrowhead. (A) Thin lamina of taenite (Ta) surrounded by oxidation products (Feox) and nearby kamacite (Ka), Backscattered electron image; (B) Iron oxidation products (Feox) covered by organic material, probably Birch tar (Org, dark) and a latest layer of adhering sediment (Sed), Backscattered electron image image. (C), (D) Scratched surface (Scr) below organic material (wood tar; Org) and sediment (Sed). Scanning electron images. Hofmann et al. (2023).

Gamma spectrometry of the arrowhead was able to detect the presence of the isotopes aluminium²⁶, potassium⁴⁰, uranium²³⁸, thorium²²⁸, cobalt⁶⁰, and cesium¹³⁷. Ramen spectroscopy of the organic material produced a signal typical of a tar-like material, which was probably birchwood tar used to attach the arrowhead to the arrow. 

The presence of aluminium²⁶ strongly supports the meteoric origin of the metal suggested by the presence of nickel, cobalt, gallium, and germanium, and the ratios of iron to nickel and nickel to cobalt. The presence of the Widmanstätten patterns and taenite rule out an origin from the Twannberg Meteorite, fragments of which have only ever been found to contain kamacite. The concentrations of nickel and germanium in the metal are consistent with the parent meteor having mostly likely been an IAB type iron meteorite, such as the Cañon Diablo Meteorite from Arizona or the Campo de Cielo meteorites from Argentina. The composition of the metal could also correspond to an IC group meteorite, although these are much rarer, with only 13 known examples, none of them from Europe. 

Aluminium²⁶ is a cosmogenic isotope, found close to the surface of iron-nickel Solar System bodies, where it is formed by cosmic rays bombarding magnesium²⁶, the element to which it also decays, with a half-life of 717 000 years. This short half-life means that aluminium²⁶ and magnesium²⁶ reach an equilibrium point, witht the proportion of aluminium²⁶ decreasing deeper within the body. The proportion in the metal of the Mörigen arrowhead implies that it was at a depth of about 40 cm when it was in the parent body, implying a meteorite with an original diameter of about 80 cm. Such a meteorite would have had a minimum mass of about two tonnes.

The metal of the arrowhead is likely to have undergone some modification since it arrived on Earth. The most obvious modification is the layer of rust (iron oxide) which has formed on its surface, but the presence of copper and arsenic, elements not usually found in nickel-iron meteorites, is probably a result of Human actions, possibly originating when the metal was worked with tools used to work on bronze, but also quite possibly a result of being stored with bronze items.

The chemical and isotopic composition of the Mörigen arrowhead suggests that it derived from an IAB type meteorite with a minimum mass of about two tonnes. Three large IAB meteorites with compositions compatible with the Mörigen arrowhead are known from Europe; the Bohumilitz Metoerite from the Czech Republic, the Retuerte de Bullaque Meteorite from Spain and the Kaalijarv Meteorite from Estonia. Of these, the Kaalijarv is known to have been particularly large, producing a series of craters, the largest of which, the Kaalijärv Crater on the island of Saarema in Estonia, is 110 m in diameter. This object is thought to have had an original mass of several hundred tonnes, most of which was destroyed during the impact, leaving only small fragments of shrapnel. A piece of shrapnel from the Kaalijarv Meteorite would be a plausible source for the metal of the Mörigen arrowhead, although it is possible that the metal was broken off a larger mass, with other iron artefacts (now lost to us) being made from the remaining material. About 10 kg of material has been recovered from the Kaalijarv Meteorite to date, with dating based upon the stratigraphic location of these fragments suggesting the meteorite fell between 1870 and 1440 BC. This Bronze Age date, combined with the parent body having been sufficiently large to produce a fragment with the aluminium²⁶ signature seen in the Mörigen arrowhead, and the fact that it fell in an area known to have been inhabited during the Bronze Age, and therefore would have been observed, makes the Kaalijarv Meteorite the most likely source for the material used to make the arrowhead. 

Kaalijärv Crater on the island of Saarema in Estonia. Kaspars Priede/Wikimedia Commons.

However, this does not rule out other meteorites, such as Bohumilitz or Retuerta de Bullaque, or even an unknown impactor, as sources of the material. The Morasko IAB strewn field in Poland, which has been dated to about 3000 BC and which produced craters up to 90 m in diameter, can be ruled out, as all recovered fragments of this meteorite have much higher levels of germanium (about 500 parts per million) than seen in the Mörigen arrowhead. The Wietrzno Axe and Czestochowa-Rakowa Bracelets are close in time to the Mörigen arrowhead, but have much higher nickel contents, suggesting that they were made with material from a different meteorite.

A search for meteoric iron artefacts near to the Twannberg strewn field produced only a single item, and this was clearly derived from a different meteorite. This suggests that Bronze Age peoples were not aware of the Twannberg Impact, and had no means of detecting and utilizing metal from buried fragments of this object. The artefact uncovered, an iron arrowhead from the Mörigen Pile Dwelling in Bern Canton appears to have been derived from the Kaalijarv Meteorite, which fell in Estonia in about 1500 BC, implying that meteoric iron was a commodity traded across Europe before 800 BC (the approximate age of the Mörigen settlement), with the arrowhead, or the metal from which it was made, apparently having been transported about 1600 km. 

Since it is highly unlikely that only a single artefact would have been made from a source such as the Kaalijarv Meteorite once people were aware of it, there is a distinct possibility that other objects made from iron derived from this source are present in archaeological collections elsewhere in Europe, and possibly beyond. While it is possible that larger objects were made from this source, the highly fragmented nature of the material makes it more likely that most artefacts were small, and out current understanding of the ability of Bronze Age people to work iron, also suggests any objects will be very flat, giving a clear set of parameters for searching archaeological collections for more objects.

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Mineralogy, geochemistry and classification of the Smolenice Meteorite.

In 2012 a suspected iron meteorite weighing 13.95 kg was found in south-western Slovakia, near the town of Smolenice. The object had a distinct colour, shape, and density different from that of the surrounding rocks. The Smolenice Meteorite consists of a single mass of elongated shape with dimensions of 255 × 135 × 130 mm. It has a rusty colour due to the oxidation of its surface. Regmaglypts are relatively uniform across the entire surface. The mass of the recovered meteorite was 13.95 kg. The meteorite name Smolenice was approved by the Nomenclature Committee on Meteorites at the Meteoritical Society in 2019. The main mass of meteorite is in the private collection of the finder (Stanislav Antalík). The type specimen is deposited in the Mineralogical Museum of Comenius University in Bratislava (24.52 g). Other samples are deposited in the Slovak National Museum (Natural History Museum) in Bratislava (28.6 g and 37.9 g).

In a paper published in the journal Geologica Carpathica on 3 June 2020, Milan Gargulák of the State Geological Institute of Dionýz Štúr, Daniel Ozdín of the Department of Mineralogy and Petrology at Comenius University, Pavel Povinec of the Department of Nuclear Physics and Biophysics at Comenius University, Stanislav Strekopytov of the Imaging and Analysis Centre at the Natural History Museum, and the National Measurement Laboratory at LGC, Timothy Jull of the Department of Geosciences at the University of Arizona, and the Isotope Climatology and Environmental Research Centre of the Hungarian Academy of Sciences, Ivan Sýkora, also of the Department of Nuclear Physics and Biophysics at Comenius University, Vladamír Porubčan of the Astronomical Institute of the Slovak Academy of Sciences, and Stefan Farslang of the Department of Earth Sciences at the University of Cambridge, present a minerological and geochemical classification of the Smolenice Meteorite.

The original shape of the Smolenice meteorite. The original dimensions were 255 × 135 × 130 mm and the weight was 13.95 kg. Milan Gargulák in Gargulák et al. (2020).

The mineral composition of the Smolenice iron is simple. It is composed predominantly of iron (kamacite) and minor phases taenite, troilite, and daubréelite. Kamacite constitutes more than 95% by volume of the meteorite. In the Smolenice Meteorite five different types of kamacite can be distinguished. Type I has lamellae separated by a thin layer of taenite, together forming a characteristic crystal lattice of the iron meteorites. Type II has lamellae which are usually considerably thinner than Type I and terminated in finger-shaped contact. Type III consists of allotriomorphic shapes of predominantly elongated type, sharply separated from other lamellae by thin taenite layer. Type IV is a matrix in which kamacite together with taenite forms a typical plessite texture. Type V is a matrix found between the individual lamellae without taenite.

Different types of kamacite in the Smolenice Meteorite. Gargulák et al. (2020).

Types I and IV are the most abundant; type V is less common and types II and III are rare. The kamacite I lamellae cross in three main directions, intersecting at angles of 66 ± 2°, 67 ± 2° and 47 ± 2°, 68 ± 2°, 69 ± 2° and 43 ± 2° respectively. The occurrence of two groups of different angles suggests that two crystal grains were captured in a studied polished section. These two grains (the crystals) are rotated approximately 1.5° relative to each other and separated by type III kamacite lamella. The average width of the dominant type I iron lamellae in the polished section is 0.25 mm; after the calculation with respect to the orientation of the polished section it is 0.22 mm (0.10–0.35 mm).

Backscattered electron microscope image of Kamacite Type I. Gargulák et al. (2020).

Neumann’s lines were not observed. The measured lamellae widths correspond to iron of the fine octahedrite type. The average nickel content of the kamacite is 6.76% by weight.

Taenite is present in the two basic forms. The dominant form are thin films with a thickness of only 2.7–11.8 μm that separate the individual lamellae of kamacite Type I. Less represented is the common occurrence of taenite and kamacite Type IV forming plessite texture. The taenite occurs in the form of allotriomorphic grains, which are approximately isometric or stretched in one direction according to the cut of the polished section. In the spaces between the parallel kamacite Type I lamellae, the taenite grains in kamacite Type IV are oriented omni-directionally and the average grain size is 19.2 μm. In the spaces of the matrix enclosed by the three kamacite Type I oblique lamellae, the taenite grains are oriented parallel to the kamacite Type I lamellae and form Widmanstätten patterns. The smallest size of the taenite grains was observed in orientated plessite textures. The grains are elongated here in a direction parallel to kamacite I; their average width is 1.0 μm and the average length is 3.7 μm.

Backscattered electron microscope image of Kamacite Type II. Gargulák et al. (2020).

The length–width ratio of individual grains varies from 1.1 to 13.3. In many cases, diffuse transition between taenite and kamacite Type IV with a gradual transition to the plessite texture can be observed at the edge of the iron lamellae. The average content of nickel in taenite is 24.54% by weight.

Backscattered electron microscope image of Kamacite Type III. Gargulák et al. (2020).

The kamacite is relatively homogeneous and its nickel content is within a narrow range of 5.16–7.36% by weight. A dispersion of 16.73–33.93% by weight nickel was found in the taenite, but high values characteristic for tetrataenite were not found. In both phases, a significant iron-nickel substitution characteristic for meteoric iron was recorded. Of the other monitored elements, significant dependencies were identified only between cobalt and copper and only in the taenite, while in the kamacite the dependencies of iron and nickel versus cobalt and copper are missing. The negative correlation between cobalt and nickel in taenite documents the substitution of these two elements for one other. The negative correlation between copper and nickel shows that copper substitutes for nickel. In contrast, iron is substituted by cobalt in the nickel irons structure with more similar ionic radius compared to iron.

Backscattered electron microscope image of Kamacite Type IV. Gargulák et al. (2020).

Depending on the total nickel content of the meteorite, kamacite is formed at a temperature range of about 500–800°C; the nickel content in the iron is increasing with a decreasing temperature. The presence of Widmanstätten patterns indicates that at higher temperatures the taenite crystals reach the size of tens of centimetres to 1 metre. The Smolenice iron does not form visible Widmanstätten patterns in the cut, but after etching, these patterns are clearly visible and discernible. The Widmanstätten patterns have a classic appearance and copy the structural surfaces of the octahedrite. The complex and polyphase structures of the kamacite and taenite point to a complex decomposition of the original kamacite at temperatures below 400°C. The absence of Neumann’s lines in the Smolenice meteorite proves that during its flight through space, no larger impact, or collision with another object happened.

Backscattered electron microscope image of Kamacite Type V. Gargulák et al. (2020).

Troilite is a rare mineral in the Smolenice meteorite and forms oval grains of up to 3 mm in the kamacite. Among the admixtures, the Smolenice iron is characterized by an increased content of chromium, which is probably due to nano-exsolutions of daubréelite. This is indicated by very thin exotic lamellae of the daubréelite. Average chemical composition of the troilite are: iron 62.38% by weight, sulphur 36.13% by weight, Nickel 0.01% by weight, copper 0.02% by weight, germanium 0.06% by weight, gallium 0.01% by weight,, silicon 0.01% by weight, chlorine 0.01% by weight, titanium 0.01% by weight. 

(a) Uniform arrangement of taenite grains; (b) parallelly oriented grains of taenite in plessite texture. Backscattered electron microscope images. Gargulák et al. (2020).

Daubréelite is a rare mineral in the Smolenice meteorite and was observed only in troilite as lamellae with a maximum width of 80 μm. Very thin exsolution lamellae are also frequent with widths up to 0.8 μm. The daubréelite lamellae are parallel to the cleavage of troilite. For this type of daubréelite found in troilite, an increased content of manganese (up to 0.42% by weight) is typical. On the other hand, the increased concentrations of chromium are characteristic for troilite. Similar textures and exsolution lamellae of the daubréelite in troilite are known from various types of meteorites. For daubréelite in troilite, the increased content of manganese is typical, and is known from both irons and EH chondrites. The manganese content in the Smolenice iron as well as in other irons where it occurs together with the troilite, are usually lower than in enstatite chondrites. This may be related to the nucleation of the troilite–daubréelite grains almost always dominated by troilite, which cannot accommodate manganese. The daubréelite, being a younger mineral, occurs in the form of exsolutions or lamellae.

Widmanstätten pattern in the Smolenice Meteorite. Three arrows show nodules of troilite. Stanislav Antalík in Gargulák et al. (2020).

Hydrated iron oxides are a product of surface weathering and usually do not penetrate deep into the meteorite. Oxidative iron alterations occur selectively along individual iron lamellae. Taenite appears to be more resistant to oxidation than kamacite. Oxidation products of terrestrial weathering penetrate along the fissures into troilite as well, being approximately perpendicular to the cleavage of troilite.

Backscattered electron microscope image of Troilite (orange), daubréelite (green) and hydrated iron oxides (blue). Gargulák et al. (2020).

The main mass of the Smolenice iron is slightly weathered. No limonite veinlets were detected in the meteorite under polarised light, nor in the electron microprobe. However, a small part of the iron meteorite is weathered on the surface and this part is typically less than 1 mm, but locally it penetrates up to several mm into the meteorite.

Backscattered electron microscope image of daubréelite inclusion (green) in troilite (orange), Gargulák et al. (2020).

The dominant composition of the two iron forms, kamacite and taenite, due to the low content of other minerals, also determines the chemical composition of the Smolenice Meteorite in which the iron plus nickel content reaches 97.30–99.97% by weight. Cobalt is present as a minor element (0.38% by weight). All other studied elements are present only in trace amounts. The bulk analysis of the Smolenice Meteorite is consistent with the meteoric iron of the IVA group. The Smolenice meteorite was classified mainly on the basis of the nickel, gallium and germanium content, which clearly ranks it into the IVA group. According to the analyses the Smolenice Meteorite falls mostly into the central part of the field of this group of iron meteorites. Similarly, this is also true for the nickel/phosphorus ratio, where the Smolenice iron analysis falls into the centre of the IVA group analyses. By comparing the ratios of gold to other elements (gallium, chromium, tungsten, iridium, arsenic, platinum), it is also possible to see a good match with the data for other IVA irons groups. Only the cobalt content has a small excess, outside the main range of analyses, but similar excess of cobalt was also found in the iron meteorites Altonah and Alvord. However, the inclusion of the Smolenice meteorite in this group is unlikely due to the low gallium content of (1.80 μg/g) and the width of the kamacite lamellae, which is several times larger in the meteorites of this group than in Smolenice. When comparing the nickel and iridium contents, the iron from Smolenice falls well within a relatively narrow field of IVA group iron metoerite analyses, but also in the part characteristic of the analyses for the IIIAB, IIIF and IAB groups. However, other classification criteria such as the gallium, iridium and germanium contents, the kamacite lamellae width as well as the characteristic minerals for these groups exclude the possibility of it being classified as IVA. Extraterrestrial iron meteorites of the IVA group come from the bodies with a radius of 8–49 km or 10–27 km, and the cooling rate was 11–500°C/million years or 40–325°C/million years, depending on the source consulted.

Backscattered electron microscope image of thin lamellae of daubréelite (blue) in troilite (green). Gargulák et al. (2020).

Two kinds of radionuclides can be found in meteorites. The first group is represented by primordial radionuclides (e.g. uranium²³⁵, uranium²³⁸, thorium²³² and their decay products). The second group includes cosmogenic radionuclides produced by interaction of cosmic-ray particles with meteoroids during their orbits in space. Gargulák et al. focus on cosmogenic radionuclides, mainly on long-lived ones (carbon¹⁴ with a half-life of 5730 years and aluminium²⁶ with a half-life of 717 000 years), as the fall of the Smolenice Meteorite was not observed and therefore all short-lived radionuclides might have already decayed during its stay at the earth surface. All these radionuclides have been produced in iron meteorites by galactic cosmic-ray protons and secondary neutrons on target nuclei of iron, nickel and aluminium.

Penetration of hydrated iron oxides (light grey) into iron (grey; polarized light). Gargulák et al. (2020).

Aluminium²⁶ has been frequently studied in stone and iron meteorites because it decays by positron emission accompanied by characteristic gamma-rays of 1808.65 kiloelectron volts, which makes its detection by a non-destructive gamma-ray spectrometry feasible. The measured aluminium²⁶ activity in the Smolenice fragment is 3.12 disintegrations per minute/kg, close to the saturation level. This value also clearly demonstrates that the analysed fragment is a meteorite. This value is consistent with the expected production rate of aluminium²⁶ from iron. Some studies have given a slightly higher production rate of 3.7 disintegrations per minute/kg, which would then suggest moderate shielding. When compared with other iron meteorites (and after appropriate corrections for self-absorption of gamma-rays in the sample), this value fits well within the expected meteoroid radius of 30±10 cm, if the terrestrial age of Smolenice is about 10 000 years.

Penetration of hydrated iron oxides (light grey) into iron (grey; polarized light). Gargulák et al. (2020).

As carbon¹⁴ is a pure beta-emitter with maximum energy of beta-electrons of only 156 kiloelectron volts, the measurements were carried out by accelerator mass spectrometry. We obtained the result of 0.95 disintegrations per minute/kg and Gargulák et al. compare this to the production-rate values for carbon¹⁴ from iron of 4.0 and 3.0 disintegrations per minute/kg. Using these values, Gargulák et al. calculate the terrestrial age directly from decay of carbon¹⁴ from the production rate ratios for carbon¹⁴/aluminium²⁶. Propagating the errors, this results in a terrestrial age of 9600-12 000 years

Potassium⁴⁰ in iron meteorites (because of its low content) is more likely to be produced by cosmic rays, while in stone meteorites it belongs to primordial radionuclides. The measured potassium⁴⁰ activity in the Smolenice Meteorite is 22.5 disintegrations per minute/kg, however, the potassium⁴⁰ gamma-ray peak (1460.8 kiloelectron volts) is also found in the spectrometer background, therefore special care is required during spectra evaluation. As Gargulák et al. cannot rule out possible contamination of the Smolenice meteorite by terrestrial potassium⁴⁰, more work is needed to solve the problem of the origin of potassium⁴⁰ (e.g. by potassium⁴⁰ analysis of other iron meteorites).

Based on the Widmanstätten patterns, chemical and mineral composition and other features the Smolenice Meteorite was confirmed as being of extra-terrestrial origin. The Smolenice Meteorite is composed predominantly of iron. Taenite lamellae, troilite nodules and daubréelite veinlets and parallel intergrowths occur rarely. According to iridium, gallium, nickel and germanium contents, the Smolenice Meteorite can be classified into the IVA Iron Meteorite group. Based on average kamacite bandwidths (0.22 mm), this iron is a fine octahedrite. 

Analyses of cosmogenic radionuclides (aluminium²⁶ and carbon¹⁴) indicate that the radius of the Smolenice Meteorite could have had an original diameter of 30 ± 10 cm and its terrestrial age of 11 000 years.

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Possible meteorite lands in field in Bihar State, India.

A probable meteorite has been handed to the Bihar Museum in Patna after falling in a paddy field in the village of Mahadeva in Madhubani District, Bihar, on Wednesday 24 July 2019. The object is described as weighing about 15 kg, light in colour, and magnetic, which suggests that it may be an iron or nickel-iron meteorites, or octahedrites, which are the most common class of meteorite, and left a hole over a metre deep where it landed. The object is currently on display in the Bihar Museum, though it is planned to send it on the the Srikrishna Science Center to be studied properly.

Villagers in Madhubani District, Bihar, with a possible meteorite that fell nearby on Wednesday 24 July 2019. NDTV.

A meteorite falling from the sky without previously being seen seems unlikely, but a small object entering the Earth's atmosphere in daytime over India would not necessarily produce a bright enough meteor to be seen (a 'meteor' is a bright light in the sky caused by an extra-terrestrial object entering the atmosphere, a meteorite is a piece of rock that is derived from such an object). The brightness of a meteor is caused by friction with the Earth's atmosphere, which is usually far greater than that caused by simple falling, due to the initial trajectory of the object. Such objects typically eventually explode in an airburst called by the friction, causing them to vanish as an luminous object. However this is not the end of the story as such explosions result in the production of a number of smaller objects, which fall to the ground under the influence of gravity (which does not cause the luminescence associated with friction-induced heating).

 Impact crater left by the 24 July 2019 Bihar meteorite. CNN.

These 'dark objects' do not continue along the path of the original bolide, but neither do they fall directly to the ground, but rather follow a course determined by the atmospheric currents (winds) through which the objects pass. Scientists that have witnessed meteors are sometimes able to calculate potential trajectories for hypothetical dark objects derived from meteors using data from weather monitoring services.

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Unusual inickel-iron meteorite discovered on Michgan farm.

A Michigan man has discovered he is in possession of a highly unusual meteorite after taking the object to a specialist to have it checked. David Mazurek from Edmore near Mount Pleasant obtained the rock with a farm he bought in 1988, and had been using it as a doorstop since, but contacted scientist Mona Sirbescu of the Department of Earth and Atmospheric Sciences at Central Michigan University, after seeing similar rocks being diagnosed as meteorites in news reports. The rock was in a barn on the property when he obtained it, with the previous owner claiming his father has seen it fall from the sky in the 1930s, and that it was still hot to the touch when found in a crater. 

The Edmore Meteorite. Central Michigan University. 

The part of the story about the discovery of the rock is probably apocryphal; meteorites are not usually hot to the touch when found, as the fragments that fall to Earth are typically from the inside of larger bodies that have exploded higher in the atmosphere. The friction between an object falling through the atmosphere and the molecules that make up the atmosphere is not enough to cause significant heating, but an object that enters the Earth’s atmosphere from space will generally be travelling at a far higher speed, due to the momentum from its own orbital path, which is enough to cause frictional heating. This friction will cause the outer layers of the object to become superheated, while the interior remains at the temperature that it was at in space, typically a few degrees above absolute zero. This temperature difference typically causes the object to burn then explode as a bright meteor, with left over pieces of the object’s interior falling to the ground as meteorites (bright lights in the sky are meteors, lumps of rock of extra-terrestrial origin are meteorites), which start out very cold and are not warmed significantly by their decent.

Sirbescu quickly concluded that the object was an nickel-iron meteorite (the first actual meteorite brought to her by a member of the public in 18 years), weighing about 10 kg, making it the sixth largest meteorite ever discovered in Michigan. Interestingly the meteorite appears to be about 88% iron and about 12% nickel, a higher proportion of nickel than most such objects (which are typically 90-95% iron and 5-10% nickel), which has aroused interest among meteorite collectors, with both the Smithsonian Institution and an independent mineral museum in Maine reportedly interested in obtaining the object (though the figure of US$100 000 widely quoted in the media is probably a little higher than the true value of the object).

Nickel-iron meteorites, or octahedrites, are the most common class of meteorite, being comprised largely of iron with a substantial amount of nickel, leading to the formation of the mineral kamacite, which has an octahedral form, giving these meteorites their name. Strictly speaking iron meteorites are less common than stony meteorites, but they are much more commonly found as they are resilient both to atmospheric entry and terrestrial weathering, and relatively easy to recognise. Only about 5.7% of objects entering the atmosphere are thought to be iron meteorites, but they comprise about 90% of objects in collections. Nickel-iron meteorites are the most common form of iron meteorite, though low-nickel meteorites hexahedrites, are also found. Both are thought to have formed within the interior of large planetesimals in the early Solar System. These bodies were large enough to develop differentiated iron cores (as is seen in the Earth today), but which were subsequently torn apart by tidal forces generated by the larger planets.

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Meteorite stolen from Virginia museum.

A meteorite has been stolen from the Science Museum of Virginia in Richmond. The rock, which was contained in a metal stand in an exhibit on the museum's first floor was removed some time between 9.30 am and 2.30 pm on Thursday 15 March 2018, by a thief who apparently used some sort of tool to partially dismantle the stand. Virginia Capitol Police are seeking the perpetrator of the crime, however the item, a nickel-iron meteorite about the size of a tennis ball, may be hard to dispose of, as it is not particularly uncommon, giving it a low value (about US$1500) and a limited number of potential buyers. The museum expects to be able to replace the meteorite cheaply and within a few days.

A meteorite that was stolen from the Science Museum of Virginia on 15 March 2018. Virginia Capitol Police.

Nickel-iron meteorites, or octahedrites, are the most common class of meteorite, being comprised largely of iron with a substantial amount of nickel, leading to the formation of the mineral kamacite, which has an octahedral form, giving these meteorites their name. Strictly speaking iron meteorites are less common than stony meteorites, but they are much more commonly found as they are resilient both to atmospheric entry and terrestrial weathering, and relatively easy to recognise. Only about 5.7% of objects entering the atmosphere are thought to be iron meteorites, but they comprise about 90% of objects in collections. Nickel-iron meteorites are the most common form of iron meteorite, though low-nickel meteorites hexahedrites, are also found. Both are thought to have formed within the interior of large planetesimals in the early Solar System. These bodies were large enough to develop differentiated iron cores (as is seen in the Earth today), but which were subsequently torn apart by tidal forces generated by the larger planets.

Anyone with information on the crime can contact the Virginia Capitol Police on (804) 786-2120. 

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