Tuesday 21 July 2020

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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