Fig. 1: Etched face of a slice of the Dronino iron. Although the many pieces recovered from this find may be covered in a 2-3 mm layer of rusty secondary iron oxides, the interior is fresh. Note the abundant, elongated inclusions of sulphide (troilite, FeS) within the fine-grained metal host.
"Rock of the Month # 233, posted for November 2020" ---
The Dronino iron meteorite from Russia is an ataxite with about 10 vol.% sulphide inclusions (Russell et al., 2004). The initial find in 2000 was a block some 40 kg in weight. Then (quoting Meteoritical Bulletin online, 27 October 2020) "In summer 2003, scientific expeditions and meteorite hunters collected more than 600 fragments (the largest is 250 kg) totaling about 3,000 kg and occurring at a depth of 0.2-2 m across an area of 0.5x1.5 km". These and other findings were summarized by Grokhovsky et al. (2005). The great meteorite shower was found near Dronino village, some 350 km east and south of Moscow.
The largest mass weighs 250 kg, while most are 0.1 to 15 kg in size. The fragments have a thick Fe oxide coat. Etching of slices reveals a fine-grained ataxitic duplex texture of two phases, kamacite and taenite, the one with some 7.0% Ni and 0.75% Co and the other with 26.3% Ni and 0.35% Co, with swirls of martensite (a hard, carbon-bearing native iron, a kind of natural steel) in kamacite. Besides the abundant sulphide (troilite), chromite and an Fe phosphate (possibly graftonite) are also present. Bulk analysis by INAA includes 9.81% Ni, 0.554% Co, 37 ppm Cr and 32 ppm Cu, 1.68 ppm Ir and 0.284 ppm Au, as well as 3.52 ppm arsenic. Shock- induced melting may explain the metal textures. The great fall most likely predates the founding of the nearby town of Kasimov in 1152 A.D.
A further study of Dronino (Zubkova et al., 2008) affirmed that the iron is mainly kamacite, plus taenite, chromite and other minerals. Some sectors are enriched in troilite and violarite. Dronino shows variable alteration due to terrestrial weathering, depending on whether a sample was found in impermeable clay or permeable sandy deposits. The more weathered samples may also contain goethite, hematite and akaganeite (Fe oxides and hydroxides), as well as the secondary sulphate nickelhexahydrite. Another secondary mineral found in cavities within shards of the weathered iron is an hydrated iron carbonate, chukanovite (Pekov et al., 2007).
Dronino appears in at least one illustrated guide to meteorites, e.g., concerning irons and stony-irons (Casado and Allepuz i Sunye, 2016, pp.149-169). It has been used for comparison in studies of other irons (e.g., Comelli et al., 2016), and in an assessment of the bulk chemistry of iron meteorites, along with well-studied irons such as Canyon Diablo, Cape York and Muonionalusta (Hidaka et al., 2019).
A brief note to place Dronino in context: After the great Sikhote-Alin meteorite shower of 1947, Dronino is the second-largest (by far) of the 44 iron meteorites recovered in Russia. Dronino is perhaps a unique iron (out of the 1,255 irons, or 1.96% of the 64,103 verified meteorites listed in Meteoritical Bulletin as of 28 October 2020). For a start, the metal bulk of the meteorite is very fine-grained, with none of the Widmanstatten pattern or Neumann lines (traces of exsolved cubic iron phases) that characterize most coarser-grained irons. This makes it an "ataxite" in textural terms. However, ataxites in iron groups IVA and IVB tend to have much higher nickel contents than Dronino. The trace element chemistry is also distinct. Thus Dronino is listed as an "ungrouped iron", whose origin and affiliations are currently unknown. It is also worth noting that the typical iron sulphide in meteorites is troilite (FeS), as is the case in Dronino. Other iron and iron-nickel sulphides that are generally or locally abundant in some terrestrial geological environments are rare or localized in meteorites, and mostly confined to some fairly unusual classes of carbonaceous chondrites and achondrites: pyrite, pyrrhotite and pentlandite.
Fig. 2: Here is a polished face of a slice of Dronino, the obverse of the face shown in Figure 1. The streaked-out troilite nodules are again prominent. The origin of the directive texture are not clear (not to me, anyway!). On Earth, igneous rocks may acquire aligned fabrics such as this during cooling, by flow, while the rock is still molten. Alternatively, deformation subsequent to the original cooling and crystallization of the rock may lead to realignment, segregation and recrystallization of the constituent minerals, as in regional metamorphism within the Earth's crust.
Casado,JV and Allepuz i Sunye,D (2016) Meteoritos: introduccion y guia de reconocimiento. Private publisher, 3rd edition, 318pp. (in Sp.).
Comelli,D, d'Orazio,M, Folco,L, El-Halwagy,M, Frizzi,T, Alberti,R, Capogrosso,V, Elnaggar,A, Hassan,H, Nevin,A, Porcelli,F, Rashed,M and Valentini,G (2016) The meteoritic origin of Tutankhamun's iron dagger blade. Meteoritics & Planetary Science 51, 1301-1309.
Grokhovsky,VI, Ustyugov,VF, Badyukov,DD and Nazarov,MA (2005) Dronino: an ancient iron meteorite shower in Russia. Lunar and Planetary Science 36, abstract 1692.
Hidaka,Y, Shirai,N, Yamaguchi,A and Ebihara,M (2019) Siderophile element characteristics of acapulcoite-lodranites and winonaites: implications for the early differentiation processes of their parent bodies. Meteoritics & Planetary Science 54, 1153-1166.
Pekov,IV, Perchiazzi,N, Merlino,S, Kalachev,VN, Merlini,M and Zadov,AE (2007) Chukanovite, Fe2(CO3)(OH)2, a new mineral from the weathered iron meteorite Dronino. Eur.J.Mineral. 19, 891-898.
Russell,SS, Folco,L, Grady,MM, Zolensky,ME, Jones,R, Righter,K, Zipfel,J and Grossman,JN (2004) The Meteoritical Bulletin, No.88, 2004 July. Meteoritics & Planetary Science 39, A215-272.
Zubkova,NV, Pekov,I, Chukanov,NV, Pushcharovsky,D and Kazantsev,SS (2008) Nickelhexahydrite from the weathered meteorite Dronino: variations of chemical composition, crystal structure, and genesis. Doklady Earth Sciences 422 no.1, 1109-1112. Original in Russian, ibid., vol. 422 no.2, 229-232, with crystal structure data, illustration and 11 references.
Graham Wilson, posted 28 October 2020, last addition 29 October 2020
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