MORPHOLOGY AND PHYSICO-CHEMICAL CHARACTERISTICS OF AN IRON FRAGMENT FROM CHACO PROVINCE I.A. BUCURICA1,2, C. RADULESCU1,3*, A.A. POINESCU4*, I.V. POPESCU1,3,5, I.D. DULAMA1, C.M. NICOLESCU1, S. TEODORESCU1, M. BUMBAC3, G. PEHOIU6*, O. MURARESCU6 1 Valahia University of Targoviste, Institute of Multidisciplinary Research for Science and Technology, 130004 Targoviste, Romania; E-mail: [email protected] 2 University of Bucharest, Faculty of Physics, Doctoral School of Physics, 050107 Bucharest, Romania 3 Valahia University of Targoviste, Faculty of Sciences and Arts, 130004 Targoviste, Romania; E-mail: [email protected] 4 Valahia University of Targoviste, Faculty of Materials Engineering and Mechanics, 130004 Targoviste, Romania; E-mail: [email protected] 5 Academy of Romanian Scientists, 050094 Bucharest, Romania; E-mail: [email protected] 6 Valahia University of Targoviste, Faculty of Humanities, 130105 Targoviste, Romania; E-mail: [email protected] *Corresponding authors: [email protected]; [email protected]; [email protected] Received October 17, 2018 Abstract. This study aims to demonstrate that the investigated sample is nickel- rich, a signature of the meteorites composed of iron, in order to establish if studied sample belongs or not to Campo del Cielo meteorite group. The discrete structure found in meteorites is a fingerprint fully justified by structural analysis using optical microscopy (OM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), as well as by elemental content using energy dispersive spectrometry (EDS) and inductive coupled plasma mass spectrometry (ICP-MS). The presence of crystalline phase’s kamacite and taenite was confirmed, with a good correlation between experimental results and standard diffraction data (i.e. ICDD card numbers of minerals). Vickers hardness tests (HV 0.1 and HV 1.0) were achieved in accordance with STAS 492/1-85 and the average values (247–255.2 N/mm2) include the fragment into iron meteorite hardness category. The isotopic signature of kamacite (δ56/54Fe = 0.15‰ and δ62/60Ni = 0.81‰) determined by ICP-MS, certify the origin of metallic fragment in Campo del Cielo meteoritic group. Key words: iron-nickel meteorite, optical microscopy, SEM-EDS, XRD, Vickers hardness, ICP-MS, isotope ratio. 1. INTRODUCTION Not once, the objects discovered near the meteorites craters, wrongly been considered fragments of meteorites. For that, many scientists from all over the world are contribute to reveal the authenticity of the outer space matter, found on Earth’s surface. As known, the meteorite is the last stage of a meteoroid transformation Romanian Journal of Physics 64, 906 (2019) Article no. 906 I.A. Bucurica et al. 2 on its way to Earth. The meteoroid originates from the decomposition of asteroids / comets, as a result of which fragments of them enter the atmosphere of the Earth and, more often than not, they break down by burning. Passing through the atmosphere of the Earth leads to the appearance of a luminous effect called a meteor / shooting star. The meteoroid falled on Earth is called the meteorite. If meteoroids are large enough, the heating effects affect only the external portion, leaving the interior unaffected [1]. Meteorites can be classified into three main groups: irons (hardness 4–5 Mohs scale), stones (hardness 6 Mohs scale) and mixed meteorites (hardness 6–7 Mohs scale). From the mineralogical point of view, meteorites consist of different amounts of iron-nickel alloys, silicates, sulphides and other phases in small quantities [2]. Considering Wasson and Kallemeyn [3] evaluations, all the irons meteorites that contain graphite and carbides are member of IAB group complex, these irons commonly contain silicates, like those in group IIE, and were not formed by fractional crystallization in metalic cores of asteroids, but in localized metallic pools by processes that are still poorly understood [4, 5]. Depending on the percentage of nickel in iron, these subdivisions are classified into: hexahedrites (< 6% Ni), octaedrites (6%–17% Ni) and ataxites (> 17% Ni). Octaedrites are the most common type of iron meteorite showing unique structure, called the Widmanstatten structure. This unique structure is the result of the combination of kamacite and taenite present in approximately equal amounts [6]. Obviously, the recognizing of meteorite is not easy at all, many of them being quite often confused with terrestrial rocks. As a distinctive sign for most of the meteorites is the melting crust. It usually has a dark or dark gray color, differing from the shading surfaces, colored lighter (usually gray), also, in the structure of the crust can be observed mainly the traces of the action of atmospheric currents. But there are meteorites where crust is transparent or rusty. In this study a metallic object, supposed to be a meteorite fragment from New Campo del Cielo group (Argentina), was investigated from the physico-chemical point of view. By deduction, this fragment was marked as possible part of the iron- based meteor group, discovered in the provinces of Chaco and Santiago del Estero since 1576. The Campo del Cielo meteorites are considered to be a silicate-bearing iron type which has been classified as belonging to the IAB Main Group [3]. Most of the Campo del Cielo samples are dominantly FeNi metal (bulk composition 6.68% Ni), and approximately 10% are silicate-rich [7]. These silicate-rich samples are mainly characterized by silicate clasts contained within metal, and a small proportion is dominated by silicate material with networks of metal veins. The silicate material in these samples is C- and S-enriched [7]. In the meteorological catalog of the Natural History Museum in London (UK), these discovered fragments are classified as belonging to coarse octaedrites category [3, 8, 9]. In this respect, several complementary techniques have been used and the obtained results were compared with the scientific literature [10–16], but also with the catalogs of the International Society for Meteorics and Planetary Science [9]. 3 Iron fragment from Chaco Province Article no. 906 2. MATERIALS AND METHODS 2.1. MATERIALS AND SAMPLE PREPARATION The dark-grey solid fragment presented in Figure 1 have a weight of 7.3 g and a melted non-uniform structure and is supposed to be some kind of out of space object. This item has no authenticity certificate and was received from an anonymous collector. From the owner testimonial it is know the fact that originates from South America, more precisely, near Chaco province in Argentina. Fig. 1 – Metallic object suppose to be a meteorit fragment from New Campo del Cielo group (Argentina). All chemical reagents were of analytical grade. Distilled deionized water (Milli-Q Water System Millipore), hydrochloric acid (37%, Merck) and nitric acid (high purity, Merck) were used for digestion process, as well for blank preparation. For hardness tests, Optical Microscopy (OM) and Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS), the sample was polished with sand paper (different granulometry) and diamond paste, and etched with 2% Nital solution (2% nitric acid in methanol). In the end, the sample was cleaned with distilled ionized water and dry with high pressure nitrogen flux (2 atm). For the XRD analysis, a piece weighing 2.06 g, overall dimensions of 14 × 11 × 3 mm, and having two distinct sides was provided. Visual examination showed a metallic appearance, without sandy or stony insertions. One side had a specific cut and treatment of an optical investigation that was previously performed, to search for corresponding characteristics such as Widmanstätten pattern etc. The second side had no treatment. This untreated side was considered to belong to the outer side of the parent meteorite body, when this was reaching the atmosphere and broke into pieces. For ICP-MS analysis, the sample (i.e. 50 mg) was digested with 3 mL HNO3 and 9 mL HCl on a hot plate by using a TOPwave Microwave-assisted pressure system (Analytik Jena). After digestion process, the PTFE-TFM vessel with sample was cooled for one hour, and then the solution was transferred with distilled water to 100 mL volumetric flasks. Finally, the clear solution sample was analyzed by Article no. 906 I.A. Bucurica et al. 4 ICP-MS. For this stage, ultra-pure argon (99.999% pure) was used as cooling, auxiliary, and nebulizer gas. 2.2. CALIBRATION STANDARDS Solutions for quantitative measurements were prepared using Certipur® Certified Reference Material ICP multi-element standard IV (~1000 mg/L in 6.5% HNO3, Merck) in 5% HNO3 without further purification. The standard multielement solution contain: Ag (1011 mg/L), Al (999 mg/L), B (990 mg/L), Ba (1009 mg/L), Bi (1000 mg/L), Ca (995 mg/L), Cd (999 mg/L), Co (998 mg/L), Cr (999 mg/L), Cu (998 mg/L), Fe (999 mg/L), Ga (997 mg/L), In (998 mg/L), K (1003 mg/L), Li (999 mg/L), Mg (1001 mg/L), Mn (1001 mg/L), Na (1003 mg/L), Ni (999 mg/L), Pb (1001 mg/L), Sr (998 mg/L), Tl (1002 mg/L), and Zn (999 mg/L). 2.3. ANALYTICAL TECHNIQUES Optical Microscopy Stemi 2000-C and Axio Imager M2m microscopes by Zeiss’s were used for optical investigations due to different approach of the study. On first stage, the stereo microscope Stemi 2000-C, designated for high volume samples that require variable working distance was used at 40x magnification range. Also, this microscope, through its attached Axiocam 105 digital video camera (5 megapixel HD), allowed real-time image acquisition. After preliminary information were obtained, Axio Imager M2m and its capabilities was considered to be more suited for reaching even closer to the surface, so, by using AxioCam ICc5 and 50x EC Epiplan-NEOFLUAR objective, were performed new series of optical analysis for more accurate surface images.