Meteorite Fall at Sadiya, India: A Raman Spectroscopic Classification

Meteorites are the only physical materials available on the Earth that allow direct study of the original dust from which the Solar System formed. The standard method for identifying meteorites is to compare the chemical composition of the sample with that of the meteoritic rock previously studied [1]. Classification of ordinary chondrite meteorites generally implies determining the chemical group by the composition in endmembers of olivine and pyroxene, and identifying the petrologic group by microstructural features [2]. Raman spectroscopy can be used as an alternative technique to determine the endmember content of olivine and pyroxene in ordinary chondrites. The distribution of Fe and Mg in olivine and pyroxene structures is reflected by the peak position in the Raman spectra because of the different size of the two ions, therefore providing a correlation between the obtained spectrum and the chemical composition [2].


Introduction
Meteorites are the only physical materials available on the Earth that allow direct study of the original dust from which the Solar System formed. The standard method for identifying meteorites is to compare the chemical composition of the sample with that of the meteoritic rock previously studied [1]. Classification of ordinary chondrite meteorites generally implies determining the chemical group by the composition in endmembers of olivine and pyroxene, and identifying the petrologic group by microstructural features [2]. Raman spectroscopy can be used as an alternative technique to determine the endmember content of olivine and pyroxene in ordinary chondrites. The distribution of Fe and Mg in olivine and pyroxene structures is reflected by the peak position in the Raman spectra because of the different size of the two ions, therefore providing a correlation between the obtained spectrum and the chemical composition [2].
We report here a recent meteorite fall that occurred on June 5, 2017 (04:30 pm IST) in Natun Balijan village of Sunpura, Sadiya (27°50'09''N; 95°51'34''E). Generally, meteorites are named from their place of find or fall. Therefore, the recent fall meteorite studied here is named as Sadiya. According to the eye witness, a single stone was fallen in paddy field with a roaring sound and it formed an impact pet about 3.5 ft depth. The stone was fully covered with fusion crust and had well rounded edges and well developed regmaglypts on its surface ( Figure 1). The weight of the meteorite was 3 kg and had specific gravity of about 3.4 g/cm 3 , typical of stony meteorites. A small part of the sample was taken for analysis and the main stone under custody of the district administration. The Sadiya meteorite fall represent the fifth 'observed fall' in North East India and it is the seventh meteorite of this region since 1846 [3]. This report demonstrates usefulness of Raman spectroscopic technique for classification of meteorite. The Raman spectroscopy could be used to analyze individual mineral grains in a completely non-destructive way, and it offers some unique capabilities as an analytical method in the study of the extent of the shock metamorphism in a chondrite, prior to the microscopical study [4]. Our preliminary compositional, spectroscopic and mineralogical analysis of the Sadiya meteorite sample show that the meteorite belongs to LL-5 type chondrite.

Methodology
Studies on bulk and mineralogical composition of meteorites provide very important constraint for testing theories of Solar System formation. As meteorites retain a record of the elements, and mineral phases that existed in the system's accretionary stages. Microanalysis was performed using scanning electron microscopy (LEO 1430vp SEM) coupled with Oxford INCA energy dispersive X-ray spectrometer at accelerating voltage 20 kV and working distance 10 mm. A part of the sample was ground in an agate mortar and used for bulk chemical analysis of major and trace elements. The Raman spectra were collected on bulk powdered meteorite samples using an Ar ion laser laser with a power of ~5 mW, which used an excitation source having wavelength 488 nm coupled with a Jobin-Yvon Horiba LabRam-HR Micro-Raman spectrometer equipped with an Olympus microscope with 10X, 50X and 100X objectives, using the method described elsewhere [5,6].

Result and Discussion
The electron microscopic study of the meteorite sample reveals olivine, pyroxene and triolite as major minerals ( Figure 2). All unweathered ordinary chondrites show these four iron-bearing minerals. Therefore, the presence of these minerals in Sadiya meteorite confirms that the meteorite under consideration is an LL-type ordinary chondrite  [6,7]. The results facilitated determination of the major elemental composition of Sadiya meteorite (Table 1), as well its comparison with the average values for H, L, LL and CI ordinary chondrites [8]. The range for chemical group in ordinary chondrite is generally expressed as endmember content. According to the definition in Van Schmus and Wood and in Hutchison [8,9], the chemical groups in ordinary chondrites are: H (Fa [16][17][18][19][20] and Fs [14][15][16][17][18] (Table 1).
However, the Raman spectroscopy has not regularly been used as a method for meteorite chemical classification. This technique has already been used for meteorite characterization, especially Lunar and Martian rocks [10,11]. The correlation between composition of olivine and pyroxene has been investigated by many authors [11][12][13][14]. Recently, the potential of Raman spectroscopy has been largely exploited for the identification of shock-induced features or shock-induced polymorphs in meteorites and classification of the metamorphic stage in low-grade chondrites by many authors [4,[15][16][17].
The Raman spectra of the sample shows ( Figure 3) two characteristics peaks of olivine at 819.95 cm -1 (Peak A); 850.20 cm -1 (Peak B) and three characteristic peaks of pyroxene at 335.34 cm -1 (Peak A); 678.49 cm -1 (Peak B) and 1004.50 cm -1 (Peak C) [5,6,[18][19][20]. The endmembers are responsible for these characteristics peak changes [5,6,[18][19][20][21]. Comparison of chemical group attribution based on theoretical Raman peak position prescribe by Pittarello [22]. The full width at half maximum (FWHM) of the peak is considered to be related to the degree of structural disorder of the crystal [4]. The FWHM values of Sadiya meteorite can be related to the degree of crystal structural disorder resulting from shock deformation. The measured FWHM averaged for the observed peaks for forsterites in the Sadiya meteorite sample is 10.42 cm -1 for the component at 819.95 cm -1 and 27.28 cm -1 for the component at 850.20 cm -1 . The well-crystallized terrestrial olivines have FWHM value 9.1 cm -1 and 9.5 cm -1 corresponding to the peak positions at 820 cm -1 and 854 cm -1 . The observed FWHM values of Sadiya meteorite are higher than those observed for well-crystallized terrestrial olivines. According to Miyamoto and Ohsumi [23], the FWHM value 10 cm -1 for poorly shocked to 21 cm -1 for strongly shocked meteorites. The FWHM value of ~10-27 cm -1 in Sadiya meteorite in accordance with the moderate to strongly shocked stage [24]. The  FWHM value 34.3 cm -1 for the pyroxene peak at 1004.50 cm -1 of Sadiya meteorite is higher than the well-crystallized terrestrial pyroxenes [4].

Conclusion
The measured Raman band positions and relative intensities are consistent with chemical composition for olivines and pyroxenes and show influence of shock. The Raman spectroscopic results are complementary to SEM-EDX and compositional analysis. Preliminary assessment suggests that the Sadiya meteorite resemble moderate to strongly shocked ordinary chondrites and it may be classified as LL-5 group.  Table 2 Chemical group attribution based on theoretical Raman peak positions compared with Sadiya meteorite [2].