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Local Structure of Iron in Tektites and Natural Glass: an Insight Through X-Ray Absorption Fine Structure Spectroscopy

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Local Structure of Iron in Tektites and Natural Glass: an Insight Through X-Ray Absorption Fine Structure Spectroscopy 288 JournalL. Wang, of Mineralogical A. Yoshiasa, andM. Okube, Petrological T. Hiratoko, Sciences, Y. Hu, Volume H. Arima 108, andpage K. 288 Sugiyama─ 294, 2013 Local structure of iron in tektites and natural glass: An insight through X-ray absorption fine structure spectroscopy * * ** * *** Ling WANG , Akira YOSHIASA , Maki OKUBE , Tatsuya HIRATOKO , Yuan HU , § § Hiroshi ARIMA and Kazumasa SUGIYAMA * Graduate School of Science and Technology, Kumamoto University, Kumamoto, Kumamoto 860-8555, Japan ** Materials and Structure Laboratory, Tokyo Institute of Technology, Yokomama, Kanagawa 226-8503, Japan ***Testing center for Gold and Jewelry of Jiangsu Province, Nanjing, Jiangsu 210016, China § Institute for materials research, Tohoku University, Sendai, Miyagi 980-8577, Japan The local structure of iron in tektites from six strewn fields, and impact- and non-impact-related glass were studied using the Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, in order to obtain quantitative data on Fe-O bond length and Fe coordina- tion number. X-ray absorption fine structure (XAFS) spectra and Fe-O bonds in standard minerals such as he- matite, fayalite, and magnetite were compared. The degree of oxidation was measured based on the valencies 3+ of iron in the samples. Tektites contain a greater proportion of ferrous than ferric iron [0.04(1)-0.13(1) Fe / 3+ ƩFe]. The ferric ratios of impact-related glass [0.18(1)-0.52(1) Fe /ƩFe], and volcanic glass [0.26(1)-0.30(1) 3+ Fe /ƩFe] are higher than that in tektites. Based on the measured Fe-O distance, it was inferred that 4- and 5-coordinated Fe exist in tektites, whereas volcanic glass contains 5- and 6-coordinated Fe. Impact-related glass possesses various local structures caused by the combination of 4-, 5-, and 6-coordinated Fe. During formation, tektites experience high temperatures and a reducing atmosphere when they were ejected into the outer space. In contrast, the impact-related glass, which was ejected into the atmosphere or which remained close to the crater, experienced a more complex environment, with air pressure, density, and temperature vary- ing across the atmospheric layers. Thus, impact-related glass presents more complicated oxidation states and structure compared to tektites. Volcanic glass, on the other hand, has a relatively stable redox condition; and thus, it undergoes only a small change in the degree of oxidation. This study indicates that the local structure and oxidation state of Fe may change due to the environment that the glass experienced during its formation. These different kinds of natural glass can be distinguished from each other using the study of the local structure. Keywords: Local structure, Iron, Tektites, Natural glass, XAFS INTRODUCTION processes, such as volcanic glass, which is the product of rapidly cooling magma, and pseudotachylite, which is Tektites and impact-related glass are formed by natural formed by frictional melting of wall rocks during rapid impact events (Barnes, and Barnes, 1973; Alvarez et al., fault movement related to seismic shocks (Price et al., 1980). Impact events happen not only on the Earth’s sur- 2012) or impact events (Reimold, 1995); Such glass often face but also in extraterrestrial environments. Therefore, experiences high temperature and/or pressure. Compari- intensive studies of collision on the Earth’s surface not son of these natural glass types can help us understand the only help us understand the Earth’s history but also in the role played by the atmosphere in the formation of the research related to extraterrestrial environments. In this glass. regard, tektites and impact-related glass, the most impor- The relation between local structures of aluminum tant products of collision, are worthy of our attention. and titanium in natural glass, and their origin have been Other types of natural glass are products of geological studied and discussed (Giuli, 2000; Farges, 1996a, 1996b, doi:10.2465/jmps.130212 1996c). Wang (2011) classified tektites from three strewn L. Wang, [email protected] Corresponding author fields using the -4 , 5- and 6-coordinated titanium and A. Yoshiasa, [email protected] also interpreted the effects of pressure, temperature, and Local structure of Fe in kinds of glasses studied by XAFS method 289 quenching rate on the tektites. Iron is also a common ele- pinite, and australite from the Australian strewn field; ment, and is sensitive to external environments. Depend- bediasites from the North American strewn field; and ing on the redox condition, iron can have a range of va- moldavites from the European strewn field. The impact- lencies (0, 2, 3, 4 and 6), and coordination numbers (4, 5, related glasses are suevite, impactite, darwin glass, and 6 and 8). Redox conditions (i.e., oxygen fugacity) at köfelsite from four impact craters. Other volcanic glass which a rock forms and evolves can be important for in- (Kilauea Volcanic glass, obsidian, perlite, and pitchstone) terpreting the rock history (Frost, 1991). Glass structure and fault rock (pseudotachylite) have been studied for (i.e., cation coordination number) is affected by pressure comparison. Fayalite, magnetite, and hematite as the most and temperature that exists during its formation (Stebbins common iron compounds are helpful in determining the and McMillan, 1989; Paris et al., 1994; Mysen and Neu- oxidation and coordinate states of iron. ville, 1995; Yarger et al., 1995). Fe K-edge XAFS measurements were performed us- XAFS method is an advanced technology used for ing a Beamline BL-9C equipped with a Si (111) double- the measurement of local structure, and its application to crystal monochromator (Photon Factory, KEK, Tsukuba, geological research has been growing. Early studies had Japan). The storage ring was operated with electron ener- confirmed that the Fe is mostly divalent in tektites. How- gy of 2.5 GeV and ring current of 450 mA. Spectra were ever, the coordination number remains disputed. Other recorded in the transmission mode at room temperature methods, such as the Mössbauer spectroscopy and molec- near the Fe K-edge from 6709.8 to 8109.5 eV, with steps ular dynamics calculations present a range of values be- from 0.855 to 7.604 eV, and 1 to 3 seconds of counting 2+ tween 4- and 5-coordinated sites for Fe (Dunlap et al., time. 1998; Rossano et al., 1999; Rossano et al., 2000). Most The EXAFS function, χ(k), was extracted from each research on local structure of impact products considers measured spectrum using the standard procedure (Maeda, either coordination number or oxidation state based on 1987). Following the technique of Lytle et al. (1989), χ(k) analysis and comparison of pre-edge features, but com- was normalized using the MacMaster coefficients. In bined analyses of EXAFS, Fe-O bonds, and distance are quantitative analyses, we employed the Fourier-filtering rare. Giuli et al. (2002) was the first study to incorporate technique and a non-linear least-squares fitting method the results of XANES and EXAFS in tektites and impact by comparing the observed χ(k)exp and calculated χ(k)calc. glass, and summarize the local structures of Fe. This man- We used the EXAFS formula in the single scattering theo- uscript includes a study on more tektites and impact glass ry with the cumulant expansion up to the fourth order samples that compliments data in the study cited earlier. term (Ishii, 1992), EXPERIMENTS The samples studied comprise of six tektites (Table 1) from different strewn fields: hainanite, indochinite, philip- (1), Table 1. Location, chemical composition (by EPMA), and colors of the studied tektites a Koeberl (1986) and references therein. b Heide et al. (2001) and references therein. c Ho and Chen (1996) 290 L. Wang, A. Yoshiasa, M. Okube, T. Hiratoko, Y. Hu, H. Arima and K. Sugiyama where NB is the coordination number of scattering atom B Edge jump was normalized to unity in all XANES at a distance RAB from the absorbing atom A, | fB(k; π)| is figures. Pre-edge, threshold, and edge crest energies gath- the backscattering amplitude of photoelectrons, and ψAB(k) er around 7110.1, 7117.6, and 7124.7 eV, respectively. In is the phase shift function. Values of the function | fB(k; π)| addition, all tektites possess a shoulder around 7120.0 eV, and ψAB(k) were calculated using the FEFF3 program though the edge crest breadth becomes increasingly nar- (Rehr et al., 1991). σn denotes the nth cumulant. The mean row from moldavite-green to bediasite-black. A detailed free path λ of the photoelectron was assumed to be de- comparison of moldavite-green and hainanite is shown in pendent on the wavenumber, λ(k) = k/η, where η is a con- Figure 1b. The first derivative of the XANES profile stant. Analyses of the XAFS data were performed using along with the photon energy, as shown in Figure 1c, ena- XAFS93 programs (Maeda, 1987), which is elaborated by bles precise examination. The main peaks are bound-state Yoshiasa et al. (1997). The single-shell fitting was carried transitions: 1s→3d, 1s→4s, and 1s→4p, but no signifi- out for each nearest-neighbor distance. Because the third- cant differences exist among the tektites. XANES spectra and fourth-order terms in the cumulant expansion were of Fe K-edge in impact-related (darwin glass, impactite, negligible, the final refinement was performed as the har- and suevite) and non-impact related glass (pseudotachy- monic model by the structural parameters RAB, σ2, η, and lite, obsidian, and perlite) are shown in Figure 2a. All im- ΔE0 values. Here, ΔE0 is the difference between the theo- pact-related glass, with the exception of darwin glass, retical and experimental threshold energies. The reliability have a clear dual-edge crest.
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