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 coordination number. X-ray absorption fine structure (XAFS) spectra and Fe-O bonds in standard minerals such as hematite, 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. XANES spectrum of darwin of fit parameters, glass shares a similar trend with tektites, possessing a dis- tinct pre-edge, shoulder (around 7120 eV), and a unitary (2) edge crest. The edge crests of suevite and impactite are at higher energy levels. Shoulders of non-impact related between the experimental and calculated EXAFS func- glass are at higher energies than impact-related glass. In tions was less than 0.035. Figure 2b, first derivative of darwin glass shows patterns similar to that of tektites, which has a high-low profile; on RESULTS AND DISCUSSION the other hand, suevite and impactite, have a high-low- low pattern. XANES spectra Pre-edge and thresholds are sensitive to oxidation state of a given element. XANES calibration of Fe in a 3+ Fe K-edge absorption spectrum comprises features of series of silicate glass (Fe /ΣFe = 0.00-0.99) (Berry, bound states transitions: 1s→3d (pre-edge), 1s→4s 2003) and chemical shift in magnetite and maghemite (threshold), and 1s→4p (edge crest) (Waychunas et al., (Okudera et al., 2012) have been used to estimate the oxi- 1983). XANES spectra of Fe K-edge (Fig. 1a) show that dation state in natural glass. Using fayalite, α-Fe2O3, and 3+ all tektites have the same pattern, which has a noticeably Fe3O4, we can simulate a curve between the Fe /ΣFe ratio 2 unique pre-edge, shoulder, and edge crest. and the threshold energy: y = 4.084x + 7117.299 (R = 0.999), as shown in Figure 3. From the combination of

Figure 1. (a) Fe K-edge XANES spectra of tektites, and (b) detailed comparison of broad crest of moldavite-green and hainanite, and (c) the corresponding first derivative spectra, dI/dE. In (a), the pre-edge and threshold position at 7110.1 eV and 7117.6 eV are shown by the solid and dotted vertical line for comparison. The crest becomes increasingly sharp from top to bottom. In (c), the three main peaks of derivation spectra are 1s→3d, 1s→4s, and 1s→4p bound state transition. Local structure of Fe in kinds of glasses studied by XAFS method 291

Figure 2. (a) Fe K-edge XANES spectra of impact-, and non-impact related glass, and (b) the corresponding first derivative spectra, dI/ dE. The short vertical line in (a) indicates the shoulder position.

3+ tity of Fe in tektites is oxidized to Fe [0.04(1)-0.13(1) 3+ Fe /ΣFe] while, impact- and non-impact related glass 3+ contain 0.18(1)-0.52(1) Fe /ΣFe and 0.26(1)-0.30(1) Fe3+/ΣFe, respectively.

Radial distribution function and local structure

EXAFS k3χ(k) functions were transformed into radial dis- −1 tribution functions (RDF) over the k range of 2.5-9 Å for Fe K-edge of tektites, impact-related glass, and non- impact related glass. Fourier transform of EXAFS is shown in Figure 4. In order to obtain further information on structural parameters, we conducted parameter fitting with analytical EXAFS formula; the obtained structural parameters are summarized in Table 3. Fe-O distances of 3+ Figure 3. Correlation between Fe /ΣFe (x) and threshold energy (y) australite and moldavite-brownish are 2.01(1) Å; whereas 2 represented by an equation y = 4.084x + 7117.299 (R = 0.999). those of moldavite-green and indochinite are 2.03(1) Å; and those of Philippine, bediasite, and hainanite are about this curve and the threshold energies of tektites and natu- 2.06 Å. Fe-O distance of impact-related glass are longer ral glass, we estimated the Fe3+/ΣFe ratio in each glass than tektites with the exception of darwin glass. EXAFS type. Table 2 shows the energy feature in tektites, impact- analysis result shows that darwin glass is a tektite-like related glass, and non-impact related glass. A little quan- impact-related glass. Non-impact related glass possesses

3+ Table 2. Energies of fitted Fe K-edge features in natural glass and estimated Fe /ΣFe ratio 292 L. Wang, A. Yoshiasa, M. Okube, T. Hiratoko, Y. Hu, H. Arima and K. Sugiyama

3 Figure 4. Fourier transformation of the Fe K-edge EXAFS oscillation function k χ(k) of tektite, impact-, and non-impact related glass. No phase shift corrections have been made. The main peaks corresponding to Fe-O bonds increase with increasing average coordination number of Fe in all samples.

Table 3. The structural parameters determined by XAFS

Uncertainties in the last decimal place are shown in parentheses. the longest Fe-O bond among all natural glasses. ish, and moldavite-green, which have a wide edge crest Estimations of the coordination states in each natural with Fe-O distance of 2.01-2.03 Å; and indochinite, glass type are shown in Figure 5. The three lines are 6-, philippinite, bediasite-black, and hainanite, which have a 5-, 4-coordinated Fe from top to bottom, respectively. narrow edge crest with Fe-O distance of 2.03-2.06 Å. 3+ 3+ Squares indicate tektites, with 0.04(1)-0.13(1) Fe /ΣFe Due to the close Fe /ΣFe ratio in tektites, we can com- and a mixture of 4- and 5-coordinated Fe. Triangles indi- pare the variation in the coordination site with pre-edge 3+ cate impact-related glass with 0.18(1)-0.52(1) Fe /ΣFe intensity. Figure 6 illustrates the relationship between pre- and 4-, 5-, 6-coordinated Fe. Crosses indicate non-impact edge intensity and Fe-O distance, showing a direct rela- 3+ related glass with 0.26(1)-0.30(1) Fe /ΣFe and mixture of tion between intensity and Fe-O distance. The fitted curve 2 5- and 6-coordinated Fe. XAFS analysis allows us to is y = 0.104x − 0.155 (R = 0.930) identify these three kinds of natural glass clearly. Overall consideration of the XANES edge crest CONCLUDING REMARKS width and the Fe-O distance enables the classification of tektites into two subgroups: Australite, moldavite-brown- The Fe K-edge XANES patterns, Fe-O distances, coordi- Local structure of Fe in kinds of glasses studied by XAFS method 293

non-impact related glass are distinguishable from the study of their local structure. Tektites and impact glass experienced high temperatures and pressures during their formation (i.e., during the impact event). Furthermore, tektites and impact glass might have experienced different atmospheric conditions, which might have caused differences in their local structures. Tektites were flung into outer space, which contains minimal oxygen; thus, ferric iron is rare in tektites. Conversely, impact-related glass samples were flung within the atmosphere over a short time and landed near the crater. Because of the higher oxygen partial pressure at the Earth’s surface, iron is easily oxidized to ferric iron. Due to the high specific heat capacity of the surrounding rock, the impact glass solidifies quickly, restricting the complete oxidization to Figure 5. Estimation of coordination state in each natural glass. ferric iron. Volcanic glasses are always formed at the sur- Three lines are 6-, 5-, 4-coordinated Fe from top to bottom, re- face of the Earth, which has a relatively stable redox con- spectively. 4-, 5-, and 6-coordinated Fe-O based on the Shannon dition; thus, it only undergoes small changes in the degree IV 2+ V 2+ VI 2+ Ironic Radii: Fe -O = 2.01 Å, Fe -O = 2.02 Å, Fe -O = IV 3+ V 3+ VI 3+ of oxidation. This study indicates that the local structure 2.16 Å, Fe -O = 1.87 Å, Fe -O = 1.96 Å, Fe -O = 2.025 Å. and oxidation state of Fe might provide insights into the atmospheric conditions prevalent during the formation of the glass.

ACKNOWLEDGMENTS

This study was performed within the Photon Factory Pro- ject PAC No. 2009G600.

REFERENCE

Alvarez, L.W., Alvarez, W., Asaro, F. and Michel, H.V. (1980) Ex- traterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208, 1095-1108. Barnes, V. and Barnes, M. (1973) Tektites. pp. 444, Dowden, Hutchinson, & Ross, New York. Figure 6. Relationship between pre-edge intensity and Fe-O dis- Berry, A., O’neill, H.ST.C., Jayasuriya, K.D., Campbell, S.J. and tance. Foran, G.J. (2003) XAENS calibration for the oxidation state of iron in a silicate glass. American Mineralogist, 88, 967- 977. nation numbers, and radial distribution functions of tek- Dunlap, R.A., Ealman, D.A. and MacKay, G.R. (1998) A Möss- bauer effect investigation of correlated hyperfine parameters tite, and impact- and non-impact related glass, have been in natural glass (Tektites). Journal of Non-Crystalline Solids, determined using EXAFS analyses. Comparisons of the 223, 141-146. glass sample with the standard minerals provided us with Farges, F., Brown, G.E. and Rehr, J.J. (1996a) Coordination chem- detailed information on formation of these kinds of natu- istry of Ti (IV) in silicate glass and melts: I. XAFS study of ral glass. titanium coordination in oxide model compounds. Geochimi- ca et Cosmochimaica Acta, 60, 3023-3038. Tektites contain a higher proportion of ferrous than Farges, F., Brown, G.E., Navrotsky, A. and Rehr, J.J. (1996b) Co- ferric iron; i.e., the ferric ratios of impact- and non-im- ordination chemistry of Ti (IV) in silicate glass and melts: II. pact related glass are higher than that in tektites. Accord- Glass at ambient temperature and pressure. Geochimica et ing to the Fe-O distance, 4- and 5-coordinated Fe exists Cosmochimaica Acta, 60, 3039-3053. Farges, F., Brown, G.E., Navrotsky, A., Gan, H. and Rehr, J.J. in tektites. On the contrary, non-impact related glass (1996c) Coordination chemistry of Ti (IV) in silicate glass - - - samples contain 5 and 6 coordinated Fe. Impact related and melts: II. Glass and melts from ambient to high tempera- glass samples possess more complex FeO with mixtures tures. Geochimica et Cosmochimica Acta, 60, 3055-3065. of 4-, 5-, and 6-coordinated Fe. Tektites, impact- and Frost, B.R. (1991) Introduction to oxygen fugacity and its petro- 294 L. Wang, A. Yoshiasa, M. Okube, T. Hiratoko, Y. Hu, H. Arima and K. Sugiyama

logic importance. Reviews in Mineralogy and Geochemistry, of the seismogenic zone. Tectonophysics, 518-521, 63-83. 25, 1-9. Rehr, J.J., Musture de Lson, J., Zabinsky, S.I. and Albers, R.C. Giuli, G., Pratesi, G., Ciproani, C. and Paris, E. (2002) Iron local (1991) Theoretical X-ray absorption fine structure standards. structure in tektites and impact glass by extended X-ray ab- Journal of the american chemical society, 113, 5135-5140. sorption fine structure and high-resolution X-ray absorption Reimold, W.U. (1995) Pseudotachylite in impact structures-gener- near-edge structure spectroscopy. Geochimical et Cosmo- ation by friction melting and shock brecciation? : A review chimica Acta, 66, 4347-4353. and discussion. Earth Science Reviews, 39, 247-265. Heide, K., Heide, G. and Kloess, G. (2001) Glass chemistry of Rossano, S., Balan, E., Morin, G., Bauer, J.-P., Calas, G. and 57 tektites. Planetary and Space Science, 49, 839-844. Brouder, C. (1999) Fe Mössbauer spectroscopy of tektites. Ho, K.-S. and Chen, J.-C. (1996) Geochemistry and origin of tek- Physics and Chemistry of Minerals, 26, 530-538. tites from the Penglei area, Hainan Province, southern China. Rossano, S., Ramos, A.Y. and Delaye, J.M. (2000) Environment

Journal of Southeast Asian Earth Science, 13, 61-72. of ferrous iron in CaFeSi2O6 glass; contributions of EXAFS Ishii, T. (1992) Note on the K extended X-ray absorption fine- and molecular dynamics. Journal of Non-Crystalline solids, structure Debye-Waller factor. Journal of Physics: Con- 273, 48-52. densed Matter, 4, 8029-8034. Stebbins, J.F. and McMillan, P. (1989) Five- and six- coordinated

Koeberl, C. (1986) Geochemistry of tektites and impact glass. An- Si in K2Si4O9 glass quenched from 1.9 Gpa and 1200 °C. nual Review of Earth and Planetary Sciences, 14, 323-350. American Mineralogist, 74, 965-968. Lytle, F.W., Sayers, D.E. and Stern, E.A. (1989) Report of the In- Wang, L., Yoshiasa, A., Okube, M. and Takeda, T. (2011) Titanium ternational Workshop on Standards and Criteria in X-ray Ab- local structure in tektite probed by X-ray absorption fine sorption Spectroscopy. Physica B, 158, 701-722. structure spectroscopy. Journal of Synchrotron Radiation, 18, Maeda, H. (1987) Accurate bond length determination by EXAFS 885-890. method. Journal of the Physical Society of Japan, 56, 2777- Waychunas, G.A., Apted, M.J. and Brown, G.E.Jr. (1983) X-ray 2787. K-edge absorption spectra of Fe minerals and model com-

Mysen, B. and Neuville, D. (1995) Effect of temperature and TiO2 pounds: near-edge structure. Physics and Chemistry of Min-

content on the structure of Na2Si2O5-Na2Ti2O5 melts and erals. 10, 1-9. glass. Geochimica et Cosmochimica Acta, 59, 325-342. Yarger, J.L., Diefenbacher, J., Smith, K.H., Wolf, G.H., Poe, B. Okudera, H., Yoshiasa, A., Murai, K., Okube, M., Takeda, T. and and McMillan, P.F. (1995) Al3+ coordination changes in high Kikkawa, S. (2012) Local structure of magnetite and maghe- pressure aluminosilicate liquids. Science, 270, 1964-1967. mite and chemical shift in Fe-K edge XANES. Journal of Yoshiasa, A., Koto, K., Maeda, H. and Ishii, T. (1997) The mean- Mineralogist and Petrological Sciences, 107, 127-132. square relative displacement and displacement correlation- Paris, E., Dingwell, D.B., Seifert, F.A., Mottana, A. and Romano, function in tetrahedrally and octahedrally coordinated ANB8−N C. (1994) Pressure-induced coordination change of Ti in sili- crystals. Japanese Journal of Applied Physics, 36, 781-784. cate glass: a XANES study. Physics and Chemistry of Miner- als, 21, 520-525. Manuscript received February 12, 2013 Price, N.A., Johnson, S.E., Gerbi, C.C. and West, D.P., (2012). Manuscript accepted July 22, 2013 Identifying deformed pseudotachylyte and its influence on Published online September 27, 2013 the strength and evolution of a crustal shear zone at the base Manuscript handled by Hiroki Okudera