Experiment title: Experiment Fe local structure in tektites and impactites by XANES number: and EXAFS 08-01-251

Beamline: Date of experiment: Date of report: BM-08 from: 8-11-2000 to: 10-11-2000 30-4-2001

Shifts: Local contact(s): Received at ESRF: 6 Pier Lorenzo Solari

Names and affiliations of applicants (* indicates experimentalists): Gabriele Giuli* Dip. Scienze della Terra and INFM, Universita’ di Camerino (I) Eleonora Paris * Dip. Scienze della Terra and INFM, Universita’ di Camerino (I) Giovanni Pratesi* Museo di Mineralogia e Litologia, , Universita’ di Firenze (I) Curzio Cipriani Museo di Mineralogia e Litologia, , Universita’ di Firenze (I)

Report: This project is focused on the structural study of glasses formed during the impact of asteroids on the earth surface, and in particular on the Fe coordination number and oxidation state. The interest on impact cratering, due to the its importance as a geological process and, partly, to the economic importance of several big impact craters (see Montanary and Koeberl, 2000 for a review), has stimulated a wealth of studies on impact glasses; however, relatively few were devoted to the structural study of these glasses (Korotayeva et al., 1985; Farges et al., 1997; Rossano et al., 1999; Giuli et al., 2000; Giuli et al., 2001). In this study the samples are studied by means of EXAFS and High resolution XANES. To get information on the Fe local environment and oxidation state we collected High-resolution XANES spectra of several Fe compounds with known Fe-coordination number and oxidation state (Fe2+ in 4-, 5-, 6-, and 8-fold coordination and Fe3+ in 6-fold coordination) to be compared with the studied 2 samples. For some of the standards we collected also EXAFS spectra in order to calibrate the S0 factor, which is needed to extract information on the coordination number from EXAFS. In the previous run (08-01-197) we collected EXAFS and High resolution XANES spectra of several tektites (a moldavite, an indochinite and an australite) and impact glasses (a suevite from the Ries crater, germany; an irzighite from the Zamanshin crater, Khazakstan; and an impact glass from the Wabar craters, Saudi Arabia). The data on tektites were analysed to get Fe-O distances and coordination number (=2.00 Å, =4.5), the results being reported in Giuli et al. (2001). In the present run (08-01-251) we collected XAS data on three tektites and three impact glasses from the Ivory Coast strewn field, a glass of presumable impact origin from Virginia, an impact glass from the Auelloul crater (Mauritania), Darwin glass from the Darwin crater, Australia), and a glass of uncertain origin from Kofel (Austria). Unfortunately, malfunctioning of the X-ray optics (after each injection, the beam was lost) prevented us to use all the beamtime allocated (6 shift), so that we were not able to collect EXAFS spectra of all the samples. However, some important results have been achieved: 1) Ivory Coast tektites represent a group of glasses chemically distinct from all other tektites. However, they are rarer than other types of tektites and much more difficult to find even in big museum collections. The samples examined here were provided By prof. C; Koeberl, who collected them on an expedition in Ghana and Ivory Coast. The data on the Fe environment are consistent with the other tektites already studied (Giuli et al., 2001), and will be published along with data at the Al and Si K- edge on the same samples (Giuli et al., in prep). These data are the first complete structural characterisation of Ivory Coast tektites; 2) in comparing the data now acquired, with those of the last run, it is important to stress that the Fe K-edge spectra of all the tektites so far analysed are very similar, whereas the spectra of impact glasses are clearly different with those of tektites. In particular, of all the impact glasses studied, irghizites (from the Zamanshin crater) are the most different from all the other in that, as shown by the analysis of the pre-edge peaks, they contain appreciable amounts of trivalent Fe.

Refences cited:

Farges F., and Brown G.E. (1997). Geochimica et Cosmochimica Acta, 61 (9), 1863-1870. Giuli G., Pratesi G., Corazza M., and Cipriani C. (2000a). American Mineralogist,85,1172-1174. Giuli G., Pratesi G., Paris E., Cipriani C; (2001) submitted to Geochimica et Cosmochimica Acta Korotayeva N.N., Polosin A.V., and Malysheva T.V. (1985). Geokhimiya, 6, 899-903. Montanari A., & Koeberl C. (2000) Impact Stratigraphy, Lecture Notes in Earth Sciences. Springer, Berlin. Rossano S., Balan E., Morin G., Bauer J.P., Calas G., Bronder C. (1999). Physics and Chemistry of Minerals, 26, 530-538.