AN INVESTIGATION OF FE OXIDATION-STATES AND SITE DISTRIBUTIONS IN A TIBETAN TEKTITE

Room temperature Fe-57 Mossbauer effect measurements have been used to investigate the local Fe environment and the Fe3+/Fe2+ ratio in a Tib etan tektite. The spectrum has been analyzed in terms of two quadrupol e splitting distributions; one corresponding to Fe2+ and one correspon ding to Fe3+. The Fe2+ sites show a mean isomer shift of +1.011 mm/s r elative to alpha-Fe and a mean quadrupole splitting of 1.845 mm/s. The Fe2+ quadrupole splitting distribution showed a large peak for positi ve values, which has been identified with Fe2+ with local octahedral c oordination, and a smaller peak for negative values, which has been id entified with Fe2+ with local tetrahedral coordination. The Fe3+ sites showed an average isomer shift of +0.20 mm/s and a single peaked quad rupole splitting distribution with a mean near zero. The Fe3+/Fe2+ rat io, based on the relative areas of the spectra components was found to be 0.015. These results are discussed in terms of those obtained usin g fitting methods based on discrete sites and other distributions of p arameters. Hyperfine parameters for the Tibetan tektite are compared w ith those obtained for tektites from other regions of the Australasian strewn field. All tektites studied show similar quadrupole distributi ons and similar mean hyperfine parameters. Fe3+/Fe2+ ratios ranged fro m 0.005 to 0.036. The relationship of the microstructure of the Tibeta n tektite with that of tektites from various regions of the Australasi an strewn field is discussed.