Local structure of iron in tektites and natural glass: An insight through X-ray absorption fine structure spectroscopy

Abstract

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 of iron in the samples. Tektites contain a greater proportion of ferrous than ferric iron [0.04(1)-0.13(1) Fe³⁺/ΣFe]. The ferric ratios of impact-related glass [0.18(1)-0.52(1) Fe³⁺/ΣFe], and volcanic glass [0.26(1)-0.30(1) Fe³⁺/ΣFe] 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 varying 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.