Tracking Glass Structure in IDPs with O-XANES

Introduction: Amorphous silicates can be formed and/or altered via different mechanisms (melting, condensation, irradiation, corrosion, collision). Different mechanisms may leave signature chemical compositions, and/or imprints in the bonding environments of specific atomic species. For example, different oxygen fugacities could be recorded as Fe oxidation, or silica polymerization could vary with formation temperature and mechanism. X-ray Absorption Near-Edge Spectroscopy (XANES) is a powerful technique for studying chemical bonding and atomic environments in amorphous systems. Here we use XANES at the oxygen K-edge to study silicates in interplanetary dust particles (IDPs), combining experimental with modeled spectra. For amorphous silicates within equilibrated aggregates (EAs) [1] and Glass with Embedded Metals and Sulfides (GEMS) [2], O-K XANES promises to address several questions: In principle, it should be able to differentiate between phases with medium-range order from those without any order which could distinguish between highly unequilibrated amorphous phases (irradiated, corroded) from more equilibrated glasses (melt formation). It may be possible to detect and study the presence of H in the silicate through the OH bonds which would be useful for identifying alteration in a nebular environment, such as solar wind implantation. Finally, just as Si-K XANES has been used to constrain the presence of silicone oil in interplanetary dust particles (IDPs)[3], OXANES can detect silicone oil contamination in IDPs, and has been used to distinguish aerogel contamination in samples returned by the Stardust mission [4]. Such contamination can be difficult to detect using other methods. The primary difficulty in using O-K XANES is that every oxygen atom contributes to the spectrum. While this means that the O-XANES is potentially very information rich, it also means that interpretation of the spectrum requires simultaneous modeling of all oxygen coordination environments present in the sample.