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Mineralogical identification of stardust particles by xanes at the advanced light source.

01 Mar 2007-Iss: 1338, pp 2273

AbstractThe Stardust cometary samples are the most technically challenging returned extraterrestrial materials to date. Friable cometary particles generally disintegrated upon impact with the aerogel and are distributed along tracks that are two to three orders of magnitude larger than the typical particle size. As a result, tracks contain up to hundreds of particles that are buried deep in the aerogel tiles and are intimately mixed with aerogel. A method for doing mineralogical surveys of these particles is urgently needed. As a first step in their analysis, entire tracks can be extracted in aerogel keystones [3] or quickstones [4]. Individual particles can then be laboriously extracted, but it is impractical to do this for more than a few particles per track. Here we describe a method for doing mineralogical analysis of particles within aerogel keystones using a combination of synchrotron x-ray fluorescence at the micron scale (μXRF), x-ray near edge structure spectroscopy (μXANES) and x-ray diffraction (μXRD). This should facilitate mineralogical surveys and efficient searches for rare minerals, such as Ca, Ti-rich CAI-like materials. We performed our analysis of Stardust particles using keystones at beam line 10.3.2 of the Advanced Light Source at Lawrence Berkeley National Lab.[1] The μXANES capability of the beam line provides an excellent keystone survey technique to spot potentially interesting particles from the diverse material typically present in a keystone because it is able to rapidly identify minerals based on their chemical environment. Individual sub-micron particles such as those found in comet Wild 2 do not always yield useful diffraction data due to random crystal orientation and their small size. Furthermore, while electron microscopy can extract diffraction data from even the finest of minerals, it cannot do so rapidly (dozens of grains/day) as the preparation techniques are the rate limiting step. Therefore, when dealing with Wild 2 samples, XANES provides the powerful combination of mineralogical identification as well as high counting statistics and is very useful as a tool for comparing Wild-2 mineralogy against meteorite classes. Methods Each Stardust XANES spectrum was compared against a linear superposition of XANES spectra from known mineralogical standards. A fit therefore yields a percentage composition of several known minerals to achieve a minimum χ fit. To obtain valid results it is necessary to have a library of XANES spectra on hand for every mineral suspected to exist in the sample. With a large standards database it is possible to make very exact mineralogical identifications in a very short time. By measuring spots in the keystone near the track, it is also possible to remove the XANES contribution from contaminants in the aerogel. Many minerals require several XANES spectra to describe them as a consequence of optical anisotropy arising from the point group of the crystal. For example, diopside can have different signatures depending on the physical orientation of the crystal. See figure 1. Luckily, any crystal can be described using a linear superposition of at most 6 basis XANES spectra. [2]

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Abstract: A study is presented of anisotropy effects in x-ray absorption in the layered compounds of $2H\ensuremath{-}\mathrm{W}{\mathrm{Se}}_{2}$ and $1T\ensuremath{-}\mathrm{T}\mathrm{a}{\mathrm{S}}_{2}$. In the measurements it was essential to separate the thickness effect from the true anisotropy effect which is dependent on the angle between the x-ray polarization and the crystal axes. The Se $K$ edge and the W and Ta $L$ edges were measured. Anisotropy in the white line of Se was found but no anisotropy was discerned in the W and Ta white lines. It is pointed out that x-ray absorption in general, and the extended x-ray absorption fine structure (EXAFS) in particular, have the anisotropy dependence of a second-order tensor and the theoretical expression for the EXAFS anisotropy at the ${L}_{2,3}$ edges is explicitly displayed. The anisotropy of the EXAFS in the Se and W absorption was measured and a good agreement with theory is found. The anisotropy of EXAFS at the ${L}_{2,3}$ edges has the new feature of a cross term between the final $s$ and $d$ states, which permits a determination from the measurements that the average contributions of the final $s$ state is 0.02 of that of the final $d$ states to the total absorption of the W ${L}_{3}$ edge. Finally, only qualitative agreement is obtained between band calculations and the near edge x-ray absorption structure, as expected theoretically.

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