Combined In Situ Micro X-Ray Diffraction and Raman Study of Carbonaceous Matter in Ureilites and Terrestrial Materials

Introduction: It has recently become possible to undertake in situ non destructive microanalytical studies of meteorites and other precious materials. Flemming [1] outlines the recent developments made in expanding the applicability of micro X-ray diffraction (μXRD) for use with Earth and planetary materials. Here we report a reconnaissance study using the complimentary techniques of in situ μXRD and Raman spectroscopy to examine carbonaceous material in two ureilite samples (Dar al Gani 976 and Dar al Gani 1023) and four terrestrial samples (impact suevites from Kara and Gardnos and carbonrich veins from Torr Mor and the Ereland volcanic complex). The μXRD analysis was conducted at the University of Western Ontario using the Bruker-AXS D8 Discover diffractometer, with a 50 μm beam in which diffracted rays are collected using a two dimensional general area diffraction detector system (GADDS), recording textural and crystal structural information. Raman microspectroscopy was carried out at the University of Aberdeen with the 514.5 nm Ar ion laser beam of a Renishaw inVia Reflex Spectrometer, using a smaller spot size (typically 1-2 μm) to characterise the carbonaceous material and to provide a proxy for the degree of structural order. Results: There is generally good agreement between the μXRD and Raman data, although crystalline graphite seems to be more readily identified using μXRD (Table 1). Graphite is anisotropic and there may be an orientation effect such that crystalline graphite will only be detected when the Raman laser is perpendicular to the stacked crystallite. The larger beam size used in μXRD means that there is potentially some overlap between phases and some patterns may be ambiguous (e.g. chaoite). Using a second technique such as Raman microspectroscopy to examine the phases identified by μXRD provides confirmation and additional information. The μXRD configuration includes an integrated optical microscope and digital camera system with a HeNe targeting laser. The laser, microscope optics and X-ray beam are aligned such that when the laser is in optical focus, the point on the sample illuminated by the laser is at the centre of diffraction. Image maps can be created prior to analysis and then be used for subsequent Raman imaging and spectroscopy. Fig. 1 shows μXRD data for one location on the Kara suevite sample. Fig. 2 shows Raman spectra for the same sample, corroborating the presence of amorphous carbon. Figure 1. Montage of Kara μXRD data showing the two dimensional record of diffracted X-rays (top), the integrated diffraction pattern (bottom) and microscope image centered on the spot location (inset). The top image shows “powder rings” indicative of a phase with crystallites in every random orientation, with 2-theta increasing from the left. The raw integrated diffraction pattern (in grey) shows a very broad, diffuse signal from ~12o-25o which is interpreted as amorphous carbon. The background subtracted pattern is shown in black and is overlain with the integrated 1D diffraction pattern. Matching peak locations for graphite are indicated in red, quartz in blue and aragonite in magenta.

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