X-ray spectroscopy and electron microscopy of planetary materials

The work described in this thesis features the study of planetary materials, including nakhlite martian meteorites, comet 81P/Wild2 grains, and asteroid Itokawa 25143 particles, using various electron microscopy techniques and X-ray spectroscopy. Transmission electron microscopy (TEM), Fe-K X-ray absorption spectroscopy (XAS) and in-situ Transmission X-ray Diffraction (XRD) have been used to determine the structure, ferric content and stoichiometry in the secondary phase mineral assemblages of the nakhlites. By measuring the energy position of the Fe-K XANES 1s→3d pre-edge transition centroid, the Fe3+-rich nature of these minerals has been shown. Analyses of the crystalline phyllosilicates in Lafayette found trioctahedral ferric saponite with a 2:1 T-O-T lattice structure and d001-spacings of 0.96 nm, as well as Fe-serpentine with a 1:1 T-O structure and d001-spacings of 0.7 nm. A ferric poorly crystalline or amorphous gel of similar composition to the phyllosilicates was found as fracture fills throughout the other nakhlites. XRD and Fe-K XAS also allow the mineralogical identification of comet Wild2 terminal grains. The terminal grains of Tracks #170 and #176 are Fe-metal, with hematite subgrain material in #170. The terminal grain of #170 also includes Cr-bearing silicate, similar to the Cr-bearing terminal grain of Track #177. Olivine was found amongst the terminal grains of Track #178 alongside magnetites. The presence of magnetite is consistent with low temperature water-rock interaction similar to a carbonaceous chondrite matrix. XAS has also been used to study Itokawa particles, finding ferrous olivines and pyroxenes, as well as studying other metals and Ni-bearing phases. Comparisons with the Tuxtuac meteorite showed the similarity of Itokawa particles to that of LL5 and LL6 chondrite materials. Measuring Fe-K XAS and XRD has proved to be an effective and non-destructive method for mineralogical characterisation of planetary samples, and determining the oxidation state of Fe-silicates, especially on a micron scale. These will be essential techniques for future sample return missions and meteorite finds.