11:45 AM - 12:15 PM
[SMP33-11] Site occupation analysis for cation in minerals by analytical electron microscopy: application of TEM-ALCHEMI for earth and planetary science
Keywords:Analytical electron microscopy, ALCHEMI, HARECXS, Site occupancy, Cation ordering
Spence and Taftø  developed the ALCHEMI (Atom Location by CHanneling-Enhanced MIcroanalysis) method in which transmission electron microscope with energy dispersive X-ray spectrometer (TEM-EDS) is used to determine the crystallographic sites of impurity elements from micrometric regions in crystals on the basis of channeling-enhanced X-ray emissions. Subsequent improvements in the ALCHEMI have led to more statistical method called HARECXS (High Angular Resolution Electron Channeling X-ray Spectroscopy) or St-ALCHEMI (statistical ALCHEMI). This method is widely applicable including minerals with low symmetric structures (reviewed by ). Recently, we successfully determined the order parameter of Al and Si in the tetrahedral sites in sillimanite (Al2SiO5) at the areas free from precipitates of mullite (3Al2O3 2SiO2) and glasses . In this study, we carried out the ALCHEMI/HARECXS method for major rock forming minerals to extend its applicability in earth and planetary science.
Sample minerals are olivine ([Mg, Fe]2SiO4), diopside (Ca[Mg, Fe]Si2O6) and sillimanite. The samples were prepared by a focused ion beam system (FIB, FEI Quanta 200 3DS or Helios NanoLab G3 CX), and examined by using TEM-EDS (JEOL JEM-2100F, JED-2300T). X-ray signals generated under illumination of 200 keV electrons were collected as a function of electron beam direction. The homogeneous areas free from dislocations or inclusions were selected as regions of interest.
The one-dimensional tilting ALCHEMI datasets under the condition exciting the particular systematic reflection row (HARECXS profiles) were successfully obtained from regions ~1 µm in diameter by automated control of beam tilting and X-ray detection following Muto and Ohtsuka . For determination of cation site occupation, we performed theoretical calculations of HARECXS profiles using ICSC code  and evaluated sample thicknesses by convergent-beam electron diffraction. The experimentally obtained HARECXS profiles were fitted by linear combinations of calculated profiles, and the site occupations of constituent cations were quantitatively determined. Obtained results were almost consistent with the results determined by single crystal X-ray diffraction experiments. Moreover, the cation distributions in annealed samples showed continuous disordering with increasing temperature. This study suggests that the ALCHEMI/HARECXS method is one of the most promising candidates for accurate analysis of minerals in particular the samples having heterogeneous textures.
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