APPLICATION OF MICRO-BEAM TO MINERALS IN A THIN SECTION OF METEORITE AND STRUCTURE REFINEMENT. K. Ohsumi1, M. Uchida2, K. Hagiya3, M. Miyamoto4 and M. Ohmasa3. 1Photon Factory, National Laboratory for High Energy Physics, Japan; 2Synchrotron Radi. Sci., Graduate School for Advanced Studies, Japan; 3Department of Life Science, Himeji Institute of Technology, Japan; 4Mineralogical Insti.,Graduate School of Science, Univ. of Tokyo, Japan.
Polychromatic SR microbeam with diameter of 1.6 (m and with divergence of 40 urad. was produced by a micro-pinhole technique for structure refinement using Laue method. This size of micro-area on the sample is comparable to those examined by optical microscope, scanning electron microscope, electron prove micro analyzer (EPMA), Raman spectroscopy and so on. The X-ray diffraction method with this micro-pinhole provides crystallographic information from the exact same micro-area as analyzed by various methods mentioned above. The micro-pinhole was installed in the Laue camera which was developed at the beam line 4B of the Photon Factory, KEK1). One of the application of this method is given below.
In crystallographic studies of meteorite which must be affected by shock as presumed from its origin, the microbeam is indispensable to search micro-area that is good enough for structure refinement, or to obtain diffraction data from submicron crystal grain with high signal to noise ratio.
The microbeam has been applied to olivine ((Mg,Fe)2SiO4) and two unknown crystals included in a thin section of ureilite (meteorite) with thickness of 30 um, in order to (1) obtain site occupancies of Mg and Fe in cation sites (M1 and M2) which indicate thermal history of the sample, (2) identify two unknown crystals in the rim of the olivine covered with carbonaceous material. The results are summarized below.
(1) Structure refinement was successfully carried out in an usual manner based on more than 300 Laue intensities, and site occupancy of Mg in M1 site was determined as 0.908(4) under the constraint of Mg and Fe abundant which was analyzed by EPMA.
(2) One of the unknown crystal is determined as diamond, and the thermal parameter(B) was refined to be 0.19(3)A2 by the least-squares method based on 12 Laue diffraction intensities. The other was revealed to be a face-centered cubic phase of iron (Fe) which was crystallized, topotaxial to the olivine, of Fe atoms from M1 and M2 sites under reduction by carbon. The thermal parameter (B) was also refined to be 0.06(1)A2 based on 8 Laue diffraction intensities.
1) K. Ohsumi et al.: Rev. Sci. Instrum., 66(2), 1448-1450, 1995.