Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.295t&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 295
Other
Scientific paper
Tetrataenite, observed in slow-cooled meteorites, is characterized by the ordered structure of FeNi and the tetragonal distortion from face-centered cubic taenite. The ordering of Fe/Ni was performed in the extra-slow cooling (e.g., 0.1-100 degrees C/m.y.) at the temperature below 300 degrees C (Clarke and Scott, 1980: Albertsen, 1981). The ordering of Fe and Ni in tetrataenite has mainly been confirmed by Mossbauer spectra (Danon et al., 1979). A tetrataenite single crystal of about 40 micrometers in diameter could be selected from the Saint-Severin meteorite (LL6). The chemical composition of the sample was determined to be FeNi by electron probe microanalysis. The lattice constants were determined on the assumption of triclinic symmetry on the diffractometer as: a = 3.581(2), b = 3.582(2), c = 3.587(2) angstrom, alpha = 90.03(3), beta = 90.04(3), gamma = 90.00(3) degrees. The lattice constants show the clear tendency of the tetragonal lattice symmetry within the experimental error. No threefold twinning along [111] was detected by high resolution X- ray diffraction. The diffraction data were collected on a four- circle diffractometer using Moka radiation. A total of 1484 reflections were measured and 696 independent reflections (>1 sigma) were used for the structure determination. Then the same crystal was supplied to the diffraction experiments by synchrotron radiation. The diffraction data were collected using the X-ray of wavelength = 1.746 angstroms. The correction parameter for anomalous scattering in the wavelength are for Fe atom f' = -6.299, f" = 0.469, and for Ni atom f' = -1.740, f" = 0.638. Because of small lattice constants and rather long wavelength, only 25 independent reflections were collected. In the ordered structure of the space group symmetry P4/mmm, all the atoms must occupy special positions. The intensities of the superstructure reflections should be interpreted only by the difference of scattering factors of Fe and Ni. But the model of P4/mmm could not give the sufficient intensities of the superstructure reflections. Intensity distributions indicate the possibilities of the space groups Pm or P1. The space group symmetry of Pm is assumed in the present structure determination. Because of the small number of reflections by synchrotron radiation, the positional and thermal parameters were refined using the reflection data by Mokalpha radiation. After several cycles of the least squares refinements, the structure converged to an R-factor of 7.5% (for 696 observed reflections). According to the results of reflection data of synchrotron radiation, it can be concluded that one Ni and one Fe positions are fully ordered and that another Ni and Fe positions are about 80% ordered as Ni0.8Fe0.2 and Ni0.2Fe0.8, respectively. The positional shifts of the atoms are within 0.04 angstroms from the average positions. Albertsen, J.F. (1981) Phys. Scripta, 17, 467-472. Clarke, R.S. Jr., and Scott, E.R.D. (1980) Amer. Mineral., 65, 624-630. Danon, J. Scorzelli, R.B., and Azvedo, I.S. (1979) Nature, 281, 469-471.
Fukuda Takahiro
Tagai Tokuhei
Takeda Hidenori
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