An X-ray single crystal study of asymmetric thermal vibrations and the positional disorder of atoms in elbaite
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A least squares refinement with a generalized structure factor expression in the Gram-Charlier expansion was applied to the structure of elbaite, XY<sub>3</sub>Z<sub>6</sub>(BO<sub>3</sub>)<sub>3</sub>T<sub>6</sub>O<sub>18</sub>A<sub>3</sub><sup>(1)</sup>A<sup>(2)</sup> (X = Na<sub>0.640</sub>Ca<sub>0.090</sub>K<sub>0.006</sub><sub>0.264</sub>, Y = Al<sub>0.415</sub>Fe<sup>2+</sup><sub>0.084</sub>Mn<sup>2+</sup><sub>0.034</sub>Mg<sub>0.005</sub>Ti<sup>4+</sup><sub>0.003</sub>Li<sub>0.458</sub>, Z = Al, T = Si, A<sup>(1)</sup> = (OH)<sub>0.750</sub>, and A<sup>(2)</sup> = F<sub>0.179</sub>(O,OH)<sub>0.071</sub>). Elbaite has space group <i>R</i>3<i>m</i>, and hexagonal cell dimensions of <i>a</i> = 15.874(3) and <i>c</i> = 7.116(3) Å. The least squares refinement of the X-ray single-crystal data measured at room temperature converged at <i>R</i> = 0.0221 for 179 variables, including terms up to the fourth order coefficients. The O1 and O2 sites showed multiple split probability density functions (pdfs), and the X site showed a circular ring-like pdf, providing evidence of positional disorder at these sites. The disorder at the O1 and O2 sites was interpreted to be due to substitutional disorder of the Y site cations. The remaining atom sites all showed unimodal pdfs with a more or less asymmetric tendency along the hexagonal <b>c</b>-axis. The asymmetry is related to the primary effect of pyroelectricity.
- Journal of Mineralogical and Petrological Sciences
Journal of Mineralogical and Petrological Sciences 102(2), 115-126, 2007-04-01
Japan Association of Mineralogical Sciences