Crystal chemical significance of chemical zoning in dissakisite-(Ce) |
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| Authors: | Mihoko Hoshino Mitsuyoshi Kimata Norimasa Nishida Masahiro Shimizu |
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| Institution: | (1) Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan;(2) Chemical Analysis Division, Research Facility Centre for Science and Technology, University of Tsukuba, Ibaraki 305-8577, Japan |
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| Abstract: | Dissakisites from Trimouns dolomite mine, France, have two kinds of single crystals: chemical-zoned and homogeneous types.
Back-scattered electron microprobe (BSE) images of these dissakisites reveal both Ca–Al rich dark zones and Fe-ΣREE rich bright
zones. Crystal structures of three dark and two bright zones in a chemical-zoned dissakisite and of a homogeneous zone in
unzoned dissakisite were refined to individual R indices (about 3.0–5.0%) based on 1,400 observed |F
0| > 4σF
0] reflections measured with MoKα X-radiation using the single crystal diffractometer. The differences in brightness between
their BSE images arise from those in coupled substitutions of the elements occupying A2 and M3 sites. The main reason for
these differences is that ten-coordinated A2 polyhedra and M3 octahedra are directly linked through their shared edge, which
creates a great potential for making this coupled substitution. This zoning indicates that formation of the whole zoned crystal,
where each zone could be grown steadily with its crystallographic axes mutually parallel to each other, may be identified
as autoepitaxy. |
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| Keywords: | Dissakisite Rare earth elements Chemical zoning Crystal structure Coupled substitution Autoepitaxy |
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