1 Crystal chemical significance of chemical zoning in dissakisite-(Ce)

Telephone number: +81-029-853-4544 Fax number: +81-029-853-7887 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 1400 observed [׀F0 ׀> 4σF0] reflections measured with MoKα X-radiation using 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. Keywords dissakisite · rare earth elements · chemical zoning · crystal structure · epitaxy 2Introduction Dissakisite-(Ce) of an ideal formula CaCeMgAl2Si3O12(OH)—a Mg analogue of allanite-(Ce) (e.g. Armbruster et al. 2006)—has been reported from some metamorphic occurrences (Grew et al. 1991