The crystal structure of monoclinic britholite-(Ce) and britholite-(Y)

Abstract

Abstract The crystal structures of the monoclinic dimorphs of natural britholite-(Ce) and britholite-(Y) have been solved in space group P21 using both annealed and unannealed samples. Like the P63 britholite atomic arrangement reported recently, the structure of monoclinic britholite is similar to the apatite atomic arrangement. The monoclinic britholite dimorph differs from its hexagonal counterpart principally in the ligation of the REE equivalent of the apatite Ca(1) site. Whereas in P63 britholite each Ca(1) equivalent has either three short or three long REE-O(3) bonds, in the P21 dimorph the Ca(1) equivalents have either one long and two short REE-O(3) bonds or one short and two long REE-O(3) bonds. Thus 3/m is removed from the P63/m apatite symmetry elements, yielding P21 symmetry. The symmetry reduction explains the common observation of biaxial optical characteristics of britholite samples.