THE CRYSTAL-STRUCTURE OF MONOCLINIC BRITHOLITE-(CE) AND BRITHOLITE-(Y)

The crystal structures of the monoclinic dimorphs of natural britholite-(Ce) and britholite-(Y) have been solved in space group P2(1) using both annealed and unannealed samples. Like the P6(3) 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 P6(3) britholite each Ca(1) equivalent has either three short or three long REE-O(3) bonds, in the P2(1) 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 P6(3)/m apatite symmetry elements, yielding P2(1) symmetry. The symmetry reduction explains the common observation of biaxial optical characteristics of britholite samples.