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

The crystal structures of the monoclinic dimorphs of natural britholit e-(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 britholi te dimorph differs from its hexagonal counterpart principally in the l igation 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 l ong REE-O(3) bonds, in the P2(1) dimorph the Ca(1) equivalents have ei ther one long and two short REE-O(3) bonds or one short and two long R EE-O(3) bonds. Thus 3/m is removed from the P6(3)/m apatite symmetry e lements, yielding P2(1) symmetry. The symmetry reduction explains the common observation of biaxial optical characteristics of britholite sa mples.