Calciobetafite (new mineral of the pyrochlore group) and related minerals from Campi Flegrei, Italy; crystal structures of polymignyte and zirkelite; comparison with pyrochlore and zirconolite

The three minerals are polymorphs of the compound: (Ca,Na, REE, Th..) 2 VIII Zr 2 VII (Ti,Nb,.) 3 VI (Fe,Ti) (super V,IV) O 14 . The crystal structures of polymignyte, zirkelite and zirconolite may be derived from that of pyrochlore. Chains, formed by distorted (Ca,REE..) cubes alternating with (Ti,Nb..) octahedra in pyrochlore, are replaced in the remaining minerals by either chains of Zr polyhedra with seven vertices or chains in which (Ti,Nb..) octahedra alternate with distorted (Fe,Ti..) tetrahedra or trigonal bipyramids. The different arrangement of these chains gives rise to the different symmetries of the three phases.--Modified journal abstract.