Abstract
Polymignyte, zirkelite and zirconolite have often been considered the same mineral. Their occurrence, with calciobetafite (a new member of the pyrocjilore group), in a “sanidinite” from Campi Flegrei has allowed their crystal-chemical study and identification as three distinct phases. The three minerals are polymorphs of the compound: . The crystal structure of zirconolite (space group C2/c) has been previously determined (Gatehouse et al., 1981) on synthetic crystals; those of polymignyte (e.g., Acam) and zirkelite (e.g., P312) are described in the present paper. 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. The crystal structures of zirkelite and zirconolite are very similar, as they differ only in the stacking of identical pairs of layers of polyhedra. The difficulties in distinguishing the three polymorphs by X-ray powder diagrams are discussed.
