希土類炭酸塩鉱物の結晶化学 The crystal chemistry of rare earth carbonate minerals

The carbonate ion with a triangle shape is always isolated from other carbonate ions in the crystal structure of a carbonate. The rare earth atoms in carbonate minerals tend to have higher coordination numbers in comparison with those in the silicates and phosphates, owing to the lower negative charge per an apical oxygen atom of the carbonate ion. The differences in coordination numbers and sizes of coordination polyhedra are observed between the smaller rare earth, e.g. Y, and larger rare earth, e.g. Ce, and the differences are directly reflected in the crystal structures of carbonates without infinite framework structures. The major part of rare earth carbonate minerals can be classified into four mineral groups, lanthanite, tengerite, bastnäsite and kozoite groups. The isomorphous substitution of rare earths with calcium can be observed in the kozoite group, as a solid solution series between kozoite-(Nd) and calcioancylite-(Nd), whereas non can be observed in the tengerite and bastnäsite groups. The bastnäsite group is unique to the other carbonate minerals because their crystal structures can accept both of the larger Ce and smaller Y ions.