R2: Crystal structure, crystal chemistry, physical properties of minerals, crystal growth and applied mineralogy

[R2-14] Crystal structures of kimuraite-(Y) and lokkaite-(Y)

*Ritsuro Miyawaki¹, Koichi Momma¹ (1. National Museum of Nature and Science)

Keywords:kimuraite-(Y), lokkaite-(Y), crystal structure

The crystal structures of kimuraite-(Y) and lokkaite-(Y) were re-examined by means of the twining and statistical ordering models with the space groups following to the individual extinction rules in their single crystal XRD data. The crystal structure of kimuraite-(Y) determined with the space group I222 is harmonic with that previously determined with the space group I2cb. The crystal structure of lokkaite-(Y) was analyzed with the monoclinic space group Cm to determine the mean structure in the C222. Each crystal structure of kimuraite-(Y) and lokkaite-(Y) consists of two types of layers: a flat calcium layer and a corrugated rare earth layer (Figure). The rare earth atoms are coordinated to 9 oxygen atoms. The 9-coordinated polyhedra are connected with the wedging triangle of carbonate anion to form the corrugated rare earth layer, as those in the crystal structure of tengerite-(Y). Calcium atoms statistically order into the Ca sites at the middle of the flat calcium layer. The C and O atoms neighboring to the Ca atoms should be shifted with the statistical occupancies. The crystal structures of kimuraite-(Y) and lokkaite-(Y) differ from each other in the sequence of stacking layers corresponding to the ratio of calcium and rare earths atoms.