Host: The Mineralogical Society of Japan
The crystal structures of two rare earths calcium hydrous carbonate minerals, kimuraite-(Y) and lokkaite-(Y), are determined with specimens from Hizen-cho, Saga Prefecture, Japan. The data were collected with MoKa radiation on a CCD detector. The structures were solved and refined with Shelx-97. The refinements converged to R1 = 5.79 and 8.47 % for kimuraite-(Y) and lokkaite-(Y), respectively. Kimuraite, CaY2(CO3)4.6H2O, is orthorhombic, with space group Iba2, a = 24.015(2)A, b = 6.0425(6)A, c = 9.2596(9), Z=4. It consists of the tengerite-type corrugated sheet of nine-coordinated Y-polyhedra and CO3 triangles, and the layer of calcium ions and water molecules. The sheet and the layer stack alternately in the crystal structure of kimuraite-(Y). Lokkaite-(Y), CaY4(CO3)7·9H2O, is orthorhombic, with space group Abm2, a = 6.1044(8)A, b = 39.176(5)A, c = 9.2193(12), Z=4. In lokkaite, a pair of the corrugated sheets is located between the Ca-H2O-layers. The symmetry of the corrugated sheet in lokkaite-(Y) is lower than that of tengerite-(Y), while the sheet in kimuraite-(Y) keeps the symmetry. These results confirm the previous prediction on the structural relationships among tengerite-(Y), kimuraite-(Y) and lokkaite-(Y).