The Canadian Mineralogist
Volume 34, pages 649-655 (1996)

THE CRYSTAL STRUCTURE OF SHOMIOKITE-(Y)

JOEL D. GRICE
Research Division, Canadian Museum of Nature, P.O. Box 3443, Station "D", Ottawa, Ontario K1P 6P4

Abstract

The crystal structure of shomiokite-(Y), Na3(Y,REE)(CO3)3•3H2O, has been determined and refined to indices R = 5.3 and Rw = 4.7%. A sample from the Poudrette Quarry, Mont Saint-Hilaire, Quebec, gave the following average electron-microprobe-determined composition: Na2O 23.25, Y2O3 19.05, La2O3 0.20, Ce2O3 1.81, Pr2O3 0.52, Nd2O3 2.79, Sm2O3 0.95, Gd2O3 1.05, Tb2O3 0.36, Dy2O3 2.81, Ho2O3 0.73, Er2O3 2.13, Eu2O3 0.00, Yb2O3 1.55, Tm2O3 0.40, Lu2O3 0.26, CO2 33.36 and H2O 13.66 total 104.88 wt.% (with proportion of CO2 and H2O determined by stoichiometry from results of crystal-structure analysis). This yields the empirical formula Na2.97(Y0.67La0.01Ce0.04Pr0.01Nd0.07Sm0.02Gd0.02Tb0.01Dy0.06Ho0.02Er0.04Tm0.01 Yb0.03Lu0.01) 1.02(CO3)3•3H2O and the simplified formula Na3(Y,REE)(CO3)3•3H2O. The structure is orthorhombic, Pbn21, with refined unit-cell parameters a 10.035(6), b 17.32(1), c 5.937(3) Å, V 1031.7(7) Å3, and Z = 4. An indexed X-ray powder-diffraction pattern is given. The structure analysis was essential in determining the number of C atoms and H2O groups in the formula. The correct orientation for the polarity of the structure was determined. The crystal structure is layered on (001), with Na(CO3)•H2O layers being cross-linked by Y atoms in [9]-coordination. The Na(CO3)•H2O layer of shomiokite-(Y) is comparable to a layer of similar composition, (Na,Y)(CO3)•H2O, in donnayite- (Y), but its Sr interlayer includes an additional (CO3) group.