Publication details

Crystal structure of the (REE)-uranyl carbonate mineral shabaite-(Nd)



Year of publication 2017
Type Article in Periodical
Magazine / Source Journal of Geosciences
MU Faculty or unit

Faculty of Science

Keywords shabaite-(Nd); uranyl carbonate; rare-earth elements; crystal structure; mineral evolution
Description Shabaite-(Nd) is a rare supergene mineral formed during alteration-hydration weathering of uraninite; its structure has remained unknown until now. Based on single-crystal X-ray diffraction data, shabaite-(Nd) is triclinic, twinned (leading to a pseudo-monoclinic diffraction pattern), space group P-1, with a = 8.3835(5), b = 9.2766(12), c = 31.7519(3) angstrom, alpha = 90.058(3), beta = 89.945(4), gamma = 90.331(4)degrees, V = 2469.3(4) angstrom(3) and Z = 4. The structure was refined from diffraction data to R = 0.060 for 8434 unique observed reflections. The structure of shabaite-(Nd) is based upon finite clusters of polyhedra, the well-known uranyl-tricarbonate cluster (referred to as UTC), the Ca-polyhedra linked to UTC and infinite sheets of Nd-polyhedra with internal Nd-O-C linkages. The infinite sheets of Nd-polyhedra are stacked perpendicular to c; to the UTC are staggered approximately perpendicular to the Nd-based sheets (thus approx. parallel to c), forming electroneutral ({Nd-2(3+)(CO3)(2-)(2)(H2O)(2)}(2+){Ca(H2O)(5)}(2+)[(UO2)(CO3)(3)](4-))(0) layers. Adjacent layers are linked by hydrogen bonds. The ratio of elements in the chemical formula of shabaite-(Nd), obtained from the structure refinement, Nd2Ca[(UO2)(CO3)(3)] (CO3)(2)(H2O)(10.5), Z = 4, was confirmed by an electron-microprobe study. The resulting empirical formula for the average of six analyses is Ca-1.01(Nd0.64Ce0.32Sm0.28Gd0.19Y0.19Pr0.13Dy0.10La0.07Tb0.05Ho0.01)(Sigma 1.98)(UO2)(CO3)(4.98)(H2O)(10.5)(based on 1 U apfu). A discussion of the crystal-structural relationships of shabaite-(Nd) and chemically related minerals is given as well as discussion of complexities of their crystal structures.
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