Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Jakub Plášil, Radek Škoda

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

Journal of Geosciences, volume 62 (2017), issue 2, 97 - 105

DOI: http://doi.org/10.3190/jgeosci.232

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) Å, α = 90.058(3), β = 89.945(4), γ = 90.331(4)°, V = 2469.3(4) ų 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 ({Nd³⁺₂(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 point analyses is Ca_1.01(Nd_0.64Ce_0.32Sm_0.28Gd_0.19Y_0.19Pr_0.13Dy_0.10La_0.07Tb_0.05Ho_0.01)_Σ1.98(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.