Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Gwladys Steciuk, Radek Škoda, Jan Rohlíček, Jakub Plášil

Crystal structure of the uranyl-molybdate mineral calcurmolite Ca[(UO₂)₃(MoO₄)₂(OH)₄](H₂O)_˜5.0: insights from a precession electron-diffraction tomography study

Journal of Geosciences, volume 65 (2020), issue 1, 15 - 25

DOI: http://doi.org/10.3190/jgeosci.297

Calcurmolite is a rare supergene U mineral formed during the alteration-hydration weathering of uraninite and hypogene Mo minerals; its structure has remained unsolved owing to a lack of crystal material suitable for conventional structure analysis. Here, single-crystal precession electron-diffraction tomography shows the calcurmolite (Rabejac, France) structure to be modulated; it is triclinic, crystallizing in the super-space group P1(α00)0, with a = 3.938 Å, b = 11.26 Å, c = 14.195 Å, α = 84.4°, β = 112.5°, γ = 133.95° and has a modulation vector q = 0.4 a*. Due to the poor quality of diffraction data, only a kinematical refinement was undertaken, although final results were reasonable: R_obs/R_all = 0.3825/0.3834 for 3953/17442 observed/all reflections. The structure of calcurmolite is based upon the infinite uranyl-molybdate sheets with baumoite topology (U : Mo ratio = 1.5) and an interlayer of 6-coordinated Ca²⁺ cations with interstitial H₂O (ligands are apical uranyl O atoms and molecular H₂O). Adjacent sheets are linked via Ca-O, as well as H-bonds. The structure formula, based on assumed occupancies in the supercell 5a × b × c, is Ca[(UO₂)₃(MoO₄)₂(OH)₄](H₂O)_~5.0 (for Z = 4).