Abstract
In this work, the structures of chemically related uranyl-oxide minerals agrinierite and rameauite have been revisited and some corrections to the available structure data are provided. Both structures were found to be twinned. The two minerals are chemically similar, and though their structures differ considerably, their unit-cell metrics are similar. Agrinierite was found to be twinned by metric merohedry (diffraction type I), whereas the structure of rameauite is twinned by reticular merohedry (diffraction type II). The twinning of the monoclinic unit cells (true cells) leads to pseudo-orthorhombic or pseudo-tetragonal supercells in the single-crystal diffraction patterns of both minerals. According to the new data and refinement, agrinierite is monoclinic (space group Cm), with a = 14.069 (3), b = 14.220 (3), c = 13.967 (3) Å, β = 120.24 (12)° and V = 2414.2 (12) Å3 (Z = 2). The twinning can be expressed as a mirror in (101) (apart from the inversion twin), which leads to a supercell with a = 14.121, b = 14.276, c = 24.221 Å and V = 2 × 2441 Å3, which is F centered. The new structure refinement converged to R = 3.54% for 6545 unique observed reflections with I > 3σ(I) and GOF = 1.07. Rameauite is also monoclinic (space group Cc), with a = 13.947 (3), b = 14.300 (3), c = 13.888 (3) Å, β = 118.50 (3)° and V = 2434.3 (11) Å3 (Z = 2). The twinning can be expressed as a mirror in (101) (apart from the inversion twin), which leads to a supercell with a = 14.223, b = 14.300, c = 23.921 Å and V = 2 × 2434 Å3, which is C centered. The new structure refinement of rameauite converged to R = 4.23% for 2344 unique observed reflections with I > 3σ(I) and GOF = 1.48. The current investigation documented how peculiar twinning can be, not only for this group of minerals, and how care must be taken in handling the data biased by twinning.
Highlights
Uranyl-oxide hydroxy-hydrates (UOHs) represent a fascinating group of minerals and synthetic phases, closely connected with the hydration–oxidation of uranium dioxide, UO2+x, as nuclear fuel or uraninite (Finch & Ewing, 1992; Wronkiewicz et al, 1996; Janeczek et al 1996; Plasˇil, 2014, 2018a)
Numerous studies focused on their structures, solubilities and thermodynamic stabilities were undertaken in the 1990s and at the beginning of the millennium (e.g. Casas et al, 1997; Finch & Murakami, 1999; Kubatko et al 2006; Klingensmith et al, 2007; Gorman-Lewis et al, 2008) due to the general importance of UOHs in nuclear waste disposal and the environmental chemistry of uranium
Agrinierite, K2(Ca0.65Sr0.35)[(UO2)3O3(OH)2]2Á5H2O (Cahill & Burns, 2000), has been so far the only naturally occurring UOH to contain an essential amount of Sr, reported by Cesbron et al (1972)
Summary
Uranyl-oxide hydroxy-hydrates (UOHs) represent a fascinating group of minerals and synthetic phases, closely connected with the hydration–oxidation of uranium dioxide, UO2+x, as nuclear fuel or uraninite (Finch & Ewing, 1992; Wronkiewicz et al, 1996; Janeczek et al 1996; Plasˇil, 2014, 2018a). Agrinierite, K2(Ca0.65Sr0.35)[(UO2)3O3(OH)2]2Á5H2O (Cahill & Burns, 2000), has been so far the only naturally occurring UOH to contain an essential amount of Sr, reported by Cesbron et al (1972) This is of interest due to the possible incorporation of 90Sr into the alteration phases of spent nuclear fuel. We report the investigation of twinning in both minerals and provide a correct description of the agrinierite unit cell and improved structure models
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