Hidden and apparent twins in uranyl-oxide minerals agrinierite and rameauite: a demonstration of metric and reticular merohedry

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Authors

PLÁŠIL Jakub PETŘÍČEK Václav ŠKODA Radek MEISSER Nicolas KASATKIN Anatoly V.

Year of publication 2021
Type Article in Periodical
Magazine / Source Journal of Applied Crystallography
MU Faculty or unit

Faculty of Science

Citation
Web https://journals.iucr.org/j/issues/2021/06/00/vb5019/
Doi http://dx.doi.org/10.1107/S1600576721009663
Keywords agrinierite; rameauite; twinning; merohedry; Jana2020
Description 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) A, ß = 120.24 (12)° and V = 2414.2 (12) A3 (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 A and V = 2 × 2441 A3, which is F centered. The new structure refinement converged to R = 3.54% for 6545 unique observed reflections with I > 3?(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) A, ß = 118.50 (3)° and V = 2434.3 (11) A3 (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 A and V = 2 × 2434 A3, which is C centered. The new structure refinement of rameauite converged to R = 4.23% for 2344 unique observed reflections with I > 3?(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.
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