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

Autor: Plášil J; Institute of Physics ASCR, v.v.i., Na Slovance 2, Prague 8, 18221, Czech Republic., Petříček V; Institute of Physics ASCR, v.v.i., Na Slovance 2, Prague 8, 18221, Czech Republic., Škoda R; Department of Geological Sciences, Masaryk University, Kotlářská 2, Brno, 61137, Czech Republic., Meisser N; Musée cantonal de géologie, Université de Lausanne, Anthropole, Dorigny CH-1015, Switzerland., Kasatkin AV; Fersman Mineralogical Museum of the Russian Academy of Sciences, Leninsky Prospekt 18-2, Moscow 119071, Russian Federation.
Jazyk: angličtina
Zdroj: Journal of applied crystallography [J Appl Crystallogr] 2021 Nov 02; Vol. 54 (Pt 6), pp. 1656-1663. Date of Electronic Publication: 2021 Nov 02 (Print Publication: 2021).
DOI: 10.1107/S1600576721009663
Abstrakt: 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) Å 3 ( 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 Å 3 , 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) Å 3 ( 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 Å 3 , 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.
(© Jakub Plášil et al. 2021.)
Databáze: MEDLINE
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