Abstract
Revisiting the structure of uranyl arsenate mineral hügelite provided some corrections to the available structural data. The previous twinning model (by reticular merohedry) in hügelite has been corrected. Twinning of the monoclinic unit cell [a= 7.0189 (7) Å, b= 17.1374 (10) Å, c= 8.1310 (10) Å and β= 108.904 (10)°], which can be expressed as a mirror in [100], leads to a pseudo-orthorhombic unit cell (a= 7.019 Å, b= 17.137 Å, c= 61.539 Å and β= 90.02°), which is eight times larger, with respect to the unit-cell volume, than a real cell. Moreover, the unit cell of chosen here and the unit cell given by the previous structure description both lead to the same supercell. A new structure refinement undertaken on an untwinned crystal of hügelite resulted in R= 4.82% for 12 864 reflections with Iobs > 3σ(I) and GOF= 1.12. The hydrogen-bonding scheme has been proposed for hügelite for the first time.
Highlights
Uranyl arsenates are, along with uranyl phosphates, the most common alteration products after oxidation weathering of uraninite (Finch & Murakami, 1999; Krivovichev & Plasˇil, 2013; Plasˇil, 2014)
Single-crystal X-ray diffraction We studied two tiny crystals of hugelite from the Labskadeposit
There is a clear relationship between the unit cell derived by Locock & Burns (2003) and the supercell found in our study
Summary
Along with uranyl phosphates, the most common alteration products after oxidation weathering of uraninite (Finch & Murakami, 1999; Krivovichev & Plasˇil, 2013; Plasˇil, 2014). The mineral hugelite, Pb2[(UO2)3O2(AsO4)2](H2O), was described originally as a lead–zinc vanadate hydrate (Durrfeld, 1913), but was redetermined later (Walenta & Wimmenauer, 1961) as a lead uranyl arsenate hydrate, structurally related to dumontite, Pb2[(UO2)3O2(PO4)2]Á5H2O (Piret & Piret-Meunier, 1988). Later on, it was investigated again, from the material originating from the type locality, which is the Michael Mine at Weiler, near Lahr in the Black Forest, Baden–Wurttemberg, Germany (Walenta, 1979). We report on the results of our analysis that might help to understand the nature of the twinning in this mineral, as well as helping in future analyses of twinned crystal structures
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