The crystal structure of uranyl-oxide mineral schoepite, [(UO2)4O (OH)6]( H2O)6, revisited

Abstract

New single-crystal X-ray diffraction experiments have revealed that the crystal structure of schoepite, one of the more common U-oxide minerals, is centrosymmetric, rather than acentric as reported in the past. Schoepite is orthorhombic, space group Pbca, a = 16.7810 (5), b = 14.7044(4), c = 14.2985(5) Å, with V = 3528.22(19) Å 3 and Z = 8. Its structure was solved by charge-flipping algorithm and refined to an agreement index (R) of 4.7 % for 4662 unique reflections collected using microfocus X-ray source. Schoepite structure, in line with its previous determination, is based upon U-O-OH sheets of the fourmarierite topol ogy and an interlayer filled only by molecular H 2 O. The complexity calculations show that the difference in complexity values between schoepite, [(UO 2 ) 4 O(OH) 6 ](H 2 O) 6 , and metaschoepite, [(UO 2 ) 4 O(OH) 6 ](H 2 O) 5 , are much smaller (about 100 bits/cell) then considered previously (about 1000 bits/cell). Such small difference is in line with the easy transformation of schoepite to metaschoepite under ambient conditions and a joint occurrence of both minerals. The online version of this article (doi: 10.3190/jgeosci.252) contains supplementary electronic material. who reported that schoepite is orthorhombic, P2 1 ca, with a = 14.337(3), b = 16.813(5), c = 14.731(4) Å, V = 3551(2) Å 3 , Z = 4; the chemical formula was given as [(UO 2 ) 8 O 2 (OH) 12 ](H 2 O) 12 , with Z = 4. Recently, we have undertaken new studies on the structures of schoepite-related minerals (Plášil et al. 2017; Olds et al. 2018), which revealed several issues, among them the fact that all related studied structures are centrosymmetric. Thus, it prompted our re-examination of the schoepite structure. Experimental SampleThe schoepite crystal used in this study was recovered from a specimen stored in the collections of the Royal Museum for Central Africa in Tervuren (Belgium): n° RGM2708. This specimen originates from the Shinkolobwe mine, the Haut-Katanga Province, Democratic Republic of Congo. The specimen is composed of a massive mixture of uranyl-oxide minerals. On the surface and fissures, there are yellow tabular to prismatic crystals of schoepite (described originally as paraschoepite) (Fig. 1), reddish-orange needle-like curite and bipyramidal soddyite. It should be noted that we found no evidence for paraschoepite on the specimen (cf. Christ and Clark 1960).

Nuclear forensic signatures and structural analysis of uranyl oxalate, its products of thermal decomposition and Fe impurity dopant

“…This was most likely a result of preferred orientation, as reflection angles were still in agreement despite the difference in intensities. It was considered unusual that all oxalates exhibited a similar XRD pattern, as in similar mechanisms, dehydration can affect the crystal structure such that a distinguishable XRD pattern is obtained [36][37][38]. This result supports the argument that UO 2 C 2 O 4 is hygroscopic, absorbing H 2 O with brief exposure to air prior to XRD analysis and reforming the trihydrate oxalate.…”

Crystallographic Insights into Uranyl Sulfate Minerals Formation: Synthesis and Crystal Structures of Three Novel Cesium Uranyl Sulfates

“…An analog of the hydroxy-hydrated uranyl oxide mineral schoepite ([(UO2)8O2(OH)12](H2O)12 [6,7]) was synthesized according to the procedure discussed in [9], and its purity was checked using powder XRD; H2SO4 (Sigma-Aldrich, 98%) and Cs2SO4 (Vekton, Russia, 99%) were used as received. The 0.2 g of synthetic schoepite (0.03 mmol), 0.12 g of cesium sulfate (0.33 mmol), and 0.01 (0.19 mmol) ml of sulfuric acid were dissolved in 10 ml of deionized water.…”

“…In this study, we presented the results of uranyl sulfate synthesis experiments that might elucidate some of the formation behavior of natural uranyl sulfates. Herein, we reported on the alteration experiment of the synthetic analog of uranyl-oxide hydroxy-hydrate mineral schoepite, [(UO2)8O2(OH)12](H2O)12 [6,7]. As the result, four different crystalline phases Cs[(UO2)(SO4)(OH)](H2O)0.25 (1), Cs3[(UO2)4(SO4)2O3(OH)](H2O)3 (2) [8], Cs6[(UO2)2(SO4)5](H2O)3 (3), and Cs2[(UO2)(SO4)2] (4) were obtained, including three novel compounds.…”

3D Electron Diffraction as a Powerful Tool to Study the Earliest Nanocrystalline Weathering Products: A Case Study of Uraninite Weathering

“…Unambiguously, its uranyl-oxide layer (structural units) belongs to the so-called fourmarierite topology (Figures and S4). − This phase is related to the mineral schoepite, [(UO 2 ) 4 O­(OH) 6 ]­(H 2 O) 6 , one of the common early-alteration phases of natural pitchblende. , Kroupaite is a mineral with the fourmarierite topology of UOH layers, structurally related to schoepite and the broader family of schoepite-related minerals.…”