Extending the mineralogy of U ⁶⁺ (I): Crystal structure of lepersonnite-(Gd) and a description of the new mineral lepersonnite-(Nd)

Abstract Two uranyl carbonate silicate minerals have been studied in detail, lepersonnite-(Gd) and the new mineral lepersonnite-(Nd). Both minerals originate from Katanga province in the Democratic Republic of Congo (Africa): lepersonnite-(Gd) from the Shinkolobwe mine (type locality) and lepersonnite-(Nd) from the Swambo mine (type locality) and Shinkolobwe mine. Each occurs as radial to acicular aggregates composed of long prismatic, thin crystals, as the products of hydration–oxidation weathering of uraninite. A detailed description of the chemical and physical properties of lepersonnite-(Nd) is provided, including the thermal analysis of the -(Gd) member. Moreover, Raman and infrared spectroscopy data are provided for both minerals. The crystal structure of lepersonnite-(Gd) has been solved based on 3D electron diffraction data. According to the best dataset at 95 K, lepersonnite-(Gd) is orthorhombic, with a = 11.838(134) Å, b = 15.822(333) Å, c = 39.147(190) Å and V = 7353(282) Å 3 ( Z = 4). The dynamic refinement of the crystal structure ( R obs / wR obs = 0.1204/0.1088 and R all / wR all = 0.1204/0.1088 for 3090/50507 observed/all reflections and 247 refined parameters) revealed a large complex sheet structure with structural units defining a new lepersonnite topology. It contains infinite sheets of uranyl polyhedra, planar CO 3 groups, protonated Si-tetrahedra, and Gd 3+ polyhedra, and a thick interlayer hosted with H 2 O and partially occupied Ca 2+ sites. Based on the structure refinement, bond-valence considerations, and new electron microprobe data, we infer that the ideal formula of lepersonnite-(Gd) is [Ca 0.5 Gd 0.5 (H 2 O) 18 (OH) 1.5 ] [Gd(UO 2 ) 12 (SiO 3 OH) 2 (CO 3 ) 4 (OH) 10 O 2 (H 2 O) 5 ]. On the basis of their closely related powder X-ray diffraction patterns and the similar behaviour of Nd and Gd, we expect the two minerals to adopt the same structures, apart from Ca, which was absent in lepersonnite-(Nd). The ideal formula for lepersonnite-(Nd) is [Nd(H 2 O) 18 (OH) 2 ][Nd(UO 2 ) 12 (SiO 3 OH) 2 (CO 3 ) 4 (OH) 10 O 2 (H 2 O) 5 ].