Date Published: June 01, 2020
Publisher: International Union of Crystallography
Author(s): Jakub Plášil, Boris Kiefer, Seyedat Ghazisaeed, Simon Philippo.
The crystal structure of the uranyl-phosphate mineral phurcalite is characterised by extensive hydrogen bonding. It comprises a less common type of H2O bonding in solids: a transformer H2O unit (with a three-coordinated O atom), which is not directly linked to any metal cation. This study documents the advantage of combining XRD data and TORQUE calculations, which are significantly less demanding of resources than DFT calculations..
Uranyl phosphates and arsenates represent a group of environmentally important minerals formed during a hydration–oxidation weathering of primary U minerals, mostly uraninite (Finch & Murakami, 1999 ▸; Krivovichev & Plášil, 2013 ▸; Plášil, 2014 ▸). Generally, due to their low solubility products (see e.g. Ilton et al., 2010 ▸; Astilleros et al., 2013 ▸; Göb et al., 2013 ▸), they can occur both in the vadose zone of the uranium deposits (Murakami et al., 1997 ▸; Finch & Murakami, 1999 ▸; Plášil et al., 2006 ▸, 2009 ▸; Göb et al., 2013 ▸) and in mine dumps, wastes and tailings (Buck et al., 1996 ▸; Roh et al., 2000 ▸; Fuller et al., 2002 ▸; Catalano et al., 2006 ▸; Cantrell et al., 2011 ▸; Maher et al., 2013 ▸). This makes uranyl phosphate and arsenate minerals essential for controlling U mobility in the environment. Nowadays, more than 50 uranyl phosphates and arsenates are known to occur in nature, some of them being discovered in the past decade (Mills et al., 2008 ▸; Plášil et al., 2010 ▸, 2018 ▸; Pekov et al., 2012 ▸).
The structure of the mineral phurcalite (calcium uranyl phosphate heptahydrate) is stabilized by an extensive network of hydrogen bonds. Phurcalite is unique among uranyl phosphates in that it shows a high Ca:U ratio (2:3) (for instance mineral autunite has 1:2) and its structure displays an unusual hydrogen bonding scheme. Structure data obtained from a XRD experiment and theoretical calculations (TORQUE) indicate that the structure of phurcalite contains a rare functional type of H2O group in the interlayer which is not linked to any metal cation directly, as it accepts one hydrogen bond from an adjacent H2O group. This H2O group thus splits the incident bond-strength (represented by one incoming hydrogen bond) into two weaker hydrogen bonds. Therefore it is a transformer H2O group with a three-coordinated O atom. Our study advances our understanding of hydrogen bonding in complex uranyl minerals and shows the synergy of experiment and theory provides new insights into the complex hydrogen bonding in uranyl phosphates and the role of H2O groups in complex oxysalt minerals. In summary, it is likely that the rare hydrogen bonding topology in phurcalite is responsible for its low abundance in nature.