Kaatialaite mineral Fe[AsO2(OH)2]5H2O from Jáchymov, Czech Republic forms white aggregates of needle-shaped crystals with micrometric size. Its structure at ambient temperature has already been reported but hydrogen atoms could not be identified from single-crystal X-ray diffraction. An analysis using 3D electron diffraction at low temperature brings to light the hydrogen positions and the existence of hydrogen disorder. At 100 K, kaatialaite is described in a monoclinic unit cell of a = 15.46, b = 19.996, c = 4.808 Å, β = 91.64° and V = 1485.64 Å3 with space group P21/n. The hydrogen sites were revealed after refinements both considering the dynamical effects and ignoring them. The possibility to access most of the hydrogen positions, including partially occupied ones among heavy atoms, from the kinematical refinement is due to the recent developments in the analysis of 3D electron data. The hydrogen bonding observed in kaatialaite provides examples of H2O configurations that have not been observed before in the structures of oxysalts with the presence of unusual inverse transformer H2O groups.

• Keywords
• 3D electron diffraction, disorder, ferric arsenate, hydrogen bonds, kaatialaite,
• Publication type
• Journal Article MeSH

The crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O, orthorhombic, a = 17.3785 (9) Å, b = 15.9864 (8) Å, c = 13.5477 (10) Å, V = 3763.8 (4) Å3, space group Pbca, Z = 8 has been refined from single-crystal XRD data to R = 0.042 for 3182 unique [I > 3σ(I)] reflections and the hydrogen-bonding scheme has been refined by theoretical calculations based on the TORQUE method. The phurcalite structure is layered, with uranyl phosphate sheets of the phosphuranylite topology which are linked by extensive hydrogen bonds across the interlayer occupied by Ca2+ cations and H2O groups. In contrast to previous studies the approach here reveals five transformer H2O groups (compared to three expected by a previous study) and two non-transformer H2O groups. One of the transformer H2O groups is, nevertheless, not linked to any metal cation, which is a less frequent type of H2O bonding in solid state compounds and minerals. The structural formula of phurcalite has been therefore redefined as {Ca2(H2[3]O)5(H2[4]O)2}[(UO2)3O2(PO4)2], Z = 8.

• Keywords
• TORQUE method, crystal structure, hydrogen bonding, phurcalite, uranyl phosphate,
• Publication type
• Journal Article MeSH