Hydrogen disorder in kaatialaite Fe[AsO(OH)]5HO from Jáchymov, Czech Republic: determination from low-temperature 3D electron diffraction.

Kaatialaite mineral Fe[AsO(OH)]5HO 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 = 15.46, = 19.996, = 4.808 Å, β = 91.64° and = 1485.64 Å with space group 2/. 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 HO configurations that have not been observed before in the structures of oxysalts with the presence of unusual inverse transformer HO groups.