A crystal structure refinement of uralolite, Ca2Be4(PO4)3(OH)3∙5H2O, from Weinebene, Austria

Abstract

The crystal structure of uralolite, Ca₂Be₄(PO₄)₃(OH)₃·5H₂O, from the spodumene deposit of Weinebene, Carinthia, Austria, has been refined with X-ray single crystal data gathered on a CCD diffractometer. Uralolite is monoclinic, space group P2₁/n, a = 6.553(1), b = 16.005(3), c = 15.979(3) Å, β = 101.63(1)°, V = 1641.5(5) ų. While previously only isotropic displacement parameters and no hydrogen atom positions were reported for uralolite, now anisotropic displacement parameters were used for non-hydrogen atoms and hydrogen atoms were located and refined yielding R₁ = 0.038 for 3909 observed reflections. Uralolite is built up from corrugated layers [Be₄(PO₄)₃(OH)₃]^4- parallel to (010), which contain Z-shaped groups of four BeO₄ tetrahedra sharing corners via three OH groups and are further crosslinked by PO₄ tetrahedra. Two Ca atoms in Ca(O_phosphate)₅(H₂O)₂ coordination and an interstitial water molecule link these layers along [010]. The OH groups and the Ca-bonded H₂O molecules are all involved in hydrogen bonds with O···O distances of 2.780(2) − 3.063(2) Å and O-H···O angles of 150(1) − 179(1)° excluding a bifurcated bond. The interstitial water molecule displays a distorted tetrahedral environment of O atoms and accepts and donates each two hydrogen bonds. The crystal structure exhibits a C2/c pseudosymmetry for the [Be₄(PO₄)₃(OH)₃]^4- layers and the Ca atoms. However, the disposition of the water molecules and an asymmetric hydrogen bond pattern involving OH groups as well as H₂O molecules are decisive for the lowering of the symmetry of the structure to the true space group P2₁/n.