Crystal structure, large distortion of the Zn tetrahedron, and statistical displacement of water molecules in skorpionite

Abstract

The crystal structures of skorpionite from the Skorpion zinc deposit in Namibia [Ca₃Zn₂(PO₄)₂CO₃(OH)₂·H₂O; monoclinic; a = 19.0715(8), b = 9.3321(3), c = 6.5338(3) Å, β = 92.6773(12)°; space group C2/c] and [a = 19.0570(14), b = 9.3346(5), c = 6.5322(4) Å, β = 92.752(2)°; space group Cc] are analyzed using single–crystal X–ray diffraction and refined to yield R values of 0.0253 and 0.0272 for 1576 and 2446 unique reflections with F_o > 4σ(F_o), respectively. Hydrogen atoms in the structure determined by the difference Fourier method. Although two space groups, C2/c and Cc, are possible, the Cc space group without center of symmetry is more likely the structure of skorpionite, which shows that skorpionite is a ferroelectric mineral. The disordered structure is induced in skorpionite by twinning and/or domain structures because of the relaxation of the natural polarization caused by the arrangement of polarized water molecules. The space group Cc model without the center of symmetry eliminates the need for statistical distribution. Bond valence sum calculations and hydrogen bond networks can be explained in detail by the model. In the complicated structure caused by the chemical composition, the local structure with a non–ideal coordination environment is observed near the Zn sites. Hydrogen atoms are continuously arranged with regular arrangements of water molecules in the tunnel structure.