were investigated by single-crystal X-ray diffraction and were refined to the R 1 values of 0.0212, 0.0282, and 0.0270 using 2800, 588, and 1128 unique reflections with F o >4σ(F o ), respectively. The chemical formula of legrandite is similar to that of adamite and paradamite, except for the presence of water molecules. In the structure of legrandite, the hydrogen atoms are distributed among the two hydroxyl and the two water molecule positions. On the basis of bond valence calculations, the hydrogen bonding in legrandite can be classified into three types: (1) one acceptor with linear normal hydrogen bonding (Type-A), (2) two acceptors with linear hydrogen bonding and one excess weak hydrogen bonding (Type-B), and (3) several acceptors with one linear hydrogen bondings and several weak hydrogen bondings by weak electrostatic interactions (Type-C). The variety of hydrogen bonding interactions provides structural stability to legrandite. The Zn3-O1 bond shows a remarkable distance of 2.341(2) Å, which is ascribed to the three-dimensional periodicity of the complex mineral structure. The local structures of adamite and paradamite violate a fundamental crystallographic law with respect to the cation coordination number and unit cell volume. The crystal structures of legrandite and paradamite are characterized by proton transfer tunnels running along the crystal axes.

The crystal structures of skorpionite from the Skorpion zinc deposit in Namibia [Ca 3 Zn 2 (PO 4) 2 CO 3 (OH) 2 •H 2 O; monoclinic; a = 19.0715(8), b = 9.3321(3), c = 6.5338(3) Å, β = 92.6773(12)°; space group C 2/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, C 2/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.