THE CRYSTAL STRUCTURE OF CAVANSITE: LOCATION OF THE H2O MOLECULES AND HYDROGEN ATOMS IN Ca(VO)(Si4O10)•4H2O

Cavansite Ca(VO)(Si4O10)center dot 4H(2)O, is a novel layer silicate, dimorphous with pentagonite. The earlier determination of its structure did not yield the location of the hydrogen atoms or clear resolution of the oxygen atoms of the H2O molecules. In this study, structures are reported from data collected at 100(2) and 296(2) K, and they clearly resolve and allow refinement of the position of the H atoms. The 100 K and 296 K structures were refined to R1 values of 0.0141 and 0.0389, respectively. Cavansite is orthorhombic with space-group symmetry Pcmn, with a 9.7518(4), b 13.6854(6), and c 9.5965(4) angstrom at 100 K, and a 9.7614(7), b 13.6666(10), and c 9.6049(7) angstrom at 296 K. Cavansite consists of two layers, a layer of silicate tetrahedra and a layer formed of irregular CaO4(OH2)(4) polyhedra and VO5 square-based pyramids that alternate in ribbons parallel to [100] in (010); the layers join laterally at the four corners of the base of the V pyramid. The silicate layer consists of four- and eight-membered rings of silicate tetrahedra with apices pointing along [010] or [0 (1) over bar0]. The four oxygen atoms in the Ca polyhedron that are not linked to the V square-based pyramids are part of H2O molecules. Adjacent ribbons in (010) are linked by hydrogen bonding.