Batisivite, the first silicate related to the derbylite-hemloite group
Armbruster, Thomas; Kadiyski, Milen; Reznitsky, Leonid Z.; Sklyarov, Evgeny V.; Galuskin, Evgeny V.
European Journal of Mineralogy Volume 20 Number 5 (2008), p. 975 - 981
published: Nov 5, 2008
ArtNo. ESP147052005023, Price: 29.00 €
The crystal structure (space group Pī1, a = 7.5208(4), b = 7.6430(4), c = 9.5724(4) Å, α = 110.204(3), β = 103.338(6), γ = 98.281(7)°, V = 487.14(7) Å3, Z = 1) of the new mineral batisivite, (V, Cr)8Ti6[Ba(Si2O7)]O22, has been determined and refined from single-crystal X-ray data to R = 2.59%. Strong and sharp X-ray reflections define a C-centred monoclinic lattice with a = 4.9878(3), b = 14.1899(16), c = 7.0813(2) Å, β = 103.598(4)°, characteristic for members of the derbylite group. Considering additional weak and slightly diffuse reflections, the pattern could be indexed with a primitive triclinic cell as cited above. There are eight metal (M) sites, with M1-M4 defining octahedral α-PbO2 double chains and M5-M8 defining octahedral columns as in the structure of V3O5. The stacking of both units defines the structure of the derbylite type with channels that may be described as chains of empty cube-octahedra running parallel to  in triclinic setting. In batisivite adjacent cube-octahedra are alternately occupied by Ba and disilicate units, lowering the symmetry to triclinic. However, Ba-disilicate order is not complete. One cubeoctahedron is statistically filled by ca. ¾ Ba and ¼ disilicate, the other is occupied by ca. ¾ disilicate and ¼ Ba. This type of disorder is in qualitative agreement with the observed slight diffusivity of the super-structure reflections. The bridging oxygen of the disilicate group occupies the same site as disordered central Ba.