Batisivite, the first silicate related to the derbylite-hemloite group
Batisivite, the first silicate related to the derbylite-hemloite group (in From field observation to experimental petrology and back; a special issue to honour Werner Schreyer, Walter Maresch (editor), Edward Grew (editor) and Friedrich Seifert (editor))
European Journal of Mineralogy (October 2008) 20 (5): 975-981
The crystal structure (space group P (super -) 1, a = 7.5208(4), b = 7.6430(4), c = 9.5724(4) Aa, alpha = 110.204(3), beta = 103.338(6), gamma = 98.281(7) degrees , V = 487.14(7) Aa (super 3) , Z = 1) of the new mineral batisivite, (V, Cr) (sub 8) Ti (sub 6) [Ba(Si (sub 2) O (sub 7) )]O (sub 22) , has been determined and refined from single-crystal X-ray data to R =2.59%. Strong and sharp X-ray reflections define a C-centered monoclinic lattice with a = 4.9878(3), b =14.1899(16), c = 7.0813(2) Aa, beta = 103.598(4) degrees , 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 alpha -PbO (sub 2) double chains and M5-M8 defining octahedral columns as in the structure of V (sub 3) O (sub 5) . 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 cube-octahedron is statistically filled by ca. 3/4 Ba and 1/4 disilicate, the other is occupied by ca. 3/4 disilicate and 1/4 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.