Zeitschrift für Kristallographie
International journal for structural, physical, and chemical aspects of crystalline materials
ISSN 0044-2968	www.zkristallogr.de
Zeitschrift für Kristallographie, 2001, Volume 216, Issue 08, p. 442 Inorganic Crystal Structures
Giovanni Ferraris, A. Gula, G. Ivaldi, Massimo Nespolo, G. Raade
Crystal structure of kristiansenite: a case of class IIB twinning by metric merohedry
The structure of the new disilicate kristiansenite, Ca2ScSn(Si2O7)(Si2O6OH), has been solved and refined from a crystal polysynthetically twinned by metric merohe-dry. The Bravais lattice is mC, with parameters a = 10.028(1), b = 8.408(1), c = 13.339(2) Å, α = 90.01(1), β = 109.10(1), γ = 90.00(1)°, but the space-group type is C1 (Z = 4). The twin law is m´, and the two components of the twin have nearly identical volumes: as a consequence, the Laue group of the twin is practically 2/m. By taking into account the twinning, an anisotropic refinement of the structure in C1 converged to R1 = 0.0242 for 259 refined parameters and 4862 observed reflections. The effects of the twinning by metric merohedry and of the volume ratio of the components on the symmetry of the diffraction pat-tern are discussed. The triclinic structure approximates within about 0.1 Å the monoclinic symmetry, the lower symmetry resulting mainly from cation ordering. Kristian-senite represents a new type of silicate structure and the first known case with the presence of protonated and nor-mal disilicate groups at the same time. The disilicate groups and the other polyhedra centred on cations lie on different alternating (101) planes.
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