title: Superspace-symmetry determination and multidimensional refinement of the incommensurately modulated structure of natural fresnoite.
abstract: The structure of natural fresnoite, Ba2TiSi2O8, from the sanbornite deposits of eastern Fresno County, California, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(alpha, alpha, 1/2)(-alpha, alpha, 1/2)0gg, cell parameters a=8.5353 (6), c=10.4128 (7) A, modulation vectors q1=0.3020 (3) (a*+b*), q2=0.3020 (3) (-a*+b*). Data collection was performed on an Xcalibur CCD diffractometer at 110 K. The structure was refined from 3452 reflections to final R=0.0123. The model includes modulation of both atomic positions and displacement parameters. As a consequence of the Ba and O positional modulation, eight-, nine- and tenfold Ba coordinations occur throughout the structure. The change of coordination around the Ba atom is clearly represented by the deformation of the pentagonal rings, as seen from a projection along [001]. The deformed pentagonal rings correspond to Ba atoms with eight- and ninefold coordinations and form octagonal clusters closely resembling those observed in the incommensurate structure of melilite-type compounds.
authors.affiliation: Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy. lbindi@geo.unifi.it
authors.forename: Luca
authors.initials: L
authors.lastname: Bindi
authors.forename: Michal
authors.initials: M
authors.lastname: Dusek
authors.forename: Vaclav
authors.initials: V
authors.lastname: Petricek
authors.forename: Paola
authors.initials: P
authors.lastname: Bonazzi