The structural relation between the new synthetic silicate K2ScFSi4O10 and narsarsukite, Na2(Ti,Fe3+)(O,F)Si4O10

During investigations of the system Sc2O3-Al2O3-TiO2-SiO2, a new, unusual silicate, K2ScFSi4O10, was synthesised as colourless pseudo-cuboctahedral crystals from a KF-MoO3 flux. The crystal structure was determined from single-crystal X-ray diffraction data (Mo-Kalpha radiation, CCD area detector, 2theta(max). = 65degrees, R-int = 1.07%) and refined in space group I4/m (no. 87) to R1 (F) = 1.46% and wR2(all) = 4.15% using 976 'observed' reflections with F-o > 4sigma(F-o). Unit-cell parameters are: a = 11.207(2), c = 8.166(2) Angstrom, V = 1025.6(4) Angstrom(3), Z = 4. The crystal studied is merohedrally twinned with twin plane {110}, and shows pseudosymmetry I4/mmm. The structure contains infinite chains of corner-sharing, nearly regular ScO4F2 octahedra [d(av)(Sc-O) = 2.071 Angstrom, d(av)(Sc-F) = 2.042 Angstrom] connected via shared F atoms along [001]. These octahedral chains share their O atoms with tubular [001] silicate chains built from four-membered silicate rings. The latter are built from one crystallographically unique, distorted[001] silicate chains built from four-membered silicate rings. The latter are built from one crystallographically unique, distorted SiO4 tetrahedron with d(av)(Si-O) = 1.617 Angstrom. The resulting three-dimensional framework hosts voids occupied by a unique, eight-coordinated K atom. K2ScFSi4O10 is isotypic with narsarsukite Na-2(Ti,Fe3+)(O,F)Si4O10, but, although their topologies are basically identical, there exist differences which cause the merohedral twinning and pseudo-holohedry (pseudo-I4/mmm) of K2ScFSi4O10: the presence of the larger K atom causes a considerable rotation of the octahedral units around [001] and a distinct rearrangement of the framework voids, both leading to the presence of a pseudo-mirror plane {110}. In narsarsukite, no such twinning is possible, as demonstrated by a careful reinvestigation of the crystal structure of a narsarsukite crystal from Mont Saint Hilaire, Canada, which confirmed a previous structure determination. The crystal-chemical relations between K2ScFSi4O10 and KTiOPO4-type AScFAsO(4) (A = Rb, Cs) and other compounds and minerals containing trans or cis MO4F2 (M = Cr, Al, Fe) octahedra are pointed out. A structure determination of K2ScFSi4O10 at 120 K revealed no significant changes of the atomic arrangement. The shrinkage was distinctly stronger along [00 1] than that along [ 100].